Flex PCB

The key points of FPC assembly, and details need to know about the Flexible PCBA

FPC assembly, Flexible PCBA-Bright PCB China FPC assembly factory

 

 

 

 

 

 

Abstract: In the trend of miniaturization of consumer electronic products, FPC is more and more widely used.

The FPC SMT process is different from the Rigid PCB. This post describes details of the FPC SMT preprocessing, Printing, SMT,
reflow, testing, sub-board processes and other key points. Bright PCB Technology is a China FPC assembly factory,
manufacturing Flexible PCBA for years, owned high reputation in this industry.

PCB is printed circuit board, referred to as Rigid board. FPC is flexible circuit board, also known as flexible circuit board or
flexible circuit, referred to as Flex PCB. The miniaturization of electronic products is an inevitable trend, a considerable
number of consumer products, surface mount, due to the relationship between the assembly space, the SMD are
mounted on the FPC to complete the assembly of the machine, FPC in the calculator, mobile phones, digital cameras ,
Digital cameras and other digital products have been widely used in the FPC on SMD surface mount SMT technology has
become one of the trends.
FPC surface SMT process’ solution is differ from the traditional Rigid PCB SMT in many details, in order to do FPC SMT
technology, the most important thing is positioning. FPC board’s hardness is not enough, softer, if you do not use a
dedicated carrier board, you can not complete the fixed and transmission, it can not complete printing, SMT, furnace and
other basic SMT processes. Following FPC SMT production on the FPC pretreatment, fixation, printing, SMT, reflow,
testing, testing, sub-board processes, such as key points were detailed.
1. FPC pretreatment
FPC board is soft, the factory is not generally vacuum packaging, in the transportation and storage process is easy to
absorb moisture in the air, to be cast in the SMT line before the pre-baking treatment, the water slowly forced discharge.
Otherwise, the reflow soldering under high temperature impact, FPC absorption of water vaporization quickly become
prominent FPC, easily lead to FPC delamination, blistering and other undesirable.

Pre-baking conditions are generally 80-100 ℃ temperature 4-8 hours, under special circumstances, you can increase the
temperature to 125 ℃ or more, but the corresponding shortening baking time. Before baking, be sure to make a small
sample test to determine if the FPC can withstand the set baking temperature, or consult the FPC manufacturer for
appropriate baking conditions. Baking, FPC stacking can not be too much, 10-20PNL more appropriate, and some FPC
manufacturers will put a piece of paper between each PNL to be isolated, to be sure that this separation of the paper can
withstand the set baking Temperature, if you do not need to take out after the separation of paper, and then baking. After
baking the FPC should be no obvious discoloration, deformation, from Alice and other undesirable, subject to the IPQC
sampling qualified to vote.

2. Special carrier board production
According to the CAD file of the circuit board, read the hole positioning data of the FPC to manufacture the high precision
FPC positioning template and the special carrier plate, make the diameter of the positioning pin on the positioning
template and the positioning hole on the carrier plate, the hole diameter of the positioning hole on the FPC match. Many
FPC because to protect part of the line or the design of the reasons is not the same thickness, and some places thick and
some places thin, and some have to strengthen the metal plate, so the board and the FPC at the junction of the need to
press The actual situation for processing grinding groove, the role is in the printing and placement to ensure that the FPC
is flat. Material requirements of thin plate, high strength, less heat absorption, heat quickly, and after several thermal
shock after warping small. Commonly used carrier plate materials are synthetic stone, aluminum, silica gel plate, special
high temperature resistant magnetized steel plate.

Ordinary carrier board: common carrier board design is convenient, proofing fast. Commonly used ordinary carrier
material for the engineering plastics (synthetic stone), aluminum, etc., engineering plastic carrier life of 3000-7000 times,
easy to operate, good stability, not easy to absorb heat, not hot, the price is 5 times the aluminum the above. Aluminum
carrier board heat absorption fast, no temperature difference between inside and outside, deformation can be simple
repair, cheap, long life, the main drawback is hot, to use insulated gloves to send.

Silicone sheet: The material has a self-adhesive, FPC directly on the stick, without tape, but also easier to remove, no
residual glue, and high temperature. Silicone sheet in use, the use of chemical process, the use of silica gel material in the
process of aging will decline in viscosity, viscosity during use will be less clean, life expectancy is short, up to 1000-2000
times, the price is relatively high.

Magnetic fixture; special high temperature (350 ℃) to strengthen the magnetization of steel processing to ensure that the
process of reflow soldering, good elasticity, good flatness, high temperature deformation. Because the strengthening of
the magnetic properties of the steel sheet has been pressed flattened FPC surface, FPC reflow soldering in the reflow
soldering to avoid the bad welding, to ensure the welding quality and improve yield. As long as not man-made damage
and accident damage can be permanently used, long life. Magnetic fixture at the same time on the FPC for thermal
protection, the board will not have any damage to the FPC. But the magnetic fixture design complexity, high unit price,
mass production only a cost advantage.

3. production process
Here we take the common carrier board as an example to describe the FPC SMT points, the use of silica gel plate or
magnetic fixture, FPC to facilitate a lot of fixing, without the use of tape, and printing, patch, welding process is the key
points the same.

1). Fixation of FPC:
Prior to SMT, you first need to accurately FPC fixed on the carrier board. In particular, the shorter the storage time
between printing, mounting and soldering, the better the FPC is fixed to the carrier board.

Carrier plates are available with locating pins and without locating pins. Without positioning pins of the carrier plate, with
the positioning pin with the positioning template supporting the use of the first set of carrier plate in the template of the
positioning pin so that the positioning pin through the positioning hole on the carrier plate exposed FPC will be a set in the
Exposed to the positioning pin, and then tape fixed, and then let the carrier board and FPC positioning template
separation, printing, patch and welding. With the positioning pin on the carrier plate has been fixed about 1.5mm spring
pin number, you can FPC one piece directly on the carrier plate spring locating pin, and then tape fixed. In the printing
process, the spring pin can be completely pressed into the stencil board, will not affect the printing results.

Method 1(single-sided tape fixed): with a thin high-temperature single-sided tape will be fixed on the FPC board on the
four sides, not to offset and Qiao FPC, tape viscosity should be moderate, easy to peel after reflow, and FPC No residual
glue. If you use automatic tape machine, can quickly cut the length of the same tape, can significantly improve efficiency,
cost savings, to avoid waste.

Method 2 (double-sided adhesive tape): first with high-temperature double-sided tape attached to the carrier board, the
effect and silica gel plate, and then paste the FPC to the carrier plate, pay special attention to tape viscosity can not be too
high, or reflow after stripping , It is likely to cause FPC tear. After repeated oven over and over, the viscosity of double-
sided tape will gradually become low, low viscosity can not be reliable fixed FPC must be replaced immediately.

This station is to prevent FPC dirty focus of the station, need to wear finger sets of operations. The carrier board should
be cleaned properly before being reused. It can be cleaned with a non-woven cloth dipped in a cleaning agent, or an anti-
static dust roller can be used to remove dust and tin beads. Take FPC should not be too Rigid, FPC is more fragile, prone to
crease and fracture.
2). FPC solder paste printing:
FPC on the composition of the solder paste is not very special requirements, the size of the solder ball particles and metal
content to FPC, there is no fine pitch IC shall prevail, but the FPC on the solder paste printing performance requirements
are higher, solder paste should have excellent Thixotropy, solder paste should be able to easily release the mold and can
be firmly attached to the FPC surface, will not appear mold release obstruction bad stencil or printing after the collapse.

Because the loading board FPC, FPC positioning with the high temperature tape, so that the plane is inconsistent, so FPC
printing surface can not be as flat as the PCB and the thickness of the Rigidness of the same, it is not appropriate to use
metal scraper, but should be used in Rigidness 80 -90 degrees of polyurethane-type scraper.

Paste printing machine with the best optical positioning system, otherwise there will be a greater impact on the print
quality, FPC although fixed on the carrier board, but between the FPC and the carrier board will always produce some
small gap, which is Rigid with the PCB The biggest difference between the board, so the device parameters set the printing
effect will have a greater impact.

Printing station is to prevent the FPC dirty focus of the station, you need to wear finger sets of operations, while
maintaining the cleanliness of the workplace, ground rubbing steel mesh to prevent solder paste pollution FPC gold finger
and gold-plated buttons

3). FPC patch
According to the characteristics of the product, the number of components and chip efficiency, the use of medium-speed
placement machine, high-speed placement machine can be mounted. Because each piece of FPC has the positioning of the
optical MARK mark, so the SMD mount on the FPC and PCB placement is not very different. Note that, although the FPC is
fixed on the carrier board, but its surface can not be as flat as the PCB Rigid board, FPC and the board will certainly exist
between the local gap, so the nozzle drop height, blowing pressure Need to be set accurately, the nozzle moving speed to
be reduced. At the same time, FPC to the majority of the board, FPC yield is relatively low, so the whole PNL contains
some bad PCS is normal, which requires placement machine with BAD MARK recognition, otherwise, in the production of
such non-whole PNL is a good board case, the production efficiency will be greatly reduced.

4). Reflow soldering of FPC
The use of forced convection hot air convection oven, so that the temperature on the FPC can be more uniform changes
in the production of welding to reduce the bad. If the use of single-sided tape, because only fixed FPC’s four sides, the
middle part of the deformation in the hot air state, the pad is easy to form tilt, melting tin (high temperature liquid tin) will
flow and empty welding, Tin beads, so that a higher rate of non-performing process.

A) Temperature curve test method:
Due to the different heat absorption of the carrier plate and the different types of components on the FPC, the
temperature rise rate during the reflow process is different and the heat absorption is different. Therefore, the
temperature profile of the reflow oven is carefully set, Great influence. The more reliable method is based on the actual
production of the carrier board spacing in the test board before and after the release of the two FPC equipped with the
carrier board, while the test board FPC mounted components, high temperature solder wire test temperature The probe
is welded to the test point, and the probe wire is fastened to the carrier plate with a high temperature tape. Note that high
temperature tape can not cover the test points. Test points should be selected in the vicinity of the solder pad on the side
of the carrier board and QFP pins, etc., such test results better reflect the real situation.
B) The temperature curve settings:
In the furnace temperature debugging, because the FPC temperature is not good, so it is best to use temperature /
temperature / return curve of the temperature curve, so that the parameters of the temperature zone is easy to control
some other FPC and components of the impact of thermal shock should be small some. According to experience, it is best
to adjust the furnace temperature to the lower limit of the technical requirements of solder paste, back to the furnace of
the wind speed are generally used by the furnace can use the minimum wind speed, back to the furnace chain stability is
better, can not shake.

5). FPC inspection, testing and sub-board:
As the carrier plate in the furnace endothermic, especially the aluminum carrier plate, baked when the temperature is
higher, it is best to increase the forced cooling fan in the mouth to help rapid cooling. At the same time, the operator must
take insulated gloves, so as to avoid burned by high temperature carrier board. FPC from the carrier board to complete
the welding, the force should be uniform, can not use brute force, so as to avoid the FPC is torn or crease.
Remove the FPC on the SMT magnifying glass over 5 times under the visual inspection, focusing on checking the surface
of residual plastic, discoloration, gold finger dip tin, tin beads, IC pins with soldering and other welding problems.
Because the FPC surface can not be very flat, so that the false positive rate of AOI is high, so FPC is generally not suitable
for AOI inspection, but by using a dedicated test fixture, FPC can complete ICT, FCT test.
FPC to the majority of the board, may be ICT ICT, FCT test before the need to do sub-board, although the use of blades,
scissors and other tools can also be completed sub-board operations, but operating efficiency and low quality, high scrap.
If it is shaped FPC mass production, it is recommended to create a special FPC stamping sub-module, the stamping
division, can greatly improve the efficiency of work, while punching out the edge of the FPC neat appearance, punching
board generated when the internal stress is low, Can effectively avoid solder joints crack.
4. summary
SMD on the FPC, the precise positioning and fixation is the key points, the key of fixing is to produce the appropriate
carrier board. Then the FPC pre-baking, printing, patch and reflow. Apparently the FPC SMT process is much more difficult
than PCB Rigid board, so the precise setting of process parameters is necessary, at the same time, strict production
process management is also important to ensure that operators strictly enforce the SOP on each of the provisions, with
the line Engineers and IPQC should strengthen the inspection, the timely detection of abnormal production lines, analyze
the reasons and take the necessary measures to the FPC SMT production line to control the quality of FPC assembly.

Bright PCB Technology is a China FPC assembly manufacturer, offers FPCBA prototy and mass producing of Flexible PCBA,
if need FPC or Flexible PCB assembly please Contact US!

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with signature and stamp.

tmliir Oct 16, 2016 FPC, PCBA&SMT&PCB Assembly 0 Comment Read More

FPC Prototype, Flex PCB Prototype, Flexible Circuit Prototyping

We Bright PCB Technology has years of producing FPC, with our experienced guys serves hunders of clients who need Flexible Circuit Board. Our products including the Single layer FPC, double sided FPC, and multilayer FPC, moreover, many clients need the FPC assembly, that is most of the SMT factory not familiar with. Fortunately this is our advantage, 80% of our FPC clients chose us to finish the FPC assembly, because we take care of every single detail about the processes of FPCBA.

Single layer FPC is most using in the lighting industry and display industry as a light provider device’s part. We take very care about the feature of these industries, the stableness, long life span, proper conductivity are the main features of the FPC used in lighting and display. A pic of our product below:

bright-pcb-fpc-black-solder-mask-white-silk-enig

 

FPC prototyping means fast producing, quick test and immediately delivery.

With our professional equipment, we can efficiently do the drilling, PTH, lines, etching, etc. And applied the flying probe machine, fast test is available. FPC prototype in 3 days is abusolutely ok for us, and this service helped our client faster their product exploring.

And self-owned lines of surface treatment helps us to control the quality of OSP, ENIG on the surface of Flexible Circuit Board.

If need FPC prototype including Single layer FPC, Double sided FPC or Multilayer FPC, pls contac us immediatly! Also FPC assembly is available. Know more about the FPC assembly pls view this post: The differences need to know between the FPC assembly and Rigid PCB assembly

Copyright Notice: All images and text on this site are protected by copyright. Can be used ONLY by written authorization with signature and stamp.

 

tmliir Jun 19, 2014 FPC, PCB 0 Comment Read More

What is Flexible Printed Circuit (FPC)-Brief Introduction

Brief Introduction of Flexible Printed Circuit (FPC)

As we all know that FPC is widely used in the mobile phones. It can connect the LCD and Mainboard, Side Key with Mainboard, which a normal rigid PCB can not achieve.
FPC, full name is Flexible Printed Circuit), is made of polyimide or mylar, an insulation material.
Flexible Printed Circuit (FPC) can be folded, bended freely. Besides, FPC is with very light weight, small volume, well heat dispersion, easy installation.
Flexible Printed Circuit (FPC), with it’s advantage of helping to make the electronic products with high density, now it’s widlely used in the area of aerospace, military, mobile communication, laptop, PDA, computer, camera, etc. Meanwhile Flexible Printed Circuit (FPC) also has some disadvantages, like not that durable, harder design and engineering, low possiblity of fix up, harder inpection, higher cost, etc. But these are not big problems compared with the advantages of Flexible Printed Circuit (FPC) can bring to us. That’s why it becomes more and more important and more often used in the electrics and telecommunication industries.

Structure of Flexible Printed Circuit (FPC)

Flexible Printed Circuit (FPC) can be single layer and multilayer.
Usually when we refer to the single layer FPC, it has only 1 layer of copper. But actually it has 5 layers (including adhesive, while the stiffiner not include), and normal double sided FPC has 1 inner layer (base film) with 1 layer of copper opn each side. But actually it has 9 layers (including adhesive, while the stiffiner not include).

–Posted by Bright PCB Technology Co., Ltd

tmliir Aug 17, 2013 FPC, PCB 0 Comment Read More

PCB History

PCB History

Development of the methods used in modern printed circuit boards started early in the 20th century. In 1903, a German inventor, Albert Hanson, described flat foil conductors laminated to an insulating board, in multiple layers. Thomas Edison experimented with chemical methods of plating conductors onto linen paper in 1904. Arthur Berry in 1913 patented a print-and-etch method in Britain, and in the United States Max Schoop obtained a patent to flame-spray metal onto a board through a patterned mask. Charles Durcase in 1927 patented a method of electroplating circuit patterns.

The Austrian engineer Paul Eisler invented the printed circuit as part of a radio set while working in England around 1936. Around 1943 the USA began to use the technology on a large scale to make proximity fuses for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army. At around the same time in Britain work along similar lines was carried out by Geoffrey Dummer, then at the RRDE.

Before printed circuits (and for a while after their invention), point-to-point construction was used. For prototypes, or small production runs, wire wrap or turret board can be more efficient. Predating the printed circuit invention, and similar in spirit, was John Sargrove’s 1936–1947 Electronic Circuit Making Equipment (ECME) which sprayed metal onto a Bakelite plastic board. The ECME could produce 3 radios per minute.

During World War II, the development of the anti-aircraft proximity fuse required an electronic circuit that could withstand being fired from a gun, and could be produced in quantity. The Centralab Division of Globe Union submitted a proposal which met the requirements: a ceramic plate would be screenprinted with metallic paint for conductors and carbon material for resistors, with ceramic disc capacitors and subminiature vacuum tubes soldered in place. The technique proved viable, and the resulting patent on the process, which was classified by the U.S. Army, was assigned to Globe Union. It was not until 1984 that the Institute of Electrical and Electronics Engineers (IEEE) awarded Mr. Harry W. Rubinstein, the former head of Globe Union’s Centralab Division, its coveted Cledo Brunetti Award for early key contributions to the development of printed components and conductors on a common insulating substrate. As well, Mr. Rubinstein was honored in 1984 by his alma mater, the University of Wisconsin-Madison, for his innovations in the technology of printed electronic circuits and the fabrication of capacitors.

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components’ leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. The patent they obtained in 1956 was assigned to the U.S. Army. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes is expensive and the protruding wires are merely cut off.

From the 1980s small surface mount parts have been used increasingly instead of through-hole components; this has led to smaller boards for a given functionality and lower production costs, but with some additional difficulty in servicing faulty boards.

Historically many measurements related to PCB design were specified in multiples of a thousandth of an inch, often called “mils”. For example, DIP and most other through-hole components have pins located on a grid spacing of 100 mils, in order to be breadboard-friendly. Surface-mount SOIC components have a pin pitch of 50 mils. SOP components have a pin pitch of 25 mils. Level B technology recommends a minimum trace width of 8 mils, which allows “double-track” – two traces between DIP pins.

tmliir Jun 25, 2012 PCB 0 Comment Read More

PCB manufacturing

PCB manufacturing consists of many steps.
PCB computer aided manufacturing

Manufacturers never use the Gerber or Excellon files directly on their equipment, but always read them into their computer aided manufacturing (CAM) system. PCBs cannot be manufactured professionally without a CAM system, which performs the following functions:

Input of the Gerber data
Verify the data; optionally DFM
Compensate for deviations in the manufacturing processes (e.g. scaling to compensate for distortions during lamination)
Panelize
Output of the digital tools (layer images, drill files, AOI data, electrical test files,.)

Panelization

Panelization is a procedure used to handle PCBs which would otherwise be too small to process. A number of identical circuits are printed onto a larger board (the panel) which can then be handled in the normal way. In some cases we may have multiple different designs of PCB in a panel, for example a product has three different boards and all of these boards are repeated 10 times in a panel, so we have total of thirty boards (three different types) in the panel. The panel is broken apart into individual PCBs when all other processing is complete. Separating the individual PCBs is frequently aided by drilling or routing perforations along the boundaries of the individual circuits, much like a sheet of postage stamps. Another method, which takes less space, is to cut V-shaped grooves across the full dimension of the panel. The individual PCBs can then be broken apart along this line of weakness.

The process of removing individual PCBs from a larger board is called Depaneling. While drilled/routed perforations and grooves were common for a number of years, today this is often done by lasers, which cut the board with no contact. This reduces the stresses on the fragile circuits caused by torque. This method is often completely automated with full boards entering the laser depaneling machine via conveyor, being cut into individual pieces by laser, and leaving the system via conveyor, and sometimes stacked, on the other side.

Copper patterning

The pattern in the manufacturer’s CAM system is usually output on a photomask (photo-tool, film) by a photoplotter and replicated via silk screen printing or by exposing on a photo-sensitive photoresist coating. Direct imaging techniques are sometimes used for high-resolution requirements.

Subtractive, additive and semi-additive processes

Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern:

1. Silk screen printing uses etch-resistant inks to protect the copper foil. Subsequent etching removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank (non-conductive) board. The latter technique is also used in the manufacture of hybrid circuits.
2. Photoengraving uses a photomask and developer to selectively remove a photoresist coating. The remaining photoresist protects the copper foil. Subsequent etching removes the unwanted copper.
3. PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a ‘PCB Prototyper’) operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format.

In additive methods the pattern is electroplated onto a bare substrate using a complex process. The advantage of the additive method is that less material is needed and less waste is produced. In the full additive process the bare laminate is covered with a photosensitive film which is imaged (exposed to light through a mask and then developed which removes the unexposed film). The exposed areas are sensitized in a chemical bath, usually containing palladium and similar to that used for through hole plating which makes the exposed area capable of bonding metal ions. The laminate is then plated with copper in the sensitized areas. When the mask is stripped, the PCB is finished.

Semi-additive is the most common process: The unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed bare original copper laminate from the board, isolating the individual traces. Some single-sided boards which have plated-through holes are made in this way. General Electric made consumer radio sets in the late 1960s using additive boards.

The (semi-)additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes to produce conductive vias in the circuit board.

Patterning method by volume

The method chosen depends on the number of boards to be produced.
Large volume

Silk screen printing–the main commercial method.
Photographic methods–used when fine linewidths are required.

Small volume

Print onto transparent film and use as photomask along with photo-sensitized boards. (i.e., pre-sensitized boards), then etch. (Alternatively, use a film photoplotter).
Laser resist ablation: Spray black paint onto copper clad laminate, place into CNC laser plotter. The laser raster-scans the PCB and ablates (vaporizes) the paint where no resist is wanted. Etch. (Note: laser copper ablation is rarely used and is considered experimental.[clarification needed])
Use a CNC-mill with a spade-shaped (i.e., a flat-ended cone) cutter or miniature end-mill to rout away the undesired copper, leaving only the traces.

Hobbyist

Laser-printed resist: Laser-print onto transparency film, heat-transfer with an iron or modified laminator onto bare laminate, touch up with a marker, then etch.
Vinyl film and resist, non-washable marker, some other methods. Labor-intensive, only suitable for single boards

Chemical etching

Chemical etching is usually done with ammonium persulfate or ferric chloride. For PTH (plated-through holes), additional steps of electroless deposition are done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.

The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates.

As more copper is consumed from the boards, the etchant becomes saturated and less effective; different etchants have different capacities for copper, with some as high as 150 grams of copper per litre of solution. In commercial use, etchants can be regenerated to restore their activity, and the dissolved copper recovered and sold. Small-scale etching requires attention to disposal of used etchant, which is corrosive and toxic due to its metal content.

The etchant removes copper on all surfaces exposed by the resist. “Undercut” occurs when etchant attacks the thin edge of copper under the resist; this can reduce conductor widths and cause open-circuits. Careful control of etch time is required to prevent undercut. Where metallic plating is used as a resist, it can “overhang” which can cause short-circuits between adjacent traces when closely spaced. Overhang can be removed by wire-brushing the board after etching.
Inner layer automated optical inspection (AOI)

The inner layers are given a complete machine inspection before lamination because afterwards mistakes cannot be corrected. The automatic optical inspection system scans the board and compares it with the digital image generated from the original design data.

Lamination

Multi-layer printed circuit boards have trace layers inside the board. One way to make a 4-layer PCB is to use a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom prepreg and copper foil. Lamination is done by placing the stack of materials in a press and applying pressure and heat for a period of time. This results in an inseparable one piece product. It is then drilled, plated, and etched again to get traces on top and bottom layers. Finally the PCB is covered with solder mask, marking legend, and a surface finish may be applied. Multi-layer PCBs allow for much higher component density.

Drilling

Holes through a PCB are typically drilled with small-diameter drill bits made of solid coated tungsten carbide. Coated tungsten carbide is recommended since many board materials are very abrasive and drilling must be high RPM and high feed to be cost effective. Drill bits must also remain sharp so as not to mar or tear the traces. Drilling with high-speed-steel is simply not feasible since the drill bits will dull quickly and thus tear the copper and ruin the boards. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or “Excellon files”. The drill file describes the location and size of each drilled hole. These holes are often filled with annular rings (hollow rivets) to create vias. Vias allow the electrical and thermal connection of conductors on opposite sides of the PCB.

When very small vias are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, the vias may be laser drilled — evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.

It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers and no outer layers.

The hole walls for boards with 2 or more layers can be made conductive and then electroplated with copper to form plated-through holes. These holes electrically connect the conducting layers of the PCB. For multilayer boards, those with 3 layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper.
Plating and coating

PCBs are plated with solder, tin, or gold over nickel as a resist for etching away the unneeded underlying copper.

After PCBs are etched and then rinsed with water, the solder mask is applied, and then any exposed copper is coated with solder, nickel/gold, or some other anti-corrosion coating.

Matte solder is usually fused to provide a better bonding surface or stripped to bare copper. Treatments, such as benzimidazolethiol, prevent surface oxidation of bare copper. The places to which components will be mounted are typically plated, because untreated bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was coated with solder by hot air solder levelling (HASL). The HASL finish prevents oxidation from the underlying copper, thereby guaranteeing a solderable surface. This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU and US, which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin, 0.7% copper, 0.05% nickel, and a nominal of 60ppm germanium.

It is important to use solder compatible with both the PCB and the parts used. An example is Ball Grid Array (BGA) using tin-lead solder balls for connections losing their balls on bare copper traces or using lead-free solder paste.

Other platings used are OSP (organic surface protectant), immersion silver (IAg), immersion tin, electroless nickel with immersion gold coating (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG) and direct gold plating (over nickel). Edge connectors, placed along one edge of some boards, are often nickel plated then gold plated. Another coating consideration is rapid diffusion of coating metal into Tin solder. Tin forms intermetallics such as Cu5Sn6 and Ag3Cu that dissolve into the Tin liquidus or solidus(@50C), stripping surface coating or leaving voids.

Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias. Silver, zinc, and aluminum are known to grow whiskers under the influence of an electric field. Silver also grows conducting surface paths in the presence of halide and other ions, making it a poor choice for electronics use. Tin will grow “whiskers” due to tension in the plated surface. Tin-Lead or Solder plating also grows whiskers, only reduced by the percentage Tin replaced. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Another coating issue is tin pest, the transformation of tin to a powdery allotrope at low temperature.
Solder resist application

Areas that should not be soldered may be covered with solder resist (solder mask). One of the most common solder resists used today is called LPI (liquid photoimageable). A photo sensitive coating is applied to the surface of the PWB, then exposed to light through the solder mask image film, and finally developed where the unexposed areas are washed away. Dry film solder mask is similar to the dry film used to image the PWB for plating or etching. After being laminated to the PWB surface it is imaged and develop as LPI. Once common but no longer commonly used because of its low accuracy and resolution is to screen print epoxy ink. Solder resist also provides protection from the environment.
Legend printing

A legend is often printed on one or both sides of the PCB. It contains the component designators, switch settings, test points and other indications helpful in assembling, testing and servicing the circuit board.

There are three methods to print the legend.

Silk screen printing epoxy ink was the established method. It was so common that legend is often misnamed silk or silkscreen.
Liquid photo imaging is a more accurate method than screen printing.
Ink jet printing is new but increasingly used. Ink jet can print variable data such as a text or bar code with a serial number.

Bare-board test

Unpopulated boards may be subjected to a bare-board test where each circuit connection (as defined in a netlist) is verified as correct on the finished board. For high-volume production, a bed of nails tester, a fixture or a rigid needle adapter is used to make contact with copper lands or holes on one or both sides of the board to facilitate testing. A computer will instruct the electrical test unit to apply a small voltage to each contact point on the bed-of-nails as required, and verify that such voltage appears at other appropriate contact points. A “short” on a board would be a connection where there should not be one; an “open” is between two points that should be connected but are not. For small- or medium-volume boards, flying probe and flying-grid testers use moving test heads to make contact with the copper/silver/gold/solder lands or holes to verify the electrical connectivity of the board under test. Another method for testing is industrial CT scanning, which can generate a 3D rendering of the board along with 2D image slices and can show details such as soldered paths and connections.

Assembly

After the printed circuit board (PCB) is completed, electronic components must be attached to form a functional printed circuit assembly, or PCA (sometimes called a “printed circuit board assembly” PCBA). In through-hole construction, component leads are inserted in holes. In surface-mount (SMT – surface mount technology) construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.

There are a variety of soldering techniques used to attach components to a PCB. High volume production is usually done with SMT placement machine and bulk wave soldering or reflow ovens, but skilled technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02 in. by 0.01 in.) by hand under a microscope, using tweezers and a fine tip soldering iron for small volume prototypes. Some parts may be extremely difficult to solder by hand, such as BGA packages.

Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques. For further comparison, see the SMT page.

After the board has been populated it may be tested in a variety of ways:

While the power is off, visual inspection, automated optical inspection. JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.
While the power is off, analog signature analysis, power-off testing.
While the power is on, in-circuit test, where physical measurements (for example, voltage) can be done.
While the power is on, functional test, just checking if the PCB does what it had been designed to do.

To facilitate these tests, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise boundary scan test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.

In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard. The JTAG test architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes. JTAG tool vendors provide various types of stimulus and sophisticated algorithms, not only to detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.

When boards fail the test, technicians may desolder and replace failed components, a task known as rework.

Protection and packaging

PCBs intended for extreme environments often have a conformal coating, which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax; modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another technique for applying a conformal coating is for plastic to be sputtered onto the PCB in a vacuum chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered extremely difficult.

Many assembled PCBs are static sensitive, and therefore must be placed in antistatic bags during transport. When handling these boards, the user must be grounded (earthed). Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components. Even bare boards are sometimes static sensitive. Traces have become so fine that it’s quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.

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PCB characteristics

Much of the electronics industry’s PCB design, assembly, and quality control follows standards published by the IPC

organization.
Through-hole technology

The first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of

the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or

more compact double-sided boards, with components soldered on both sides. Horizontal installation of through-hole parts with

two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction,

inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part’s

mechanical strength), soldering the leads, and trimming off the ends. Leads may be soldered either manually or by a wave

soldering machine.[28]

Through-hole PCB technology almost completely replaced earlier electronics assembly techniques such as point-to-point

construction. From the second generation of computers in the 1950s until surface-mount technology became popular in the late

1980s, every component on a typical PCB was a through-hole component.

Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and limits the available routing

area for signal traces on layers immediately below the top layer on multilayer boards since the holes must pass through all

layers to the opposite side. Once surface-mounting came into use, small-sized SMD components were used where possible,

with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or

mechanical limitations, or subject to mechanical stress which might damage the PCB.

Surface-mount technology
Main article: Surface-mount technology

Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid-

1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto

the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement

on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies

with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and

greatly increasing production rates. Components can be supplied mounted on carrier tapes. Surface mount components can be

about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper;

prices of semiconductor surface mount devices (SMDs) are determined more by the chip itself than the package, with little

price advantage over larger packages. Some wire-ended components, such as 1N4148 small-signal switch diodes, are actually

significantly cheaper than SMD equivalents.
Circuit properties of the PCB

Each trace consists of a flat, narrow part of the copper foil that remains after etching. The resistance, determined by width and

thickness, of the traces must be sufficiently low for the current the conductor will carry. Power and ground traces may need to

be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for

shielding and power return. For microwave circuits, transmission lines can be laid out in the form of stripline and microstrip with

carefully controlled dimensions to assure a consistent impedance. In radio-frequency and fast switching circuits the inductance

and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; but they can be

used as a deliberate part of the circuit design, obviating the need for additional discrete components.
Materials

Excluding exotic products using special materials or processes all printed circuit boards manufactured today can be built using

the following four materials:

Laminates
Copper-clad laminates
Resin impregnated B-stage cloth (Pre-preg)
Copper foil

Laminates

Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermoset resin to form

an integral final piece of uniform thickness. The size can be up to 4 by 8 feet (1.2 by 2.4 m) in width and length. Varying cloth

weaves (threads per inch or cm), cloth thickness, and resin percentage are used to achieve the desired final thickness and

dielectric characteristics. Available standard laminate thickness are listed in Table 1:

table 1

Notes:

Although this specification has been superseded and the new specification does not list standard sizes, these are still the

most common sizes stocked and ordered for manufacturer.

The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate’s type designation (FR-4,

CEM-1, G-10, etc.) and therefore the characteristics of the laminate produced. Important characteristics are the level to which

the laminate is fire retardant, the dielectric constant (er), the loss factor (tδ), the tensile strength, the shear strength, the glass

transition temperature (Tg), and the Z-axis expansion coefficient (how much the thickness changes with temperature).

There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the

requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well

known prepreg materials used in the PCB industry are FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4

(woven glass and epoxy), FR-5 (woven glass and epoxy), FR-6 (matte glass and polyester), G-10 (woven glass and epoxy),

CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven

glass and epoxy), CEM-5 (woven glass and polyester). Thermal expansion is an important consideration especially with ball

grid array (BGA) and naked die technologies, and glass fiber offers the best dimensional stability.

FR-4 is by far the most common material used today. The board with copper on it is called “copper-clad laminate”.
Copper thickness

Copper thickness of PCBs can be specified as units of length (in micrometers or mils) but is often specified as weight of copper

per area (in ounce per square foot) which is easier to measure. One ounce per square foot is 1.344 mils or 34 micrometres

thickness.

The printed circuit board industry defines heavy copper as layers exceeding 3 ounces of copper, or approximately 0.0042

inches (4.2 mils, 105 μm) thick. PCB designers and fabricators often use heavy copper when design and manufacturing circuit

boards in order to increase current-carrying capacity as well as resistance to thermal strains. Heavy copper plated vias transfer

heat to external heat sinks. IPC 2152 is a standard for determining current-carrying capacity of printed circuit board traces.
Safety certification (US)

Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as components in devices or

appliances. Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat

deflection, and direct support of live electrical parts.
Multiwire boards

Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting

matrix (often plastic resin). It was used during the 1980s and 1990s. (Kollmorgen Technologies Corp, U.S. Patent 4,175,816

filed 1978) Multiwire is still available in 2010 through Hitachi. There are other competitive discrete wiring technologies that have

been developed (Jumatech, layered sheets).

Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach allowed designers to forget

completely about the routing of wires (usually a time-consuming operation of PCB design): Anywhere the designer needs a

connection, the machine will draw a wire in straight line from one location/pin to another. This led to very short design times (no

complex algorithms to use even for high density designs) as well as reduced crosstalk (which is worse when wires run parallel

to each other—which almost never happens in Multiwire), though the cost is too high to compete with cheaper PCB

technologies when large quantities are needed.

Cordwood construction

Cordwood construction can save significant space and was often used with wire-ended components in applications where

space was at a premium (such as missile guidance and telemetry systems) and in high-speed computers, where short traces

were important. In “cordwood” construction, axial-leaded components were mounted between two parallel planes. The

components were either soldered together with jumper wire, or they were connected to other components by thin nickel ribbon

welded at right angles onto the component leads. To avoid shorting together different interconnection layers, thin insulating

cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next

interconnection layer. One disadvantage of this system was that special nickel-leaded components had to be used to allow the

interconnecting welds to be made. Differential thermal expansion of the component could put pressure on the leads of the

components and the PCB traces and cause physical damage (as was seen in several modules on the Apollo program).

Additionally, components located in the interior are difficult to replace. Some versions of cordwood construction used soldered

single-sided PCBs as the interconnection method (as pictured), allowing the use of normal-leaded components.

Before the advent of integrated circuits, this method allowed the highest possible component packing density; because of this,

it was used by a number of computer vendors including Control Data Corporation. The cordwood method of construction was

used only rarely once semiconductor electronics and PCBs became widespread.

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What is PCB?

A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductors on different layers are connected with plated-through holes called vias. Advanced PCBs may contain components – capacitors, resistors or active devices – embedded in the substrate.

Printed circuit boards are used in all but the simplest electronic products. Alternatives to PCBs include wire wrap and point-to-point construction. PCBs require the additional design effort to lay out the circuit but manufacturing and assembly can be automated. Manufacturing circuits with PCBs is cheaper and faster than with other wiring methods as components are mounted and wired with one single part. Furthermore, operator wiring errors are eliminated.

When the board has only copper connections and no embedded components it is more correctly called a printed wiring board (PWB) or etched wiring board. Although more accurate, the term printed wiring board has fallen into disuse. A PCB populated with electronic components is called a printed circuit assembly (PCA), printed circuit board assembly or PCB assembly (PCBA). The IPC preferred term for assembled boards is circuit card assembly (CCA), for assembled backplanes it is backplane assemblies. The term PCB is used informally both for bare and assembled boards.

The world market for bare PCBs reached nearly $60 billion in 2012.

(from Wikipedia)

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