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.
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.
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