PCB characteristics

  tmliir   Aug 25, 2011   PCB   0 Comment

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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