Printed circuit board:BASIC EXPLAINATION
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.
Components – capacitors, resistors or active devices – are generally soldered on the PCB. Advanced PCBs may
contain components embedded in the substrate.
PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layer (outer and inner
layers). Conductors on different layers are connected with vias. Multi-layer PCBs allow for much higher
component density.
glass epoxy is the primary insulating substrate. A basic building block of the PCB is an panel with a thin
layer of copper foil laminated to one or both sides. In multi-layer boards multiple layers of material are
laminated together.
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 no embedded components it is more correctly called a printed wiring board (PWB) or etched
wiring board. However, 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),[1] and for assembled backplanes it
is backplane assemblies. The term PCB is used informally both for bare and assembled boards.
Plating and coating:
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.[23] This solder was a
tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the
EU, 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 60 ppm 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.
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.
Components – capacitors, resistors or active devices – are generally soldered on the PCB. Advanced PCBs may
contain components embedded in the substrate.
PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layer (outer and inner
layers). Conductors on different layers are connected with vias. Multi-layer PCBs allow for much higher
component density.
glass epoxy is the primary insulating substrate. A basic building block of the PCB is an panel with a thin
layer of copper foil laminated to one or both sides. In multi-layer boards multiple layers of material are
laminated together.
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 no embedded components it is more correctly called a printed wiring board (PWB) or etched
wiring board. However, 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),[1] and for assembled backplanes it
is backplane assemblies. The term PCB is used informally both for bare and assembled boards.
Plating and coating:
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.[23] This solder was a
tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the
EU, 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 60 ppm 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.
