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Exposed Conductor Plating and Coating

  Time:2012-06-04 11:50

To Remove the unneeded underlying copper, PCBs are plated with solder, tin, or gold over

nickel as a resist.The soldermask is applied to the pcb after it is etched and then rinsed

with water,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 bounding surface or stripped to bare

copper. The treatment such as benzimidazolethiol can prevent surface oxidation of bare

copper. The place to which components will be mounted are typically plated, because

untreated bare oxidizes quickly, and therefore is not readily solderable. Traditionally, any

exposed copper was coated with solder by hot air solder levelling(hasl).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 compiance 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% nickle,and a nominal of 60ppm

germanium.

It is important to use solder compatible with 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 led-free solder paste.

Other platings used are OSP(organic surface protectant), immersion silver (IAG), immersion

tin, electroless nickel with immersion gold coating(ENIG), 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 paid diffusion of coating metal into tin

solder.Tin forms intermetallic 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.