Changes

Electroless deposited nickel coatings with an additional immersion layer of gold gaining increased importance in the coating of printed circuit boards (PCBs). The process sequence is shown in <xr id="fig:Electroless Deposition of Nickel Gold"/><!--(Fig. 7.2)--> using the example of the DODUCHEM process.

<figure id="fig:Electroless Deposition of Nickel Gold">

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After the pre-cleaning (degreasing and etching) a palladium sulfate activator is used, which activates the exposed copper surfaces on the printed circuit board and thus facilitates the nickel deposition. The electroless working chemical nickel electrolyte contains – besides other ingredients – Sodium-hypophosphite, which is reduced to phosphorus in a parallel occurring process and incorporated into the nickel deposit. At the temperature of 87 – 89°C a very homogeneous nickel-phosphorus alloy layer with approx. 9 wt% P is deposited with layer thicknesses > 5 μm possible. During a consecutive processing step, a very thin and uniform layer (< 0.1 μm) of gold is added in an immersion electrolyte. This protects the electroless nickel layer against corrosion, achieving a solderable and well bondable surface for thick or fine aluminum bond wires.

<figure id="fig:Coating composition of a printed circuit board">

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