Changes

The electroless metal plating with adding reduction agents to the electrolyte isbased on the oxidation of the reducing agent with release of electrons whichthen in turn reduce the metal ions. To achieve a controlled deposition from suchsolutions the metal deposition has to happen through the catalytic influence ofthe substrate surface.

Otherwise a “wild” uncontrollable deposition would occur. In most casespalladium containing solutions are used for the activation which seed thesurfaces with palladium and act as catalysts in the copper and nickelelectrolytes.

The electrolytes contain besides the complex ion compounds of the metals tobe deposited also stabilizers, buffer and accelerator chemicals, and a suitablereduction agent.

These electrolytes are usually operating at elevated temperatures (50° – 90°C).The deposits contain besides the metals also process related foreign inclusionssuch as for example decomposition products of the reduction agents.The electroless processes are used mainly for copper, nickel, and gold

Electroless deposited nickel coatings with an additional immersion layer of goldare seeing increased importance in the coating of printed circuit boards (PCBs).The process sequence is shown in ''(Fig. 7.2)'' using the example of theDODUCHEM process.

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

It is possible to enhance this layer combination further by adding a immersionpalladium layer between the electroless nickel and the gold coating(DODUBOND process). This Pd layer acts as a diffusion barrier and allows theusage of this surface combination also for gold wire bonding.

As an alternative, for gold wire bonding applications a thicker gold layer of 0.2 –0.5 μm can be applied using an electroless process. Typical electrolytes work ata temperature of approx. 80°C with deposition rates of 0.3 – 0.4 μm per 30minutes. There are however limitations with these electroless electrolytesconcerning their stability and the robustness of the process compared to otherelectroplating processes which reduces their wider usage ''(Fig. 7.3)''.

Coating compositionof a printed circuit board withreductively enhanced gold

A tin coating by ion exchange is usually not possible since copper is the moreprecious metal. By adding thio-urea the electro-chemical potential of copper isreduced to a level (approx. 450 mV, significantly lower than tin) that allows theexchange reaction. Using a suitable electrolyte composition and enhancersolutions like with the DODUSTAN process ''(Fig. 7.4)'' tin coatings can beproduced that, even under usually unfavorable conditions of copperconcentrations of 7 g/l in the electrolyte, are well solderable.

The immersion tin deposition is suitable for the production of a well solderablesurface on printed circuit boards and electronic components. It is also used asan etch resist against ammonia based solutions or as corrosion and oxidationprotection of copper surfaces.