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SnAg plating has produced Silver-rich balls on the coating surface

November 29, 2009

My name is Adam Zhang and currently working in a company in Nagoya Japan as a electrochemist. Recently I was assigned a SnAg (Ag3.5%) plating project but struggling to achieve satisfying finishing.

My baths were MSA-based which were purchased from two different suppliers (one japan based, one EU based), but both of them returned coatings with surface Ag-rich balls of a few microns high. As the surface shape is extremely important in my industry so such a finishing was obviously unacceptable. The number of the Ag-rich ball on the surface varied from a few to about ten (by naked eye) depending on the plating conditions but they were always presence. The coating thickness was 20 µm and current density deployed was 3 ASD. Reducing thickness and lowering the Ag concentration in the bath eased the problem to some extent but can't completely eliminate it.

Initially I thought it was caused by the preferential deposition of Ag under limited current condition in some areas which returned the dendritic Ag-rich deposition but later it's confirmed not such a case.

I just wonder if anyone here had similar problem before or anyone know the possible reasons behind this? Any suggestions/feedback would be highly appreciated. I am completely blocked at this stage. Many thanks

Adam Zhang
plating shop employee - Nagoya, Japan

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In the lab, experiment with increasing the amount of MSA towards the maximum. This will slow down the plate rate some.

My original thought is that you need better filtration of the tanks and better rinsing. You also should check the parts after cleaning for microscopic burs and/or parts that are not clean enough.

James Watts
- Navarre, Florida
December 1, 2009

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December 9, 2009

Adam,
I am interested in knowing how you know it is not due to a limiting current, because that would be my first thought too. You may want to try doing a Hull Cell test to see what the best current density is for your desired system; this will also tell you how the bath behaves under different conditions.

It sounds like you have silver nodules forming, so you should consider how you are pretreating your substrate and checking to see if there are any nucleation sites on its surface.

You may also try using Pulse Plating or Periodic Reverse Pulse Plating,as this too may help eliminate nodule growth.

It would be good to know your plating conditions and bath composition, pretreatments and substrates.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK