Letter 11803

Poor solderability using gold finish - gold too thick? 

Hi,

I'm a qc manager for an electronics manufacturing. One of our products is pcba, pure smt, for cell phones. It has a gold finish and the problem is that the solderability looks really bad. The solder is very rough and not so shiny. All process parameters have been checked and the same parameters yield good results with hasl pcbs. I suspect that the pcb gold finish is too thick. Could this be what's causing the problem? Thanks.

Jay Chavez
- Manila, Philippines

Jay,

I have a lot experience with solderability. Most my experience is for silver, tin and tin-lead alloy on surface mount components. Let me see if I can help.

It is highly unlikely that too thick of a gold plating can cause your solderability problem. In fact, too thin of a plating will show failure after steam age solderability. Solderability has two failure modes which are dewetting and non wetting. Dewetting can be caused by oxidation on the base metal material that you are soldering to, surface contamination underneath the final coating, or final protect coating is too thin or doesn't provide the protection during the steam age process. Non wetting is the top coating (gold) doesn't wet or take solder. This could be by surface contamination of the gold surface. In this case, a good activated flux or surface cleaning can fix this problem. Other things to check for, is to make sure your solder pot is in spec and free from contamination.

Hope this helps!

Karl Weyermann
- Lebanon, KY

Hi Karl,

Doesn't the gold eventually become a contaminant in the solder pot? So long term, thick gold plating may end up being the limiting factor for the life of the solder pot, "causing", in a way, the solderability problem. If the pot remained within specification for gold contamination, then the only problem is wasted gold.

Tom Pullizzi Platronica.com Falls Township, PA

 

Tom,

Yes, eventually gold will become a contaminant in a solder pot, and it may affect solderability. However after this point, everything will start failing solderability using the same solder pot. Not just parts with thick gold. Solder pots need to be changed regularly. In fact, it is cheaper to change most solder pots, with fresh solder, than to perform analysis to check for foreign contamination.

Karl Weyermann
- Lebanon, KY

Perhaps a good starting point is to define is how thick is the gold? Then is it too thick? Since gold is soluble in a "solder pot" assume tin/lead....60/40 ...90/10 The gold is only there to protect the solderability of the underlying base metal.i IF the underlaying base metal is free of oxides and in a "solderable" condition then the gold is the Saran Wrap to preserve it. Gold does enter the equation as it is soluble and can contribute to brittle solder joints but that gets to be blah blah blah to this situation. Dull solder from poor cooling or bad wetting angle all are another subject.

Jon Quirt
- Fridley, MN

I think, as the first poster, that it is a contamination on the surface. Usually it will be removed when running the pcb:s in an acid cleaner.

Peter Fogel
- Sweden

Hi
I'm a qc inspector, I facing the problem with the gold plating with solderability.
If gold plating got stick with mold flash it will become non wetting after solderability test?

Liew Siew Ling
electronics - Singapore

A few years ago, I requested gold plating on one of our PCBs as part of changing over to surface-mount technology. The solderability was terrible. I was told that gold plating above a certain thickness creates bad joints because the gold dissolves in the solder and creates a brittle, nasty-looking joint. The next iteration, I went with "Immersion gold" which is much thinner. Solderability was excellent, and we have done all boards with immersion gold since then.

Paul Boisseau
- Lexington MA USA