Poor solderability -- is gold plating too thick?

A. 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 de-wetting and non wetting. de-wetting 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!

A. 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.

A. 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.

A. 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.

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

A. 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.

A. For solderability,from my experience, electroplated nickel is better compared to electroless nickel. And for electroless nickel, the higher the P content, the lower the solderability. Good luck.

A. With standard tin/lead solder, gold dissolves in the melt to form intermetallic compounds, particularly AuSn6. On cooling, this forms acicuclar (needle like) crystals which give a grainy appearance to the solder surface. They are also brittle and thus greatly decrease the strength of the joint.
If the gold is thin it can completely dissolve before the intermetallic concentration is enough to cause problems. On thick gold, intermetallic formation is enhanced by too high soldering temperature and/or time.
Specifying thick gold and trying to do a 'better' soldering job are both counter productive. 15-50 microns is far too much. 5 microns or less is more normal, possibly one over 5 microns of nickel would be better. Immersion gold, which is too thin to measure, is fine provided there is good. Control the solder bath/iron at about 200 °C. If you cannot achieve a good joint in under a second you need better flux.
coverage of the nickel to prevent it oxidising.
Mould flash is a disaster. You cannot solder onto plastic!

A. Yes. The gold thickness is way too high for a solder joint. It does effect the strength of the solder joint.
From ASTM B488 [affil link] Standard Specification of electrodeposited coatings of gold for Engineering use, X1.1.4 Solderability - Type 1 golds are preferred for solder application. Oxidation of co-deposited metals can make soldering more difficult. It is strongly recommended that gold coating thicknesses not exceed 0.5 micron for solder application. There is evidence to show that when the gold content in the tin-lead solder joints exceeds about 3% by weight, embrittlement occurs leading to potential solder joints failure.