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-----Corrosion of soldered joints at indium/gold interface
Q. Does anyone have any information on the corrosion of joints where indium solder has been used to join gold leaded components on PCBs.
Vincent Moran- Bradford, Yorkshire, UK
A. Was the joint prepared using some kind of flux? What was it, and was it completely removed before the part was put into service? What are the service conditions of the part? Hermetic seal, salt fog, humidity, industrial, etc.
Tom Pullizzi
Falls Township, Pennsylvania
A. Vincent, is your problem just corrosion or are you looking at the total failure of the solder joint? Your problem, if your joint failed may be more than just corrosion. It may be due to gold leaching. Many indium alloys are gold leaching. This is especially true of pure indium. We use indium alloy solders extensively in our instruments because of their good thermal properties. (Our instruments are cooled to 20 Kelvin.) There are alloys that specifically are used with gold-plated surfaces. The one I use the most is 70%In, 30%Pb with a flux containing NO rosin, NO zinc or ammonium chlorides. It is used in aerospace applications and is water soluble. If the joint is properly cleaned and inspected after the soldering operation, it is extremely reliable.
Other indium alloys that are recommended for gold surfaces:
50 In, 50 Pb
60 Pb, 40 In
81 Pb, 19 In
92.5 Pb, 5 In, 2.5 Ag
92.86 Pb, 4.76 In, 2.38 Ag
The last two are for higher temperature applications. Higher temperatures will increase gold leaching. This is especially true if you do not have a barrier layer under your gold.
Ronna EricksonRadio Astronomy - Amherst, Massachusetts
Q. I am designing a metallic seal for an underwater sample container that must maintain its seal for decades. We are presently using silver-plated Inconel C-rings, but I'm concerned about tarnishing and sulphate attack ruining the seal over time. We tried gold plating, but it was not as effective as silver at forming a seal. I'm considering Indium now, but its position on the galvanic table worries me. Does anyone have any experience with galvanic (or other) corrosion of indium plating?
Brendan SimonsProject Engineer - Hamilton, Ontario
April 20, 2012
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