Buy a small thin piece of AuSn eutectic. This is used for lid sealing purposes on ceramic packages. Then develop a working knowledge of its properties: bend it, try soldering to it, try to stretch it, etc. This is the material you are making when you get too much gold into the mix. You CAN actually solder with this stuff if you use a board which can withstand about a 300C reflow.
The most frequent direct effect I normally see of too much gold is you can tell that the melting point shot up as gold was absorbed, and the joint stops flowing. Hence from a visual standpoint, you end up with cold, poorly flowed joints, with plenty of crack initiation spots and a very brittle structure. If you can actually see the entire joint, then if it is nice and shiny and nicely shaped, then you probably have a reliable joint. But if you can't see the whole joint, such as beneath a ceramic leadless part, or some other kind of LGA, then you will need to do some kind of destructive testing to make sure you aren't making the bad stuff.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vladimir
Sent: Wednesday, November 23, 2016 9:27 AM
To: [log in to unmask]
Subject: Re: [TN] Gold embrittlement
Hi Ioan,
Au and Sn form intermetallic platelets and cracking goes at their interface with the matrix.
The fact that your product did pass the test doesn't mean there were no cracking in solder and the product will last a as long as it supposed to in field (it will depend on how good a reliability model was).
I'd definitely suggest you to section joints after the test.
Regards,
Vladimir
SENTEC
Sent from my BlackBerry 10 smartphone on the Rogers network.
Original Message
From: Ioan Tempea
Sent: Wednesday, November 23, 2016 12:02
To: [log in to unmask]
Reply To: TechNet E-Mail Forum
Subject: [TN] Gold embrittlement
Dear Technos,
I have SnPb37 joints with 4-8% Au that did not fail thermal cycling, no cracks, pass electrical tests like a charm. This kinda contradicts J-STD-001 and common knowledge, so I would appreciate some insider info regarding Au embrittlement, to help me figure this one out.
I know one could write volumes on this, but I'm sure the questions below can be answered in a brief manner.
* How did Au embrittlement end up being a reliability issue? Real catastrophic failures?
* What is the effect on the long term reliability of solder joints?
* What screening method creates catastrophic failure of embrittled joints (mechanical shock, thermal cycling, vibration, etc.)?
* It looks like the embrittlement process goes on as long as there's Au to combine with Sn. What is the speed of this reaction? Otherwise said, how long does it take for a joint with high Au content to become brittle?
* Has anybody screen for embrittlement? Or, anyone tried to maximise embrittlement (e.g. 100 hours @ 150 C) and then screened the hardware?
* Anything else I should know?
Thanks,
Ioan Tempea, P. Eng.
Manufacturing Engineer, Satellite Systems
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