Ed,

We actually did thermal shock testing of untinned LCCs (ceramic parts, right) on controlled CTE PWAs (best case) to try to determine how "bad" not tinning was.  Answer:  It was bad.  Most parts failed at <100 cycles (normal parts were ~1000 cycles).  We often go around on this (typically 1-2 parts per year we "just can't tin" and we re-visit this issue).  We've always gone to tinning, even as far as recalling hardware to do it.  Note that the Au "limit" is ~4% by weight (~1.8% by volume in Sn63); beyond that is BAD.).  Also note that just a gross volume calculation (Au vs. Solder volume) can be misleading as the Au doesn't always evenly distribute in solder, it's the locallized concentrations that kill ya.

I have some detailed info (too large to Technet) but if you can accept ~1 meg inputs I can mail to you.  Let me know.

Jim
---------------------------------------------------------
Jim Maguire
Boeing Phantom Works
E/E M&P
Phone (253)657-9063
fax       (253)657-8903
Pager  (206)982-3737
email:  [log in to unmask]
---------------------------------------------------------

> ----------
> From:         Knowles, Ed[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Knowles, Ed
> Sent:         Monday, April 05, 1999 2:28 PM
> To:   [log in to unmask]
> Subject:      [TN] Gold Embrittlement
>
> Please don't blast me on bringing this subject up again.  I have searched
> the TechNet archives and found lots of good information, and also noted that
> it is a somewhat sensitive subject, having been around for 30+ years.  We
> have a condition that I believe is clearly unreliable based on what I have
> read.  The condition is:
>
> - 4 lead LCCC's (leadless with castellations)
> - Approximately 50 microinches of glod plating on untinned castellations.
> - Untinned parts mass reflow soldered onto PWB's (parts were supplied on
> tape and reel, so subcontractors did not see that they were untinned).
> - Microsectioning shows a 50 to 100 microinch layer of Au/Sn intermetallic
> after soldering, on top of copper/nickel surface.  Au concentration averages
> about 25% in this intermetallic region.
> - These assemblies will be subjected to high vibration and long term (years)
> thermal cycling in actual use.
>
> Replacement of these parts would be expensive since they are on many boards
> installed in closed and tested boxes. For this reason, mangement is hoping
> that testing will qualify this condition.  I hate to see the affected
> program waste time and money, and am afraid they might even get a warm fuzzy
> and fly these parts.
>
> Can any of the numerous industry experts in this group give me something
> concrete to present to the program such as actual experiences or obtainable
> reports?  Is the above condition a no-brainer or is it not as bad as it
> looks to me?  I tried getting copies of the 2 reports listed in the
> J-STD-001 Handbook, but struck out.
>
> Thank you for your time.
>
> Ed Knowles
> Lockheed Martin
> [log in to unmask]
>
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