Mike,
        sorry to butt in on your question to Andrew but this is one of my
pet issues.

        The issue that I believe you are concerned with on your NASA spec is
properly called intermetallic embrittlement. It is an issue between Gold and
Tin/Lead solder whereby the mix of the Gold and Solder causes a crystalline
structure which is not mechanically sound. The problem is further aggravated
by thermal excursions and vibration. In it's worst case I have seen demos of
an FR4 board being flexed moderately and lots of components falling right
off the board to the floor. The problem is worst when the percentage of Gold
dissolved in the Solder is greater then 4% by weight (approx. 30-40
microinches of Gold for a typical SMT solder joint). This threshold has been
shown in some studies to be sufficient for common board assemblies. For Nasa
and their stress, thermal cycles and other issues, they are probably only
comfortable if they avoid the issue all together.

        There are many studies and articles on this issue but they are not
readily found through any one source. I have a number of printed paper
articles that I have collected over the years but nothing I could readily
send to you. Let me know if you have any specific questions that I could
answer, or did I already give you enough for your enquiry.

Sincerely,

Brad Velander
Lead PCB Design
Norsat International Inc.
#100 - 4401 Still Creek Dr.,
Burnaby, B.C., Canada.
V5C6G9.
voice: (604) 292-9089 (direct line)
fax:    (604) 292-9010
email: [EMAIL PROTECTED]
www: www.norsat.com


> -----Original Message-----
> From: Mike Reagan [mailto:[EMAIL PROTECTED]]
> Sent: Tuesday, April 17, 2001 1:55 PM
> To: Protel EDA Forum
> Subject: Re: [PEDA] Gold Emersion
> 
> 
> Andrew.
> Are you involved with developing flight boards?   From time 
> to time I will
> do a flight design for NASA/GSFC in Md.  GSFC has a spec S-312, that
> prohibits anything except  tin/lead hasl finish for flight 
> boards. Gold /
> white tin, etc are strictly prohibited .   When we spec a 
> board out tin/
> lead is the only choice.   I am not sure about the chemical /molecular
> interaction between gold and tin but, I know they don't like 
> gold plated
> boards for flight.   There is apparently  migration from the 
> gold to the
> tin, and somehow this translates into reduced reliability ( can anyone
> elaborate on this?)   I'm not even sure if this a specific 
> problem to the
> environment in space or just an interaction between gold and 
> solder.   For
> that reason, we don't use gold or white tin plating.
> 
> Interestingly, many component leads will be gold plated on 
> flight approved
> hardware.  Go figure that one out
> 
> 
> Mike Reagan
> EDSI


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