Thanks for the insight Brad

Mike Reagan
EDSI



----- Original Message -----
From: Brad Velander <[EMAIL PROTECTED]>
To: 'Protel EDA Forum' <[EMAIL PROTECTED]>
Sent: Tuesday, April 17, 2001 5:25 PM
Subject: Re: [PEDA] Gold Emersion


> 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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