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June 2000

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Subject:
From:
Brooks Bill <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 8 Jun 2000 12:19:03 -0700
Content-Type:
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text/plain (182 lines)
Sorry, I see that I posted to the wrong guy... I meant that message for
Ralph Richart... I believe he was the originator... I concur with your
findings however. The thing that amazes me is that the buyer or QC dept of
whatever company is purchasing these boards is trying to backtrack to the
gold immersion stage of the process after the board has already been
assembled correctly... Obviously they were successful in assembling the
board with HASL as a preparatory coating and have no further need of any
other processes to complete the assembly. It was probably not a requirement
to conformal coat the board, in the first place... they just replaced a
process that is intended to prepare the surface of the board to receive
components...with another that is totally acceptable in most all
applications, HASL....,  which obviously, has been successfully
accomplished.... they have populated the boards...
 They will ruin the boards if they try to plate gold over the soldered
component lands and leads....the exposure to the chemicals will oxidize and
etch the surface of the tin-lead and any vulnerable components.. and make
the boards rejectable due to bad solder and damaged components.....a no-win
scenario.....

Bill Brooks
Sr. PCB Designer



-----Original Message-----
From: Alain Savard [mailto:[log in to unmask]]
Sent: Thursday, June 08, 2000 11:58 AM
To: [log in to unmask]
Subject: Re: [TN] still confused


Sorry Bill,
but my post was accidental.. I accidently clicked the send button before
typing my answer.

Conformal coating prevents corrosion at that point, NOT gold. Adding gold
over the solder will NOT significantly increase corrosion-resistance, but
the chemicals used to gold-plate are very likely to damage the components.

Alain Savard, B.Sc.
Chemical Process Analyst
CAE Electronics Ltd.


-----Original Message-----
From: Brooks Bill [mailto:[log in to unmask]]
Sent: June 8, 2000 2:26 PM
To: [log in to unmask]
Subject: Re: [TN] still confused


Alain, I feel for your situation.... It sounds to me like a messed up note
on the drawing. Gold - OVER - solder has never been a recommended procedure
in any documentation I have ever seen in my 30 years of doing boards...
especially after the board has been populated.
The only use of gold I have seen is those repair kits for adding gold to
edge connector fingers like the type used on PCI cards that may have worn
the gold down over time...
IF I WERE YOU... I would make an effort to get past the buyer to the
designer behind the board and get him to straighten out the buyer....
Tough situation.... good luck.
- Bill Brooks



-----Original Message-----
From: Alain Savard [mailto:[log in to unmask]]
Sent: Thursday, June 08, 2000 10:09 AM
To: [log in to unmask]
Subject: Re: [TN] still confused


-----Original Message-----
From: Ralph Richart [mailto:[log in to unmask]]
Sent: June 8, 2000 12:55 PM
To: [log in to unmask]
Subject: [TN] still confused


We have answers....

So I had a long talk with my customer this morning to provide input
based on the invaluable advice from you all.

I said - "is there a specific reason for choosing 3-8u" of immersion
gold over tin/lead solder?"
He said - "absolutely, the gold provides corrosion resistance, the gold
does not"
I said - "even such a thin amount - only 3-8u"?"
He said - "definitely, all we need is the coverage, the thickness is
irrelevant"

Is this true?

Needless to say, we are rebuilding the parts as we speak.  Rework sounds
difficult to say the least, although I may try to come up with some
scheme for fun as they have a bunch of boards that are no good right
now.

thanks for all the help.....

ralph

PS - more on the larrylobster story later; it's more silly than anything
else.  Paul's poem may soon become our own, however.  Very funny!

Off to build some boards....

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