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October 2009

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Subject:
From:
Stewart McCracken <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stewart McCracken <[log in to unmask]>
Date:
Thu, 8 Oct 2009 11:24:15 +0100
Content-Type:
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text/plain (199 lines)
Hi Justin,

The purple discolouration which you refer to on the surfaces of the
connector pins (photo labelled gold terms previous batch) may well be
gold-tin intermetallic compounds. 

We have analysed something similar in the past. At the solder wetting
front (edge of the solder as it wets over the pins), the ratio of gold
to tin is very different from that at the "bulk" solder fillet. As a
result, you can form a wide range of intermetallics with varying
stochiometry (ratio of gold and tin atoms) which produce various
different colours when viewed using light. 

The best way to confirm if you (or your supplier) need to is to get the
parts examined using SEM+EDX (scanning electron microscope and energy
dispersive x-ray analysis). If you are careful, you can also see these
thin IMC layer when prepared in x-section - requires really good prep
technique! 

Be happy to provide further background if you need it - get back to me
if we can be of further help. 

Kind regards,

Stewart

Stewart McCracken
MCS Ltd.
Centre House
Midlothian Innovation Centre
Roslin
Midlothian
EH25 9RE
 
m. +44(0)7711 541735 
t.   +44(0)131 440 9090
f.   +44(0)131 440 9085
e.  [log in to unmask]
w. www.themcsgroup.co.uk



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Justin Braime
Sent: 08 October 2009 08:46
To: Administrator
Subject: Re: [TN] gold connector discoloration

Thanks for your input guys.

The appearance of the connectors is fairly consistent across all the
PCBs in
the batch. So either there was some touching up done very consistently,
or
they've increased solder paste volume - the contractor uses the Mydata
'solder jet' technology rather than stencil printing.

Apart from one pin on that one PCB, there's no solder on the gold mating
surfaces, but the purple/black discoloration (it's not very clear on the
photos compared to the actual view through the stereo microscope)on
lower
parts of the mating surfaces is widespread - not sure if this is flux
wicking up the lead and causing the gold to tarnish, or even nickel
passivation following gold dissolution? Either way, I've got misgivings
about the reliability, so back the PCBs will go!

Many thanks,
 
Justin

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: 07 October 2009 18:14
To: [log in to unmask]
Subject: Re: [TN] gold connector discoloration

I agree with Victor on the solder-on-Au-connector issue...

Looks to me that someone either reworked or touched-up the connectors
after
SMT or they hand soldered the connectors in...

Adding flux to the connector and touching the SJs to clear a solder
short or
(most likely) to add solder to the joint will cause solder to wet up
into
the contact area on low profile connectors...Extra heat starts it but
too
much solder and flux does it every time...

You'll have to ask the CM to remove and replace the connectors...

Paul

Paul Edwards
Surface Art Engineering


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: Wednesday, October 07, 2009 10:06 AM
To: [log in to unmask]
Subject: Re: [TN] gold connector discoloration

Beware of fretting with the solder on the gold plated contacts.

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gumpert, Ben
Sent: Wednesday, October 07, 2009 9:34 AM
To: [log in to unmask]
Subject: Re: [TN] gold connector discoloration

Justin,

It looks like you have some solder that has made its way up the gold.
The picture of the prior board looked as if there was less solder used
to attach the connector, as there is still evidence of gold on the leads
(where solder has not covered the leads as it has done in the 'new
batch' image).

Maybe they installed connectors that had been tinned, so there was
additional solder in the process.

Ben

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Justin Braime
Sent: Wednesday, October 07, 2009 10:12 AM
To: [log in to unmask]
Subject: [TN] gold connector discoloration

Hi Technetters,



We've just received a new batch of boards from one of our
subcontractors,
and the Hirose DF12 header sockets have discoloration on the mating
terminals.

On previous batches, we have seen a little discoloration adjacent to the
solder joints, but never up in the connector mating area itself. The
process
hasn't (allegedly) changed.



I've posted some pics at:

http://gallery.me.com/jbraime/100005



Any ideas as to the cause? Any detriment to functionality?



Thanks in advance,



Justin



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