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February 2011

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TechNet E-Mail Forum <[log in to unmask]>, Amol Kane <[log in to unmask]>
Date:
Mon, 14 Feb 2011 15:12:16 +0000
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TechNet E-Mail Forum <[log in to unmask]>, Stewart McCracken <[log in to unmask]>
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Stewart McCracken <[log in to unmask]>
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Hi Amol,

Just noted an important typo in my last mail. 

"The formation of the IMC's can also cause dramatic increase in fluidity of the solder preventing normal spread a- could tie in with your observations on heel fillet formation." 

should have read...

"The formation of the IMC's can also cause dramatic DECREASE in fluidity of the solder preventing normal spread - could tie in with your observations on heel fillet formation." 

Kind regards,

Stewart




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


Hi Amol,

Could be that the issue you describe is related to formation of gold-tin intermetallics (IMC's). We have seen similar in the past, the net result being a range of colours developed on pins, ranging from orange through purple. Investigation at the time confirmed that a wide range of IMC's had formed on the pins. The problem was particularly obvious at the leading edge of solder flow on the pin, where the ratio of gold-tin is locally much higher than you might expect from a bulk calculation of % gold in the solder volume deposit. Be good to see some images, but note that IMC's can form many different shapes and forms, and can produce a wide range of colours. The formation of the IMC's can also cause dramatic increase in fluidity of the solder preventing normal spread a- could tie in with your observations on heel fillet formation. 

Kind regards,

Stewart
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Amol Kane
Sent: 14 February 2011 14:16
To: [log in to unmask]
Subject: [TN] Surface Mount connector discoloration

Dear Technetters,
We have a SMT mounted connector:  copper alloy base, Ni underplate and gold flash on the surface. The connector is perpendicular to the direction of travel in reflow (LF board, 240C peak). One side of the connector solders with good heel fillets without any discolored pins, but the pins on the other side get discolored (reddish purple hue) and has more inconsistent solder flow on the heel (some acceptable, some barely acceptable). The connector is rated to 260C reflow capable.

The connector can only go in one way, and it's always the leading edge in reflow that has the hue.  It could be a part issue or a reflow issue. I wanted to know if anybody else experienced this with the specific connector material set and what the root cause may be

We have a run coming up and I can/will post pictures. Any suggestions before the run with just the explanation  given above will be greatly appreciated. I have contacted the manufacturer and await their reply. Just thought I would also e-mail the experts on the group about this.

Regards,
Amol


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