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September 2003

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Subject:
From:
Bev Christian <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 3 Sep 2003 09:01:03 -0400
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text/plain (69 lines)
Lance,
Well for a first timer, you sure got a lot of responses!  Good first question!

I have gone back and reread you original query, Howard's expansion and all the replies.  So what do we know:
1) Applies to ON Semi diodes only
2) Happens on secondary side
3) Parts pass solderability testing
4) Only happens on one of the two leads on the part
5) Chain conveyor and there are cooling zones 
6) Happens "inconsistently"

I doubt very much you have a cold solder joint.

We definitely need answers to Werner's questions as to the orientation of the parts and is it always the trailing lead (if this is the orientation)?
Is there a difference in CTE between this part and equivalent parts from other suppliers? What is the difference between the heat sinking abilities of the different pads to which the part is soldered? If it is a disturbed joint on these parts but not others, the presence of a CTE difference, with or without additional heat sinking, is the only reason I can come up with so far to explain the difference.
If it is not a disturbed joint and the condition is truly random, then I would wonder whether:
1) ON Semi got a questionable lot of lead material feedstock/solderable surface with a SMALL amount of impurities in the metal(s,) which are showing up on the surface of the joint (but not enough to affect solderability testing - if so, who cares?) 
2) The slight difference in cooling of one end compared to the other (heat sinking, thermal shadowing) is allowing slight differences in surface morphology that gives different light reflection properties.  On TechNet we have received several questions about rainbow, iridescent colors on solder joint surfaces and that's all it is.

Keep digging, Lance, and let us know more!  I am very curious to see how this plays out.

regards,
Bev Christian
Research in Motion

-----Original Message-----
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: September 3, 2003 7:55 AM
To: [log in to unmask]
Subject: Re: [TN] pic's


Hi Lance,
I would venture to say, that the CTE of these diodes is significantly
different than the PCB. I would further hazard the guess,  that the diodes with
disturbed SJs are predominantly those oriented in the conveyor belt direction. What
I think is happening is that the leading edge SJ solidifies, the trailing
edge one is still pasty, and the differential contraction on cooling causes the
trailing SJ to be disturbed.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com

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