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October 2012

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Subject:
From:
Steve Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory <[log in to unmask]>
Date:
Fri, 5 Oct 2012 12:18:42 -0400
Content-Type:
text/plain
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Phil,

Send the photo my way, I get it posted...

Steve

-----Original Message----- 
From: Phil Nutting
Sent: Friday, October 05, 2012 9:55 AM
To: [log in to unmask]
Subject: [TN] solder splash under SMT caps

Here is my question for today.

We have an SMT board that measures 16 inches by 17 inches be 0.062 thick (it 
is well supported in the end application) with 32 voltage multiplier 
circuits so the board develops 12.5kV.  On the 2225 capacitors we had 
failures (arcing) under the parts and our customer removed some caps only to 
find solder splashes.  We used the standard IPC pad configuration for a 2225 
part (3.7mm gap) with no solder mask under the capacitor, ENIG finish and I 
suspect tin/lead paste.  I have a photo I will post if Steve Gregory is 
still offering his site.

Our customer wants us to use a specific capacitor by AVX and the AVX pad 
layout of 4.6mm gap. Our contract manufacturer wants us to open up the gap 
between pads by 1mm (essentially the 4.6mm recommended by AVX).

My theory on this failure is the following; (could be any one or a 
combination of any items listed below)
Too much solder paste
Incorrect reflow pre-heat
Incorrect liquidus temp

I have read John Maxwell's white paper and see that he recommends a larger 
space, but he didn't see significant performance enhancement above the 1808 
part.  Refer to 
http://johansondielectrics.com/technical-notes/application-notes/new-impact-of-pad-design-and-spacing-on-ac-breakdown-performance.html.

I agree that this greater distance will help with arcing, but will it help 
with the solder splash?

Will separating the pads have that much affect that the solder will be drawn 
out to the outside visible pads?

Inquiring minds want to know....

Phil Nutting
Design for Manufacturing Engineer
Kaiser Systems, Inc.
126 Sohier Road
Beverly, MA 01915
Phone: 978-922-9300 x1310
Fax: 978-922-8374
e-mail: [log in to unmask]<mailto:[log in to unmask]>
www.kaisersystems.com<http://www.kaisersystems.com>
www.linkedin.com/in/philnutting<http://www.linkedin.com/in/philnutting>


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