Toby,

This is where the photo I have is useful.  The solder splashes, in the photo, are about 1/3 to ½ the width of the pad gap, one splash per capacitor.  This is seen on about 8 to 10 out of 17 capacitors that were removed.  Splash is usually shaped as a half circle close to the right hand pad in all instances.

Phil

From: Toby Carrier [mailto:[log in to unmask]]
Sent: Friday, October 05, 2012 11:52 AM
To: TechNet E-Mail Forum; Phil Nutting
Subject: Re: [TN] solder splash under SMT caps

What are the size of the solder splash in comparison to the area? Are there numerous small splashes or just one large slpash? Are you finding the splashes under every 2225 capacitor, or just a certain area of capacitors?

Toby

On Fri, Oct 5, 2012 at 9:55 AM, Phil Nutting <[log in to unmask]<mailto:[log in to unmask]>> wrote:
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.
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Toby

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