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Solder resist causes short circuits?

Flyback

Well-Known Member
Thread starter #1
Hello,
We have just found out that our LED footprint is wrong. We have had 200 LED MCPCB’s made up (100 LEDs on each one) with solder resist between the LED pads. –Now we find out that there should be no solder resist between the LED pads.
Do you think we must throw all these LED MCPCBs away?
The LED is the "WICOP2 Z8 Y19" LED by Seoul Semiconductor.
Page 8 of the Seoul Semiconductor App Note called “Wicop” says that there should be no solder resist between the pads of the LED. They say that it is because flux residue can cause short circuit between the LED pads and the flux residue more readily settles on top of solder resist.

Page 8 of the following....("PSR" = Photo Solder Resist")
http://www.seoulsemicon.com/_upload/Goods_Spec/WICOP_Application_Note_Rev2.0.pdf

So Since when did solder resist *cause* short circuits?
 

jpanhalt

Well-Known Member
Most Helpful Member
#2
There is a recent thread on another forum about conductive flux residue. Apparently some of the no-clean and/or water-washable fluxes can conduct. I do not see the connection between that and solder resist. Without solder resist, wouldn't the flux still be there unless it is cleaned off? Can the boards by cleaned ultrasonically?

John

Edit: Didn't read everything in the link. But what I read seemed to say that resist alone can cause faults in the soldering (e.g., balling), which can cause shorts. Since it is mentioned several times, I would take it seriously. Moreover, there doesn't seem to be a way to fix such things once they happen. So, it is an and/or situation.

If the problem is caused only by conductive flux, then cleaning (if possible) might help.
 
Last edited:

Flyback

Well-Known Member
Thread starter #3
Thanks, i thought solder balling due to "solder resist on pad" was addressed by having a sufficient margin between the "edge" of the solder resist and the edge of the pad?..i thought with a 0.15mm clearance ...ie the solder resist is 0.15mm outside the pad edge....then you were fine?
 
#4
I think the issue is tiny 0.3mm gap between pads. If leveled by resist rather than having a recess from the copper thickness, there is more chance of solder balls holding the LED away from the substrate. Personally, I wouldn't think it a serious issue.
 

Flyback

Well-Known Member
Thread starter #5
Thanks yes, but then you would have to apply the same reasoning to 0402 and 0603 resistors...and avoid having solder resist between their pads?
 

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