Solder resist between LED pads causes short circuit between these pads?

[treez]

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

Since when did solder resist *cause* short circuits?

 

[Kajunbee]

I did not think that flux would be conductive either.

 

[betwixt]

I think the issue is the 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. Perhaps for the next batch of boards you should have it removed but still use the ones you already have.

Of course if you really want to throw them away, I'll send you directions to my bin :grin:

Brian.

 

[c_mitra]

http://www.seoulsemicon.com/_upload/...ote_Rev2.0.pdf

The document referenced also says

The performance of an WICOP can be impacted by the color of the silk screen used on the PCB.

on page 11. I am impressed.

 

[marce]

I would always use solder resist, had to many shorts in production when solder resist is not used...

 

[c_mitra]

I would always use solder resist, had to many shorts in production when solder resist is not used...

I too strongly agree; if the manufacturer does not agree, they should give a convincing explanation. The solder resist has a smoother and more hydrophobic surface (compared to the etched and exposed board).

 

[Kajunbee]

The document suggest that removing psr makes it less susceptible to flux shorting. I fail to see how this would make a difference. Seems to me that removing psr would make a common area for flux to pool together. Seems to me that removing the flux after reflow we be and issue. Is this a common problem with flip chip technology. Meaning that since the led covers the pads it cannot be cleaned thoroughly or inspected for other problems such as solder bridging etc.

 

[marce]

I always recommend boards are cleaned, there is no such thing as a no clean flux, that is just an advertising slogan...