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16 mil pitch QFPs / solder paste type 4 & 5?


K

#24034

16 mil pitch QFPs / solder paste type 4 & 5? | 7 April, 2003

I have just started an assembly which uses a 16 mil pitch QFP. We ran on Saturday, using a type 3 solder paste. (working Sat's really sucks)

We really struggled with paste release after the stencil sat for 15 minutes without printing. Apertures clogged up and were difficult to clean. We tried 3 different solder pastes with similar results. (Kester R562, Alpha WS-609, and AIM WS-483)

Our apertures are 8 mil wide.

1. When aperture widths drop below 9-10 mil is a type 4 or 5 paste required?

2. If we go with a type 4 or 5 paste, will we create other process challenges?

Interested to hear your thoughts/experiences.

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RDR

#24035

16 mil pitch QFPs / solder paste type 4 & 5? | 7 April, 2003

How thick is your stencil? We print 16mil every day with type 3 paste 2 of which you have tried (Alpha and Kester) Stencil should be 5 mil.

Russ

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#24036

16 mil pitch QFPs / solder paste type 4 & 5? | 7 April, 2003

Russ is correct. A 5 thou thick stencil should give good release.

Here's how you run through it: * You want at least 5 solder particles across the smallest aperture. So in your case, 8 thou wide, which equals to 0.20mm gives us 0.20/5=40 microns. That means that you should be fine with a standard Type 3 paste. * You need to consider area and apect ratios on your stencil to get Russ's 5 mil number. [Search the fine SMTnet Archives for background.]

So, tell us about your stencil underside wiping approach. As you reduce the width of your apertures, wipe frequency needs to increase.

Compared to Type 3, the Type 4 & 5 pastes you're discussing are more: * Expensive * Slumpy

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K

#24041

16 mil pitch QFPs / solder paste type 4 & 5? | 7 April, 2003

Sorry, I forgot to include the stencil thickness. It is 5 mil.

Thanks for the input, everbody.

I just checked with our stencil supplier & I thought that Electro-polish was standard with lazer, but I now find out it is not. Our stencil did not get the electro-polish! So that could be part of the problem.

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