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Kester paste problems

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Kester paste problems | 22 May, 2007

We now have an AOI in our process after printing and have noticed that all the small aperatures are getting clogged by solder paste. Mainly on all fine pitch parts. We are using Kester ws R562 SN63/PB37 mesh -325+500 90% metal. It was manufactured in April of 07. We have tried adjusting the stencil printer but had no effect on it. We also had the stencil printer Manuf. in our facility and he tried to adjust it also and had no luck. He recommended a smaller mesh size. Just wondering if anyone else has any ideas or the same problems as I do. Thanks for any input.

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Kester paste problems | 22 May, 2007

I would consider calling Kester and telling them they need to send their people out. I could not really see mesh size being an issue on anything 20mil and above.

There are several other issues that can cause aperture clogging as well...:

Incorrect stencil aperture design. (The area and aspect ratio must be correct.) Stencil seperation speed Underside board support Temperature and humidity Squeegee pressure name a few.

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Kester paste problems | 22 May, 2007

What are the area and aspect ratio of the apertures that clog?

When you print, you should be able to fit at least 4 of the mean sized solder sphere across the width or diameter of your smallest apertures.

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Kester paste problems | 24 May, 2007

Here's some quick things to look for:

1. Does your solder paste feel like concrete, does it roll-up into your squeegee blades, and stick to your stencil? If so, your problem is the solder paste. The mixture of powder and flux, or metal load is out of wack. Your particle size is a type 3, which is pretty common for standard 20-mil pitch technology.

If it's your paste, call your rep or salesman and threaten that you're going to go to a different manufacturer if you don't get quick resolution. Have them check the lot code, etc. and/or send back a jar and have them check viscosity.

2. Aspect the other fellas pointed out. That's your aperture/stencil thickness. There are varying guidelines for different technologies. Some rules-of-thumb, for 20-mil pitch, the range is 0.81 - 1.27, and for 16-mil pitch, 0.62 - 1.12.

Have you measured this yet?

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Kester paste problems | 24 May, 2007

We have a similiar problem with kester em907, a pb free paste. You say that your AOI system picks up blocked apertures, do you mean that the printers own camera looks at the stencil and sees apertures blocked? Or, is it picking up insufficient paste on the pads? When we set up our printer to check the stencil after a print, we seem to always get some blocked apertures, up to 40-50%, but have noticed that the print quality is never really effected by this(assuming we never leave the paste long enough to dry into the apertures). If this is the case you may just want to turn off your stencil inspection, I think it's a rarely used option anyway(if the apertures really are blocked you'll get insufficients )

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