| | | Helo, | | Is it common for actual print height of solder paste to exceed the stencil thickness. For example If the stencil is 6mill thick and the actual height of the print is apx. 10 or 11mill. What causes the paste height to be greater than the stencil thickness, and what possible remedy can you sugest? | | Thanks.. | Gary: I'd be suprised if there wasn't variation in the height of your solder paste deposits after printing.
The action of the metal squeegee blade on the metal stencil develops a weak electrostatic field across the aperature openings. This field is too weak to bridge aperatures greater than 25 mils. At 25 mils and below, the field is a fact of life and it is strongest at the "far end" of the aperature ... that protion of the aperature that last has contact with the print squeegee blade. This is sort of a field compression effect that is common in electromagnetics. The flood blade couter-balances this somewhat, but it's not really worth talking about. So when you lift the stencil from the board (substrate) you're printing on, the field will tend to pull the paste from the pad as it is moved away from the board. What you're seeing, in the height variation is called field effect peaking (FEP), by the experts. (Grin ;-)) Did I get ya goin' on that?
Seriously Gary, the real issue is solder paste volume, not height. Peaks and valleys will show on your paste "brick" for each pad, due to:
1 rheology of the paste 2 characteristics of the aperature walls 3 slump of the paste 4 method and speed of lifting the stencil
One more point, one popular laser guided paste height measuring machine that costs over $10k performs poorly in gauge R&R studies. Check it out.
TTYL
Dave F
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Helo,
Is it common for actual print height of solde...
- Feb 10, 1999
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