Paste Penetration in Via for Paste Through Hole

Help! We have been doing some trials on Paste Through Hole and have encountered a problem. The screen is designed (as is suggested in technical info re PTH) to give double the paste volume required to fill the void between the pin and via. The problem we are encountering is that a) we are finding it very difficult to get the calculated volume into the via, and b) approximately 1/5th of the volume is then displaced onto the pin end after insertion. I guess that the latter is best solved by putting more volume around the pin rather than in the via. When the board is reflowed we therefore have only about 50% fill in the via's. This, unfortunately does not conform to our customers inspection standards. Is there a trick that we are missing ie different print blade angles, screen design etc?

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| Help! | We have been doing some trials on Paste Through Hole and have encountered a problem. The screen is designed (as is suggested in technical info re PTH) to give double the paste volume required to fill the void between the pin and via. The problem we are encountering is that a) we are finding it very difficult to get the calculated volume into the via, and b) approximately 1/5th of the volume is then displaced onto the pin end after insertion. I guess that the latter is best solved by putting more volume around the pin rather than in the via. When the board is reflowed we therefore have only about 50% fill in the via's. This, unfortunately does not conform to our customers inspection standards. | Is there a trick that we are missing ie different print blade angles, screen design etc?

One thing to remember is that solder wets all wettable surfaces well and likes to flow downhill. With sufficient paste volume on surface through hole pads, and some in the holes and on pins, effective wetting will occur. Of course it's a littl more scientific that this, but you get the basic picture. Earl Moon

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