Causes of SMT Assembly missing paste printing and less tin 

1. The principle of solder paste printing

The solder paste is squeezed into the stencil through a scraper so that the solder paste contacts and adheres to the PCB surface. Then the solder paste that adheres to the PCB surface during demolding overcomes the resistance of the stencil wall and is transferred to the PCB surface.

2. Observe, think & compare

A>Although part of the substrate around the pad is covered by the stencil opening during printing, the solder paste at the bottom of the stencil opening is still difficult to contact the PCB pad and the surrounding substrate. As a result, it is not enough to overcome the hole wall during demolding. Resistance (Eg. Only a small amount of solder paste on the pad)?

B>There is a 35 um deep annular pit between the pad and the solder mask. Does the solder paste with the stencil opening on the pit not touch the bottom of the pit?

C>Why are other pads connected to the circuit not easy to miss?

3. Bare copper plate printing verification

The solder paste of 5 different brands of 4# powder can be stably tinned (laser + electro-polished stencil ) on a circular hole with a thickness of 0.1 and an opening diameter of 0.28.

Solutions of SMT Assembly missing paste printing and less tin

1. Find all the pads that are not connected to the outer circuit, change the size of these pads from the original circle with a diameter of 0.27 to a circle with a diameter of 0.31; Also, reduce the area of the deep pit around the pad, and make the original opening area on the deep pit become on the copper foil of the pad, so that the gap between the opening area on the deep pit and the bottom of the stencil is reduced. After the small batch verification is OK, the original stencil is used for mass production, and the soldering of the soldering pads that are difficult to solder is good (the area of the soldering pad is increased, and no poor soldering is found in the batch verification);

2. Reduce the PCB solder mask thickness and the influence of the higher height solder mask layer on the circuit near the pad. Generally, it is recommended that the PCB solder mask thickness should be less than 25um.

3. The new PH stencil is used to eliminate printing gaps to the maximum extent. PH stencil is introduced.

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▪ The maintenance engineers have sophisticated experience so that those defective boards caused by all kinds of smt soldering can be repaired. Meanwhile, ensuring the connectivity rate of each circuit board also make sense.

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