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Cause And Improvement of Tombstone And Misalignment in SMT


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Tombstone in SMT

Cause: PCB design - size of the two pads is not symmetrical, pad distance is too large or too small, one end of the component can not reach the pad.

Improvement: Design according to the pad design principle of CHIP components, pay attention to the symmetry of pad and pad distance.

Cause: Pick& Place quality - position shifting. Component thickness is not set correctly; Z axis height of the nozzle head is too high, component is dropped from height when placing to PCB (low placement pressure).

Improvement 1: Improve the placement accuracy. Precisely adjust the accuracy of the first article.

Improvement 2: Correct the placement coordinates when the position is found to be shifting during continuous production.

Improvement 3: Set correct component thickness and mounting height.

Cause: Component quality - oxidation or contamination of the solder pin or poor adhesion of the pin. The pin of the components is not wetted or the pin falls off during welding.

Improvement 1: Reinforce incoming inspection system.

Improvement 2: Reinforce first soldering article inspection.

Improvement 3: Check quality after replacement component. If any components are found to be defective, replace the components in time.

Cause: PCB quality - Pad is contaminated (silkscreen, solder mask on Pad or oxidation etc).

Improvement: Reinforce incoming inspection system, scrape off silkscreen and soldermask on PCB pads when necessary.

Cause: paste printing process - the amount of solder paste on two pads is inconsistent.

Improvement 1: Remove the solder paste from the stencil hole, clean the bottom of the stencil regularly.

Improvement 2: If the stencil opening is too small, increase the size of the opening.

Cause: whether the conveyor belt vibration caused the components position to shift.

Improvement 1: Conveyor belt is too loose, remove 1 to 2 chain links.

Improvement 2: Check whether the height and distance of the guide rails at the entrance and exit matching well.

Improvement 3: Carefully handle of PCBs in every process.

Cause: whether the hot air speed in reflow oven is too large.

Improve: Adjust the hot air speed.

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