Analysis of the causes of PCBA SMT defects

In the production process of PCBA SMT, due to the influence of operating errors, it is easy to cause PCBA SMT defects, such as: virtual soldering, short circuit, warping, missing parts, solder beads, warped feet, floating height, wrong parts, cold welding, reverse Toward, white/reverse side, offset, component damage, less tin, too much tin, gold finger sticking tin, glue overflow, etc. It is necessary to analyze these defects and make improvements to improve the quality of the product.

Analysis of the causes of PCBA SMT defects
  1. Air welding

The specificity of red glue is weak; the opening of the stencil is poor; the distance between copper and platinum is too large or the copper is pasted on small components; the scraper pressure is high; the flatness of the components is poor (legs warped, deformed) in the preheating area of the reflow furnace The temperature rises too fast; the copper and platinum in the PCB is too dirty or oxidized;

The PCB board contains moisture; the machine patch is offset; the red glue printing is offset; the machine’s splint track is loose, causing patch offset; the MARK point is mistakenly illuminated, causing components to be deflected, resulting in empty soldering;

  1. Short circuit

The spacing between the stencil and the PCB board is too large, causing the red glue to be printed too thickly and causing a short circuit; the height of the component patch is set too low, which will squeeze the red glue and cause a short circuit; the reflow oven heats up too quickly; the component patch is offset; the network Poor board openings (too thick, too long lead openings, too large openings); red glue cannot bear the weight of the component; deformation of the screen or scraper causes red glue printing to be too thick; red glue has strong specificity ; The roll-up of the sealing tape at the empty mounting points causes thicker red glue printing on peripheral components; the reflow soldering vibration is too large or not level;

  1. Stand upright

The copper and platinum on both sides are of different sizes, resulting in uneven pulling force; the preheating temperature rise rate is too fast; the machine patch is offset; the thickness of the red glue printing is uniform; the temperature distribution in the reflow furnace is uneven; the red glue printing is offset; the machine track The splint is loose, causing the patch to shift; the machine head shakes; the red glue is too specific; the furnace temperature is improperly set; the distance between copper and platinum is too large; the MARK point is mistakenly illuminated, causing the component patch to shift.

  1. Missing parts

The vacuum pump carbon chip is defective and the vacuum is insufficient, resulting in missing parts; the suction nozzle is clogged or defective; the component thickness test is improper or the detector is defective; the patch height is improperly set; the suction nozzle blows too much or does not blow; the suction nozzle vacuum setting Improper setting (applicable to MPA); special-shaped components are placed too fast; the head air pipe is broken; the gas valve sealing ring is worn; there are foreign objects on the side of the reflow furnace track and the components on the wiper board;

  1. Tin beads

Insufficient preheating of reflow soldering and heating up too quickly; the red glue is not reheated after being refrigerated; the red glue absorbs moisture and causes splashing (the indoor humidity is too high); there is too much moisture in the PCB board; excessive diluent is added; the stencil is opened Improper hole design; uneven tin powder particles.

  1. Offset

The positioning reference point on the circuit board is unclear; the positioning reference point on the circuit board is not aligned with the reference point on the stencil; the fixed clamp of the circuit board in the printing machine is loose, and the positioning mold ejector pin is not in place; the optical positioning system of the printing machine There is a fault; the solder paste is leaking out of the stencil opening and does not match the design document of the circuit board.

To improve the defects of PCBA patching, strict inspections need to be carried out in every link to prevent problems from the previous process from flowing to the next process as little as possible.

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