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SMT quality problems: common defects and solutions in the point glue process


Wire drawing /Drag tail

Wire drawing / Drag tail is a common defect in point glue, and the common causes are: the inside diameter of the nozzle is too small, the point glue pressure is too high, the distance between the nozzle and the PCB is too large, the expired or bad quality of the adhesive, The viscosity is too good, it fails to return to room temperature after taking out from the refrigerator, the amount of point glue is too large, etc.

Solution: Change the glue tip with a larger inner diameter; reduce the point glue pressure; adjust the "stop" height; change the glue, choose the glue of the appropriate viscosity; after removing the patch glue from the refrigerator, return to room temperature (about 4h) Put into production; adjust point glue volume.

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glue mouth Clogged

The failure phenomenon is that the glue nozzle has a small amount of glue or no glue spots come out. The reason is generally that the pinholes are not completely cleaned; impurities are mixed in the patch glue, and there is a phenomenon of hole plugging; incompatible glue is mixed.

Solution: change the clean needle; change the good quality glue;

Runaway

The phenomenon is that only the point glue action is performed, but there is no glue output. The reason is that the patch glue is mixed with air bubbles; the glue mouth is blocked.

Solution: The glue in the syringe should be degassed (especially the glue installed by yourself); replace the glue tip.

Component shift

The phenomenon is that the components are displaced after the chip adhesive is cured, and the component pins are not on the pads in serious cases. The reason is that the amount of adhesive produced by the chip adhesive is uneven, such as one more and one less in two points of chip components; When the component is shifted or the initial adhesive strength of the patch is low; the PCB is left for too long after dispensing, and the glue is semi-cured.

Solution: Check the glue nozzle for blockage to eliminate uneven glue; adjust the working state of the placement machine; change the glue; the PCB placement time after dispensing should not be too long (less than 4h)

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Will drop after wave soldering

The phenomenon is that the bonding strength of the components after curing is not enough, lower than the specified value, and sometimes the chip will be dropped when touched by hand. The reason is that the curing process parameters are not in place, especially the temperature is not enough, the component size is too large, and the heat absorption is large; Light curing lamp is aging; insufficient glue; component / PCB is contaminated.

Solution: Adjust the curing curve, especially increase the curing temperature. Generally, the peak curing temperature of the thermal curing adhesive is about 150 , and the failure to reach the peak temperature will easily cause the film to fall off. For the light curing adhesive, it should be observed whether the light curing lamp is aging. Whether the tube is blackened; the amount of glue and whether the component / PCB is contaminated are issues that should be considered.

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Floating / shifting of component pins after curing

The phenomenon of this kind of failure is that the component pins float or shift after curing, after soldering, the solder will enter the pad, and in severe cases, short circuits and open circuits will occur. The main causes are uneven patch glue and patch glue amount. Excessive or component offset when mounting.

Solution: Adjust the dispensing process parameters; control the amount of dispensing; adjust the placement process parameters.


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