PCBA Tech - Summary of some problems in SMT patch processing quality

Common defects and solutions in the dispensing process

Drawing/tailing

Wire drawing/tailing is a common defect in dispensing. The common causes are that the inner diameter of the glue nozzle is too small, the dispensing pressure is too high, the distance between the glue nozzle and the PCB board is too large, the patch glue is out of date or the quality is not good, and the patch glue The viscosity is too good, it fails to return to room temperature after being taken out of the refrigerator, the amount of glue is too large, etc.

Solution: Change the glue nozzle with a larger inner diameter; reduce the dispensing pressure; adjust the "stop" height; change the glue and choose a glue with a suitable viscosity; the patch glue should be returned to room temperature (about 4h) after being taken out of the refrigerator Put into production; adjust the amount of glue.

Clogged glue nozzle

The fault phenomenon is that the amount of glue from the glue nozzle is too small or there is no glue point. The cause is generally that the pinhole is not completely cleaned; impurities are mixed in the patch glue, and there is a plugging phenomenon; incompatible glue is mixed.

Solution: Change a clean needle; change a good quality patch glue; the brand of patch glue should not be mistaken.

Empty play

The phenomenon is that there is only dispensing action, but no amount of glue. The cause is that the patch glue is mixed with bubbles; the glue nozzle is blocked.

Solution: The glue in the injection cylinder should be debubbled (especially the glue installed by yourself); replace the glue nozzle.

Component shift

The phenomenon is that the components are shifted after the patch glue is cured, and the component pins are not on the pads in severe cases. The cause is that the amount of glue out of the patch glue is uneven, for example, the two-point glue of the chip component is more than one and the other is less; When the component shifts or the initial adhesion of the patch adhesive is low; the PCB is placed for too long after the glue is dispensed, and the glue is semi-cured.

Solution: Check whether the glue nozzle is blocked and eliminate the uneven glue output; 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)

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 there will be chipping 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; The light curing lamp is aging; the amount of glue is not enough; the component/PCB is contaminated.

Solution: Adjust the curing curve, especially increase the curing temperature. Usually the peak curing temperature of the thermal curing adhesive is about 150 degree Celsius, and the peak temperature can not reach the peak temperature, which may cause the film to fall. For the light curing adhesive, it should be observed whether the light curing lamp is aging., Whether there is blackening of the lamp; the quantity of glue and whether the components/PCB are polluted are all issues that should be considered.

Floating/shifting of component pins after curing

The phenomenon of this kind of failure is that the component pins float or shift after curing, and the tin material will enter under the pad after wave soldering. In severe cases, there will be short circuits and open circuits. The main reasons are uneven patch glue and the amount of patch glue. Too much or component offset during placement.

Solution: Adjust the dispensing process parameters; control the dispensing volume; adjust the patch process parameters.

Solder paste printing and chip quality analysis

Analysis of solder paste printing quality

Common quality problems caused by poor solder paste printing are as follows:

Insufficient solder paste (partially lacking or even overall lacking) will lead to insufficient solder joints of components after soldering, components open circuits, components offset, and components stand up.

The adhesion of the solder paste will cause the circuit to be shorted and the components to be offset after welding.

The overall offset of solder paste printing will cause poor soldering of the entire board components, such as less tin, open circuit, offset, vertical parts, etc.

The tip of the solder paste can easily cause a short circuit after soldering.

Main factors leading to insufficient solder paste

When the printing press was working, the solder paste was not added in time.

The quality of the solder paste is abnormal, and foreign objects such as lumps are mixed in it.

The solder paste that has not been used up has expired and is used twice.

The quality of the circuit board, there is an inconspicuous covering on the pad, such as the solder resist (green oil) printed on the pad.

The fixed clamping of the circuit board in the printing machine is loose.