PCB Tech - Common defects and solutions in PCB dispensing process

Common defects and solutions in PCB dispensing process:

1. Wire drawing/tailing

1.1. Wire drawing/tailing is a common defect in glue 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the dispensing pressure is too high, the distance between the glue nozzle and the PCB is too large, the patch glue is out of date or the quality is not good, and the reason is too small. The viscosity of the sheet glue is too good, the glue cannot be restored to room temperature after being taken out of the refrigerator, and the amount of glue is too large.

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

2. The glue nozzle is blocked

2.1. 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 phenomenon of plugging; incompatible glue is mixed.

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

3. Empty play

3.1. The phenomenon is that there is only dispensing action, but no glue output. The reason is that the patch glue is mixed with air bubbles; the glue nozzle is blocked.

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

4. PCB components shift

4.1. The phenomenon is that the components are shifted after the patch glue is cured, and the component pins are not on the pads in serious 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; The component shifts during patching or the initial adhesion of the patch glue is low; the PCB is placed for too long after the glue is placed, and the glue is semi-cured.

4.2. Solution: Check whether the glue nozzle is blocked and eliminate 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)

5. The chip will drop after wave soldering

5.1. 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 PCB component size is too large, and the heat is absorbed The quantity is large; the light curing lamp is aging; the amount of glue is not enough; the component/PCB is contaminated.

5.2. 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 may not reach the peak temperature and it will easily cause the film to fall.

For light-curing adhesives, it is necessary to observe whether the light-curing lamp is aging and whether the lamp tube is blackened; the quantity of glue and whether the components/PCB are polluted are all issues that should be considered.

6. Float/shift of component pins after curing

6.1. The phenomenon of this failure is that the component pins float or shift after curing, and the tin material will enter under the pad after wave soldering, and short circuit or open circuit will occur in severe cases. The main causes are uneven patch glue, excessive patch glue, or component deviation during patching.

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