SMT quality problem summary

Common defects and solutions in dispensing process

Drawing / trailing

Drawing/trailing is a common defect in dispensing. The common causes are too small inner diameter of the nozzle, too high dispensing pressure, too large distance between the nozzle and PCB, expiration or poor quality of the adhesive, too good viscosity of the adhesive, failure to recover to room temperature after being removed from the refrigerator, too much dispensing, etc.

Solutions: Change the nozzle with larger inner diameter; reduce the dispensing pressure; adjust the "stop" height; change the glue, select the suitable viscosity of the glue; the glue should be removed from the refrigerator to room temperature (about 4 hours) and put into production; adjust the amount of dispensing.

Plugging of glue mouth

The reason is that the pinholes are not cleaned completely, the impurities in the glue are mixed into the glue and there is plugging phenomenon, and the insoluble glue is mixed.

Solution: replace the clean needles; replace the adhesive with good quality; the adhesive label should not be mistaken.

Empty beat

The phenomenon is that there is only glue dispensing, but there is no glue output. The reason is that the glue is mixed into the bubble, and the rubber plug is blocked.


Solution: the glue in the syringe should be deactivated (especially for its own glue).

Component shifting

The phenomenon is that the components shift after the adhesive solidifies, and the pins of the components are not on the pad when the adhesive is serious. The reason is that the adhesive quantity of the adhesive is not uniform, such as one more or one less in the two-point glue of the chip components, the component shift or the initial adhesive force of the adhesive is low when the adhesive is mounted, and the PCB is placed for too long after the adhesive is semi-solidified.

Solution: Check whether the glue nozzle is blocked, eliminate the glue non-uniform phenomenon; adjust the working state of the mounter; change the glue; after dispensing PCB should not be too long (less than 4 hours)

After wave soldering, the film will fall off.

The phenomenon is that the bonding strength of the cured components is not enough, which is lower than the prescribed value. Sometimes the chip will fall off when touched by hand.

Solution: Adjust the curing curve, especially increase the curing temperature. Usually the peak curing temperature of the heat curing adhesive is about 150 C, which can not reach the peak temperature and easily cause chip dropping. Problem.

After solidification, component pins float / shift.

This fault is caused by the floating or shifting of the pin after solidification, the solder will enter the pad after wave soldering, and short circuit and open circuit will occur in serious cases.

The solution is to adjust the dispensing process parameters, control the dispensing quantity, and adjust the technological parameters of the patch.

Quality analysis of solder paste printing and placement

Analysis of solder paste printing quality

The quality problems caused by poor solder paste printing are common in the following aspects:

Insufficient solder paste (partial or even whole) will lead to insufficient solder solder joint, open circuit, offset and erection of components.

Solder paste adhesion will result in short circuit and partial deflection after welding.

Overall offset of solder paste printing will lead to poor welding of the whole board components, such as less tin, open circuit, offset, vertical parts and so on.

Solder paste can easily cause short circuit after welding.

Main factors leading to insufficient solder paste

The printing machine did not replenish the solder paste when working.

Solder paste is of unusual quality, mixed with hard lumps and other foreign matter.

Previously unused solder paste has expired and been used for two times.

Circuit board quality problems, solder pad with an inconspicuous cover, such as the solder inhibitor printed on the pad (green oil).

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

Solder paste ointment screen uneven thickness.

Solder paste leaks contaminants from printed screen boards or circuit boards (e.g. PCB packaging, screen wiping paper, floating foreign bodies in the ambient air, etc.).

Solder paste scraper damage, mesh damage.

Equipment parameters such as pressure, angle, speed and stripping speed of solder paste blade are not set properly.

After the solder paste printing was finished, the human factor was accidentally knocked off.

Main factors leading to adhesion of solder paste

The design defect of PCB is too small.

The problem of web board is not properly positioned.

The net board is not wiped clean.

The problem of net plate makes solder paste fall off badly.


Solder paste has poor performance, viscosity and failure.

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

Equipment parameters such as pressure, angle, speed and stripping speed of solder paste blade are not set properly.

After solder paste printing is finished, artificial factors are squeezed and adhered.

Main factors leading to solder paste printing overall deviation

The positioning datum points on the PCB are not clear.

The positioning datum points on the circuit board and the datum points of the grid board are not positive.

The clamping of the circuit board is loose in the printing machine. The locating thimble is not in place.

Failure of optical positioning system of printing machine.

The solder paste web print opening does not conform to the design document of the circuit board.

The main factors leading to the printing of solder paste

Solder paste viscosity and other performance parameters are problematic.

There is a problem in setting the release parameters when the circuit board is separated from the print screen.

The hole wall of burr holes is burr.

Patch quality analysis

The common quality problems of SMT patches include missing parts, side parts, turning parts, deviation parts, damaged parts and so on.

Main factors causing patch leakage

Component feeding rack (feeder) feeding is not in place.

The gas inlet of the element suction nozzle is blocked, the suction nozzle is damaged, and the suction nozzle height is not correct.

The vacuum gas path of the equipment is jammed.

Poor incoming circuit boards, resulting in distortion.

No solder paste or solder paste is too small on the pad of the circuit board.

The quality of components is the same.