PCB Quality Control
Sampling inspection should be carried out on all batches of PCBs prior to solder paste printing. Inspection items include:
· Whether deformation occurs on PCB;
· Whether oxidation takes place on PCB pad;
· Whether scratches, shorts and copper exposure takes place on PCB surface;
· Whether printing is evenly smooth or not.
In the process of process control on PCB performance, sufficient attention should be paid from the beginning to the end. Firstly, gloves have to be worn when picking up PCB boards. Secondly, when visual inspection is applied, the distance between naked eyes and inspected boards should be in the range from 30cm to 45cm with the angle about 30° to 45°. PCB boards should be gently dealt with during inspection to avoid collision or falling and they mustn’t be stacked up or kept upright to stop circuits from being cut. Meanwhile, location holes on boards should be inspected to ensure that stencil openings are compatible with pads on PCB.
Application and Storage of Solder Paste
In the process of SMT assembly, validity of solder paste has to be rigorously monitored to maintain its high reliability. Overdue solder paste mustn’t be applied and purchased solder paste should be kept in cold closet of refrigerator. Uncovered solder paste has to be used within one week. In the process of solder paste application, the workshop temperature should be controlled at approximately 25℃ and RH (Relative Humidity) should be controlled within 35% to 75%. Temporarily-not-used solder paste should be placed away from workshop to stop it from being mixed up with in-use solder paste. When “new” solder paste has to be mixed up with “old” solder paste, the mixing ratio should be 3:1.
Some Control Measures in Solder Paste Printing
Successful solder paste printing should be compatible with the following requirement:
· Printing should be complete;
· No bridging takes place;
· Printing thickness should be evenly smooth;
· No turn-down edge occurs on pad;
- No deviation occurs in printing.
If solder paste printing is found incomplete, PCB board, stencil and scraping blade should be adjusted to make it complete. If bridging takes place in solder paste printing, chips should be inspected with the finest pitch, normally CPU. If solder paste printing isn’t done evenly smooth, scraping pressure should be adjusted. If turn-down edge is found at pad, stencil opening should be inspected to ensure that no blocking is available. If deviation is found in solder paste printing, stencil position should be in time adjusted.