Since printing solder paste is a key process to ensure the quality of SMT assembly, the quality of printing solder paste must be strictly controlled.
When there is a narrow pitch (lead center distance is less than 065mm), it must be fully inspected.
When there is no narrow pitch, it can be tested regularly (such as once every hour), or it can be sampled according to the sampling rules shown in the figure below.
I. Inspection method
The inspection methods are mainly visual inspection and solder paste inspection machine inspection.
1. Visual inspection, use 2 ~ 5 times magnifier or 3.5 ~ 20 times microscope.
2. Use a paste inspection machine (SPI) for inspection at narrow pitches.
The solder paste printing process is very unstable in PCB production compared to other processes. According to research by many companies and universities, the maximum change in this process is 60%. This is because there are many related process parameters involved in the solder paste printing process. About 35 parameters need to be controlled. These parameters include solder paste type, environmental conditions (temperature, humidity, etc.), template types (chemical corrosion, laser cutting, Laser cutting and polishing, electroforming), stencil thickness, opening shape, aspect ratio, area ratio, printer model, scraper, print head technology, printing speed, etc. These factors greatly reduce the repeatability of printing.
Generally, 2DSPI detection is sufficient. Can be tested on the whole board or locally. The test points for the whole board test should be selected from the upper, lower, left, right and middle points on the printed surface.
Inspection requires that the thickness of the solder paste range from -10% to + 15% of the thickness of the template.
For narrow pitch QFP, CSP, 01005, POP and other packages, 3DSPI solder paste inspection machine should be used for inspection.
The inspection standards shall be implemented in accordance with the enterprise standards formulated by the unit or other standards (such as IPC standards or SJ / T10670-1995 general technical requirements for surface assembly processes).
All PCBA processes must pay attention to the control of details, and the printing of solder paste is the starting point of the entire PCBA or SMT processing. A good start is the beginning of everything. When we do each step, we leave it to the customer A quality product is not far away.
The above is the pcb manufacturing knowledge provided by fastpcba technicians, I hope to help you!