Circuit board assembly foundries BGA has a short circuit problem, the result of the analysis is caused by excessive amount of solder paste, and too much solder paste is because the "green paint (solder mask) and silk screen layer (silkscreen)" printing thickness too Thick caused by.
In theory, the inconsistency of the thickness of the green paint and silk screen layer is likely to cause the difference between the solder paste thickness and the amount of solder paste, because in all solder paste printing conditions are the same, thicker green paint and silk screen layer Resulting in steel (stencil) and circuit board (PCB) of the gap is too large, the amount of printed solder paste will increase.
PCB circuit board
Do not know if you have touched this similar problem?
The following is a basic size of the BGA specifications:
Ball pitch: 0.65mm
Ball diameter: 0.36 to 0.46 mm
Ball height: 0.23 to 0.33mm
According to the analysis of the SMT processing plant, because this board has a total of two different suppliers, and the actual measurement of the two suppliers of green paint (solder mask) and silk screen layer (silkscreen) after printing found Its thickness difference of 27.1um, so it will cause the difference in the amount of solder paste.
In all the conditions of solder paste printing are the same, the actual use of these two different circuit board manufacturers to produce the circuit board to print solder paste, measured the amount of solder paste and found the two of the solder paste volume A difference of nearly 16.6%. So SMT factory to infer the short circuit problem in the PCB manufacturers. How many of these inferences have some questions?
First of all, if the solder paste printing for the BGA short circuit has such a significant impact, why the beginning of the production, SPI (solder paste inspection machine) did not catch out, but to wait until Reflow to see the results of X-Ray? This is not a bit too late?
Second, the amount of solder paste this work is too much margin is too small, PCB manufacturers can be so precise control of green paint and silk screen layer thickness? After the production is not or will continue to appear similar problems?
In fact, the difference between 16.6% of the amount of solder paste can be used to adjust the speed of the scraper and scraper pressure back, after all, just caused by the gap, the speed of the scraper or increase the pressure can increase the difference between the amount of solder paste. The focus is that the SPI does not really grasp the focus of the grasp, for example, the overall amount of solder paste should be controlled in what range, that is, can not only look at the area of solder paste printing, but also calculate the thickness of solder paste and calculate The entire pad of tin, so as to be able to control the quality.
In addition, the PCB manufacturers of green paint and silk screen thickness of the printing capacity should also be standard down, when the feed will be included in the inspection project, so as not to cause future production rate of yield impact.