What should be considered in the assembly of PCBA

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The solder paste printing process mainly solves the problem of solder paste printing volume consistency (filling and transfer), not the problem of solder paste demand for each solder joint. In other words, the solder paste printing process solves the problem of fluctuating soldering flux, not the problem of high or low flux. To solve the problem of high and low through rate, the key lies in the distribution of solder paste. Through the optimization and matching design of pads, solder masks and stencil windowing, the amount of solder paste is allocated to each solder joint as required. Of course, the consistency of the amount of solder paste is also related to the design, and the different process capability indexes provided by different designs of PCB solder masks are different.

1. Area ratio
Area ratio refers to the ratio of the opening area of the steel mesh to the opening wall area

2. Transfer rate
The transfer rate refers to the ratio of the solder paste deposited on the pads in the stencil opening window during printing. It is expressed by the ratio of the actual amount of solder paste transferred to the stencil opening volume.

3. Effect of area ratio on transfer rate
Area ratio is an important factor affecting solder paste transfer. Generally, the area ratio is required to be greater than 0.66 in engineering. Under these conditions, a transfer rate of more than 70% can be obtained.

4. Area comparison design requirements
The area ratio has requirements on the design of the stencil, which mainly affects fine-pitch components. In order to ensure the area ratio requirement of the fine pad stencil opening window, the thickness of the stencil must meet the area ratio requirement. In this way, for components that require a large amount of solder paste, the amount of solder paste needs to be increased by increasing the window area of ​​the stencil—this requires space around the pad deformation, which is a major consideration in component pitch design.

on sum up, PCB assembly activities can be categorized in a number of ways according on component kinds, manufacturing volume, automation level, and location. The majority of small and medium-sized businesses use outsourced assembly as a way to cut costs. Mass production uses automated SMT lines with high volume. For big components, through-hole assembly is still common, while surface-mount technology is more popular for smaller parts. Assembly procedures and partners must change together when products go from prototypes to large-scale manufacturing in order to create high-quality boards at competitive prices.