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This article will continue to explain PCBA processing and SMT related knowledge for you. The following is about the relationship between manufacturability design and manufacturing can be summarized into the following two points.
(1) PCBA's manufacturability design determines the PCBA's soldering through rate level. Its influence on soldering yield is congenital, and it is difficult to compensate by optimizing on-site process.
(2) Design for manufacturability determines production efficiency and production costs. If the PCBA process design is unreasonable, additional trial production time and tooling may be required. If it cannot be solved, it must be completed by repair. These have reduced production efficiency and increased costs.
The following is an example of 0.4mmQFP.
0.4mmQFP is a widely used package, but it is also the top ten package with poor soldering. The main poor welding performance is bridging and open welding.
0.4mmQFP bridging and open welding phenomenon
The reason why 0.4mmQFP is prone to bridging is that the spacing between the leads is relatively small, generally only 0.15~0.20mm, and it is more sensitive to changes in the amount of solder paste. If the solder paste printing is slightly thicker, it may cause bridging. Therefore, the usual improvement measures are to reduce the thickness of the stencil for solder paste printing, but this may result in more open soldering. If a relatively large solder paste volume process window can be provided, then the soldering yield can be effectively improved.
From the perspective of process design, two problems need to be solved: one is how to control the change in the amount of solder paste; the other is how to reduce the influence of the amount of solder paste on bridging. If these two problems can be solved, the welding quality of 0.4mmQFP can be well controlled.
The following describes the solder joint structure of 0.4mmQFP and the principle of solder paste printing.
The molten solder spreads on the surface of the pad and the pin, and the width of the pad determines the amount of molten solder absorbed. The influence of the thickness of the solder mask on the sealability between the stencil and the pad-if the solder mask is thicker, the amount of solder paste will increase.
After understanding these two points, the 0.4mmQFP process design can be carried out. Specifically, through the integrated design of the pad, solder mask and stencil, it can effectively control the fluctuation of the solder paste volume and reduce the sensitivity of the solder paste volume to bridging. Spend.
If the PCB pad is designed to be wider, the stencil window is designed to be narrower, and the solder mask between the pads is removed, then a stable amount of solder paste can be obtained (the effect of the solder mask on the printing thickness of the solder paste is removed ), a weld structure that can adapt to changes in the amount of solder paste (wide and narrow stencil openings), thereby achieving the process goal of less or no bridging. Practice has proved that such a design can completely solve the bridging problem of 0.4mmQFP.
Of course, the design is just an idea, and other designs can be carried out according to the capabilities of the PCB factory.
Through the above content, it is explained that we should pay attention to process design, give process design the same status as hardware design, and create high quality products.
