In PCB processing, chip capacitors, BGA and other components have one thing in common, that is, they are afraid of the effect of "stress", especially the effect of multiple stress (such as installing multiple screws) and overstress (such as falling). Therefore, we can classify them as stress change sensitive components.

During the PCBA soldering process, one-handed boards, plug-in pressing, board slicing, manual pressing, mounting screws, ICT testing, FA testing, turnover transmission and other links may generate additional stress, causing the solder joints of these components to crack or "break".

The design of assembly reliability can mainly be optimized through the network layout to reduce the generation of environmental stress or improve the ability to resist stress damage. It should be noted that the design improvement is only one aspect, and the effective method is to increase the strength of the solder joints and reduce the generation of stress. Therefore, the design/recommendations in this section are for general reference only.

(1) Place stress-sensitive components away from pcb components to avoid their possible bending.

For example, in order to eliminate the deformation of the bending structure during the assembly of the daughter board, the connector that connects the daughter board to the mother board should be placed on the edge of the daughter board as much as possible, and the distance between the screws should not exceed 10mm.

For another example, in order to avoid stress fracture of BGA solder joints, BGA layout should be avoided where PCB assembly is easy to bend.

(2) Analyze and strengthen the four-corner model of the large-size BGA

When the PCB is bent, the solder joints of the four corners of the BGA receive the greatest force and are most likely to crack or break. Therefore, the analysis and reinforcement of the BGA four-corner model is very important and effective to prevent the cracking of the corner solder joints.

It can be reinforced with special glue or SMT patch. This requires space for component layout, and the requirements and methods of reinforcement work should be indicated on the process technical documents.

These two suggestions are mainly considered from the design aspect. In addition, the assembly process should be improved to reduce the generation of stress, such as avoiding the use of single-handed plates, mounting screws and supporting tooling. Therefore, the design of assembly reliability should not be limited to the improvement of the layout of the components, and more importantly, we should start from the enterprise to reduce the stress of assembly by choosing appropriate methods and tools to strengthen the staff of the SMT chip processing plant. Training, standardized management and technical actions, only through this, students can effectively solve the problem of solder joint failure that can occur in the assembly stage.

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