Problems caused by mechanical process factors of BGA solder joints
A product is shown in Figure 7-53. The PCB surface treatment is OSP, and the lead process and lead-free process are used successively. The BGA in the picture has a virtual soldering of about 3/1000, and the position is fixed. They are all located in the figure. Location.
1) Analysis of process conditions
Peak temperature: 238-240℃;
Time above 220℃: 58-60.7s;
Total furnace time: 300s;
Heating rate of reflow welding: 2.5℃/s.
From the welding temperature curve, there is no major problem, and from the normal solder joint slice diagram, the shape of the solder joint is also very good. The place where the cracked solder joint appears is not the common BGA four-corner part, but a middle position closer to the fixed edge, as shown in Figure 7-54.
2) Analysis of assembly and welding process
There are two conditions for the fracture of BGA solder joints, one is weak solder joint strength, and the other is stress. Check the assembly and welding process, the link that may cause stress is the ICT test.
According to the test fixtures used, the tested single boards were separately counted on defects, and it was found that all the problematic boards came from the same test fixture. After further analysis, it was confirmed that the cause of the BGA solder joint fracture was the indenter near the fractured solder joint in the test fixture, as shown in Figure 7-55. Remove the indenter to solve the problem.
During single board assembly and welding, steps such as screw assembly, testing, and turnover may generate relatively large stresses, posing a threat to the accessory BGA.
In fact, many fractures of BGA solder joints do not occur during the welding process, but occur during assembly, turnover, and transportation. The “operations” of these processes are difficult to reproduce and determine, and it is difficult to identify the cause. However, in most cases, it can be inferred from the occurrence stage and operation of the failed board.
The above is the information that Jingbang provides to you about the problems caused by the mechanical process factors of the BGA solder joints in the SMT chip processing plant.