Failure Analysis for Vibration Stress on Ball Grid Array Solder Joints

During the research and development of a new type of train control system, the trial products continued to fail during the vibration test process and later assembly process. After the circuit location analysis, it is confirmed that the circuit is blocked due to a chip area on the circuit board. This paper mainly analyzes the root cause of circuit blocking and puts forward improvement measures. First, Nondestructive analysis including Appearance inspection, C-mode Scanning Acoustic Microscope (C-SAM), X-Ray and Computed Tomography (CT) was performed on the failure sample, but no abnormality was found. Then, the result of cross section analysis showed that the some solder joints under the chip were cracked, and more closer the corner, more serious for cracking. With the help of scanning electron microscope (SEM), it was suspected that chip side corner solder joints were subjected to large stress and caused cracking. The source of specific stress for solder joint cracking is further analyzed. Shadow moire analysis of ball grid array (BGA) chip and printed circuit board (PCB) showed that both of them had slight warping during reflow soldering process. However, the warping of both parts was small and the direction of warping was the same. It would not cause the huge stress of solder joints cracking during heating or cooling. The random vibration test shows that the overall vibration frequency of the chip is not consistent with that of the control panel, so the stress caused by resonance can also be excluded. Combined with the above analysis, we could get rid of solder joint cracking due to improper design, improper soldering process control or excessive warpage. According to the failure background, it was speculated that the soldering point cracking is mainly caused by the mechanical stress over the tolerance of the solder joints caused by the vibration test, and the anti-vibration protection of the BGA chip was needed. On the premise of not changing the layout of the plate and the selection of the device, the stress caused by the vibration process could be relieved by the integral encapsulation of the components, the filling of the bottom of the chip or the local glue, thus improving the application reliability of the solder joints. Finally, through the comprehensive analysis, it is determined that the four corner glue was used under the chip. The vibration test results of batch products showed that the failure rate was significantly reduced to acceptable level. It was proved that the cracking of the solder joints was mainly caused by mechanical vibration, and the shock absorption effect was obviously played by the spot glue, and the reliability of the practical application stage of the high speed iron could be ensured.