Development of Highly Reliable BGA and Flip-Chip Structures by Using Cu-Cored Solder Ball

A Cu-cored solder joint is a micro-joint structure in which a Cu sphere is encased in solder. It results in a more accurate height and has low thermal and electrical resistance. In a previous paper, we examined the thermal fatigue life of a Cu-cored solder ball grid array (BGA) joint through actual measurements and crack propagation analysis. As a result, we found that the thermal fatigue life of a Cu-cored solder BGA joint is about twice as long as that of a conventional joint. In this paper, we describe the impact strength of a Cu-cored solder BGA joint determined by conducting an impact bending test. This test is a technique to measure the impact strength of a micro-solder joint. This method was developed by Yaguchi et al., and they confirmed that it is an easier and more accurate method of measuring impact strength than the board level drop test. First, we simulated the impact bending test by finite element analysis (FEA) and calculated solder strains of both Cu-cored solder joints and conventional joints. The results indicated that the maximum solder strain of a Cu-cored solder joint during the impact bending test was slightly smaller than that of a conventional joint. The solder volume of the Cu-cored solder joint was also smaller than that of a conventional joint. On the other hand, joint stiffness of the Cu-cored solder joint was larger than in a conventional joint. The former increases the solder strain of the Cu-cored solder joint, and the latter decreases it. By balancing these phenomena, it is possible to obtain a maximum solder strain in the Cu-cored solder joint that is slightly smaller than in a conventional joint. Based on these phenomena, the impact strength of the Cu-cored solder joint is predicted to be the same as or higher than that of a conventional joint. Therefore, we measured the impact strengths of a Cu-cored solder joint and a conventional joint using the impact bending test. As a result, we confirmed that the impact strength of the Cu-cored solder joint was the same as or higher than that of a conventional joint. Accordingly, a Cu-cored solder BGA joint is a micro-joint structure that makes it possible to improve thermal fatigue life without decreasing impact strength. Moreover, we investigated whether the use of Cu-cored solder in a flip-chip (FC) joint improved its reliability. As a result, we found that the stress of an insulating layer on a Si die surface was reduced by using a Cu-cored solder FC joint. This is because bending deformation of the Cu land occurs, and the difference in thermal deformation between the Si chip and the Cu land becomes small. Accordingly, the Cu-cored solder FC joint is a suitable structure for improving reliability of a low-strength insulating layer.