THE EFFECT OF VARIATIONS IN THE SILVER LEVEL AND MICROALLOYING ON THE IMPACT STRENGTH OF TIN-SILVER-COPPER BGA SPHERES
Authors: Keith Sweatman, Shoichi Suenaga, Masuo Koshi, and Tetsuro Nishimura Company: Nihon Superior Co., Ltd Date Published: 2/12/2009
Pan Pacific Symposium
Abstract: It is now well recognized that area array packages attached with near-eutectic tin-silver-copper (“SAC”) spheres are vulnerable to failure when portable devices in which they are mounted are dropped. Although the failure may be in the laminate rather than the solder joint itself the fact that the problem does not occur to the same extent with similar packages attached with tin-lead solder suggests that it is some aspect of the solder itself that is the cause of the increased sensitivity to impact loading. Two aspects of the solder are considered to be the main factors in determining the behaviour of the solder in this situation- its flow stress and the characteristics of the interfacial intermetallic layer. The flow stress is important because it determines the extent to which a shock load is transmitted through the solder rather than being absorbed by its deformation. The character of the intermetallic can determine whether unrelieved stress generated by the shock load will initiate cracking and low energy crack propagation along that interface. In this paper the authors will report work directed at exploring the effect of variation in these two aspects of the solder on impact strength. Flow stress falls with silver content and a range of tin-copper based solders with reduced and zero silver in the form of solder spheres reflow soldered to solder-mask-defined copper pads were subjected to impact loading in shear and tension. There is increasing evidence that the properties of the interfacial intermetallic can be modified by microalloying additions to the solder and the effect of the addition of such elements to these low silver and silver-free alloys on impact strength was measured. Evidence will be presented for the stabilizing effect of a microalloying addition on the mechanical integrity of the interfacial intermetallic.