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14 SMT007 MAGAZINE I JULY 2019 Feature by Keith Sweatman NIHON SUPERIOR CO. LTD. Abstract The need to minimize thermal damage to components and laminates, reduce warpage- induced defects to BGA packages, and save energy is driving the electronics industry towards lower process temperatures. For sol- dering processes, the only way that tempera- tures can be substantially reduced is by using solders with lower melting points. Because of constraints of toxicity, cost, and performance, the number of alloys that can be used for elec- tronics assembly is limited, and the best pros- pects appear to be those based around the eutectic in the Bi-Sn system, which has a melt- ing point of about 139°C. Experience so far indicates that such Bi-Sn alloys do not have the mechanical proper- ties and microstructural stability necessary to deliver the reliability required for the mount- ing of BGA packages. Options for improving mechanical properties with alloying additions that do not also push the process temperature back over 200°C are limited. An alternative approach that maintains a low process temper- ature is to form a hybrid joint with a conven- tional solder ball reflowed with a Bi-Sn alloy paste. During reflow, there is mixing of the ball and paste alloys. But it has been found that to achieve the best reliability, a proportion of the ball alloy has to be retained in the joint, par- ticularly in the part of the joint that is sub- jected to maximum shear stress in service— which is usually the area near component side. The challenge is then to find a reproducible method for controlling the fraction of the joint thickness that remains as the original solder ball alloy. Empirical evidence indicates that, for a par- ticular combination of ball and paste alloys and reflow temperature, the extent to which the ball alloy is consumed by mixing with the paste alloy depends on the volume of paste deposited on the pad. If this promising method of achieving lower process temperatures is to be implemented in mass production without compromising reliability, it would be neces- sary to have a method of ensuring the opti- mum proportion of ball alloy left in the joint after reflow can be consistently maintained.

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