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A Guide to Lead-free SoldersWetting and Joint Formation

A Guide to Lead-free Solders: Wetting and Joint Formation Chapter 3 In Chapter 1, we discussed the general properties and performance of solder alloys. This was followed by a discussion of electronic packaging architecture and assembly technology and their relationship to the solder joint. We now turn our attention to the formation of a solder joint. Solder-joint formation involves the interactions of several complex metallurgical processes. These pro- cesses include heat, mass and momentum transfer across interfaces. In addi- tion, metallurgical reactions may take place at the interface, producing addi- tional phases. Let us consider the flip-chip joint that uses a Sn-based alloy on a gold-plated, nickel over copper pad. Initially, as the assembly enters reflow, the solder in the paste undergoes melting, under the protection of a blanket of flux. The melt must then wet the solder pads on the printed wiring board and the pads on the chip itself must, of course, remain wetted. The solder wets the pad by spreading outward along the pad. During wetting, the melt will dissolve the protective gold layer and also a portion of the underlying nickel, up to the solubility limit. This dissolution process is controlled by diffusion of the Au and Ni into the melt and the diffusion http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png

A Guide to Lead-free SoldersWetting and Joint Formation

Editors: Engelmaier, Werner
A Guide to Lead-free Solders — Jan 1, 2007
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Publisher
Springer London
Copyright
© Springer-Verlag London Limited 2007
ISBN
978-1-84628-309-3
Pages
53 –78
DOI
10.1007/978-1-84628-310-9_3
Publisher site
See Chapter on Publisher Site

Abstract

Chapter 3 In Chapter 1, we discussed the general properties and performance of solder alloys. This was followed by a discussion of electronic packaging architecture and assembly technology and their relationship to the solder joint. We now turn our attention to the formation of a solder joint. Solder-joint formation involves the interactions of several complex metallurgical processes. These pro- cesses include heat, mass and momentum transfer across interfaces. In addi- tion, metallurgical reactions may take place at the interface, producing addi- tional phases. Let us consider the flip-chip joint that uses a Sn-based alloy on a gold-plated, nickel over copper pad. Initially, as the assembly enters reflow, the solder in the paste undergoes melting, under the protection of a blanket of flux. The melt must then wet the solder pads on the printed wiring board and the pads on the chip itself must, of course, remain wetted. The solder wets the pad by spreading outward along the pad. During wetting, the melt will dissolve the protective gold layer and also a portion of the underlying nickel, up to the solubility limit. This dissolution process is controlled by diffusion of the Au and Ni into the melt and the diffusion

Published: Jan 1, 2007

Keywords: Solder Joint; Solder Alloy; Lead Free Solder; Molten Solder; Joint Formation

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