Electromigration of SnBi Solder for Second-Level Interconnect, Phase 2

Statement of Work & Project Statement

Statement of Work (Version 2.2; February 11, 2025)
Project Statement (Verson 2.1; February 19, 2025)

Project Motivation

High-density microelectronic packaging has the tendency to thermally warp when assembled using relatively high-melting-temperature, RoHS-compliant SAC (SnAgCu) solders. Thermal warpage results in defects such as head-on-pillow (HoP), solder bridging and non-wet opens. In addition, high-temperature soldering can have an associated high cost of operation with the solder reflow equipment and a corresponding negative impact on the environment. Low-melting-temperature lead-free solders are attractive alternatives to SAC solders.

However, although SnBi has a low melting temperature, it also has shortcomings that need to be addressed such as a high propensity for electromigration and segregation at the anode under high current density conditions, which can lead to potentially brittle solder joints with high electrical resistance with time in field. There is a need to explore ways to mitigate these shortcomings and develop SnBi solder joints that maintain their performance and reliability over the life of the products in which they are used. Taking advantage of back stress is one potential way of reducing the rate of electromigration in solder joints. The chemical gradient arising from electromigration may be another push-back mechanism. Either way, there is potential to reduce electromigration in low-height solder joints by taking advantage of mechanical and/or chemical back stresses.

Project Overview

The Electromigration of SnBi Solder for Second-Level Interconnect Project was organized to investigate the key factors driving electromigration in SnBi solder joints, understand various potential pathways to reduce the electromigration rate, and determine the role of back stress in reducing the overall detrimental effects of electromigration. Key parameters such as PCB finishes, solder joint height, temperature, and current density will be studied in Phase 2 for their impact on solder joint reliability in terms of joint electrical resistance rise and degradation of joint mechanical shock resistance.

In Phase 1 of this multi-phase project, the project team:

Established appropriate test vehicles for the electromigration study of solder joints, including a convenient means of achieving various joint heights
Investigated the rate of electromigration of Sn-Bi solder joints as a function of current density, temperature, joint height and surface finish
Investigated how adding minor amounts of various elements can affect EM resistance within SnBi alloys

The overall objective of Phase 2 is to determine the boundaries of the envelope formed by joint height, temperature, current density and PCB finishes within which the solder joint will operate reliably over the product life. The project team plans to:

Study the mechanical shock reliability of bottom-terminated component (BTC) SnBi solder joints as a function of the extent of electromigration
Investigate the role of back pressure in reducing electromigration in eutectic SnBi solder joints by determining the critical length (L) and critical product (J x L)
Investigate the effect of underfill on reducing electromigration