Reliable Solder Columns to Replace Solder Balls in Large 2.5D Heterogeneous Packages

Abstract

Solder balls are well suited for interconnecting normal sized ball grid array (BGA) components to printed circuit boards (PCB). However, solder balls are prone to cracking (failure) caused by excessive stresses inherently found in advanced packaging such as large-sized heterogeneous 2.5D packages where multiple dies are placed side-by-side. The growth trend is to make larger and larger BGA processors exceeding 60x60mm in size for AI and data centers. A plethora of new processor substrate and interposer designs are coming on line such as Chip-on-Wafer-on-Substrates (CoWoS). For that reason, more instances of solder ball delamination and cracking on large-sized BGA packages are probable. Solder columns were produced more than 50 years ago to reduce cracking in large ceramic packages, especially for use in harsh environments found in aerospace and defense applications. However, such legacy solder column technology is limited to Tin-Lead (SnPb) alloys. The challenge is to make Lead Free solder columns that satisfy demand for RoHS compliance, while still providing adequate stress relief (compliancy) for extending the operational life of large sized heterogeneous 2.5D processors for mission critical applications. This paper describes our ongoing work to innovate non-collapsible, compliant Lead-Free Braided Copper Solder Columns to reduce strain caused by the Coefficient of Thermal Expansion (CTE) mismatch between large BGA packages and FR4 PCBs. A braid, consisting of 16 fine copper alloy wires, serves as an exoskeleton sleeve around a solder column to provide mechanical support as well as compliancy. This novel solder column technology supports market trends to scale up the size of chip packaging while maintaining reliability. Our focus was to quickly characterize Lead-Free solder column core materials and braiding alloys by employing “Fail-Fast” destructive accelerated mechanical bend testing in our lab. Since there is an ongoing interest in Tin-Lead alloys, this paper also discusses data on Tin-Lead alloys for comparative purposes.