Disentangling bulk and interfacial factors in solder joint reliability of fine pitch packages

• Published in: Microelectronics and reliability Vol. 171; p. 115791
• Main Author: Jang, You-Cheol
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.08.2025
• Subjects: IMC; Pad finish; SAC alloy; Solder joint reliability; Thermal aging; SAC alloy; Thermal aging; Solder joint reliability; IMC; Pad finish
• ISSN: 0026-2714
• DOI: 10.1016/j.microrel.2025.115791

This study evaluates the effects of solder alloy composition and pad surface finish on the mechanical strength and thermal aging behavior of solder joints in fine pitch BGA packages. Three lead-free solder alloys—Sn–1.2Ag–0.5Cu–0.05Ni (SAC1205N), Sn–1.0Ag–0.5Cu (SAC105), and Sn–3.0Ag–0.5Cu (SAC305)—were combined with three surface finishes (CuOSP, ENEPIG, and NiAu) to fabricate nine joint configurations. Mechanical integrity was assessed using ball shear testing (BST) and ball pull testing (BPT) both before and after isothermal aging at 150 °C (0–192 h). BST results indicated that solder composition—particularly Ag content—was the dominant factor in bulk joint strength, with SAC305 consistently demonstrating the highest shear resistance due to its Ag3Sn-reinforced microstructure. In contrast, BPT results emphasized the critical role of pad finish, with NiAu delivering superior interfacial strength through the formation of uniform Ni3Sn4 intermetallic layers. Statistical correlation analysis reinforced these trends, revealing a strong correlation between solder alloy and BST (r = 0.781, p = 0.013), and between pad finish and BPT (r = 0.695, p < 0.0001). Thermal aging accelerated strength degradation across all configurations; however, Ni-doped and high-Ag solders showed improved resistance to intermetallic coarsening. Fracture mode analysis and cross-sectional imaging further demonstrated that cohesive bulk failure in SAC305–NiAu joints correlated with stable interfacial morphology, while interfacial separation was more prevalent in CuOSP-based joints due to brittle IMCs and void formation. These insights highlight the stress-mode-dependent nature of solder joint degradation. These results elucidate the differentiated contributions of bulk solder alloy and interfacial pad finish to joint performance under distinct mechanical loading conditions. In particular, the SAC305–NiAu and SAC1205N–CuOSP combinations demonstrated superior reliability, rendering them promising candidates for fine-pitch interconnects in high-performance applications. Overall, this study provides material-level design guidance for optimizing solder–pad combinations tailored to shear- or tensile-dominant loading in fine-pitch, high-performance electronic packaging. •Dual-mode testing reveals solder vs. interface-driven failure divergence.•NiAu-induced Ni₃Sn₄ IMC suppresses thermal degradation across solder types.•SAC305 excels in bulk reliability, while pad finish dictates long-term endurance.•CuOSP accelerates Kirkendall voiding, compromising interfacial integrity.•Findings support material pairing strategies for fine-pitch reliability design.