Linking microstructure and high-frequency electrical performance in low-temperature SnBiIn-based solders for advanced packaging

Abstract

The present study evaluates the microstructural development and GHz electrical response of two solder systems: a multicomponent SnBiInZnAg (SBIZA) alloy and a ternary SnBiIn alloy. Microstructural analysis revealed that SBIZA joints formed an approximately 1.7 µm thin, continuous intermetallic compound (IMC) layer, whereas SnBiIn joints exhibited an approximately 3.5 µm thick, irregular IMC accompanied by significantly higher Sn/In oxide accumulation. These interfacial distinctions strongly influenced high-frequency performance. At 10 GHz, SBIZA exhibited lower insertion loss (–0.35 dB vs. −0.44 dB) and improved return loss (< −14 dB vs. −11.3 dB), together with a smaller insertion-loss deviation (< 0.20 dB) and a flatter group-delay response. Nonlinear radio frequency testing further showed that SBIZA reduced third-order intermodulation distortion by approximately 29 dB relative to SnBiIn. High-speed digital measurements demonstrated that SBIZA maintained a clean eye opening up to 56 Gbps, achieved a rise-time distortion of approximately 28 ps (compared with approximately 45 ps for SnBiIn), and supported bit error rate levels below 10⁻¹². Overall, the results indicate that SBIZA provides superior microstructural stability and electrical reliability, positioning it as a promising candidate for next-generation low-temperature interconnects.