Book Excerpt: The Printed Circuit Assembler’s Guide to... Low-Temperature Soldering, Vol. 2, Chapter 3

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Mechanical Shock and Thermomechanical Reliability of Low-temperature Solder Hybrid Joints

HRL3 uses the individual and combined effects of solid solution strengthening, precipitate and dispersion hardening, grain refinement and diffusion modifiers to balance strength, toughness, stiffness, and ductility in their respective alloy compositions. Electronic systems under normal operating conditions are exposed to a diverse range of temperatures and mechanical loads, depending on operational environment and usage. Proxy tests such as mechanical drop shock and thermal cycling are often used to predict thermal and mechanical reliability performance of solder joints in actual devices.

Solder joints are cycled between cold and hot zones during thermal cycling tests and the materials forming the solder joint—the package, the solder, the interfacial intermetallic layer, and the substrate—will expand and contract at different rates. This is generally known as coefficient of thermal expansion (CTE) mismatch, in which these variations in CTE generate shear stresses at the interfaces between these materials and potentially lead to thermomechanical fatigue. Failures through fatigue can be divided into three stages: crack initiation, crack propagation, and sudden fast failure. There are competing mechanisms promoting crack growth and closure. While stronger materials resist crack initiation better, crack propagation depends on the plastic zone ahead of the crack tip, i.e., the alloy microstructure. Thus, the need to achieve a balanced alloy composition combining strength and microstructure optimization is required for improved fatigue life and optimal performance.

A solder paste formulated with HRL3 was evaluated for drop shock and thermal cycling performance using a Cu-OSP test vehicle and SAC305 CTBGA84 packages. Therefore, upon using the low-temperature solder paste to attach these packages to a PCB, a hybrid (or heterogeneous) LTS/SAC305 solder joint is formed. The solder paste was screen printed on a PCB by using two stencil apertures, 11 and 14 mil. Table 3.1 shows the corresponding paste volume to ball volume ratios (0.6 and 0.8) that enabled investigating the effect of paste volume on the mechanical and thermal reliability of their solder joints.

These PCBs were reflowed using the profiles shown in Figure 3.1. The HRL3 alloy was evaluated using two peak temperatures, 165°C and 175°C, while SAC305 was reflowed using its typical 245°C (all within +/-5°C) peak profile.

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