Optimizing Mixed Alloy Solder Pastes for Low Voiding and High PCB Reliability

Estimated reading time: 2 minutes

Solder joint voiding remains a critical reliability concern in electronic packaging, particularly for high-performance and safety-critical applications such as automotive electronics. Excessive voiding can adversely affect both thermal conductivity and mechanical strength, leading to elevated junction temperatures, accelerated fatigue failure under thermal cycling, and ultimately reduce product lifetime.

To address these concerns, industry standards such as IPC-7095 provide general acceptance criteria for voiding in ball grid array (BGA) packages. For bottom-terminated components (BTCs), including quad flat-pack no-leads (QFNs), IPC-A-610, and J-STD-001 are typically applied. The DKE automotive addendum from the IPC-A-610H and J-STD-001H further harmonizes evaluation methods by establishing consistent definitions and standardized procedures for void area measurement, enabling comparability of results across laboratories and studies.

Multiple factors contribute to void formation during the surface mount assembly process, including stencil thickness, reflow profile, and solder paste formulation. In this study, solder joint voiding was evaluated in accordance with the DKE automotive addendum to ensure standardized measurement practices. The key factors investigated included surface finish (OSP vs. ImSn), nano-coated stencil thickness (4 mil vs. 5 mil), reflow profile (Profile A vs. Profile B), and solder paste. Both overall void percentage and maximum individual void size were used as response variables.

The objective of this work is to provide a clear, data-driven ranking of factor importance for voiding control in QFN assemblies. The goal is to reduce voiding, improve assembly reliability, and support robust electronics production for demanding automotive environments. In light of voiding concerns, attempts have been implemented to determine if the newly developed mixed alloy solder paste (DFHR: SnAgCuBiSbInNi) could deliver voiding performance comparable to, or even lower than, SAC305, the most commonly used lead-free solder.

Solder Pastes
The solder pastes selected in the current experiment are shown in Table 1. SAC305, Flux A, type 4, 88.5 wt.% solder paste (Paste 1) was used as a control to compare to the selected high-reliability (hi-rel) paste, and DFHR + Flux B, Type 4, 89.0 wt.% solder paste (Paste 2).

Two halogen-free flux formulations were evaluated in this study. Flux A is a robust formulation designed to enhance stencil transfer efficiency, minimize voiding, and extend stencil life. Flux B is optimized for reduced voiding while maintaining excellent printability and formulation stability. Halogen-free fluxes are the industry’s preferred path forward for meeting modern environmental, safety, and reliability requirements, especially in high-reliability or no-clean applications. Halogen-containing fluxes are still used in legacy or specialty cases but carry trade-offs in regulatory exposure.

Continue reading this article in the October 2026 issue of SMT007 Magazine.