The impact of paste alloy, paste volume, and surface finish on solder joint

The reliability of solder joints is significantly influenced by the microstructure of SAC (Sn-Ag-Cu) solders, which is affected by various factors, including paste alloy, paste volume, and surface finish. This study explores the impact of these factors on the microstructure, thickness of the intermetallic compound (IMC) layer, hardness, and macro void presence in as-reflowed joints. Three lead-free solder alloys, namely SAC305 (Sn - 3.0Ag - 0.5Cu), SAC-Bi (Sn - 3.0Ag - 3.0Bi - 0.8Cu), and SAC-Bi-Sb (Sn - 3.4Ag - 3.2Bi - 3.0Sb - 0.7Cu), were tested with varying solder paste-to-sphere ratios with electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) surface finishes. ENIG involves applying a thin layer of gold over a layer of nickel on the copper surfaces. Whereas OSP is a thin organic coating designed to protect copper surfaces from oxidation. The evaluation incorporated the analysis of Ag3Sn particles, IMC thickness, and voids through scanning electron and optical microscopy and X-ray images. Additionally, microhardness was assessed by indenting seven solder joints using the Phase II Model 900-391,391 micro-Vickers hardness tester. The study revealed that the SAC305 exhibited a higher presence of Ag3Sn particles than SAC-Bi and SAC-Bi-Sb. A direct proportionality was observed between paste volume and the quantity of Ag3Sn particles. Conversely, an inverse relationship was identified between paste volume and IMC layer thickness, resulting in a thinner IMC layer with higher paste volume, regardless of the paste alloy used. Furthermore, the use of ENIG led to a reduction in IMC thickness, attributed to the inhibitory effect of the Ni barrier. Doped alloys, specifically SAC-Bi and SAC-Bi-Sb, displayed superior microhardness compared to SAC305, owing to the strengthening and hardening effects of Bi and Sb. Regarding solder voiding, a noteworthy observation indicated that an increase in the quantity of solder paste resulted in the formation of larger voids.