Solder Paste Additives for Thermal Expansion Control

Most electronic failures that occur in equipment are due to stresses induced by differences in the Coefficient of Thermal Expansion (CTE) of the different materials in a Printed Circuit Board Assemblies (PCBA). During a thermal cycle, the incompatibility of CTE between the PCB and the components induces shear fatigue that may affect the reliability of the solder interconnections on the PCB, which can eventually lead to fracture and failure of the joints and the PCB. Due to the advancement in the electronic components industry, interest from the researcher’s point of view has grown in studying the influence of additives in the solder alloys, in relation to microstructure, physical and mechanical properties and, mainly in the CTE. In this work two types of additives (Bi and graphite powder) were tested in order to reduce the CTE of a lead-free solder (SAC305) solder paste for reflow soldering. Because the selected additives have different characteristics, namely different densities, a different method of SAC305 solder additivation was tested for each type of additive. For Bi addition in SAC305 alloy (up to 6.5 wt.%), after a mechanical mixing of it, with the solder paste, a fusion technique (with a thermal cycle similar to the used on the reflow soldering) was used. For composites with graphite (addition up to 0.1 wt.%) a double-printing method was used in order to achieve a homogeneous additive distribution, so that graphite remained in the final ingot. These additivated solder alloys were chemically analyzed and characterized for thermogravimetric properties. Samples microstructure were characterized by SEM/EDS analysis, and also they were tested for their electrical resistivity. With graphite addition there is a slight increase on the initial alloy melting temperature (∼1.5°C) and with Bi an almost linear decrease was obtained (∼16 °C for the higher tested Bi addition). Composites with bismuth have a decrease trend, with the additive increase content until close to 5%. The CTE value decreases almost linearly ((from 25 to ∼14.5 μm/(m·°C); R2 = 0.9905). However, the sample of SAC305 + 6.5% Bi does not follow this trend, which may indicate that for these and higher amounts of bismuth, the composite CTE stabilizes. For composites with graphite there is a reduction of CTE (from 25 to ∼17 μm/(m·°C) for 0.04 wt. % graphite addition). For higher graphite additions the CTE seems to increase. The obtained results show that both additives can be used in order to achieve a CTE target value close to the PCB copper PAD (17 μm/(m·°C). However, the mixing method used for graphite mixing on solder paste cannot be directly transposed to the reflow soldering technique.