How does the metal content of solder paste affect solder joint strength and conductivity?
Publish Time: 2025-09-04
In modern electronics manufacturing, especially in SMT (surface mount technology) processes, solder paste is a critical connecting material, and its properties directly impact the mechanical strength, electrical conductivity, and long-term reliability of the solder joint. Metal content, a key parameter in solder paste formulation, has a decisive influence on the physical and electrical properties of the final solder joint. While this parameter may seem simple, its profound impact on microstructure formation, interfacial reactions, and service performance far exceeds its surface value.Metal content typically refers to the weight percentage of alloy powders, such as tin-silver-copper (SAC), in solder paste, typically ranging from 85% to 93%. Even slight variations in this ratio can significantly alter solder paste behavior during the reflow process, affecting the density and structural uniformity of the solder joint. When the metal content is low, the flux content in the solder paste is relatively high. During reflow soldering, excess flux evaporates due to heat, easily forming pores, voids, or inclusions within the solder joint. These microscopic defects not only weaken the structural integrity of the solder joint but also reduce the effective conductive cross-section, thereby reducing conductivity and increasing the risk of resistive heating. Particularly in high-power or high-frequency circuits, such defects can cause localized overheating, accelerate solder joint degradation, and even lead to open-circuit failure.In contrast, solder pastes with a higher metal content, due to the higher density of the alloy powder, form denser solder joints after reflow, significantly reducing voiding. This not only improves the tensile and shear strength of the solder joint but also enhances its resistance to thermal fatigue and mechanical shock. When subjected to temperature cycling or vibration stress, the dense solder joint structure distributes stress more evenly, reducing the risk of crack initiation and propagation. Experimental data shows that, under the same process conditions, increasing the metal content of a solder paste from 88% to 92% can increase the average shear strength of the solder joint by over 15% and reduce the voiding rate by 30% to 50%, significantly improving connection reliability.Metal content is also crucial for electrical conductivity. The conductive properties of a solder joint depend primarily on the presence of continuous metal phase channels. A high metal content means more alloy particles fuse into complete, low-resistance conductive paths after melting. However, low-metal content solder paste, due to the high amount of residual organic matter (such as flux residue), may form insulating or semi-insulating regions within the solder joint, hindering current transfer. This effect is particularly pronounced in small solder joints or high-density interconnect structures, potentially leading to signal attenuation and impedance mismatch, compromising circuit performance.It is important to note that a higher metal content is not necessarily better. Excessively high metal content (e.g., exceeding 93%) significantly increases solder paste viscosity, affecting printing performance and mold release from stencils, leading to insufficient solder fill or uneven edges, ultimately compromising solder joint quality. Furthermore, high-metal content solder paste is more sensitive to ambient humidity and requires stricter temperature and humidity control during storage and use. Otherwise, powder oxidation and moisture absorption can occur, indirectly impacting solder joint quality.From a microstructural perspective, metal content also indirectly affects the growth of intermetallic compounds (IMCs) at the interface between the solder joint and the pad. Dense, uniform solder joints offer more stable thermal conductivity, promoting the formation of a thin, uniform IMC layer. However, loose, porous solder joints can lead to excessive IMC growth due to localized overheating or uneven reaction, potentially becoming crack initiation sites.The metal content of solder paste profoundly influences its mechanical strength and electrical conductivity by regulating the density, defect rate, and microstructure of the solder joint. In practical applications, the optimal metal content ratio must be selected based on product requirements, process conditions, and component characteristics. This ensures optimal printing performance while maximizing the solder joint's structural integrity and electrical reliability, achieving an optimal balance between performance and process.
