| dc.description.abstract | In this study, we developed a slurry contained Sn58Bi solder powder and fluxing epoxy resin. During the reflow process, the molten solder droplets self-assemble to form solder joints, then epoxy resin surrounding formed solder joints cures to serve as underfill. This process achieves flip-chip bonding and underfilling in single step, and we term this new material self-assembly solder resin.
The fluxing epoxy resin significantly impacts the self-assembly behavior of molten solder droplets during reflow. Therefore, we selected a curing system based on dicarboxylic acid monomers and diepoxide monomer to ensure the effective removal of surface oxides of solder powder and wettability of molten solder on pads during reflow. We used differential scanning calorimetry (DSC) to analyze the curing reaction process of various dicarboxylic acid-epoxy monomer systems. Additionally, we explored using latent catalysts to adjust the curing process of the epoxy resin to minimize its’ impact on the formation of self-assembling solder joints.
Dynamic DSC analysis revealed that the tin carboxylate salts formed between the solder powder and the flux significantly affect the epoxy resin′s curing process. The coexisting of Lewis base catalyst and tin carboxylate salts will further accelerate the curing reaction. We used X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to confirm the formation of tin carboxylate salts and deduced their chemical properties to establish the mechanism by which they catalyze the epoxy resin curing process. By understanding these mechanisms, we successfully formulated a fluxing epoxy resin with good mechanical properties after curing and creating a self-assembly solder resin for initial reflow tests.
Using branching theory calculations, we optimized the self-assembly process of the solder joints formed by self-assembly solder resin. The theoretical derivation indicated that the gelation conversion rate of this epoxy curing system is approximately 0.78-0.8. This gelation point conversion rate helps determine the optimal relative positions of the exothermic peak of curing reaction and the endothermic peak of the solder melting on the DSC thermogram, minimizing the impact of the epoxy resin curing process on the self-assembly behavior of molten solder droplets.
For the self-assembly solder resin to be applicable in fine pitch flip-chip bonding, the use of ultra-fine solder powder (2-8 μm) is necessary. However, during the early stages of reflow, ultra-fine solder powder tends to sediment and agglomerate before melting, forming dense structures that hinder the fluxing epoxy resin from fully wetting the powder surface and removing surface oxides. To address this, we reacted a diamine monomer with the epoxy monomer during the initial curing process to form microgels, creating space within the packed structure of the unmelt solder powder, preventing solder powder particles from approaching each other. Branching theory calculations determined the maximum amount of diamine monomer that can be added without forming a large network structure, and DSC and rheometer tests confirmed the formation of microgels. Final reflow tests showed that samples containing diamine monomers effectively disrupted the dense packing structure of the unmelt solder particles, allowing the fluxing epoxy resin to wet the solder powder surface and remove oxides. In contrast, samples without diamine monomers failed to achieve this.
After optimizing self-assembly solder resin, we conducted reflow and reliability tests. The reflow tests showed that all LED samples functioned normally, with Sn58Bi solder fully self-assembling onto the solder pads without open circuits or bridges. Energy-dispersive X-ray (EDX) analysis confirmed the formation of Au-Sn intermetallic compounds (IMC) at the interface, with minimal Ni diffusion, and the epoxy resin completely encapsulated the solder joints. The shear test showed an average strength of 127 g, 2.54 times higher than sample reflowed with traditional solder paste. Reliability tests indicated a 40-50% reduction in shear strength, still higher than that of samples reflowed with traditional solder paste. The decrease in shear strength after high-temperature and high-humidity testing was attributed to delamination at the epoxy resin-solder joint interface and a decrease in epoxy resin hardness. After temperature cycling tests, the reduction in shear strength was due to cracks at the interface, caused by differences in the coefficients of thermal expansion. | en_US |