Study on Microstructure and Mechanical Strength of Nanoparticles Reinforced Solder Joints under Thermal Aging or Current Stressing


Student thesis: Doctoral Thesis

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Award date12 Sep 2018


Solder joint technology has been adopted as the major technique for the interconnection of chips to the printed circuit board (PCB) in electronic products. Due to the environmental concerns, the traditional Tin-Lead (Sn-Pb) solders have been prohibited and replaced with lead-free solders. Several lead-free solder systems have been proposed, such as Tin-Copper (Sn-Cu), Tin-Silver-Copper (Sn-Ag-Cu), Tin-Bismuth (Sn-Bi), Tin-Indium (Sn-In), Tin-Zinc (Sn-Zn), etc. - among which the most widely adopted one in industry is Sn-Ag-Cu solder, typically having the composition Sn3.0Ag0.5Cu (in weight percentage). Cost concerns have driven the industry either to lower the content of Ag in solder or seek alternatives that can be processed at lower temperatures. Solder interconnects are subjected to many reliability challenges, for example, degraded mechanical strength, tin whiskers, excessive growth of interfacial intermetallic compounds (IMCs), mass migration due to high electric fields (electromigration, EM), thermal gradients (thermomigration, TM), or stress gradients (stress migration, SM), etc. Doping nanoparticles (NPs) into solder materials has been found to improve the performance of solder interconnects successfully - by reinforcing the mechanical strength, refining the microstructure, inhibiting the IMCs growth, and stopping mass migration. In this study, low-melting-temperature Sn58Bi, low-silver-content Sn0.7Cu, and Sn0.7Cu0.3Ag solders have been selected as the main research materials. Three kinds of NP (ZrO2, Ag, CNTs) have been added into the solder matrix to study the performance of composite solders under thermal aging and current stressing.

As compared with Sn58Bi solder, the ZrO2 NP-doped composite Sn58Bi-ZrO2 solder joints showed a thinner interfacial IMCs layer, with 18.9% decrease in the thickness – after being thermally annealed at 100 0C for 300 hours. Phase coarsening in the matrix was also observed to be inhibited in the composite solder. The mechanical strength was reinforced in the composite Sn58Bi-ZrO2 solder joints, with ball shear strength 19.2% larger than that of the plain ones. The EM tests indicated that the composite Sn58Bi-ZrO2 solder had smaller Cu6Sn5 IMCs layers growth rates than the plain one. Phase coarsening was reduced in the composite Sn58Bi-ZrO2 solder. The EM rate was decreased by 21.5% as the thickness of Bi rich layers accumulated at the anode was decreased from 5.92 µm (Sn58Bi) to 4.65 µm (Sn58Bi-ZrO2). The enhanced EM reliability was due to the disordered orientations caused by the finer microstructure, which altered the migration of Bi atoms towards the anode more frequently. 

The effectiveness of the nanodoping methods was evaluated by comparing the performance of Sn0.7Cu0.3Ag (prepared by alloying pure Sn, Cu, and Ag metals), Sn0.7Cu+0.3Ag (prepared by doping Ag NPs into Sn0.7Cu), and Sn0.7Cu solder. Both Ag modified solder joints had higher mechanical strength and stronger resistance to EM than Sn0.7Cu, which was due to the dispersed Ag3Sn IMCs that refined the bulk solder, stopped dislocations and hindered atomic migration. The shear strength and microhardness of Sn0.7Cu0.3Ag were respectively 12.79% and 1.01% higher than those of Sn0.7Cu+0.3Ag. The EM rate, characterized with the products of diffusivity and effective charge number DZ*, were in the following order: Sn0.7Cu > Sn0.7Cu+0.3Ag > Sn0.7Cu0.3Ag. The superior performance of Sn0.7Cu0.3Ag was due to the smaller size and wider distribution of Ag3Sn IMCs in the solder matrix, which indicated that the alloying method produced greater improvements than the doping method on Sn0.7Cu solder.

The effect of the NP size on the reinforcement was studied by doping carbon nanotubes (CNTs) with three different diameter ranges (10-20, 40-60, and 60-100 nm) into Sn0.7Cu0.3Ag solder. All the CNT-doped composite solder joints displayed refined microstructure and inhibited interfacial IMC growth. The mechanical strength was reinforced due to the refined microstructure and increased dislocation density. The adsorbed CNTs destroyed the integrity of the interfacial IMCs, leading to reduced growth rate. Among these composite solders, CNTs with a diameter of 40-60 nm provided superior performance in refining the microstructure and controlling the IMC growth - reinforcing the ball shear strength by 15.3% and the hardness by 16.1%. The smaller sized CNTs I suffered from agglomeration and adsorption due to high surface tension. The larger sized CNTs III suffered from ineffective refining of microstructure. This size effect of NPs suggests that the NPs with moderate length will have the best improvement on the composite solder. 

The effect of EM was in-situ observed on cross-sectioned Cu/Sn0.7Cu/Cu line-type interconnects. The serious mass migration that occurred during EM was characterized by measuring the surface vertical variation using a step-profiler. The EM rate was estimated using two distinct methods: 1) in-situ mark movement and 2) the growth of anode IMCs layer. The atomic flux under a current density of 4.77×103 A/cm2 and a temperature of 60 0C is measured to be 5.241×1012 cm-2s-1 and 4.114×1012 cm-2s-1 based on mark movement and IMCs growth, respectively. The initial scalloped IMCs in the as-reflowed interconnect evolves towards different directions as a serrated IMC layer with deep grooves forms on the cathode side, while a thick planar IMC layer forms on the anode side, which is defined as polarized evolution. The serrated cathode IMCs is due to the fast diffusion of Cu atoms along the IMC grain boundaries. The formation of the planar IMC layer at the anode is due to the stacking of Cu atoms, which fill the grooves by reacting with Sn to form Cu6Sn5 IMCs. This polarized evolution threatens the reliability of interconnects, since a thick planar IMC layer tends to be brittle, while a thin serrated IMC layer may suffer from accumulated voids. 

The effect of alternating current (AC) stressing was studied using line-type Cu/Sn0.7Cu0.3Ag solder/Ni interconnects. Linear interfacial IMC growth behaviours were discovered, while the solder/Cu side is thicker than the solder/Ni side. The atomic migration rate of Cu atoms in Sn0.7Cu0.3Ag solder under a thermal gradient of 206.77 0C/cm was calculated to be 3.76 × 108 cm-2s-1. Serious degradation in mechanical strength happened as voids and cracks were observed in the solder/Ni and solder/Cu interfaces, respectively. Nano-indentation results showed that the mechanical strength in different solder areas was in the following order: central solder > solder/Ni interface > solder/Cu interface. The damage mechanisms under AC stressing are summarized below: 1) joule heating induced grain coarsening and redistribution of dislocations; 2) thermal fatigue induced local over-aging and thermal stress accumulation; 3) TM induced mass migration and strength redistribution.