Cheng-Ying Ho’s research while affiliated with National Tsing Hua University and other places

A new multilayer metallization, ENEPIG (Electroless Ni(P)/Electroless Pd/Immersion Au) with ultrathin Ni(P) deposit (ultrathin-ENEPIG), was designed to be used in high frequency electronic packaging in this study because of its ultra-low electrical impedance. Sequential interfacial microstructures of commercial Sn–3.0Ag–0.5Cu solders reflowed on ultarthin-ENEPIG with Ni(P) deposit thickness ranged from 4.79 μm to 0.05 μm were first investigated. Accelerated thermal aging test was then conducted to evaluate the long-term thermal stabilization of solder joints. The results showed that P-rich intermetallic compound (IMC) layer formed when the Ni(P) thickness was greater than a critical vale (about 0.18 μm). Besides, it is interesting to mention that the growth of (Cu,Ni)6Sn5 and (Cu,Ni)3Sn IMCs was suppressed with the formation of P-rich layer, i.e., Ni3P and Ni2Sn1+xP1−x phase, even though the electroless-plated Ni(P) layer was exhausted at initial stage of reflow process. The atomic Cu flux in solder joints without P-rich layer was calculated to be several times larger than that with P-rich layer formation after calculation, which implies that the P-rich layer and ultrathin Ni(P) deposit in ENEPIG served as diffusion barrier against rapid Cu diffusion.

This study aims to investigate the microstructure, β-Sn crystallography, micro-hardness and impact reliability of both Sn–3.0Ag–0.5Cu/Cu (SAC/Cu) and Sn–3.0Ag–0.5Cu/Ni (SAC/Ni) solder joints under various reflow processes. During the solidification step of the reflow process, solder joints were annealed at 210 °C for 50 s and 100 s, respectively. Network-type precipitations formed within the SAC/Cu joint, while dot-type precipitations distributed within the SAC/Ni joint. With the increase of annealing time, these precipitations grew larger; the interfacial intermetallic compounds (IMCs) became slightly thicker, and the hardness of solder alloys gradually decreased. Electron backscatter diffraction (EBSD) analysis indicates that the β-Sn grain structure depended on the distribution of precipitations. A high speed shear tester was used to evaluate the impact toughness of solder joints. Noteworthily, the short-time annealing can improve the impact reliability of solder joints. After annealing for 50 s, the average impact toughness of both SAC/Cu and SAC/Ni solder joints was enhanced, and the percentage of ductile fracture increased significantly. However, the growth of (Cu,Ni)6Sn5 at the SAC/Ni interface degraded the impact toughness as the SAC/Ni joint was annealed for 100 s. The variation of impact toughness in SAC/Cu and SAC/Ni is correlated to the variation of microstructure and hardness in solder joints.

ENEPIG with various thicknesses of submicron Ni(P) deposit (Ultrathin-ENEPIG) was adopted to evaluate the electrical impedance behavior, microstructural comparison, and high-speed impact resistance prior to and after isothermal aging process. The results show that when extending the aging time to 1000 h, the high-speed impact energy of ultrathin-ENEPIG with 0.31 μm Ni(P) layer deteriorated significantly. In contrast, mechanical performance maintained well for ultrathin-ENEPIG with 0.18 μm Ni(P) deposit. The loss of adhesion might be attributed to the Kirkendall voids resulting from the unbalanced diffusion for Sn and Cu in Ni3P crystalline. In ultrathin-ENEPIG with 0.18 μm Ni(P) deposit, on the other hand, the migration of Sn in Ni2Sn1+xP1−x was faster than in Ni3P. Besides, secondary-(Cu,Ni)6Sn5 layer slowed down the out-diffusion of Cu. Thereby, the unbalanced diffusion can be eliminated. The optimization of the Ni(P) deposit thickness selection in ultrathin-ENEPIG would be discussed and proposed based on the observations of the electrical behavior, joint strength evolution, and interfacial thermal stability.

The interfacial reaction and crystallographic orientation of Ni(P)/Au/SnAgCu/Cu and Ni(P)/Pd/Au/SnAgCu/Cu assembled solder joints were investigated. With the aid of crystallographic analysis, the Pd influence on the grain structure and preferred growth orientation of intermetallic compound (IMC) on both Cu and Ni(P) substrates were revealed. On the Ni(P) substrate, the enhanced nucleation of (Cu,Ni,Pd)6Sn5 resulted in the random grain texture and needle-like morphology in the Pd-doping assembled joints. This random texture of (Cu,Ni,Pd)6Sn5 could hinder the crack propagation directly through (0 0 0 1) plane of adjacent grains. As for the Cu substrate, the granular (Cu,Ni,Pd)6Sn5 formed near the substrate and subsequently grew into a layer-like morphology after thermal aging. The granular grains of (Cu,Ni,Pd)6Sn5 exhibited the concentrated texture after aging, whereas (Cu,Ni)6Sn5 stilled revealed the random texture. Most important, the absence of high angle grain boundary among (Cu,Ni,Pd)6Sn5 grains on the Cu substrate could further guarantee the lifetime of solder joint during the reliability test. Finally, the detailed correlation between microstructure variation and grain orientation will be investigated and discussed as well as the possible mechanism.

Near eutectic SnAg and SnAgCu solders with various solder bump heights were adopted to characterize the microstructure in solder alloys. The eutectic structure was refined and the secondary dendrite arm spacing of β-Sn decreased monotonously with the shrinkage of solder joints. Linear relationship between the secondary dendrite arm spacing and natural logarithm of the solder bump height was demonstrated in this study. The hardness and the creep deformation resistance increased with respect to the increase of solder dimension and microstructural variation due to the frequent interaction between dislocation and the precipitations in solder alloys.

The criteria of mechanical reliability in solder joints can be identified and described by comparative evaluation via drop test and high speed pendulum impact test. Systematic samples of assembly and attachment joints with various Pd additions were employed and investigated in this study. The statistical values of mechanical performances were calculated and compared. Better high speed impact performance of SAC305/ENEPIG attachment joints with 0.06 μm Pd layers was confirmed owing to the single Cu6Sn5 phase growth. However, the comparative measurement of the better performance on drop testing exhibited in ENEPIG/SAC305/immersion Sn assembly joints with 0.1 μm Pd layers deposit resulted from the thinner and layer-type IMC growth. The correlation between the cracks propagation and Pd addition was established on the basis of the elemental X-ray color mapping via Field-Emission Electron Probe Microanalyzer (FE-EPMA). It is expected that through comparison between impact and drop test in mechanical reliability, a criterion of joints reliability can be established. Besides, the optimal Pd layer deposit for the ENEPIG surface finish in the attachment and assembly solder joints was demonstrated and confirmed.

In the Sn–3.0Ag–0.5 Cu/Ni (wt%; SAC305/Ni) solder joint, the (Cu,Ni)6Sn5 spalling was suppressed by altering the microstructure of Ni films. Ni films with intertwined and straight grain boundaries (GBs) were fabricated by the diverse target powers of the direct current magnetron sputter. The intertwined GB and the straight GB of Ni films determined the degree of the (Cu,Ni)6Sn5 spalling. Ni films with straight GB accelerated the (Cu,Ni)6Sn5 spalling due to the fast diffusion path of Ni and Cu in straight GB of Ni films. In contrast, the winding grain structure of Ni film, called intertwined GB-Ni film, can suppress the interdiffusion of Cu and Ni. Thus, the (Cu,Ni)6Sn5 spalling was successfully retarded by the Ni structure with intertwined GB.

Electroless Ni-P/electroless Pd/immersion Au (ENEPIG) with ultra-thin Ni-P deposit serve as a potential replacement of traditional ENEPIG surface finish because of its superior electrical performance in flip chip solder joints interconnection. However, the interfacial reaction and mechanical reliability of solder joints in ENEPIG with ultra-thin Ni-P layer is not yet well evaluated. In this study, we investigated the characteristic microstructure of interfacial intermetallic compounds and high-speed impact responses of Sn-3.0Ag-0.5Cu/ENEPIG attachments with 4.8, 0.3, and 0.05 mu m Ni-P deposit. ENEPIG with Ni-P layer of 0.3 mu m exhibited the eutectic structure dispreading in the solder alloys and layer-type P-rich IMCs at solder/metallization interface, while there was (Cu,Ni)(6)Sn-5 precipitation in the solder but no P-rich IMCs layer formed in ENEPIG with 0.05 mu m Ni-P layer. Slower interfacial reaction rate in ENEPIG with 0.3 mu m Ni-P layer was attributed to the effect of electroless Ni-P diffusion barrier layer, which would further provide better impact resistivity than that of ENEPIG with 0.05 mu m Ni-P deposit. Moreover, breach in P-rich IMCs and underneath (Cu,Ni)(6)Sn-5 patch were observed in ENEPIG with 0.3 mu m Ni-P layer. The growth mechanism was closely related to the Ni diffusion from surface finish and element redistribution.

Sequential wetting reactions of molten Sn–3.0Ag–0.5Cu solder on electrolytic Ni and Ni–xPd (x=4 and 10 wt%) surface finish were investigated using the in situ monitoring of force-time measurement via the wetting balance. Significantly different kinetics was documented and the correlation between the characteristic wetting stages and the formation of interfacial intermetallic compounds were addressed and discussed. Besides, the addition of Pd tends to decrease the wetting time while increase the wetting force during soldering reaction. The wetting enhancement may be attributed to the drop of activation energy as well as interfacial tension and the change in microstructure of intermetallic compounds between solder and substrates.