| Sn-based solder alloys are extensively used in electronic devices to form interconnects between different components to provide mechanical support and electrical path. The formation of a reliable solder interconnects fundamentally relies on the metallurgic reaction between the molten solder and solid pad metallization in reflowing. The resultant IMC layer at the solder/pad metallization interface can grow continuously during service or aging at an elevated temperature, uplifting the proportion of IMCs in the entire solder joint. However, the essential mechanical properties of interfacial IMC (i.e. Cu6Sn5, Cu3Sn) layers, such as Young’s modulus and hardness, are drastically different in comparison with Sn-based solder and substrate. Therefore, the increasing fraction of interfacial IMCs in the solder joint can lead to significant deformation incompatibility under exterior load, which becomes an important reliability concern in the uses of solder joints for electronic interconnects.Furthermore, the growth of IMC layer during aging is accompanied by the volume shrinkage. In a miniaturized solder joint, the volume ratio of interfacial IMC layer could be significantly high. Hence, the volume shrinkage induced by the growth of IMC could further result in the dimensional change of solder joint, build-up of residual stress, and alternating the macro deformation behaviour and fracture behaviour of solder joints. In the past decades, extensive research works have been implemented and reported regarding the growth of interfacial IMC layers and its effect on the mechanical integrity of solder joints. But there are few papers focused on the reliability issued induced by the volume shrinkage due to the growth of IMC layer.The main focus of this work is the growth of IMCs in Sn-based lead free solder joints, the volume shrinkage induced by the growth of IMCs and the resultant evolution of residual stress at the solder/substrate interface. Furthermore, the effects of the volume shrinkage induced by the growth of IMCs on the macro compression creep and tensile fracture behaviour of solder joint were also investigated. The main findings and results are summarised as following.The growth of IMCs at the Sn-1%Cu solder/Cu interface parallel and perpendicular to the interdiffusion direction was investigated in this work. After aging at175℃for1006.5hours, the planar growth of interfacial IMC layers was found to follow parabolic law with aging durations, hIMC=0.27t1/2+4.6, hCu6Sn5=0.16t1/2+4.1and hCu3Sn=0.17t1/2, where t is the aging duration in hour; h is the thickness of IMC layer in μm. At the same aging temperature, the relation between the height of perpendicular IMCs and aging durations can be expressed as y=0.11(?)t, where t is aging time in hour, y is the height of perpendicular IMCs μm.Furthermore, the perpendicular IMCs also consists of two layers, the outer Cu6Sn5layer and the inner Cu3Sn layer, but the Cu3Sn was formed prior to Co6Sn5, which was reverse to the formation sequence in planar IMCs. During aging, both the planar and perpendicular growth of IMCs follows parabolic relation with aging durations, which indicates that the growth of IMCs along the two directions were governed by solid-state diffusion between Cu and Sn during aging. But, due to the longer diffusion distance in the perpendicular growth of IMCs, the perpendicular growth rate was significantly lower that the planar growth rate.The volume shrinkage induced by the growth of interfacial IMC layer during aging was also studied in this work. In order to characterise the dimensional change due to the shrinkage, the specimen was designed to enable precise measurement of the height change by Zygo. It was found that the solder part of Sn-1%Cu/Cu solder joints collapsed approximately1.2μm after aging at175℃for1132.5h. The decrease in joint height followed parabolic law with aging duration under experimental conditions, which can be expressed asâ–³h=-0.031×(?)t. The dimensional change coefficient (decrease in joint height/thickness of IMC layer) can then be derived to be αexperiment=-0.114. Based on the assumption that both Cu6Sn5and Cu3Sn are isotropic and dense, the theoretical relation between the decrease in joint height and aging duration isâ–³htotal=-(0.04-0.004x)(?)t, where x is the contribution fraction of the reaction,9Cu+Cu6Sn5â†'5Cu3Sn, to the produced Cu3Sn, and t is aging duration in hour. And the corresponding theoretical coefficient is αideal=0.147+0.0147x. It can be found that the numerical calculation has a close approximation to the experimental results.Residual stress could also be induced by the growth of interfacial IMC layers during aging, since the volume shrinkage that accompanies the growth of IMCs is subjected to the constraint from the adjacent solder and substrate. Therefore, the residual stress within Sn-1%Cu solder, Cu6Sn5layer, Cu3Sn and Cu substrate was investigated by depth-controlled nanoindentation after progressively extended aging at175℃. The obtained stress evolution with aging duration indicates that the evolution of residual stress depends on the location of a specific layer within the solder joint:(1) During aging, the Cu substrate close to the Cu3Sn/Cu interface was under compression both in the middle and at the edge of the solder joint with the average stress of about560MPa;(2) The compression stresses in the solder close to the solder/Cu6Sn5interface were70MPa at the centre and90MPa at the edge of the solder joint;(3) During aging, the compression stress within the interfacial Cu6Sns layer increased to4GPa at the centre of solder joint and3GPa at the edge;(4) In contrast, tension stress grew significantly within the interfacial Cu3Sn layer at the centre and the edge of solder joint, reaching1.7GPa and0.5GPa, respectively.From the comparison of stress-time (S-t) curves from two adjacent parts in solder joints, it can be found that:(1) there is no notable correlation between the S-t curve from interfacial Cu6Sn5layer and the curve from adjacent solder;(2) At the centre of solder joint, the stress within Cu substrate and the stress within interfacial Cu3Sn layer evolved along the opposite direction with prolonged aging, which indicates that the volume shrinkage of Cu3Sn layer was restrained by the adjacent Cu substrate;(3) At the edge of solder joints, the interfacial Cu3Sn layer and Cu6Sn5layer were mutually constrained according to the reverse trend in the S-t curves from these two parts.The compression creep of SAC305/Cu solder joints was also studied with the built creep test device. It was found that when the compression stresses were17.8MPa～22.9MPa and the temperature were433K～463K, the strain of the steady compression creep of SAC305/Cu solder joint was within0.6%～1.4%, and the corresponding creep deformation was2.5μm～5.8μm. During this steady creep stage, the thickness of interfacial IMC layer increased about1μm, based on calculation. Therefore, the height reduction induced by the growth of IMCs was0.05μm-0.16μm, which was about1%～4%of the measured steady compression creep strain. Consequently, the volume shrinkage induced by the growth of interfacial IMC layer has negligible effect on the macro compression creep of solder joints.The tensile fracture characteristics of Cu3Sn5and Cu3Sn at the Sn-1%Cu/Cu interface were also investigated by micro cantilever bending tests on micro Cu6Sn5 and Cu3Sn pillar prepared by FIB. During the micro cantilever bending tests, both micro Cu6Sn5and Cu3Sn pillars remained elastic under applied load until the fracture. From the observation of the fracture surface after the tests, both cleavage fracture and intergrannular fracture can be identified from the fracture surface of micro Cu6Sn5pillars, while only intergrannular fracture can be observed from the fracture surface of micro Cu3Sn pillars. The difference in fracture modes of IMC pillars was dominated by the microstructure of corresponding Cu6Sn5layer and Cu3Sn layer in solder joints.Furthermore, the tensile fracture behaviour of Cu/SAC305/Cu solder joints was also investigated. It was found that the fracture modes of the solder joint before aging was ductile fracture, and the corresponding fracture strength was70.8±9.0MP. When the IMC particles congregated along the Sn grain boundaries in the solder matrix, the tensile fracture strength of solder joint decreased to58.9±6.8MPa because of the residual stress and defects resulted from the growth of IMCs due to the resultant volume shrinkage. When the aging extended to81hours, the solder joint failed within interfacial IMC layer, and the fracture strength fell to57.8±8.4MPa. The change in fracture modes and fracture strength of Cu/SAC305/Cu solder joints was because of the abnormal distribution of IMC particles or the build-up of residual stresses and voids induced by the volume shrinkage during the growth of IMC layer. Moreover, it was found that the fracture strength of the micro IMC pillars was significantly higher than the fracture strength of solder joints when they fractured within interfacial IMC layer. It shows that the volume shrinkage induced by the growth of IMC layer can lead to the aggregation of residual stress and the formation of void, which can notable deteriorate the mechanical reliability of the entire solder joints. |
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