Wettability of molten Sn-Bi-Cu solder on Cu substrate

The wetting behavior of a new Sn-Bi-Cu Pb-free solder on Cu substrate was investigated by sessile drop method under an Ar-H₂ flow in the temperature range from 493 K to 623 K. The contact angle curves tested at 548 K and 623 K are found to fit exponential rule very well. However, the contact angle curve tested under 493 K is not well consistent with exponential rule, for which the spreading course may be classified into three stages. Equilibrium contact angles between Sn-Bi-Cu solder and Cu substrate decrease monotonously with the increase in temperature, which are 28°, 24° and 18° at 493 K, 548 K and 623 K, respectively. The results show that 69.5Sn-30Bi-0.5Cu exhibits good wettability on Cu substrate. Intermetallics formed at the 69.5Sn-30Bi-0.5Cu/Cu interface are identified as Cu₆Sn₅ adjacent to the solder and Cu₃Sn adjacent to the Cu substrate, respectively. Formation of intermetallic seems to improve strong wetting of the substrate by the solder.     

Reliability of lead-free solder joints in CSP device under thermal cycling

Finite element method and Garofalo–Arrheninus creep model were combined and used to evaluate the reliability of different lead-free solder joints (SnAgCu, SnAg, SnSb and SnZn) and SnPb solder joints in chip scale package (CSP) 14 × 14 device under thermal cyclic loading. The results show that von Mises stress and equivalent creep strain in each of the four lead-free solder joints and SnPb solder joints were strongly different, increasing in the order SnPb < SnAg < SnSb < SnZn < SnAgCu. It is found that maximum stress–strain concentrates on the top-surface of corner solder joints in the CSP device for all solder joints, and SnAgCu solder joints shows the highest fatigue life among those five kinds of solder joints.     

时效处理对Sn-Bi系无铅焊料的影响研究

为解决Sn-Bi焊料在焊接时容易产生凝固偏析、降低焊点力学性能的问题,采用对焊点进行时效处理的方法来消除Bi的粗化结晶,从而增强焊料的机械性能.研究表明,焊点在125℃时效处理16 h,粗大块状的Bi全部消融,呈颗粒状均匀分布,焊料中的Sn与焊盘上的Cu生成一层厚度约为3μm的Cu6Sn5金属间化合物,使焊点的剪切强度由40 MPa升高到54 MPa.研究还发现,时效处理后的焊点在100℃下长时间放置,微观组织不再发生变化,说明时效处理提高了焊料基体的抗热性能,因此,可以将125℃/16 h的时效处理作为表面贴装生产工艺流程的一道必要工序,使Sn-Bi焊料真正满足实用要求.     

Thermomigration in solder joints

In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10 μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1 °C across a micro-bump, the temperature gradient will be 1000 °C/cm, which can cause thermomigration at the device operation temperature around 100 °C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds.     

Laser probe measurements of quality evolution of solder joints during thermal cycling ageing tests

We present the results of a non-destructive measuring method allowing us to characterize the evolution of solder joints during thermal cycling ageing tests. The method uses a high resolution optical probe to detect selectively pure Joule and Peltier thermal responses of the solder joint subject to a given current pulse. The results show the Peltier and Joule responses to be good indicators for the evaluation of the age and the degradation of solder joints.     

The coupling effects of thermal cycling and high current density on Sn58Bi solder joints

Currently, one of the serious challenges in microelectronic devices is the miniaturization trend of packaging. As the decrease of joint dimension, electromigration (EM) and thermomechanical fatigue become critical issues for fine pitch packaging. The independent mechanisms of EM and thermomechanical fatigue are widely investigated and understood. However, the coupling effect of both conditions needs further exploration. The current study established the correlation between resistance and microstructure evolution of solder joint under the combination effect of thermal cycling and high current density and illustrated the different contributions of these two factors to the reliability of the joint through the comparison monopoly tests. The results revealed that cracks had more impact on resistance increase than phase segregation. The resistance evolution could be divided into three stages. First, the resistance mitigated due to the phase coarsening. Second, Joule heating effect made the resistance increase slowly. Third, EM led to the resistance increase rapidly. The high current density can help to improve the reliability of the solder joint under the coupling effect of thermal cycling and EM at the initial stage, but harmful to the consequence process.     

激光喷射钎料球键合焊点热循环试验研究

为研究激光喷射钎料球键合焊点的可靠性,用Sn3.0Ag0.5Cu钎料球对Au/Cu焊盘进行了激光喷射钎料键合试验,采用微强度测试仪、扫描电子显微镜、能谱分析仪研究了热循环条件对接头强度以及界面微观组织演变的影响.结果表明：采用激光喷射钎料键合技术焊盘表面Au层不能完全溶入钎料中,导致在界面处形成AuSn2＋AuSn4多...     

Influence of thermal cycling on fatigue strength of Cu/Sn/Cu solder joints

Abstract

The influence of thermal cycling on the fatigue life of Cu/Sn/Cu solder joints has been examined. Copper plates were bonded with tin foil (with a solder thickness of 60 µm) and suffered thermal cycling in a temperature range of 55 or 125 K. Then they were subjected to fatigue testing at a shear stress amplitude of 2 MPa and a frequency of 3.6 Hz. With the increasing number of the thermal cycles, the fatigue life decreased from 3.0×10⁵ to 5.0×10⁴ at thermal cycle 6000. However, the fatigue life did not change so much during thermal cycling in different temperature ranges. When the solder joints suffered the thermal cycling, the η phase at the bonding interface coarsened and elongated, and its arrangement became irregular. After larger numbers of thermal cycles, fine cracks appeared in the η phase parallel to the interface. After fatigue testing, circular patterns were observed inside the bonded region on a fracture surface, and their shape and size became irregular and larger with the increasing number of thermal cycles, respectively. These showed that the reduction in fatigue life was caused by improved propagation of the fatigue crack following changes in the morphology and arrangement of the η phase during thermal cycling.     

Standardization of a shear test method for lead-free solder paste chip joints

The electronics industry is moving to replace Pb-based solder with Pb-free solder because of the growing environmental regulations governing the use of lead. Solder joints made from Pb-free solder paste do not yet have an evaluation method to classify its mechanical properties such as shear strength. In this study, we reflowed solder joints from Sn–3.0Ag–0.5Cu solder paste. To standardize the shear test method, we measured the shear strength of the solder joint of a 2012 ceramic chip at a shear rate of 3–60 mm/min and a shear height of 10–380 μm using different shaped shear jigs. We statistically analyzed the optimum number of shear tests by calculating the accumulative average value, standard deviation, and width of the confidence interval. The fracture surface was examined by scanning electron microscope and discussed in terms of the shear conditions.     

Electromigration issues in lead-free solder joints

As the microelectronic industry advances to Pb-free solders due to environmental concerns, electromigration (EM) has become a critical issue for fine-pitch packaging as the diameter of the solder bump continues decreasing and the current that each bump carries keeps rising owing to higher performance requirement of electronic devices. As stated in 2003 International Technology Roadmap for Semiconductors (ITRS), the EM is expected to be the limiting factor for high-density packages. This paper reviews general background of EM, current understanding of EM in solder joints, and technical hurdles to be addressed as well as possible solutions. It is found that the EM lifetimes of Pb-free solder bumps are between the high-Pb and the eutectic composition under the same testing condition. However, our simulation results show that the electrical and thermal characteristics remain essentially almost the same during accelerated EM tests when the Pb-containing solders are replaced by Pb-free solders, suggesting that the melting points of the solders are likely the dominant factor in determining EM lifetimes. The EM behavior in Pb-free solder is a complicated phenomenon as multiple driving forces coexist in the joints and each joint contains more than four elements with distinct susceptibility to each driving force. Therefore, atomic transport due to electrical and thermal driving forces during EM is also investigated. In addition, several approaches are presented to reduce undesirable current crowding and Joule heating effects to improve EM resistance.     

Preparation of soft solder joints

The preparation of solder joints in electronic applications is not easy because the solder is soft and often surrounded by hard and brittle materials. Smearing, scratching, and structural changes caused by the preparation as well as destruction of the specimens during preparation due to their filigree geometries make the procedure demanding. A sequence has been developed that enables the preparation of soft solder under these difficult circumstances. The preparation and the development of the phases found in solder is explained step by step and illustrated with examples.     

Thermal deformation measurement of the solder joints in electronic packages using electron moiré method

In this paper, electron moiré method is used to measure the thermal deformation of electronic packages. In order to observe the electron moiré fringe, a holographic grid was replicated on the cross section of the BGA type package at 150°C. This grid was fabricated on a glass plate using a moving point holographic system, and was replicated to measured area at high temperature. Under SEM, the holographic grid(specimen grid) and a programmed electron beam scan(master grid) interfere and form electron moiré patterns. The shear strain measurement technique using electron moiré method is described. Using the electron moiré method, the shear strains in the different solder joints were measured and analysed. Some useful results were obtained.     

Solderability of SnBi-nano Cu solder pastes and microstructure of the solder joints

The solderability of the Sn58Bi (SnBi)-nano Cu solder pastes and the microstructure of the solder joints were investigated. Experimental results indicated that the addition of the nano Cu particles in the SnBi solder paste shows limited effect on the solidus. The liquidus of the SnBi-3nano Cu solder paste was 1 °C higher than the SnBi solder paste. Solid Cu₆Sn₅ intermetallic particles formed in the SnBi-3nano Cu solder paste during the heating process. The Cu₆Sn₅ intermetallic particles decreased the mobility and wettability of the molten solder. Meanwhile, the Cu₆Sn₅ nano particles worked as nucleation sites for the formation of Bi grains and Sn–Bi eutectic phase during the cooling process and led to the grain refinement of the solder bulk. The SnBi-1nano Cu solder paste showed the smallest grain size in this research. Additionally, the SnBi-3nano Cu/Cu solder joint showed a eutectic microstructure of Sn–Bi system at the center of the solder bulk but a hypereutectic microstructure with polygon Bi grains near the margin in the solder bulk.     

Decrease in fatigue life of Sn - 3.8 wt-%Ag - 1.2 wt-%Cu alloy solder joints due to thermal cycling

Abstract

Copper plates joined with a thin solder layer (60 μm thick) of Sn - 3.8 wt-%Ag - 1.2 wt-% Cu alloy were subjected to heat treatments: a thermal cycling of a temperature range between 321 K and 381 K (Δ T = 60 K) and an isothermal heating at 357 K, and then subjected to a fatigue test at 6 MPa stress amplitude. Solder joints made with a thin solder layer of Sn - Pb eutectic alloy were also examined for comparison. After heat treatments, the η phase developed and dispersed at the bonding interface of the solder joints with increasing numbers of thermal cycling and with increasing time of isothermal heating. Small voids also appeared in the η phase after heat treatments. Fine cracks appeared in the η phase after thermal cycling for 2000 cycles and higher, but no cracks were observed after isothermal heating. There was no large difference in fatigue lifetime after thermal cycling between Sn - Ag - Cu alloy solder joints and Sn - Pb eutectic alloy solder joints. The fatigue lifetime of Sn - Ag - Cu alloy solder joints and Sn - Pb eutectic alloy solder joints was 2 - 3 × 105 with no thermal cycling and was greatly reduced to 0.1 - 0.6 × 10⁵ after 8000 thermal cycles. The fatigue lifetime was also decreased to 0.6 - 1.0 × 10⁵ after isothermal heating for 16 000 min, but the decrease in fatigue lifetime was gradual compared to that after thermal cycling. The decrease in fatigue lifetime after smaller numbers of thermal cycles is explained by coarsening of the η phase, and the large decrease in fatigue lifetime after a large number of thermal cycles is explained by the appearance of cracks in the η phase during thermal cycling.     

Application of push-off shear test for evaluation of wetting-interface structure-bonding relationship of solder joints

The relationship between wetting behavior, interface structure and mechanical properties of solder/substrate couples has been studied on example of Sn-alloys and Cu substrates. The sessile drop method was used to investigate the solder wetting and spreading on polished Cu substrates in vacuum at a temperature of 503 K. The sessile drop samples after solidification were bisected perpendicularly to the substrate at the mid-plane of the contact circle. The first half of each sample was used for structural characterization of interfaces and evaluation of their mechanical properties by improved push-off shear test. The second half was used for investigation of the effect of thermocycling on structural stability and corresponding mechanical behavior of model solder/Cu joints. A comparison with the results obtained on standard solder joints has shown the usefulness of the improved push-off shear test performed directly on solidified sessile drop samples as an express test for evaluation of technological and mechanical compatibility of solder/substrate couples, particularly at the first stage of solder candidate selection.     

Failure behaviors of BGA solder joints under various loading conditions of high-speed shear test

Failure behaviors of ball grid array (BGA) solder ball joints under various loading conditions of high-speed shear test were investigated with an experimental and non-linear 3-dimensional finite element modeling work. A representative Pb-free solder alloy, Sn-3.0Ag-0.5Cu, was employed in this study. Far greater shear forces were measured by high-speed shear test than by low-speed shear test. The shear force further increased with shear speed mainly due to the high strain-rate sensitivity of the solder alloy. Brittle interfacial fractures can be more easily achieved by high-speed shear test, especially in higher shear speed. This was discussed with the relationship between the strain-rate and work-hardening effect and resulting stress concentration at interfacial regions. Shear force decreased with shear height, and it could be found that excessively high shear heights unfavorably affected the test results leading to unexpectedly high standard deviation values or shear tip sliding from the solder ball surface.     

Fracture behavior of Cu-cored solder joints

Copper-cored solder can be regarded as the next-generation solder for microelectronic semiconductors exposed to harsh operating conditions owing to its excellent sustainability under extreme thermal conditions, e.g., in microelectronic semiconductors used in transportation systems. Cu-cored solder joints with two different coating layers, Sn–3.0Ag and Sn–1.0In, were compared with the baseline Sn–3.0Ag–0.5Cu solder. The fracture strength and failure mode were examined using the high-speed ball-pull and normal-speed shear tests. The Cu-cored solder joint with the Sn–1.0In plating layer exhibited the highest ball-pull and shear strengths. In addition, it showed a much lower percentage of interface fracture between the Cu-core and plating layer than the interface fracture percentage in the Sn–3.0Ag plating layer due to the improved wettability between the Cu-core and Sn–1.0In plating layer.     

Mechanical and microstructural properties of SnAgCu solder joints

Mechanical and microstructural properties of SnAgCu solder joints with hypoeutectic, eutectic and hypereutectic compositions were studied. Eutectic SnPb joints were used as the reference. Reflowed lap shear specimens made of FR-4 glass epoxy printed circuit boards with OSP and NiAu surface finishes were used in the tests. Mechanical properties and microstructural features of the joints were examined in the as-reflowed condition and after isothermal aging at 85 °C for 1000 h. Both the composition and PCB surface finish had a notable effect on the mechanical behaviour of the SnAgCu solder joints. The shear strength value of SnAgCu solder joints was mainly dependent on the size and distribution of Ag₃Sn dispersions. The coarseness of the dispersions depends strongly on the amount of Ag in the solder alloy, the cooling rate after the reflow and the aging history of the solder joints.     

Mixed-mode fracture load prediction in lead-free solder joints

Double cantilever beam (DCB) fracture specimens were made by joining copper bars with both continuous and discrete SAC305 solder layers of different lengths under standard surface mount (SMT) processing conditions. The specimens were then fractured under mode-I and various mixed-mode loading conditions. The loads corresponding to crack initiation in the continuous joints were used to calculate the critical strain energy release rate, J_ci, at the various mode ratios using elastic–plastic finite element analysis (FEA). It was found that the J_ci from the continuous joint DCBs provided a lower bound strength prediction for discrete 2 mm and 5 mm long joints at the various mode ratios. Additionally, these J_ci values calculated from FEA using the measured fracture loads agreed reasonably with J_ci estimated from measured crack opening displacements at crack initiation in both the continuous and discrete joints. Therefore, the critical strain energy release rate as a function of the mode ratio of loading is a promising fracture criterion that can be used to predict the strength of solder joints of arbitrary geometry subject to combined tensile and shear loads.