Solder Joint Morphology Research Articles

A review: effect of copper percentage solder alloy after laser soldering

PurposeThis study aims to review the effect of copper percentage in Sn-based solder alloys (Sn-xCu,x= 0–5 Wt.%) on intermetallic compound (IMC) formation and growth after laser soldering.Design/methodology/approachThis study reviews the interfacial reactions at the solder joint interface, solder joint morphology and the theory on characterizing the formation and growth of IMCs. In addition, the effects of alloying and strengthening mechanism, including wettability, melting and mechanical properties are discussed.FindingsThis paper presents a comprehensive overview of the composition of tin-copper (Sn-Cu) solders with a potential to enhance their microstructure, mechanical characteristics and wettability by varying the Cu percentage. The study found that the best Cu content in the Sn-xCu solder alloy was 0.6–0.7 Wt.%; this composition provided high shear strength, vibration fracture life value and ideal IMC thickness. A method of solder alloy preparation was also found through powder metallurgy and laser soldering to improve the solder joint reliability.Research limitations/implicationsThis study focuses on interfacial reactions at the solder joint interface, solder joint morphology, modelling simulation of joint strength and the theory on characterising the formation and growth of IMC.Originality/valueThe paper comprehensively summarises the useful findings of the Sn-Cu series. This information will be important for future trends in laser soldering on solder joint formation.

Thermomechanical reliability of a Cu/Sn-3.5Ag solder joint with a Ni insertion layer in flip chip bonding for 3D interconnection

Thermal compression bonding (TCB) has been widely used in flip chip bonding processes for three-dimensional integrated packaging. Therefore, studying the TCB process and solder joint structure is significant for improving reliability. In this study, a Ni layer was deposited between the Cu/Sn-3.5Ag solder bumps of upper and lower chips, and TCB process tests were carried out at different bonding temperatures, forces and times. The solder height at the top and bottom of the copper pillar were recorded. Through the establishment of a mathematical analysis model, the relationship between the solder joint height and maximum stress was analyzed theoretically, and the trend of the shear stress and solder joint height was obtained from die shear tests. Then, the solder joint morphology, strength and microstructure composition of the microbumps were observed after aging. Our results showed that the growth rate of intermetallic compounds (IMCs) for Cu/Sn-3.5Ag microbumps with a 25 μm diameter was 2.5 times those with a 100 μm diameter and that the Ni barrier layer prevented the diffusion of Cu in Sn-3.5Ag solder. In addition, the Ni layer improved the strength of the Cu/Sn-3.5Ag joints after aging. Finally, with extended aging time, the fracture site was found to move from the solder region to the Cu6Sn5 layer for the Cu/Sn-3.5Ag samples, while it remained at the Sn-3.5Ag/IMC interface for the Cu/Ni/Sn-3.5Ag samples.

Effect of the Curing Properties and Viscosities of Nonconductive Films on the Solder Joint Morphology and Reliability of Chip-On-Board Packages Using Cu-Pillar/Sn–Ag Bumps

In this article, flip chip assembly using nonconductive films (NCFs) was evaluated for chip-on-board (COB) packages using Cu/Sn–Ag hybrid bumps. In general, significant thermomechanical stress can be induced at the Cu-pillar/Sn–Ag solder joint because of a large coefficient of thermal expansion (CTE) mismatch between the Si chip and printed circuit board (PCB). If a concave-shaped solder joint morphology is generated at the solder joint, thermomechanical stress could be concentrated at the solder joint, resulting in an early failure of T/C reliability. To obtain the solder joint morphology without a concave-shaped solder joint, NCFs’ curing properties and viscosity should be controlled. In particular, the NCFs’ viscosity at the solder melting temperature, at which metallurgical bonding between molten solder and Cu pads occurs, will significantly affect the solder joint morphology. For this reason, solder joint formation was investigated at each viscosity of NCFs and bonding temperature during the thermo compression (TC) flip chip bonding process. In addition, specific viscosities at certain bonding temperatures during the flip chip bonding process were obtained from measured degrees of cure and viscosity approximation. Finally, the material properties of NCFs were optimized without NCF traps at the solder joint and concave-shaped solder joint, and T/C reliability was evaluated for each silica powder content. As a result, it was confirmed that the COB packages using Cu/Sn–Ag hybrid bumps and optimized NCFs show stable resistance changes within 10% of the initial resistance after the T/C reliability test.

Development of Stacking Process for 3D TSV (Through Silicon Via) Structure using Laser

Abstract TSV chips were designed and fabricated to investigate the effects of the types of the stacking process on the manufacturability. Two processes were compared: thermocompression and laser bonding. The pitch of UBMs, solder bumps, and TSVs were 50μm. A fluxing underfill was developed and used as a pre-applied underfill during the stacking process. The bonding profiles and the solder joint morphology based on the thermocompression and the laser bonding were compared.

Effects of Thermocompression Bonding Parameters on Cu Pillar/Sn-Ag Microbump Solder Joint Morphology Using Nonconductive Films

In this paper, wafer-level preapplied nonconductive films (NCFs) were used to interconnect the Cu pillar/Sn-Ag microbumps for 3-D through silicon via vertical interconnection. Thermocompression bonding is a common method to interconnect chips to substrates using NCFs, and thermocompression bonding time should be reduced to increase the bonding productivity. Therefore, isothermal bonding method without heating and cooling process in bonding profile was introduced to reduce the bonding time. Solder joints bonded by an isothermal bonding method were compared to those joints bonded by a conventional ramp-up bonding method that was consist of heating, bonding, and cooling process. Final joint gap was decreased using an isothermal bonding method due to higher heating rate, and solder joint morphology was also changed according to the final solder joint gap. Furthermore, solder joint should have enough contact area to substrate metal pads without solder wetting on the Cu pillar sidewall to avoid reliability problems by Sn consumption at the solder joint. Effects of isothermal bonding parameters were investigated in terms of the bonding pressure, temperature, and time to optimize the isothermal bonding parameters for good solder joint. As bonding pressure and bonding temperature increased, solder joint gap decreased because final joint gap was determined by bonding pressures and dynamic viscosity of NCFs. Isothermal bonding times can be reduced to 10 s, because the degree-of-cure of NCFs could be over 90% after 2.2 s. As a summary, solder joint bonded with NCFs using optimized isothermal bonding parameters showed excellent bump joint resistances and solder wetting on substrate metal pads.

A Study on the Failure Mechanism and Enhanced Reliability of Sn58Bi Solder Anisotropic Conductive Film Joints in a Pressure Cooker Test Due to Polymer Viscoelastic Properties and Hydroswelling

A study of the failure mechanism and the enhanced reliability performance of SnBi58 solder ACF joints were investigated by considering the total influence of polymer viscoelastic and hygroscopic properties on the cross-sectional morphologies and the electrical properties of Sn58Bi solder joints in a pressure cooker test (PCT) for 120 h. The Sn58Bi solder failure mechanism is demonstrated that, when polymer resin absorbed moisture, the Tg values of polymer resins decreased, and the coefficient of thermal expansion (CTE) values of polymer resins increased, due to a hydroswelling effect. Considering the total influence of polymer viscoelastic and hygroscopic properties on the cross-sectional morphologies and the reliability of Sn58Bi solder joints in a PCT for 120 h, four typical polymer resins were evaluated. As a result, cationic epoxy (128 ppm/°C) with the smallest CTE mismatch (112 ppm/°C) with SnBi58 solder joints (16 ppm/°C) in a PCT aging test (121°C, 100% humidity, and 2 atm) showed a stable electrical property in terms of joint contact resistance and remained a complete cross-sectional Sn58Bi solder joint morphology after PCT reliability for 120 h.

Effects of Bonding Pressures and Bonding Temperatures on Solder Joint Morphology and Reliability of Solder ACF Bonding

In this paper, in order to improve the reliability of anisotropic conductive film (ACF) interconnections, solder ACF joints were investigated in terms of solder joint morphology. ACFs are film-type interconnection adhesive materials that consist of polymer adhesive resins and randomly dispersed conductive particles. Recently, in order to obtain high reliability of ACF joints, solder ACFs that use solder particles as conductive particles of ACFs have been introduced combined with an ultrasonic bonding method. However, further researches are needed in the area of crack initiation and the propagation of solder ACF joints at high temperature and high humidity conditions due to the stress generated by hygroscopic expansion of ACF resin. Therefore, in order to solve the crack initiation of solder ACF joints, solder ACF joint morphology should be controlled and optimized to minimize the solder crack-related reliability problems. In this paper, solder ACF joint reliability was investigated depending on the solder morphologies of ACF joints bonded with various bonding pressures and temperatures. According to the results, as bonding pressure increased from 2 to 6 MPa, aspect ratio (joint diameter/joint gap) increased due to the increased joint area and decreased joint gap. In the pressure cooker test (PCT) reliability tests, as solder aspect ratio increased, electrical resistances were more stable after 60 h of the tests due to higher joint strength. In the target bonding temperature profile of 250 °C, solder joints showed a concave shape. However, joints bonded at in the target bonding temperature profile of 200 °C showed a convex shape. It is mainly due to the lower degree of cure of resin when ACF temperature reached to solder Melting Point (MP) in the target bonding temperature profile of 250 °C compared with that in the target bonding temperature profile of 200°, because ACF temperature reached to solder MP in shorter time in the target bonding temperature profile of 250 °C. Concave-shaped solder joints showed higher PCT reliability than the convex-shaped solder joints due to the lower stress concentration. These results indicate that solder ACF joint morphology was a significantly important factor for highly reliable solder ACF joints in high temperature and high humidity reliability conditions.

Highly Reliable Solder ACFs FOB (Flex-on-Board) Interconnection Using Ultrasonic Bonding

In this study, in order to improve the reliability of ACF interconnections, solder ACF joints were investigated interms of solder joint morphology and solder wetting areas, and evaluated the electrical properties of Flex-on-Board (FOB) interconncections. Solder ACF joints with the ultrasonic bonding method showed excellent solder wetting by broken solder oxide layers on solder surfaces compared with solder joints with remaining solder oxide layer bonded by the conventional thermo-compression (TC) bonding method. When higher target temperature was used, Sn58Bi solder joints showed concave shape due to lower degree of cure of resin at solder MP by higher heating rate. ACFs with epoxy resins and SAC305 solders showed lower degree of resin cure at solder MP due to the slow curing rate resulting in concave shaped solder joints. In terms of solder wetting area, solder ACFs with <TEX>$25-32{\mu}m$</TEX> diameters and 30-40 wt% showed highest wetted solder areas. Solder ACF joints with the concave shape and the highest wetting area showed lower contact resistances and higher reliability in PCT results than conventional ACF joints. These results indicate that solder morphologies and wetting areas of solder ACF joints can be controlled by adjustment of bonding conditions and material properties of solder and polymer resin to improve reliability of ACF joints.

Effect of Gold Content on the Microstructural Evolution of SAC305 Solder Joints Under Isothermal Aging

Au over Ni on Cu is a widely used printed circuit board (PCB) surface finish, under bump metallization (UBM), and component lead metallization. It is generally accepted that less than 3 wt.% Au in Sn-Pb solder joints inhibits formation of detrimental intermetallic compounds (IMC). However, the critical limit for Au content in Pb-free solder joints is not well established. Three surface-mount package platforms, one with a matte Sn surface finish and the others with Ni/Au finish, were soldered to Ni/Au-finished PCB using Sn-3.0Ag-0.5Cu (SAC305) solder, in a realistic manufacturing setting. The assembled boards were divided into three groups: one without any thermal treatment, one subjected to isothermal aging at 125°C for 30 days, and the third group aged at 125°C for 56 days. Representative solder joints were cross-sectioned and analyzed using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) to investigate the evolution of the solder joint morphology as a function of Au content and isothermal aging. It was found that, if Cu is available to dissolve in the solder joint, the migration of AuSn4 from the bulk to the interface as a result of thermal aging is mitigated.