Effect of rapid thermal shock cycle on the thermomechanical reliability of 20Sn-80Pb solder bumps

The influence of rapid thermal shock(RTS) cycles on 20Sn-80Pb solder bumps was studied. In the study, 20Sn-80Pb solder bumps were prepared by desktop nitrogen lead-free reflow soldering machine. The prepared 20Sn-80Pb solder bumps were used for RTS test in the temperature rang of 0°C ~ 150°C. One cycle of RTS is 24 seconds, and the temperature rise and fall rate of RTS is 12.5 C/s. The result indicated that when the cycle of RTS reached 1500T (here T is cycle, the same below), the shear strength of Sn-80Pb solder bump dropped by drastically 48.6%. Whereas, when the cycle of RTS reached 5500T, 20Sn-80Pb solder bumps’ shear strength decreased to 18.35 MPa, which increased by 7.5% compared with that of l6.97 MPa at 4500T. With the increase of RTS cycles, 20Sn-80Pb solder bumps’ shear strength was a decreasing trend and the fracture mechanism changed from ductile fracture to ductile-brittle mixed fracture, which could be subject to the thickening of the interfaical IMCs and the stress concentration caused by the growth of interfacial IMCs. To understand the changes of the mechanical properties of 20Sn-80Pb solder bumps, the influences of RTS on the crack and interfacial IMC of 20Sn-80Pb solder bumps were studied in details.

A Surface Energy Approach to Developing an Analytical Model for the Underfill Flow Process in Flip-Chip Packaging

This paper presents a new approach to formulating an analytical model for the underfill process in flip-chip packaging to predict the flow front and the filling time. The new approach is based on the concept of surface energy along with the energy conservation principle. This approach avoids the need of modeling the flow path to predict the flow front and the filling time and thus it is suitable to different configurations of solder bumps, including different shapes and arrangements of solder bumps in flip-chip packaging. An experiment along with the CFD simulation was performed based on a proprietarily developed testbed to verify the effectiveness of this approach. Both the experimental and simulation results show that the proposed approach along with its model is accurate for flip-chip packages with different configurations besides the configuration of a regular triangle arrangement of solder bumps and a spherical shape of the solder bump.

Microstructural Analysis of Solder Bump Fabricated by Sn Electroplating on a PCB Substrate

To manufacture finer solder bumps, the SR and DFR patterns were filled using a Sn electroplating process instead of the microball process currently used in BGA technology, and the solder bump shape was fabricated through a reflow process. The microstructure of the solder bump was investigated by EBSD and TEM measurements. The EBSD results showed that the grain size of the Sn structure became finer after the reflow treatment and a scallop shape of Cu₆Sn₅ was formed on the Cu UBM. However, the Cu₃Sn phase was difficult to measure in the EBSD measurement. The Cu₃Sn compound could be investigated with TEM analysis. The Cu₃Sn phase also existed in the Sn region, with a size of several tens of nanometers, due to the eutectic reaction. The volume fraction of the Cu₆Sn₅ phase in the Sn region could be calculated from the TEM image, and the concentration of copper dissolved in the liquid tin during the reflow process could be estimated from the volume fraction. It was possible to observe the Cu₃Sn and Cu₆Sn₅ lattice images through high resolution TEM analysis, but it was difficult to observe the lattice coherency between the two phases because both were polycrystalline. Based on the results of this study, it is expected that solder bumps with a diameter of less than 100 µm can be robustly manufactured through the Sn electroplating process.

Property of Sn-37Pb solder bumps with different diameter during thermal shock

Purpose The impact strength of solder joint under high strain rate was evaluated by board level test method. However, the impact shear test of single solder bump was more convenient and economical than the board level test method. With the miniaturization of solder joints, solder joints were more prone to failure under thermal shock and more attention has been paid to the impact reliability of solder joint. But Pb-free solder joints may be paid too much attention and Sn-Pb solder joints may be ignored. Design/methodology/approach In this study, thermal shock test between −55°C and 125°C was conducted on Sn-37Pb solder bumps in the BGA package to investigate microstructural evolution and growth mechanism of interfacial intermetallic compounds (IMCs) layer. The effects of thermal shock and ball diameter on the mechanical property and fracture behavior of Sn-37Pb solder bumps were discussed. Findings With the increase of ball size, the same change tendency of shear strength with thermal shock cycles. The shear strength of the solder bumps was the highest after reflow; with the increase of the number of thermal shocks, the shear strength of the solder bumps was decreased. But at the time of 2,000 cycles, the shear strength was increased to the initial strength. Minimum shear strength almost took place at 1,500 cycles in all solder bumps. The differences between maximum shear strength and minimum shear strength were 9.11 MPa and 16.83 MPa, 17.07 MPa and 15.59 MPa in φ0.3 mm and φ0.4 mm, φ0.5 mm and φ0.6 mm, respectively, differences were increased with increasing of ball size. With similar reflow profile, the thickness of IMC decreased as the diameter of the ball increased. The thickness of IMC was 2.42 µm and 2.17 µm, 1.63 µm and 1.77 µm with increasing of the ball size, respectively. Originality/value Pb-free solder was gradually used to replace traditional Sn-Pb solder and has been widely used in industry. Nevertheless, some products inevitably used a mixture of Sn-Pb and Pb-free solder to make the transition from Sn-Pb to Pb-free solder. Therefore, it was very important to understand the reliability of Sn-Pb solder joint and more further research works were also needed.

A Novel Method for Characterization of Ultralow Viscosity NCF Layers Using TCB for 3D Assembly

For die-to-wafer (D2W) stacking of high-density interconnects and fine-pitch microbumps, underfill serves to fill the spaces in-between microbumps for protection and reliability. Among the different types of underfill, nonconductive film (NCF) has the advantages of fillet and volume control. However, one of the challenges is the solder joint wetting. An NCF must have good embedded-flux activation to mitigate Cu UBM pad oxidation due to the repeated TCB cycles that accelerate oxidation on neighboring dice. The flux in the NCF also helps in wetting the solder bumps. To realize efficient solder wetting, one must also understand the NCF deformation quality, which is a function of its viscosity. This parameter has direct impact on the deformation of solder bumps. High-viscosity NCF would be difficult to deform, thus preventing solder contact to pad during TCB reflow temperature. High bond force is required and could lead to reduced alignment accuracy. For a low viscous NCF, it requires low bond force. Solder joint wetting is a challenge with excessive squeezeout due to fast and instantaneous deformation. We seek to demonstrate in this article a creative methodology for NCF material characterization, considering the factors of NCF viscosity, deformation, and solder squeezeout. We use TCB tool position-tracking data to define the deformation curve of the NCF as a function of temperature and time at very fast profile of TCB. We use the NCF viscosity curve as reference in relation to the actual deformation, and predict dynamic deformation in three different configurations. Deformation test configurations were performed on chips with and without microbumps bonded with a rigid flat glass surface and with a bottom Cu UBM pad. The experiments were performed with different heating ramp rates at target above Sn reflow of ~250°C interface temperature. As validation, we applied the optimized TCB process (force, temperature, and ramp rate) on a test vehicle with 20 and 40 μm pitch daisy chains and obtained very good connectivity with good joint and IMC formation.

Experimental and Theoretical Studies of Cu-Sn Intermetallic Phase Growth During High-Temperature Storage of Eutectic SnAg Interconnects

In this paper, the growth of intermetallic compound (IMC) layers is considered. After soldering, an IMC layer appears and establishes a mechanical contact between eutectic tin-silver solder bumps and Cu interconnects in microelectronic components. Intermetallics are relatively brittle in comparison with copper and tin. In addition, IMC formation is typically based on multi-component diffusion, which may include vacancy migration leading to Kirkendall voiding. Consequently, the rate of IMC growth has a strong implication on solder joint reliability. Experiments show that the intermetallic layers grow considerably when the structure is exposed to heat. Mechanical stresses may also affect intermetallic growth behavior. These stresses arise not only from external loadings but also from thermal mismatch of the materials constituting the joint, and from the mismatch produced by the change in shape and volume due to the chemical reactions of IMC formation. This explains why in this paper special attention is being paid to the influence of stresses on the kinetics of the IMC growth. We develop an approach that couples mechanics with the chemical reactions leading to the formation of IMC, based on the thermodynamically sound concept of the chemical affinity tensor, which was recently used in general statements and solutions of mechanochemistry problems. We start with a report of experimental findings regarding the IMC growth at the interface between copper pads and tin based solder alloys in different microchips during a high temperature storage test. Then we analyze the growth kinetics by means of a continuum model. By combining experiment, theory, and a comparison of experimental data and theoretical predictions we finally find the values of the diffusion coefficient and an estimate for the chemical reaction constant. A comparison with literature data is also performed.

A novel method for characterization of Ultra Low Viscosity NCF layers using TCB for 3D Assembly

For die to wafer bonding of high-density interconnects and fine pitch microbumps developing and characterizing suitable underfill materials are required. In general, underfill serve to fill the spaces in-between microbumps for protection and reliability. Among the different types of underfill, Non-Conductive Film (NCF) has the advantages of fillet and volume control, and a built-in flux to aid wetting. However, challenges arise for thin dies and microbumps with fine pitches on film lamination, voiding, transparency, filler percentage, dicing compatibility and most importantly, deformation behavior and possibility to improve solder joint wetting. In a Die-to-Wafer D2W stacking with a Sn solder bump interconnect to Cu UBM, concern is high on the Cu pad oxidation due to the repeated TCB cycles that accelerate oxidation on neighboring dies. Process mitigation is needed to help reducing the oxidation. But even so, an NCF must have good embedded flux activation. Another main factor for an NCF to have efficient TCB process with good solder joint wetting, is the NCF deformation quality in which is a function of its viscosity. This parameter has direct impact on the deformation of solder bumps. High viscosity NCF would be difficult to deform, thus preventing solder contact to pad during TCB reflow temperature. High bond force is required and could lead to reduced alignment accuracy. Filler entrapment is also a subsequent concern for high filler loading, high viscosity NCF. For a low viscous NCF, careful attention in process characterization is needed in TCB with low bond force. Solder joint wetting is a problem with excessive squeeze-out due to fast and instantaneous deformation. With low viscosity, not only the bond force applied should be low, but the deformation behavior should also be understood to enable an effective NCF. We seek to demonstrate in this paper a creative methodology for Non-Conductive Film (NCF) material characterization, considering the factors of NCF viscosity, deformation, and solder squeeze-out. Characterizing NCF viscosity at fast TCB profiles is challenging considering deformation behavior of both the NCF itself and the solder bumps that shaped the solder squeeze-out and wetting. Furthermore, in this paper we use TCB tool position tracking to define the deformation curve of NCF film as a function of temperature and time at very fast profile of TCB. We use material viscosity curve as reference in relation to the actual deformation, and predict dynamic deformation based on Reynold’s equation within TCB profile duration. The experiments were performed with different heating ramp rates at target above Sn reflow of ~250C interface temperature. The deformation analysis is not limited to thin film sandwiched between parallel plates. Deformation test was performed on chips with and without microbumps and with rigid flat glass surface and its combinations. Deformation of underfill is recorded in the readout of TCB tool. As validation, we applied the optimized TCB process (force, temperature, and ramp rate) on a test vehicle with 20 and 40um pitch daisy chains and obtained close to 95% electrical yield with good joint and IMC formation. The cross-section SEM images show good wetting, revealing good activation of built-in flux when the optimized TCB profile was used.