Reflow Soldering Research Articles

Gold-Tin Solder Wetting Behavior for Package Lid Seals

This study examined the cause of nonwetted regions of the gold (Au) finish on iron-nickel (Fe–Ni) alloy lids that seal ceramic packages using the 80Au-20Sn solder (wt %, abbreviated Au–Sn) and their impact on the final lid-to-ceramic frame solder joint. The Auger electron spectroscopy (AES) surface and depth profile techniques identified surface and through-thickness contaminants in the Au metallization layer. In one case, the AES analysis identified background levels of carbon (C) contamination on the surface; however, the depth profile detected Fe and Ni contaminants that originated from the plating process. The Fe and Ni could impede the completion of wetting and spreading to the edge of the Au metallization. The Au layer of lids not exposed to a Au–Sn solder reflow step had significant surface and through-thickness C contamination. Inorganic contaminants were absent. Subsequent simulated reflow processes removed the C contamination from the Au layer without driving Ni diffusion from the underlying solderable layer. An Au metallization having negligible C contamination developed elevated C levels after exposure to a simulated reflow process due to C contamination diffusing into it from the underlying Ni layer. However, the second reflow step removed that contamination from the Au layer, thereby allowing the metallization to support the formation of lid-to-ceramic frame Au–Sn joints without risk to their mechanical strength or hermeticity.

Interfacial behavior and joint strength of Sn–Bi solder with solid solution compositions

To obtain the interfacial behaviors and the joint strength of Sn–Bi solid solution solder, Sn–2.5Bi and Sn–5Bi solders were selected to be investigated in this work with pure Sn/Cu solder joints as a reference. The results showed that the growth behaviors of intermetallic compound (IMC) at Sn–Bi solder joints were different under reflow soldering and isothermal aging, which was related with the presence of Bi atoms and the diffusion rate of Cu atoms. Furthermore, the Cu6Sn5 particles in solder matrix and the Bi precipitation could change the diffusion path of Cu atoms, which caused the change on the distribution and size of Cu6Sn5 in pure Sn, Sn–2.5Bi and Sn–5Bi solder matrix after isothermal aging. The size and distribution of Cu6Sn5 in solder matrix finally affected the shear strength of solder joints. The shear strength of Sn–2.5Bi/Cu solder joints increased compared with pure Sn/Cu solder joints, and there was no obvious change with the aging times for Sn–2.5Bi/Cu solder joints due to the effect of the reinforcement of Bi atoms. However, the shear strength of Sn–5Bi/Cu solder joints increased firstly and then decreased with the prolongation of aging times. This phenomenon was attributed to the segregation of Bi and the large amounts of Cu6Sn5 particles distributed in solder matrix during the aging condition. Besides, the IMC thickness in Sn–5Bi/Cu solder joint played the important role on the shear strength due to the brittle nature of IMC. In pure Sn solder matrix, the size of Cu6Sn5 particles were larger than that in Sn–Bi solder matrix, which deteriorated the shear strength on pure Sn solder. From the observation on the fracture morphologies, Sn/Cu and Sn–2.5Bi/Cu solder joints showed the ductile fracture occurred in the solder matrix, while Sn–5Bi/Cu solder joints exhibited a mixed fracture.

Effects of copper content in Sn-based solder on the intermetallic phase formation and growth during soldering

This research has studied the effects of copper contents in the Sn-based solder (i.e. Sn-0.7Cu, Sn-1.0Cu, and Sn-3.0Cu) on the intermetallic phase formation and the growth during soldering. In the experiments, reflow soldering was performed at 350 °C with the Cu substrate by using three solders for various soldering times of 10, 20, 40, 60, 120, 240 and 480 s. XRD data have shown that the η-Cu6Sn5 and ε-Cu3Sn phases with hexagonal crystal structure were present between the solder and the substrate. The presence of ε-Cu3Sn phase was found only when the substrate was soldered for at least 240 s. The Cu content in solder had significant effects on the intermetallic growth with long term soldering time. In addition, the diffusion coefficient was correlated to the Cu content in the solder.

Numerical investigation on the effect of condensate layer formation around large-size components during vapour phase soldering

The reflow soldering process of large size components was always problematic in microelectronics manufacturing due to the possibility of component displacement failures after soldering; like tombstone formation or skewing, which can be traced back to the different heating of the opposite component sides. During vapour phase soldering, the efficiency of heat transfer highly depends on the thickness of the condensate layer. In this paper, the inhomogeneity of condensate layer formation and its effects were investigated at large size components during vapour phase soldering by numerical simulations. For this purpose, a 3D computational fluid dynamic model was established. According to the condensate layer formation in different cases, the onset differences in the melting of the solder alloy at the opposite leads of the component were calculated. By the results, the risk of the component displacement during reflow soldering was analysed. It was found, that the congestion of the condensate layer around the large size components can cause considerable differences in the onset of the solder alloy melting, which can yield in component displacement failures after soldering. The extent of difference in the onset of melting depends on the location of the component on the board and on the applied soak temperature. Keep-out zones on the board were suggested to reduce the possibility of the component displacement failures during the vapour phase soldering process.

Investigation on nano-reinforced solder paste after reflow soldering part 1: Effects of nano-reinforced solder paste on melting, hardness, spreading rate, and wetting quality

In this study, the effects of three distinctive contents of metal oxide [Iron Nickel Oxide (Fe2NiO4), Iron Oxide (Fe2O3), Nickel Oxide (NiO), and Indium Tin Oxide (ITO)] and carbon nanostructure [diamond (C)]-reinforced solder paste on melting behavior, hardness, spreading rate, and wetting angle after reflow soldering were differentiated. Each nanoparticle was mechanically mixed with SAC 305 solder paste to produce homogeneous nanocomposite solder alloys. Based on the results, the addition of nanoparticles slightly increased the melting point; except for the SAC 305-ITO and SAC 305-Diamond composite solder pastes. The SAC 305-ITO composite solder showed negligible effects, but the SAC 305-Diamond composite solder reduced the melting point. The SAC 305-Diamond composite solder exhibited a very high increment in solder hardness compared with other nanocomposite solder pastes. Reinforcement of 0.5 wt% diamond nanoparticles increased solder viscosity higher than other metal oxide nanoparticles. The flow rate of SAC 305-Diamond molten solder on Cu substrate decreased upon the nanoparticle addition during reflow soldering. The reduction in the spreading caused the enlargement in the wetting angle of solder paste. However, very low percentage of carbon nanostructure addition into molten solder alloys reduced the melting point and increased solder hardness with tolerable spreading rate and wetting quality, which enhanced solder reliability after reflow.

Reliability performance of tin–bismuth–silver (Sn57.6Bi0.4Ag) solder joints with different content of carbon nano-tubes (CNTs) or nickel (Ni)-modified CNTs

In this study, the reliability performance of Sn57.6Bi0.4Ag solder joints containing different amounts (wt%) of CNTs and Ni-coated CNTs (Ni–CNTs) has been investigated. Thermal shock testing was used, as a means of harsh environmental stressing. The theory of the reinforcing effects for these two dopants was discussed. The difference of the dopant redistributing process during reflow soldering was also analysed. The formation mechanisms of cracks in the doped solder joints were elucidated through morphological analyses. Ball shear testing results were reported to substantiate the mechanical strength improvements of such joints. The characteristics of the fracture surfaces were analysed to clarify and correlate the changes in the related performance. Sn57.6Bi0.4Ag solder joints containing 0.03 wt% Ni–CNTs have been shown to offer the optimal reliability performance, as evidenced by its highest resistance to thermal shock. Ni coated ones showed a positive effect on the distribution of CNTs. However, solder joints containing excessive dopants of > 0.05 wt% yielded certain undesirable defects in the thermal shock testing, as evidenced by its deterioration in mechanical properties. The relative significance of such defects, e.g. aggregation of CNTs, formation of loose CNT-alloy structural phases and rapid propagation of cracks, varied considerably depending of the type and amount of dopants.

Effect of thermal misfit stress on steam-driven delamination in electronic packages

In this study, circular interfacial defects or delaminations between the die-attach layer and the substrate in electronic packages are analytically modeled to investigate the effect of thermal misfit stress on the steam-driven delamination during the solder reflow process. Based on Love-Kirchhoff plate theory, analytical solutions of strain energy and strain energy release rate of circular delaminations under the combined action of steam pressure and thermal stress are derived. By comparing the energy release rates of circular delamination under steam pressure with and without thermal stress, the contribution of thermal stress to the strain energy release rate is assessed quantitatively. Further, the contribution of thermal stress to the extension of elliptic delaminations is studied numerically. This study brings new understanding on the role that thermal stress plays in facilitating steam-driven delamination during the solder reflow process in electronic packages.

A Review on Water Vapor Pressure Model for Moisture Permeable Materials Subjected to Rapid Heating

This paper presents a comprehensive review and comparison of different theories and models for water vapor pressure under rapid heating in moisture permeable materials, such as polymers or polymer composites. Numerous studies have been conducted, predominately in microelectronics packaging community, to obtain the understanding of vapor pressure evolution during soldering reflow for encapsulated moisture. Henry's law-based models are introduced first. We have shown that various models can be unified to a general form of solution. Two key parameters are identified for determining vapor pressure: the initial relative humidity and the net heat of solution. For materials with nonlinear sorption isotherm, the analytical solutions for maximum vapor pressure are presented. The predicted vapor pressure, using either linear sorption isotherm (Henry's law) or nonlinear sorption isotherm, can be greater than the saturated water vapor pressure. Such an “unphysical” pressure solution needs to be further studied. The predicted maximum vapor pressure is proportional to the initial relative humidity, implying the history dependence. Furthermore, a micromechanics-based vapor pressure model is introduced, in which the vapor pressure depends on the state of moisture in voids. It is found that the maximum vapor pressure stays at the saturated vapor pressure provided that the moisture is in the mixed liquid/vapor phase in voids. And, the vapor pressure depends only on the current state of moisture condition. These results are contradictory to the model predictions with sorption isotherm theories. The capillary effects are taken into consideration for the vapor pressure model using micromechanics approach.

The influence of Fe2O3 nano-reinforced SAC lead-free solder in the ultra-fine electronics assembly

This paper presents the influence of Fe2O3 nanoparticles on the microstructure and fillet height for the ultra-fine electronics assembly in the reflow soldering process. Lead-free SAC solder paste was reinforced with different weighted percentages of Fe2O3 nanoparticles (i.e., 0.01, 0.05, and 0.15 wt.%) using a mechanical solder paste mixer. A new form of nano-reinforced lead-free solder paste was applied to assemble the ultra-fine capacitor (i.e., 01005 size) onto the printed circuit board by applying the reflow soldering method. Focused ion beam (FIB), high-resolution transmission electron microscope (HRTEM) system equipped with energy-dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (EDS), and nanoindentation tester, were all used to examine the microstructure, hardness, and the fillet height of the solder joints. The experimental results revealed that nano-reinforced solder with the content of 0.01, 0.05, and 0.05 wt% yielded small changes in the intermetallic layers. Furthermore, applying an increment of Fe2O3 to 0.05 wt% also improved the fillet height. The mechanism of the agglomeration of Fe2O3 in the bulk solder is discussed in this study. Moreover, simulation analysis using the volume of fluid (VOF) and discrete phase method (DPM) was both employed to describe the mechanism of nanoparticle distribution in the solder and the reflow soldering process. The findings are expected to provide profound knowledge and further reference towards the reflow soldering process of the miniaturised electronic package.

Soldering surface mount components on screen-printed Ag patterns on paper and polyimide substrates for hybrid printed electronics

The development of printed electronics on flexible substrates is increasing rapidly, where the main benefits are the large area and low cost. However, the functionality and performance of printed circuits cannot compete with standard silicon based microprocessors or integrated circuits, though the functionality and performance of printed circuits are increasing. Therefore, in this work we investigate the possibilities of using Sn42/Bi57.6/Ag0.4 low-temperature solder paste together with a reflow solder oven and hot air solder iron to mount regular surface mount device (SMD) components on screen-printed silver tracks. It was found that it is possible to solder standard Si SMD packages onto screen-printed Ag ink tracks on paper substrate; however, the component bonding strength to polyimide was not satisfactory. The resistance of the solder joints was found to be no more than 240 mΩ. The bond strength was found to be higher using the manual hot air solder iron than the reflow solder oven. Bending tests show that the bonding strength is higher for the hot air soldered components. Reference samples on FR-4 based copper printed circuit board show a bond strength ∼10 times higher but we conclude that it is possible to solder regular Si SMD components onto Ag-printed conductors on paper substrate with good results. The process could be used to fabricate hybrid printed electronics in a standard solder process line.

Optimising pin-in-paste technology using gradient boosted decision trees

PurposeThis paper aims to present a robust prediction method for estimating the quality of electronic products assembled with pin-in-paste soldering technology. A specific board quality factor was also defined which describes the expected yield of the board assembly.Design/methodology/approachExperiments were performed to obtain the required input data for developing a prediction method based on decision tree learning techniques. A Type 4 lead-free solder paste (particle size 20–38 µm) was deposited by stencil printing with different printing speeds (from 20 mm/s to 70 mm/s) into the through-holes (0.8 mm, 1 mm, 1.1 mm, 1.4 mm) of an FR4 board. Hole-filling was investigated with X-ray analyses. Three test cases were evaluated.FindingsThe optimal parameters of the algorithm were determined as: subsample is 0.5, learning rate is 0.001, maximum tree depth is 6 and boosting iteration is 10,000. The mean absolute error, root mean square error and mean absolute percentage error resulted in 0.024, 0.03 and 3.5, respectively, on average for the prediction of the hole-filling value, based on the printing speed and hole-diameter after optimisation. Our method is able to predict the hole-filling in pin-in-paste technology for different through-hole diameters.Originality/valueNo research works are available in current literature regarding machine learning techniques for pin-in-paste technology. Therefore, we decided to develop a method using decision tree learning techniques for supporting the design of the stencil printing process for through-hole components and pin-in-paste technology. The first pass yield of the assembly can be enhanced, and the reflow soldering failures of pin-in-paste technology can be significantly reduced.

Intermetallic Compound Growth between Electroless Nickel/Electroless Palladium/Immersion Gold Surface Finish and Sn-3.5Ag or Sn-3.0Ag-0.5Cu Solder

Using an electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish with a thick palladium–phosphorus (Pd-P) layer of 1 μm, the intermetallic compound (IMC) growth between the ENEPIG surface finish and lead-free solders Sn-3.5Ag (SA) or Sn-3.0Ag-0.5Cu (SAC) after reflow soldering and during solid-state aging at 150°C was investigated. After reflow soldering, in the SA/ENEPIG and SAC/ENEPIG interfaces, thick PdSn4 layers of about 2 μm to 3 μm formed on the residual Pd-P layers (~ 0.5 μm thick). On the SA/ENEPIG interface, Sn was detected on the upper side of the residual Pd-P layer. On the SAC/ENEPIG interface, no Sn was detected in the residual Pd-P layer, and Cu was detected in the interface between the Pd-P and PdSn4 layers. After 300 h of aging at 150°C, the residual Pd-P layers had diffused completely into the solders. In the SA/ENEPIG interface, an IMC layer consisting of Ni3Sn4 and Ni3SnP formed between the PdSn4 layer and the nickel–phosphorus (Ni-P) layer, and a (Pd,Ni)Sn4 layer formed on the lower side of the PdSn4 layer. On the SAC/ENEPIG interface, a much thinner (Pd,Ni)Sn4 layer was observed, and a (Cu,Ni)6Sn5 layer was observed between the PdSn4 and Ni-P layers. These results indicate that Ni diffusion from the Ni-P layer to the PdSn4 layer produced a thick (Pd,Ni)Sn4 layer in the SA solder case, but was prevented by formation of (Cu,Ni)6Sn5 in the SAC solder case. This causes the difference in solder joint reliability between SA/ENEPIG and SAC/ENEPIG interfaces in common, thin Pd-P layer cases.

Intelligent parametric design for a multiple-quality-characteristic glue-dispensing process

For double-sided circuit boards, a wave soldering carrier is generally used to shield the devices mounted on the surface of the first side of the printed circuit board (PCB), so that the solder joints are not melted again through exposure to tin wave, causing the devices to deviate or fall as a result of flushing. However, carrier adoption increases production costs. This study proposes a glue-dispensing process to replace the wave soldering carrier. In addition, glue curing and reflow soldering were performed simultaneously to enhance production efficiency. An ecofriendly glue-dispensing process using low-cost CEM-1 substrates and a glue materials featuring a low curing temperature helps reduce energy consumption and carbon emissions. The Taguchi method was used to plan and execute this experiment. The quality characteristics of assembly reliability and manufacturing costs were considered in terms of glue thrust strength and per-PCB manufacturing cost, respectively. An intelligent parametric design applying PCA statistical methods and artificial neural networks (ANN) model was proposed. Results of a confirmation test indicated that the optimal parameter combination suggested by the ANN model was superior. The most satisfactory procedure parameter combination obtained comprised GMIR-130HF for the glue material, a curing temperature of $$140\,^{\circ }\hbox {C}$$ , a 1.1 m/min conveyor velocity, and a 0.09 Mpa dispensing pressure.

SAC–xTiO2 nano-reinforced lead-free solder joint characterizations in ultra-fine package assembly

PurposeThis paper aims to investigate the characteristics of ultra-fine lead-free solder joints reinforced with TiO2 nanoparticles in an electronic assembly.Design/methodology/approachThis study focused on the microstructure and quality of solder joints. Various percentages of TiO2 nanoparticles were mixed with a lead-free Sn-3.5Ag-0.7Cu solder paste. This new form of nano-reinforced lead-free solder paste was used to assemble a miniature package consisting of an ultra-fine capacitor on a printed circuit board by means of a reflow soldering process. The microstructure and the fillet height were investigated using a focused ion beam, a high-resolution transmission electron microscope system equipped with an energy dispersive X-ray spectrometer (EDS), and a field emission scanning electron microscope coupled with an EDS and X-ray diffraction machine.FindingsThe experimental results revealed that the intermetallic compound with the lowest thickness was produced by the nano-reinforced solder with a TiO2 content of 0.05 Wt.%. Increasing the TiO2 content to 0.15 Wt.% led to an improvement in the fillet height. The characteristics of the solder joint fulfilled the reliability requirements of the IPC standards.Practical implicationsThis study provides engineers with a profound understanding of the characteristics of ultra-fine nano-reinforced solder joint packages in the microelectronics industry.Originality/valueThe findings are expected to provide proper guidelines and references with regard to the manufacture of miniaturized electronic packages. This study also explored the effects of TiO2 on the microstructure and the fillet height of ultra-fine capacitors.

Influences of soldering time on wettability and intermetallic phase between Sn-3.0Cu solder and copper substrate

In this paper, the influences of soldering time on the wettability and intermetallic phase between Sn-3.0Cu lead-free solder and copper substrate were investigated. Reflow soldering was performed at 350 ° C under variable soldering times of 10, 20, 40, 60, 120, 240 and 480 s. The results indicated that the wettability and intermetallic growth depend on the soldering time. In addition, the Cu6Sn5 and Cu3Sn intermetallic phases with a hexagonal crystal structure were found between the lead-free solder and the copper substrate. The growth of intermetallic phases increased with soldering time, and the growth of intermetallic phases remarkably depended on grain boundary diffusion and was volume diffusion-controlled for Cu6Sn5 and Cu3Sn, respectively.

The garage fab [Resources

Electronics enthusiasts like being able to make things themselves. In IEEE Spectrum's Hands On column, we've detailed how readers can make their own solder reflow ovens, conductive ink, and synthetic aperture radars. But making DIY integrated circuits seemed impossibly out of reach. After all, building a modern fab is astronomically expensive: For example, in 2017 Intel announced it was investing US $7 billion to complete a facility for making chips with 7-nanometer- scale features. But Sam Zeloof was not deterred. This 17-year-old high school student has started making chips in his garage,albeit with technology that's a few steps back along the curve of Moore's Law. Zeloof says he has been working on his garage fab, located in his home near Flemington, N.J., for about a year. He began thinking about how to make chips as his "way of trying to learn what's going on inside semiconductors and transistors. I started reading old books and old patents because the newer books explain processes that require very expensive equipment."

An Ultrahigh Vacuum Packaging Process Demonstrating Over 2 Million Q-Factor in MEMS Vibratory Gyroscopes

Micro resonators and vibratory gyroscopes could reach their fundamental thermo-elastic damping (TED) limit in an ultravacuum condition where the viscous damping is eliminated. This article presents a reliable and repeatable high vacuum sealing process as a standalone approach for performance evaluation of dynamic micro devices. We demonstrated a micro-Torr vacuum level in a ceramic leadless chip carrier package and achieved the Q-factor over 2million on a quad mass gyroscope, approaching the TED limit of the device by effectively eliminating the viscous damping. The long-term vacuum stability was characterized over 1 year and demonstrated that the Q-factor does not degrade over time and even continues improving. The long-term ultrahigh vacuum condition is enabled by surface desorption prior to sealing, pumping of residual gases by passive getters, and defect-free solder reflow in the sealing area. The experimental results confirmed the reliability and repeatability of the proposed sealing procedure, which can be adapted for other Micro Electro-Mechanical Systems (MEMS) with ultrahigh vacuum requirements.

A novel interface strengthening layer: Nanoscale AuCu super-structure formed during Au80Sn20/Cu rapid solidification soldering process

Phase composition and orientation of intermetallic compounds layers determine the functional and mechanical properties of solder joints. In this paper, interfacial reaction and strengthening mechanism of Au80Sn20/Cu joint by rapid solidification reflow soldering were studied. The Au80Sn20/Cu solder joint by rapid solidification reflow soldering exhibit a superior shear property, and the shear strength reaches up to 90MPa. It is found that a new nanoscale AuCu super-structure formed in the interfacial intermetallic compounds layer, which is coherent with the [001] Cu substrate. The orientation relationship of Cu matrix/interfacial intermetallic compounds is determined to be001Cu∥001AuCu∥2¯42¯3AuCu5Sn and200Cu∥200AuCu∥101¯0AuCu5Sn. Nanoscale AuCu super-structure influences the interface slip direction and provides interface misfit dislocation, hinder the dislocation movement, lead to the interface strengthening effect. Our finding on nanoscale AuCu super-structure formed during Au80Sn20/Cu soldering process not only sheds light on future research direction but also provides practical guidance to the engineers in solder industry.

Applying Monte Carlo Simulation to Analyze the Open Scenario in the Assembly of Electronic Components

The emergence of environmental awareness, particularly regarding green packaging, has caused SAC305, a tin-silver–copper alloy (Sn 96.5%, Ag 3%, and Cu 0.5%), to become a widely used lead-free solder material. The use of SAC305 increases in temperature requirements for reflow soldering roughly 15 °C–35 °C compared with that of the use of Sn63Pb37, a conventional tin-lead solder. However, component and printed circuit board (PCB) warpage may occur when subject to high temperature, leading to open solder joints and other defects. The ball grid array (BGA), quad flat pack, and dual-flat no-lead (DFN-8) packaging components of server PCBs were tested in this paper. First, the height of the component contacts, printing thickness of the solder paste, and PCB and component warpage were examined, and geometric principles were applied to establish an open solder joint model for nonflat solder joints. Subsequently, the Monte Carlo simulation method was used to sample the aforementioned parameters randomly and calculate the distance between the contacts at the bottom of the electronic component and the surface of the solder paste. A threshold value was used to determine whether the contacts at the bottom of the components were connected to the surface of the solder paste and independently evaluate the occurrence of open solder joints before and during reflow soldering.

Effects of Solder Volume and Reflow Conditions on Self-Alignment Accuracy for Fan-Out Package Applications

The effects of solder volume and reaction time between molten solder and a metal pad at the peak temperature of reflow on the self-alignment effect have been investigated in flip chip bonding. A glass die with two different pad designs and a flame retardant-4 (FR-4) organic substrate were used. Sn-3.0Ag-0.5Cu and Sn-3.5Ag solders were formed on Cu-organic solderability preservation (Cu-OSP) and electroless nickel electroless palladium immersion gold (ENEPIG) pads on FR-4 substrates using the stencil printing method. To assess the effect of solder volume, the thickness and opening size of the stencil mask were controlled. Reflow experiments were performed at 250°C with wetting times of 40 s, 55 s, 65 s, and 75 s. After flip chip reflow soldering, the bonding areas were cross-sectioned to inspect the shape of the interconnected solder using scanning electron microscopy. The results revealed that using an insufficient solder volume on the pad was responsible for die shifts larger than 1 μm, while a sufficient solder volume on the pad and a stable solder joint shape could ensure misalignment less than 1 μm. The Sn-3.0Ag-0.5Cu solder showed a lower die shift value than the Sn-3.5Ag solder because the Sn-3.0Ag-0.5Cu solder has stronger surface tension than the Sn-3.5Ag solder. Using a longer wetting time between the solder and the pad at the peak temperature also improved the die shift value because the increased reaction time changed the interconnected solder shape between the die and substrate from concave to convex, moving the die to a more accurate position. Furthermore, the restoring forces on die self-alignment influenced the die shift value. A stronger solder surface tension and a larger volume of solder on the pad produced stronger restoring forces for die self-alignment, thereby improving the die shift value.