Reflow Profile Research Articles

Investigation of the Role of Void Formation at the Cu-to-Intermetallic Interface on Aged Drop Test Performance

Chip-scale packages (CSPs) are widely used in portable electronic products. Mechanical drop testing is a critical reliability requirement for these products. With the switch to lead-free solder, new reliability data must be generated. Most drop test reliability data reported for CSPs are for the as-built condition. However, the mechanical shock reliability over the life of the product is equally important. This paper provides a systematic study of surface finish (immersion Sn and immersion Ag) and reflow profile (cool down rate) on the drop test reliability of CSP assemblies. A limited experiment was also performed with organic solderability preservative (OSP)-coated boards. The Sn finish provides an initial Cu-Sn intermetallic layer, while the Ag finish and OSP coating allows the formation of the initial Cu-Sn intermetallic during the reflow cycle. Drop test results for assemblies as-built and as a function of aging at 125 degC are correlated with cross-sectional analysis of the solder joints. The mean number of drops to failure decreases by approximately 80% with aging at 125 degC through 480 h. Voids develop at the Cu-Sn intermetallic-to-Cu interface during high-temperature aging, but the crack path is through the intermetallic layer and does not propagate from void-to-void. Thus, it can be concluded that the voids do not contribute to the decrease in drop test survivability observed in this study

Advances in Vapor Pressure Modeling for Electronic Packaging

Two advanced techniques have been developed for modeling vapor pressure within the plastic IC packages during solder reflow. The first involves the extension of the wetness technique to delamination along multimaterial interface and during dynamic solder reflow. Despite its simplicity, this technique is capable of offering reliable and accurate prediction for packages with high flexural rigidity. For packages with low flexural rigidity, the new decoupling technique that integrates thermodynamics, moisture diffusion, and structural analysis into a unified procedure has been shown to be more useful. The rigorous technique has been validated on both leadframe-based as well as laminate-based packages. With high accuracy and computational efficiency, these dynamic modeling tools will be valuable for optimization of package construction, materials, and solder reflow profile against popcorn cracking for both SnPb and Pb-free solders

The effect of reflow profile on SnPb and SnAgCu solder joint shear strength

Purpose – The purpose of this work is to study the effect of the reflow peak temperature and time above liquidus on both SnPb and SnAgCu solder joint shear strength.Design/methodology/approach – Nine reflow profiles for Sn3.0Ag0.5Cu and nine reflow profiles for Sn37Pb have been developed with three levels of peak temperature (230°C, 240°C, and 250°C for Sn3.0Ag0.5Cu; and 195°C, 205°C, and 215°C for Sn37Pb) and three levels of time above solder liquidus temperature (30, 60, and 90 s). The shear force data of four different sizes of chip resistors (1206, 0805, 0603, and 0402) are compared across the different profiles. The shear forces for the resistors were measured after assembly. The fracture interfaces were inspected using scanning electron microscopy with energy dispersive spectroscopy in order to determine the failure mode and failure surface morphology.Findings – The results show that the effects of the peak temperature and the time above solder liquidus temperature are not consistent between differe...

Minimizing flux spatter during lead‐free reflow assembly

PurposeThe leaching of lead from electronic components in landfills to ground water is harmful to health and to the environment. Increasing concern over the use of lead in electronics manufacturing has led to legislation to restrict its use as a joining material. Consequently, significant recent research efforts have been geared to identification of suitable lead‐free solder pastes. Typically, lead‐free solder pastes contain a very active flux in an effort to improve wetting. These aggressive fluxes have the tendency to explode (or burst) and create flux spatter, causing many process problems with sensitive electronic components. The purpose of this paper is to propose solution procedures to minimize/eliminate these flux spatters, particularly, on gold fingers in memory modules when lead‐free solder pastes are used.Design/methodology/approachFour no‐clean, lead‐free Sn‐Ag‐Cu (SAC) alloy‐based solder pastes consisting of four different flux systems from three different vendors were evaluated. Two types of reflow profiles (linear and ramp‐soak‐ramp) were also evaluated. Experiments were also conducted to optimise the soak temperature and soak time to determine a broader process window for lead‐free volume production with minimal flux spatter on the contact fingers of memory modules. In order to validate our findings the recommended profile and paste was adopted in production. Additional experiments on a board with a different surface finish were also carried out to validate the recommendations.FindingsFlux spatter can be reduced/eliminated through proper selection of flux chemistry and reflow profile optimisation. The experimental study conducted indicates there is a reduction in the occurrence of flux spatter when a ramp‐soak‐ramp profile is used with lead‐free solder pastes.Originality/valueDemonstrates that flux spatter can be reduced/eliminated by carefully choosing a soak profile and appropriate flux chemistry.

Reliability Investigation of Mixed BGA Assemblies

The effect of different reflow profiles on the reliability of lead-free (LF) Sn-3.0 Ag-0.5 Cu (wt.%) (SAC 305) ball grid array (BGA) devices assembled with a SnPb eutectic paste was investigated. The memory modules in a back-to-back configuration were reflowed on standard graphic cards finished with immersion silver (IAg) or hot air solder leveling (HASL) coatings. The reflow peak temperatures ranged from 209 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C to 227 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C, while the time above liquidus (TAL) varied from 45 to 80 s. Depending on the reflow conditions, the solder interconnects displayed varied degrees of SnPb and LF solders intermixing. It was established that in order to receive a homogeneous solder alloy, the reflow peak temperature had to be in the 218 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C–222 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C range. The reliability of solder interconnects of memory modules was assessed by subjecting the cards to 1500 cycles of accelerated thermal-cycling with a profile from 0 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C to 100 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C. It was found that the control SnPb/SnPb assemblies displayed superior reliability to that of the mixed assemblies. Regardless of the degree of homogeneity of the BGA balls, the predominant failure mode of the mixed solder joints was interfacial cracking through a Pb-rich phase near the intermetallic layer. In contrast, only partial cracks propagating diagonally through the bulk solder were present on the control boards. It was concluded that a combination of state of stress and segregation of the Pb-rich phase at the interface was responsible for the shortened thermal–mechanical fatigue life of the mixed solder interconnects.

Effect of Compatibility between Fluxes and Cyanate Ester Underfill on Lead Free Flip Chip Assembly and Reliability

The quality and reliability of flip-chip assembly is severely impacted by the compatibility between various materials used in the package. Currently, no-clean fluxes are widely used for the assembly of flip chips. Poor compatibility between the flux residue and the underfill can lead to the formation of voids, and consequently, reliability problems. Therefore, a major concern for flip-chip assembly is the compatibility between the flux residues and the underfill. The principal objective of this research was to develop a flux for lead-free packaging, which would be compatible with high performance moisture resistant cyanate ester-based underfills. During this study, commercially available fluxes, along with tailor made epoxy-based flux, were tested for their compatibility with the underfill. The results indicated that the assembly process window for the rosin-based fluxes was much wider than the epoxy-based fluxes. Synthetic flux had poor soldering performance and relatively poor compatibility with the cyanate ester underfill than rosin-based flux. Epoxy flux exhibited narrow process window and the soldering performance was sensitive to flux thickness, reflow profile, substrate pad surface finish and topography. For smaller packages with die size of 10 mm by 10 mm, the compatibility between the cyanate ester underfill and the different fluxes was comparable. However, for the packages with die size of 22 mm by 22 mm, the compatibility between the epoxy-based flux and the cyanate ester underfill was relatively better than that with the other two flux chemistries.

Lead-Free Chip Scale Packages: Assembly and Drop Test Reliability

Three underfill options compatible with lead-free assembly have been evaluated: capillary underfill, fluxing underfill, and corner bond underfill. Chip scale packages (CSPs) with eutectic Sn/Pb solder were used for control samples. Without underfill, lead-free and Sn/Pb eutectic drop test results were comparable. Capillary flow underfills, dispensed and cured after reflow, are commonly used in CSP assembly with eutectic Sn/Pb solder. With capillary flow underfill, the drop test results were significantly better with lead-free solder assembly than with Sn/Pb eutectic. Fluxing underfill is dispensed at the CSP site prior to CSP placement. No solder paste is printed at the site. The CSP is placed and reflowed in a standard reflow cycle. A new fluxing underfill developed for compatibility with the higher lead-free solder reflow profiles was investigated. The fluxing underfill with lead-free solder yielded the best drop test results. Corner bond underfill is dispensed as four dots corresponding to the four corners of the CSP after solder paste print, but before CSP placement. The corner bond material cures during the reflow cycle. It is a simpler process compared to capillary or fluxing underfill. The drop test results with corner bond were intermediate between no underfill and capillary underfill and similar for both lead-free and Sn/Pb eutectic solder assembly. The effect of aging on the drop test results with lead-free solder and either no underfill or corner bond underfill was studied. Tin/lead solder with no underfill was used for control. This test was to simulate drop performance after the product has been placed in service for some period of time. There was degradation in the drop test results in all cases after 100 and 250 h of storage at 125/spl deg/C prior to the drop test. The worst degradation occurred with the lead-free solder with no underfill.

Lead-free 0201 manufacturing, assembly and reliability test results

The trend towards smaller, faster and cheaper electronic devices has led to an increase in the use of 0201 ( L ∼ 0.02 in.; W ∼ 0.01 in.) and even smaller sized passive components. The size advantages of the 0201 component make it a popular choice among design engineers but not among manufacturing engineers. From a manufacturing perspective, the size of the 0201 package poses significant challenges to the printed circuit board (PCB) assembly process. The many challenges with 0201 assembly can be attributed to the solder paste volume, pad design, aperture design, board finish, type of solder paste, pick-and-place and reflow profile. If these factors are not optimized, they will introduce undesirable manufacturing defects. The small size of 0201 packages and undetected manufacturing defects will also raise concerns about their second level interconnect reliability, especially for lead-free solder alloys and surface finishes, with new processes and higher reflow requirements. To determine the optimum conditions, a design-of-experiment (DOE) study was carried out to investigate the effects of these parameters on assembly defects and solder joint reliability. This paper presents the test results and comparative literature data on the influence of a few key manufacturing parameters and defects associated with the 0201 component using lead-free and tin–lead solder alloys. Data pertaining to component shear strength before and after isothermal aging at 150 °C and intermetallic growth up to 500 h of aging are presented. A number of test vehicles were also subjected to thermal cycling (1500 cycles) in the range of −55/100 °C to determine the solder fatigue behavior. Shear test results for test vehicles subjected to thermal cycling is also presented. In addition, optical microscopy analysis of solder joint behavior during thermal cycling showing the progress of the solder damage and cross-sectional photos taken at 1500 cycles is included.

Soldering Properties and Interfacial Microstructure of CSP Solder Joints with Lower Melting Lead-free Solder

In this paper we investigate the appropriate reflow profiles for simulated CSP solder joints using Sn-Ag-Bi-In solder, which has a lower melting point than Sn-Ag-Cu solder. We have examined the relationship between the interfacial microstructure and mechanical characteristics of Sn-Ag-Bi-In solder at the solder joints compared with those of Sn-Zn-Bi solder. When soldering Sn-3Ag-0.5Cu CSP balls on a Cu/Ni/Au pad, Sn-8Zn-3Bi showed high joint strength at 503 K or higher, whereas Sn-3.5Ag-0.5Bi-8In showed strength at the lower temperature of 493 K. This implies that Sn-Ag-Bi-In solder is more appropriate for soldering at lower temperatures. On the joint interface, a stratified Ni-Sn layer was formed when the Sn-3Ag-0.5Cu CSP ball was soldered on the Cu/Ni/Au phases are pad using Sn-3.5Ag-0.5Bi-8In at 483 ~ 493 K. At 503 K or higher, clumped (Cu,Ni)6Sn5 unevenly formed on the joint interface, resulting in lower strength. These results suggest the appropriate reflow thermal profile for Sn-3.5Ag-0.5Bi-8In solder joints with the Sn-3Ag-0.5Cu CSP ball on the Cu/Ni/Au pads should be in the range of 483 ~ 493 K. Even if the reflow peak temperature is over 493 K, slowing down the cooling speed by 2 K/s can prevent loss of joint strength.

Study of Interfacial Delamination by Considering the Effect of Temperature and Moisture during Solder Reflow for QFN Package

Experiment has been conducted to measure the solubility Csat of mold compound and to obtain the moisture weight loss curves at various temperature. Moisture desorption modeling has been conducted to calculate the moisture diffusivity for desorption D(T) by matching with the experimental results. Finite Element Analysis (FEA) has also been conducted to calculate the transient development of strain energy release rate (ERR) at the interfacial crack tip due to thermal stress only (Gt), hygrostress only (Gh), and the combined effect (Gtotal) during solder reflow. ERR is computed based on the Virtual Crack Closure Technique (VCCT). It is found that Gh is significantly smaller than Gt, however the effect of hygrostress significantly increases the total strain energy release rate when combined with the thermal effect. The maximum Gtotal occurs at the peak of the solder reflow profile. The effects of crack size and geometrical parameters have been studied. The result imply that the interfacial crack is unstable and has a high tendency of growing to a significant extent. ERR is strongly influenced by the thickness of the package while length between the edge of die and pad has a moderate affect on the ERR.

A quantitative method of reliability estimation for surface mount solder joints based on heating factor Qη

A novel method of reliability analysis on thermal fatigue failure for surface mount solder joints, based on the heating factor Q η , is presented, by which quantitative reliability estimation and prediction of solder joints suffering from cyclic thermal stress can be done. Based on the typical lifetime data of thermal cycling test, the relationship of the mean time to failure (MTTF) as well as the reliability of solder joints as an explicit function of Q η is deduced and presented in a unified mathematic form. Numerical calculations are performed, and the result shows that the MTTF decreases quickly with the increases in heating factor and then slowly approximates to a constant value when Q η ⩾ 1500 s °C. The solder joint reliability in terms of thermal cycle degrades in an analogical fashion for different heating factors. For any given thermal cycle, calculation suggests that to obtain a higher reliability, a lower heating factor should be controlled during soldering. The presented method gives an applicable solution and can be used for online reflow control in industry. On the one hand, an ideal reflow profile can be achieved by properly controlling heating factor during soldering to meet the given reliability goal. On the other hand, the life expectancy of solder joints can be approximately estimated and predicted from a known reflow profile with a specified heating factor. Finally, for a specified reliability goal, how to properly choose and control heating factor during soldering to achieve reliable solder joints is discussed.

Effects of process conditions on reliability, microstructure evolution and failure modes of SnAgCu solder joints

In this study, microstructure evolution at intermetallic interfaces in SnAgCu solder joints of an area array component was investigated at various stages of a thermal cycling test. Failure modes of solder joints were analyzed to determine the effects of process conditions on crack propagation. Lead-free printed-circuit-board (PCB) assemblies were carried out using different foot print designs on PCBs, solder paste deposition volume and reflow profiles. Lead-free SnAgCu plastic-ball-grid-array (PBGA) components were assembled onto PCBs using SnAgCu solder paste. The assembled boards were subjected to the thermal cycling test (−40 °C/+125 °C), and crack initiation and crack propagation during the test were studied. Microstructure analysis and measurements of interface intermetallic growth were conducted using samples after 0, 1000, 2000 and 3000 thermal cycles. Failures were not found before 5700 thermal cycles and the characteristic lives of all solder joints produced using different process and design parameters were more than 7200 thermal cycles, indicating robust solder joints produced with a wide process window. In addition, the intermetallic interfaces were found to have Sn–Ni–Cu. The solder joints consisted of two Ag–Sn compounds exhibiting unique structures of Sn-rich and Ag-rich compounds. A crystalline star-shaped structure of Sn–Ni–Cu–P was also observed in a solder joint. The intermetallic thicknesses were less than 3 μm. The intermetallics growth was about 10% after 3000 thermal cycles. However, these compounds did not affect the reliability of the solder joints. Furthermore, findings in this study were compared with those in previous studies, and the comparison proved the validity of this study.

Microstructural investigation of lead‐free BGAs soldered with tin‐lead solder

PurposeThe goal of this work is to evaluate the feasibility of soldering tin‐silver‐copper balled BGAs using tin‐lead‐based solder and to investigate the influence of different production parameters on the microstructure of the solder joint.Design/methodology/approachThe soldering of the BGAs was done with various temperature profiles and two conveyor speeds under a nitrogen atmosphere in a full convection oven. One specimen from each temperature/time combination was cross‐sectioned. The cross sections were analysed with optical microscopy, scanning electron microscopy with energy dispersive X‐ray spectroscopy (SEM/EDS) at 30 kV and focused ion beam microscopy (FIB).FindingsThe cross sections show a metallurgical bond between the solder and the tin‐silver‐copper balls of the BGA, even at a peak reflow temperature of 210°C. However, the balls alloy only partially with the solder, as the liquidus of tin‐silver‐copper balls is 217°C. As soon as the peak temperature exceeds the liquidus of the ball, the solder is totally dissolved in the material of the ball. A reflow profile with a peak temperature of about 230°C on the BGA gives a homogenous reaction of the solder with the ball with a minimum formation of voids.Research limitations/implicationsThe dependence of varying reflow parameters on reliability requires detailed study. Especially the effect of a partially melted ball on the degradation of the solder joint needs to be investigated.Originality/valueFrom the findings, it can be said that soldering lead‐free balls with tin‐lead solder is possible. This is useful during the transitional period that the industry is in at the moment. More and more component manufacturers are changing their components to lead‐free, often without notice to the customer. If a production line is still running a tin‐lead process it is essential to know how to process these components with tin‐lead solder.

An investigation of the recommended immersion tin thickness for lead‐free soldering

PurposeThis paper describes the different thickness measurement techniques that enable reliable thickness assessments, and the determination of the recommended immersion tin thickness for lead‐free soldering.Design/methodology/approachImmersion tin layers were prepared with systematically varying layer thicknesses. The samples were annealed at different reflow profiles, used in assembly for tin/silver/copper (SAC‐alloy) soldering. The layers were characterized with X‐ray fluorescence, electrochemical stripping coulometry, and by examining the cross sections using a scanning electron microscope. The solderability of the samples was determined with a solder balance (Solderability Tester Menisco ST60) using a SAC‐alloy (melting point 217°C) with T(max) at ΔT=28°C and ΔT=43°C above melting.FindingsIf all pure tin is converted into the Sn/Cu IMC, so that no pure tin is left as solderable layer, the wetting behaviour will decrease dramatically. Especially for multiple soldering processes, two times reflow followed by wave soldering, it is essential to have a pure tin layer covering the Sn/Cu IMC before going to the final soldering process. The required amount of residual pure tin over the Sn/Cu IMC is detailed in several papers. It is stated that a minimum of 0.2 μm of pure tin over the Sn/Cu IMC is absolutely necessary to ensure reliable wetting and solder joint formation. With the current immersion tin thickness recommendation of 1 μm, based on the needs of lead containing solder pastes, a residual pure tin layer will not be evident or thick enough to ensure reliable assembly for multiple soldering with lead‐free temperature profiles.Originality/valueHelps to enable reliable thickness assessments, and the determination of the recommended immersion tin thickness for lead‐free soldering.

A systematic procedure for the selection of a lead‐free solder paste in an electronics manufacturing environment

PurposeHistorically, tin‐lead solder has been a commonly used joining material in electronics manufacturing. Environmental and health concerns, due to the leaching of lead from landfills into ground water, have necessitated legislation that restricts the use of lead in electronics. The transition from tin‐lead solder to a lead‐free solder composition is imminent. Several alternative solder alloys (and their fluxes) have been researched for electronics assembly in the last few years. The objective of this research was to develop a systematic selection process for choosing a “preferred” lead‐free solder paste, based on its print and reflow performance.Design/methodology/approachAfter a detailed study of industry preferences, published experimental data, and recommendations of various industrial consortia, a near eutectic tin‐silver‐copper (SAC) composition was selected as the preferred alloy for evaluation. Commercially available SAC solder pastes with a no‐clean chemistry were extensively investigated in a simulated manufacturing environment. A total of nine SAC pastes from seven manufacturers were evaluated in this investigation. A eutectic Sn/Pb solder paste was used as a baseline for comparison. While selecting the best lead‐free paste, it was noted that the selected paste has to perform as good as, if not better than, the current tin‐lead paste configuration used in electronics manufacturing for a particular application. The quality of the solder pastes was characterized by a series of analytical and assembly process tests consisting of, but not limited to, a printability test, a solder ball test, a slump test, and post reflow characteristics such as the tendency to form voids, self‐centring and wetting ability.FindingsEach paste was evaluated for desirable and undesirable properties. The pastes were then scored relative to each other in each individual test. An aggregate of individual test scores determined the best paste.Originality/valueThis paper summarizes a systematic approach adopted to evaluate lead‐free solder pastes for extreme reflow profiles expected to be observed in reflow soldering lead‐free boards.

Lead-free Solder Joint Reliability – State of the Art and Perspectives

There is an increasing demand for replacing tin-lead (Sn/Pb) solders with lead-free solders in the electronics industry due to health and environmental concerns. The European Union recently passed a law to ban the use of lead in electronic products. The ban will go into effect in July of 2006. The Japanese electronics industry has worked to eliminate lead from consumer electronic products for several years. Although currently there are no specific regulations banning lead in electronics devices in the United States, many companies and consortiums are working on lead-free solder initiatives including Intel, Motorola, Agilent Technologies, General Electric, Boeing, NEMI and many others to avoid a commercial disadvantage. The solder joints reliability not only depends on the solder joint alloys, but also on the component and PCB metallizations. Reflow profile also has significant impact on lead-free solder joint performance because it influences wetting and microstructure of the solder joint. A majority of researchers use temperature cycling for accelerated reliability testing since the solder joint failure mainly comes from thermal stress due to CTE mismatch. A solder joint failure could be caused by crack initiation and growth or by macroscopic solder facture. There are conflicting views of the reliability comparison between lead-free solders and tin-lead solders. This paper first reviews lead-free solder alloys, lead-free component lead finishes, and lead-free PCB surface finishes. The issue of tin whiskers is also discussed. Next, lead-free solder joint testing methods are presented; finite element modeling of lead-free solder joint reliability is reviewed; and experimental data comparing lead-free and tin-lead solder joint reliability are summarized. Finally the paper gives perspectives of transitions to totally lead-free manufacturing.

A simplified reflow soldering process model

Established models of temperature development during the reflow soldering process have generally used commercially available finite difference (FD) or computational fluid dynamics (CFD) modelling tools to create detailed representations of the product and reflow furnace, and of their interactions. Such models have been shown to achieve a high degree of accuracy in predicting the temperatures which a particular printed circuit board (PCB) design will achieve during reflow, but are complex to generate and analysis times are unacceptably long. With the move to adopt lead-free soldering technology, and the consequently higher reflow process temperatures, optimisation of the reflow profile will gain a renewed emphasis, increasing the industrial value of effective process models. This paper reports the development of a less complex approach to the modelling of the process, using simplified representations of both the product and the process, in order to achieve an accuracy comparable with that of more detailed models. In this simplified model the product is represented using a two-dimensional mesh of thermal conductances linking thermal masses the values of which are calculated based on the averaged properties of the PCB material and attached components within the area of each of the elements. In order to establish an accurate representation of the specific reflow furnace being simulated, a reflow data logger equipped with suitable sensors is used to make measurements of the temperature and level of thermal convection at each point along the length of the furnace for a small number of carefully chosen reflow profiles. The behaviour for any other reflow profile may then be predicted from these measurements, thereby avoiding the necessity of making detailed measurements of the furnace geometry and air flow rates.

Reflow profile study of the Sn‐Ag‐Cu solder

A study of the Sn‐Ag‐Cu lead‐free solder reflow profile has been conducted. The purpose of the work was to determine the Sn‐Ag‐Cu reflow profile that produced solder bumps with a thin intermetallic compound (IMC) layer and fine microstructure. Two types of reflow profiles were studied. The results of the experiment indicated that the most significant factor in achieving a joint with a thin IMC layer and fine microstructure was the peak temperature. The results suggest that the peak temperature for the Sn‐Ag‐Cu lead‐free solder should be 230°C. The recommended time above liquidus is 40 s for the RSS reflow profile and 50‐70 s for the RTS reflow profile.

Influence of the Interfacial Reaction Layer on Reliability of CSP Joints Using Sn-8Zn-3Bi Solder and Ni/Au Plating

In this study, we used a solder paste of Sn-8Zn-3Bi. The structure and specification of the CSP used in this study are shown in Fig. 1. The composition of the solder ball of the CSP is Sn-3.0Ag-0.5Cu. The CSPs were reflow-soldered on the Cu pad of the FR-4 PCB using Sn-8Zn-3Bi solder paste in air. The surface finish on the Cu pad was Ni-P(5 mm)/ Au(0.05 mm), Ni-P(5 mm)/Au(0.5 mm), or heat-resistant preflux (Fig. 2). The Ni-P layer was formed by electroless plating and the Au layer was formed by substitution plating. Figure 3 schematically illustrates thermal process for reflowsoldering. The reflow profile was composed of preheat at 403423 K for 60 s, followed by 20 s at a temperature above

Effects of intermetallic morphology at the metallic particle/solder interface on mechanical properties of Sn-Ag-based solder joints

Mechanical incorporation of metallic particles in the Sn-Ag-based solder resulted in various intermetallic compound (IMC) morphologies around these particles during reflow. Unlike with the Ni particles, the IMCs formed around Cu and Ag particles are relatively insensitive to reflow profiles employed. The IMC formed around the Ni particles ranges from “sunflower” morphology to “blocky” morphology with increasing time and temperature above liquidus during the heating part of the reflow profile. Mechanical properties, such as simple shear strength and creep behavior, of these composite solders were affected by the IMC morphologies in the composite solders investigated. Sunflower-shaped IMC formed around an Ni particles resulted in higher simple shear strength and better creep properties.