Temperature profile effects in accelerated thermal cycling of SnPb and Pb-free solder joints

Accelerated thermal cycling (ATC) has been widely used in the microelectronics industry for reliability assessment. ATC testing decreases life cycle test time by one or more of the following means: increasing the heating and cooling rate, decreasing the hold time, or increasing the range of the applied temperature. The relative effect of each of these cycle parameters and the failure mechanisms they induce has been the subject of many studies; however uncertainty remains, particularly regarding the role of the heating and cooling rate. In this research, three conditions with two ramp rates (14 °C/min and 95 °C/min) and two temperature ranges (ΔT = 0–100 °C and −40 to 125 °C) were applied to resistor 2512 and PBGA 256 test vehicles assembled with SnPb and Pb-free solders. The test results showed that the higher ramp rate reduced the testing time while retaining the same failure modes, and that the damage per cycle increased with the temperature difference. For the resistors, the Pb-free solder joints lasted longer than the SnPb joints at the smaller ΔT, but were inferior at the larger ΔT. In contrast, the Pb-free solder joints in the PBGA test vehicles lasted longer than the SnPb solder under both conditions.     

Creep of thermally aged SnAgCu-solder joints

The creep behaviour of Sn96.5Ag3.5- and Sn95.5Ag3.8Cu0.7-solder was studied specifically for its dependence on technological and environmental factors. The technological factors considered were typical cooling rates and pad metallizations for solder joints in electronic packaging. The environmental factors included microstructural changes as a result of thermal aging of solder joints. Creep experiments were conducted on three types of specimens—flip–chip joints, PCB solder joints and bulk specimens. flip–chip specimens were altered through the selection of various under bump metallizations (Cu vs. NiAu), cooling rates (40 K/min vs. 120 K/min), and thermal storage (24 h, 168 h, and 1176 h at 125 °C). PCB solder joints were studied by using a copper pin soldered into a thru-hole connection on a printed circuit board having a NiAu metallization. Bulk specimens contained the pure alloys. The creep behaviour of the SnAg and SnAgCu solders varied in dependence of specimen type, pad metallization and aging condition. Constitutive models for SnAg and SnAgCu solders as they depend on the reviewed factors are provided.     

Thermally induced deformation of solder joints in real packages: Measurement and analysis

The paper presents a hybrid experimental and analytical approach to track the deformation of solder joints in an electronic package subject to a thermal process. The solder joint strain is directly measured using a computer vision technique. The strain measurement is analyzed following an approach that is devised based on an established solder constitutive relation. The analysis leads to the determination of the solder joint stress and in turn, to the separation of the elastic, plastic and creep strain from the measured total strain. The creep strain rate and stress–strain hysteresis loop are also obtained. Two case studies are presented to illustrate the applications and to show the viability of the approach. Each case involves a resistor package with SAC (Sn95.5Ag3.8Cu0.7) solder joints, which is subjected to a temperature variation between ambient and 120 °C. The results confirm that shear is a dominant strain component in such solder joints. The shear strain varies nearly in phase with the temperature whereas the shear stress exhibits a different trend of variation due to stress relaxation. The peak shear stress of around 10 MPa to 15 MPa are found, which occur at near 70 °C in both cases, when the temperature ramps up at approximately 3 °C/min. The creep shear strain goes up to 0.02 and accounts for over 80% of the total shear strain. The creep strain rate is in the order of magnitude of 10⁻⁵ s⁻¹. Responding to the temperature cycling with such moderate rate, the creep strain shows modest ratcheting while the stress–strain hysteresis stabilizes in two cycles.     

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.     

Microstructure and creep behaviour of eutectic SnAg and SnAgCu solders

The paper presents creep data, that was gained on specimens of different microstructures. The three specimen types have been flip chip solder joints, pin trough hole solder joints and standard bulk solder specimens. The bulk solder specimen was a dog-bone type specimen (diameter=3 mm, LENGTH=117 mm). The pin trough hole solder joint consisted on a copper wire that was soldered into a hole of a double sided printed circuit board (thickness 1.5 mm). The flip chip solder joint specimen consisted of two silicon chips (4 mm × 4 mm), which were connected by four flip chip joints (one on each corner). SnAg and SnAgCu flip chip bumps (footprint 200 μm × 200 μm, joint height 165–200 μm, centre diameter 90…130 μm) were created by printing solder paste.Constant–load creep tests were carried out on all three specimen types at temperatures between 5 and 70 °C. Creep data was taken for strain rates between 10⁻¹⁰ and 10⁻³ s⁻¹. The specimens were tested in “as cast” condition and after thermal storage.The microstructural properties of the bulk specimens and real solder joints were examined using metallographic sectioning, optical microscopy techniques, and SEM-microprobe analysis. The results of the microstructural analysis were related to the investigated mechanical properties of the solders. Models of SnAg3.5 and SnAg4Cu0.5, that can be used with the ANSYS FEM software package, will be presented.     

The effect of solder paste composition on the reliability of SnAgCu joints

As the electronics industry is moving towards lead-free manufacturing processes, more effort has been put into the reliability study of lead-free solder materials. Various tin–silver–copper-based solders have become widely accepted alternatives for tin–lead solders. In this study, we have tested three different SnAgCu solder compositions. The first consisted of a hypoeutectic 96.5Sn/3.0Ag/0.5Cu solder, the second of a eutectic 95.5Sn/3.8Ag/0.7Cu solder, and the third of a hypereutectic 95.5Sn/4.0Ag/0.5Cu solder. A eutectic SnPb solder was used as a reference. The test boards were temperature-cycled (−40 to +125 °C) until all samples failed. The results of the temperature cycling test were analyzed, and cross-section samples were made of the failed joints. Scanning electron and optical microscopy were employed to analyze the fracture behavior and microstructures of the solder joints. The reliability of lead-free solders and the effect of microstructures on joint reliability are discussed.     

Fatigue analysis of miniaturized lead-free solder contacts based on a novel test concept

The paper presents the method of generating lifetime-prediction-laws on special prepared very stiff specimen. The combination of thin- and thick-film technology allows building up test samples on ceramic very similar to electronic packages including the measurement issues. Influences of pad surface metallurgy, microstructure of solder, ineutectic solder alloys and assembly process parameter are regarded now. The investigation objects provide monitoring of electrical and mechanical damage process of SnAgCu solder bump. Different thermo-mechanical loads will be applied in temperature ranges of 0 to +80 °C, −40 to +125 °C and −50 to +150 °C, where the temperature gradient and cycle frequency also vary. A Variation of four different chip sizes allows the determination of fatigue laws for each temperature profile, to be able to compare in between them. The results of these tests will give universal lifetime-prediction laws for SnAgCu base solder joints. Main goals are to find coefficients for lifetime prediction models such as Coffin–Manson- or Norris–Landzberg-relation, which are transferable in between different electronic packages.     

Thermal fatigue endurance of collapsible 95.5Sn4Ag0.5Cu spheres in LTCC/PWB assemblies

The behavior of thermomechanically loaded collapsible 95.5Sn4Ag0.5Cu spheres in LTCC/PWB assemblies with high (LTCC/FR-4; ΔCTE 10 ppm/°C) and low (LTCC/Arlon; ΔCTE < 10 ppm/°C) global thermal mismatches was studied by exposing the assemblies into two thermal cycling tests. The characteristic lifetimes of the LTCC/FR-4 assemblies, tested over the temperature ranges of 0–100 °C and −40 to 125 °C, were 1475 and 524 cycles, respectively, whereas the corresponding values of the LTCC/Arlon assemblies were 5424 and 1575 cycles. According to the typical requirements for the industrial lifetime duration of solder joints, the former values are inadequate, whereas the latter are at an acceptable level in a few cases. Furthermore, the global thermal mismatch affected the thermal fatigue behavior of the 95.5Sn4Ag0.5Cu spheres in the temperature range of −40 to 125 °C.     

Failure Morphology After Drop Impact Test of Ball Grid Array (BGA) Package With Lead-Free Sn–3.8Ag–0.7Cu and Eutectic SnPb Solders

High strain-rate drop impact tests were performed on ball grid array (BGA) packages with solder compositions of (in wt%) Sn-3.8Ag-0.7Cu (SnAgCu) and eutectic Sn-37Pb (SnPb). Solder balls were joined to the metallizations of plated Ni on the device side and plated Cu on the board side. The BGA packages were tested at 1500 g within 0.5 ms, resulting in an imposed bending strain of 0.2-0.3%. Both SnAgCu and SnPb joints failed at the interface at the device side but the detailed failure morphology differed significantly. The crack location for the eutectic SnPb was primarily through the solder and seldom extended through an entire bump. The SnPb joints also exhibited bulk solder deformation. The SnAgCu joints showed extremely brittle behavior with an interfacial failure at the (Ni,Cu)₃Sn₄ intermetallics/Ni under bump metallization (UBM) interface. The strain rate sensitivity of bulk solder defines the drop test performance and the eutectic SnPb solder showed better drop impact performance due to a less strain rate sensitivity     

Effect of solder creep on the reliability of large area die attachment

High temperature solders have been widely used for power device die attachment. One typical solder is Pb92.5In5Ag2.5, which is a ternary eutectic alloy with a eutectic temperature of 310°C. Such a Pb-based solder has a low Young's modulus, a low yield strength, and a high strain prior to failure. So it can be used to attach large size silicon die to mismatched substrates. In this paper, stresses and strains have been studied on a large size power MOSFET attachment using the Pb92.5In5Ag2.5 solders. A commercial finite element analysis software is employed as the simulation tool. Three types of substrates, pure copper, copper–tungsten composite, and pure molybdenum are used in the study, where molybdenum has the closest coefficient of thermal expansion to silicon. In addition to the plastic deformation simulation of the solder, a creep model of the solder was incorporated due to the low melting temperature of the solder alloy. Firstly, stresses and strains are calculated during the cooling cycle after attachment. It is found that the creep strain is the dominant plastic strain at low cooling rate (10°C/min). Also, the maximum Von Mises stress in the Si chip is decreased from 174 to 62.7 MPa after adding creep strain. As expected, the maximum creep strain happens to the die-to-copper substrate attach. Simulation on temperature cycling is done from −55°C to +150°C. The peak Von Mises stress occurs at the low temperature extreme and holds steadily during the soaking period, indicating insignificant contribution from creep. The Von Mises stress at the high temperature extreme is much lower and decreases with holding time. Significant plastic deformation of the solder layer is observed in cooling cycles. For silicon to copper substrate attach, its plastic deformation increases with each cycle. For all three substrates used, considerable solder creep is observed at heating cycles. The creep strain is much larger than the rate-independent plastic strain in the solder alloy for all three types of substrates. It is concluded that solder creep is the dominant factor affecting long term reliability of power semiconductor die attachment.     

Electromigration of Sn–37Pb and Sn–3Ag–1.5Cu/Sn–3Ag–0.5Cu composite flip–chip solder bumps with Ti/Ni(V)/Cu under bump metallurgy

We examine electromigration fatigue reliability and morphological patterns of Sn–37Pb and Sn–3Ag–1.5Cu/Sn–3Ag–0.5Cu composite solder bumps in a flip–chip package assembly with Ti/Ni(V)/Cu UBM. The flip–chip test vehicle was subjected to test conditions of five combinations of applied electric currents and ambient temperatures, namely, 0.4 A/150 °C, 0.5 A/150 °C, 0.6 A/125 °C, 0.6 A/135 °C, and 0.6 A/150 °C. The electrothermal coupling analysis was employed to investigate the current crowding effect and maximum temperature in the solder bump in order to correlate with the experimental electromigration reliability using the Black’s equation as a reliability model. From available electromigration reliability models, we also present a comparison between fatigue lives of Sn–37Pb solder bumps with Ti/Ni(V)/Cu and those with Al/Ni(V)/Cu UBM under different current stressing conditions.     

Lead-free solder flip chip-on-laminate assembly and reliability

This paper examines the assembly process for flip chip die with SnAgCu solder bumps and the results of liquid-to-liquid thermal shock testing. The SnAgCu alloy required a thicker dip layer of flux to achieve good wetting compared to the SnPb eutectic alloy. A liquid spray flux yielded more consistent solder wetting with the SnAgCu alloy. With both fluxes, a nitrogen reflow atmosphere was necessary with the SnAgCu alloy. A peak reflow temperature of 246°C was used for the assembly of the SnAgCu thermal shock test vehicles. A lower peak temperature of 235°C did not yield sufficient solder wetting. Liquid-to-liquid thermal shock testing was performed from -40°C to +125°C. The SnPb alloy performed slightly better than the SnAgCu and the dip flux was better that the spray flux. The degree of delamination with the SnAgCu alloy was significantly higher than with the SnPb alloy. Cracks in the underfill between adjacent solder balls were observed. The SnPb alloy extruded into these cracks more readily than the SnAgCu and created electrical shorts     

Thermal cycling reliability of SnAgCu and SnPb solder joints: A comparison for several IC-packages

This paper deals with a comparison study between SnPb and SnAgCu solder joint reliability. The comparison is based on non-linear finite element modelling. Three packages have been selected: silicon CSP, underfilled flip chip and QFN package. Also the effect of thermal cycling conditions has been investigated. Comparing the induced inelastic strains in the solder joint, the lead-free SnAgCu generally scores better thanks to the lower creep strain rate. On the other hand for the CSP and flip chip package, SnAgCu scores worse for the more extreme loading conditions when the inelastic dissipated energy density is selected as damage parameter. The main reason is that due to the lower creep strain rate, the stresses become higher for SnAgCu resulting in higher hysteresis loops with more dissipated energy per cycle. For the QFN package, SnAgCu scores much better.     

Properties of lead-free solder SnAgCu containing minute amounts of rare earth

Because of excellent wetting and mechanical properties, SnAgCu solder alloys have been regarded as the most promising Pb-free substitutes for the SnPb solder. The Sn-3.8Ag-0.7Cu solder has garnered attention because of its creep resistance. However, under the drives of increasingly finer pitch design and severe service conditions, novel lead-free solders with higher creep performance may be needed. Adding a surface-active element to an alloy is an effective way to improve the high-temperature performance of the solder. The present work focuses on the effect of rare earth (RE) on the physical properties, spreading property, and mechanical properties of SnAgCu solder. Results show that the creep-rupture life of SnAgCu solder joints at room temperature could be notably increased by adding a minute amount of RE, up to 7 times more than that of SnAgCu solder joints when containing 1.0wt.%RE. The differential scanning calorimetry (DSC) curves indicated that the melting temperature of SnAgCu solder with RE increased a little, and no lower melting-temperature, eutectic endothermal peak appears on the DSC curve. The electrical conductivity of the solder decreased slightly, but it is still superior to the SnPb eutectic solder. Compared to that of SnPb solder, the coefficient of thermal expansion (CTE) of SnAgCu (RE) is closer to copper, which usually serves as the substrate of printed circuit boards (PCBs). It is assumed that this will comparably reduce the thermal stress derived from thermal mismatch between the solder and the PCBs. The RE had no apparent effect on the spreading property, but when RE added up to 1.0 wt.%, the spreading area of the solder on the copper substrate decreased, obviously, because of mass oxide. The RE improved the ultimate tensile strength little, but it increased the elongation up to 30%. However, as the content of the RE increases, the elongation of the solder gradually decreased to the level of SnAgCu when the RE exceeds 0.25 wt.%. Additionally, RE made the elastic modulus of SnAgCu solder increase, so the resistance to elastic deformation of the solder is enhanced. The microstructure of SnAgCuRE led to a refining trend as the RE content increased. The RE compounds appeared in the solder when RE was 0.1 wt.%. This deteriorates the mechanical properties of the solder. The fractography of the tensile specimen containing 0.1 wt.% indicated a superior ductility to Sn-3.8Ag-0.7Cu bulk solder. However, as RE is increased to 1.0 wt.%, the fractography shows less ductile characteristics, which is believed to serve as the reason that the elongation of solder degrades as RE increases. Summarily, the most suitable content of RE is within 0.05–0.5 wt.% and is inadvisable beyond 1.0 wt.%.     

Time-independent elastic–plastic behaviour of solder materials

For a long time, constitutive modelling of solders has focused onto the elastic and creep properties. Indeed, the creep model describes the behaviour of solder joints under thermal cycling quite properly. However, in applications such as hand held electronic devices or automotive products, the pure mechanical impact like shock, bending and twisting may even matter more than sole thermo-mechanical fatigue.Therefore the time-independent behaviour of SnPb37, SnAg3.5 and SnAg4Cu0.5 has been investigated on flip chip solder joints. In the experimental tests a cyclic triangular strain wave with constant frequency but different amplitudes was used as the load function. This way the test enables to account for Bauschinger effects. The strain wave amplitudes ranged from Δ=0.25% to 4%, the strain wave frequency was fixed at f=1 Hz. The test temperature ranged from T=5 to 50 °C.The test specimen consisted of two silicon chips (3.3 × 3.3 mm²) bonded by 4 flip chip joints (one at each corner). A specially designed Micro Shear Tester has been used for the experiments with this type of specimen. In contrast to similar setups, it is actively compensated for its finite stiffness. Therefore, it is able to record force–displacement hysteresis with a resolution of better than 1 mN and 20 nm, respectively. Based on these measurements, the parameters of the constitutive equations have been evaluated by FEM analysis. This way, the complex stress state within the sample during the test has been considered precisely providing for high accuracy of the parameter extracted.As a typical application, a three point bending experiment has been simulated by FEM applying different constitutive models for the solder material. Comparing the results, it becomes clear: All the three contributions, i.e., the elastic, the creep, and the time-independent plastic material behaviour, are required in the model. Otherwise it would be incomplete and hence insufficient for assisting in the design of today's electronics packages even with respect to the most frequent load cases.     

Interconnect resistance characteristics of several flip-chip bumping and assembly techniques

Several flip-chip interconnection methods were compared by measuring interconnect resistance before and after exposure to environments including pre-conditioning, 85°C/85% RH exposure, 150°C storage, and 0–100°C temperature cycling. The goal was to determine an acceptable low-cost, reliable method for bumping and assembling chips to flexible or rigid substrates using flip-chip assembly techniques. Alternative flip-chip bumping methods are compared to a traditional wafer solder bumping method. Flip-chip interconnection methods evaluated included high lead content solder, silver filled conductive adhesive, and gold stud bumps. Under bump metallurgies evaluated included bare aluminum, evaporated Cr/Cr–Cu/Cu, and electroless nickel plating.     

Properties of low temperature Sn–Ag–Bi–In solder systems

The metallurgical and mechanical properties of Sn–3.5 wt%Ag–0.5 wt%Bi–xwt%In (x = 0–16) alloys and of their joints during 85 °C/85% relative humidity (RH) exposure and heat cycle test (−40–125 °C) were evaluated by microstructure observation, high temperature X-ray diffraction analysis, shear and peeling tests. The exposure of Sn–Ag–Bi–In joints to 85 °C/85%RH for up to 1000 h promotes In–O formation along the free surfaces of the solder fillets. The 85°C/85%RH exposure, however, does not influence the joint strength for 1000 h. Comparing with Sn–Zn–Bi solders, Sn–Ag–Bi–In solders are much stable against moisture, i.e. even at 85 °C/85%RH. Sn–Ag–Bi–In alloys with middle In content show severe deformation under a heat cycles between −40 °C and 125 °C after 2500 cycles, due to the phase transformation from β-Sn to β-Sn + γ-InSn₄ or γ-InSn₄ at 125 °C. Even though such deformation, high joint strength can be maintained for 1000 heat cycles.     

Reliability and RF performance of BGA solder joints with plastic-core solder balls in LTCC/PWB assemblies

In this paper board-level reliability of low-temperature co-fired ceramic (LTCC) modules with thermo-mechanically enhanced ball-grid-array (BGA) solder joint structure mounted on a printed wiring board (PWB) was experimentally investigated by thermal cycling tests in the 0–100 °C and −40 to 125 °C temperature ranges. The enhanced joint structure comprised solder mask defined (SMD) AgPt pad metallization, eutectic solder and plastic-core solder balls (PCSB). Similar daisy-chained LTCC modules with non-collapsible 90Pb10Sn solder spheres were used for a reference test set. The reliability of the joint structures was analyzed by resistance measurements, X-ray microscopy, scanning acoustic microscopy (SAM) and SEM/EDS investigation. In addition, a full-wave electromagnetic analysis was performed to study effects of the plastic-core material on the RF performance of the LTCC/BGA package transition up to millimeter-wave frequencies. Thermal cycling results of the modules with PCSBs demonstrated excellent fatigue performance over that of the reference. In the harsher cycling test, Weibull’s shape factor β values of 7.9 and 4.8, and characteristic lifetime θ values of 1378 and 783 were attained for the modules with PCSBs and 90Pb10Sn solder spheres, respectively. The primary failure mode in all test assemblies was fatigue cracking in eutectic solder on the ceramic side.     

Effect of alloying elements on properties and microstructures of SnAgCu solders

Recent years, the SnAgCu family of alloys has been found a widely application as a replacement for the conventional SnPb solders in electronic industry. In order to further enhance the properties of SnAgCu solder alloys, alloying elements such as rare earth, Bi, Sb, Fe, Co, Mn, Ti, In, Ni, Ge and nano-particles were selected by lots of researchers as alloys addition into these alloys. Rare earth (RE) elements have been called the ‘‘vitamin” of metals, which means that a small amount of RE elements can greatly enhance the properties of metals, such as microstructure refinement, alloying and purification of materials and metamorphosis of inclusions. In addition, a small amount of Zn addition has the ability to reduce undercooling efficiently and suppress the formation of massive primary Ag₃Sn plates, and Bi/Ga has the ability to enhance the wettability of SnAgCu alloys as well as Ni. Moreover, adding Co/Fe/Ge can effectively refine microstructure, modify interfacial Cu-Sn compounds and increase the shear strength of joints with Cu. This paper summarizes the effects of alloying elements on the wettability, mechanical properties, creep behavior and microstructures of SnAgCu lead-free solder alloys.     

Failure mechanism of lead-free solder joints in flip chip packages

The failure mechanisms of SnAgCu solder on Al/Ni(V)/Cu thin-film, underbump metallurgy (UBM) were investigated after multiple reflows and high-temperature storage using a ball shear test, fracture-surface analysis, and cross-sectional microstructure examination. The results were also compared with those of eutectic SnPb solder. The Al/Ni (V)/Cu thin-film UBM was found to be robust enough to resist multiple reflows and thermal aging at conditions used for normal production purposes in both SnAgCu and eutectic SnPb systems. It was found that, in the SnAgCu system, the failure mode changed with the number of reflows, relating to the consumption of the thin-film UBM because of the severe interfacial reaction between the solder and the UBM layer. After high-temperature storage, the solder joints failed inside the solder ball in a ductile manner in both SnAgCu and SnPb systems. Very fine Ag₃Sn particles were formed during multiple reflows in the SnAgCu system. They were found to be able to strengthen the bulk solder. The dispersion-strengthening effect of Ag₃Sn was lost after a short period of thermal aging, caused by the rapid coarsening of these fine particles.