Reliability study of board-level lead-free interconnections under sequential thermal cycling and drop impact

The sequential thermal cycling (TC) and drop impact test are more reasonable to evaluate the reliability of lead-free solder interconnections compared with separate TC test or drop impact test. In this paper, sequential TC (−40 °C/125 °C, 13 min of soak time, 12 min of dwell time, totally 50 min of cycle time) and drop impact test (a sine impact pulse with a peak acceleration of 1500g and a duration time of 0.5 ms) were conducted to study the failure mechanism of solder interconnections under sequential TC and drop impact test. The TC load has larger effect on the Cu/solder interface at the PCB side than that of Ni (P)/solder interface at the component side. For the thermally cycled samples, the failure location of solder interconnections under drop impact has changed from initiation and propagation along the thin reaction layer (mode 1) between intermetallic compound (IMC) layer and Ni (P) pad at the component side to initiation at the bulk solder and propagation along the Cu₃Sn IMC layer (mode 2) or entirely through the bulk solder (mode 3) at the PCB side. The failure mechanism has also changed from the entirely brittle crack to the mixture of fatigue crack and brittle crack.     

Failure mechanisms of lead-free chip scale package interconnections under fast mechanical loading

The reliability of chip scale package (CSP) components against mechanical shocks has been studied by employing statistical, fractographic, and microstructural research methods. The components having high tin (Sn0.2Ag0.4Cu) solder bumps were reflow soldered with the Sn3.8Ag0.7Cu (wt.%) solder paste on Ni(P)|Au- and organic solderability preservative (OSP)-coated multilayer printed wiring boards (PWBs), and the assemblies were subjected to the standard drop test procedure. The statistically significant difference in the reliability performance was observed: the components soldered on Cu|OSP were more reliable than those soldered on Ni(P)|Au. Solder interconnections on the Cu|OSP boards failed at the component side, where cracks propagated through the (Cu,Ni)₆Sn₅ reaction layer, whereas interconnections on the Ni(P)|Au boards failed at the PWB side exhibiting the brittle fracture known also as “black pad.” In the first failure mode, which is not normally observed in thermally cycled assemblies, cracks propagate along the intermetallic layers due to the strong strain-rate hardening of the solder interconnections in drop tests. Owing to strain-rate hardening, the stresses in the solder interconnections increase very rapidly in the corner regions of the interconnections above the fracture strength of the ternary (Cu,Ni)₆Sn₅ phase leading to intermetallic fracture. In addition, because of strain-rate hardening, the recrystallization of the as-soldered microstructure is hindered, and therefore the network of grain boundaries is not available in the bulk solder for cracks to propagate, as occurs during thermal cycling. In the black pad failure mode, cracks nucleate and propagate in the porous NiSnP layer between the columnar two-phase (Ni₃P+Sn) layer and the (Cu,Ni)₆Sn₅ intermetallic layer. The fact that the Ni(P)|Au interconnections fail at the PWB side, even though higher stresses are generated on the component side, underlines the brittle nature of the reaction layer.     

Thermal Fatigue Endurance of Lead-Free Composite Solder Joints over a Temperature Range of −55°C to 150°C

The thermal fatigue endurance of two lead-free solder/plastic-core solder ball (PCSB) composite joint structures in low-temperature co-fired ceramic (LTCC) modules was investigated using a thermal cycling test over a temperature range of −55°C to 150°C. The investigated solder alloys were Sn-7In-4.1Ag-0.5Cu (SAC-In) and 95.5Sn-4Ag-0.5Cu (SAC). Three failure mechanisms were observed in the test joints. Transgranular (fatigue) cracking mixed with minor intergranular cracking was the dominant failure mechanism at the outer edge of the joints in both test assemblies, whereas separation of the solder/intermetallic compound (IMC) interface and creep cracking occurred in the other parts of the test joints. The propagation rate of the transgranular crack was lower in the SAC-In joints compared with in the SAC joints. Furthermore, the SAC solder seemed to be more prone to separation of the solder/IMC interface, and more severe intergranular (creep) cracking occurred in it compared with in the SAC-In solder. In the thermal cycling test conditions, the better thermal fatigue endurance of the SAC-In solder composite joints resulted in a 75% higher characteristic lifetime compared with the SAC composite joints.     

Impact of Thermal Cycling on Sn-Ag-Cu Solder Joints and Board-Level Drop Reliability

The electronic packaging industry uses electroless nickel immersion gold (ENIG) or Cu-organic solderability preservative (Cu-OSP) as a bonding pad surface finish for solder joints. In portable electronic products, drop impact tests induce solder joint failures via the interfacial intermetallic, which is a serious reliability concern. The intermetallic compound (IMC) is subjected to thermal cycling, which negatively affects the drop impact reliability. In this work, the reliability of lead-free Sn-3.0Ag-0.5Cu (SAC) soldered fine-pitch ball grid array assemblies were investigated after being subjected to a combination of thermal cycling followed by board level drop tests. Drop impact tests conducted before and after thermal aging cycles (500, 1000, and 1500 thermal cycles) show a transition of failure modes and a significant reduction in drop durability for both SAC/ENIG and SAC/Cu-OSP soldered assemblies. Without thermal cycling aging, the boards with the Cu-OSP surface finish exhibit better drop impact reliability than those with ENIG. However, the reverse is true if thermal cycle (TC) aging is performed. For SAC/Cu-OSP soldered assemblies, a large number of Kirkendall voids were observed at the interface between the intermetallic and Cu pad after thermal cycling aging. The void formation resulted in weak bonding between the solder and Cu, leading to brittle interface fracture in the drop impact test, which resulted in significantly lower drop test lifetimes. For SAC/ENIG soldered assemblies, the consumption of Ni in the formation of NiCuSn intermetallics induced vertical voids in the Ni(P) layer.     

Ceramic packages for liquid-nitrogen operation

To evaluate their compatibility for use in a liquid-nitrogen computer, metallized ceramic packages with test chips using controlled-collapse solder (Pb-Sn) technology were cycled between 30°C and liquid-nitrogen temperature. Room-temperature electrical resistance measurements were made at regular intervals of cycles to determine whether solder failure accompanied by a significant resistance increase had occurred. For the failed solder joints characterized by the highest thermal shear strain amplitude of 3.3%, it was possible to estimate the number of liquid-nitrogen cycles needed to produce the corresponding failure rate using a room-temperature solder lifetime model. Cross-sectional examination of the failed solder joints using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis indicated solder cracking occurring at the solder-ceramic interface. Chip-pull tests on cycled packages yielded strengths far exceeding the minimal requirement. Mechanisms involving the formation of intermetallics are proposed to account for the observed solder fracture modes after liquid-nitrogen cycling and after chip pull. SEM examination of pulled chips in cycled packages found no apparent sign of cracking in quartz and polyimide for chip insulation     

Impact of Isothermal Aging on Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects: Surface Finish Effects

The interaction between isothermal aging and the long-term reliability of fine-pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt.%) solder ball interconnects was investigated. In this study, 0.4-mm fine-pitch packages with 300-μm-diameter Sn-Ag-Cu solder balls were used. Two different package substrate surface finishes were selected to compare their effects on the final solder composition, especially the effect of Ni, during thermal cycling. To study the impact on thermal performance and long-term reliability, samples were isothermally aged and thermally cycled from 0°C to 100°C with 10 min dwell time. Based on Weibull plots for each aging condition, package lifetime was reduced by approximately 44% by aging at 150°C. Aging at 100°C showed a smaller impact but similar trend. The microstructure evolution was observed during thermal aging and thermal cycling with different phase microstructure transformations between electrolytic Ni/Au and organic solderability preservative (OSP) surface finishes, focusing on the microstructure evolution near the package-side interface. Different mechanisms after aging at various conditions were observed, and their impacts on the fatigue lifetime of solder joints are discussed.     

Failure study of Sn37Pb PBGA solder joints using temperature cycling,random vibration and combined temperature cycling and random vibration tests

In this paper, the tin-lead (Sn-37wt%Pb) eutectic solder joints of plastic ball grid array (PBGA) assemblies are tested using temperature cycling, random vibrations, and combined temperature cycling and vibration loading conditions. The fatigue lives, failure modes for the solder joints and the typical locations of the failed solder joints for single-variable loading and combined loading conditions are compared and analyzed. The results show much earlier solder joint failure for combined loading than that for either temperature cycling or pure vibration loading at room temperature. The primary failure mode is cracking within the bulk solder under temperature cycling, whereas the crack propagation path is along the intermetallic compound (IMC) layer for vibration loading. The solder joints subjected to combined loading exhibit both types of failure modes observed for temperature cycling and vibration loading; in addition, cracking through the IMC and the bulk solder is observed in the combined test. For temperature cycling and vibration loading, the components in the central region of the printed circuit board (PCB) have more failed solder joints than other components, whereas for combined loading, the number of failed solder joints in the components in different locations of the PCB is approximately the same.     

The Reliability of Microalloyed Sn-Ag-Cu Solder Interconnections Under Cyclic Thermal and Mechanical Shock Loading

In this study, the performance of three microalloyed Sn-Ag-Cu solder interconnection compositions (Sn-3.1Ag-0.52Cu, Sn-3.0Ag-0.52Cu-0.24Bi, and Sn-1.1Ag-0.52Cu-0.1Ni) was compared under mechanical shock loading (JESD22-B111 standard) and cyclic thermal loading (40 ± 125°C, 42 min cycle) conditions. In the drop tests, the component boards with the low-silver nickel-containing composition (Sn-Ag-Cu-Ni) showed the highest average number of drops-to-failure, while those with the bismuth-containing alloy (Sn-Ag-Cu-Bi) showed the lowest. Results of the thermal cycling tests showed that boards with Sn-Ag-Cu-Bi interconnections performed the best, while those with Sn-Ag-Cu-Ni performed the worst. Sn-Ag-Cu was placed in the middle in both tests. In this paper, we demonstrate that solder strength is an essential reliability factor and that higher strength can be beneficial for thermal cycling reliability but detrimental to drop reliability. We discuss these findings from the perspective of the microstructures and mechanical properties of the three solder interconnection compositions and, based on a comprehensive literature review, investigate how the differences in the solder compositions influence the mechanical properties of the interconnections and discuss how the differences are reflected in the failure mechanisms under both loading conditions.     

Impact of Isothermal Aging on Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects: Die Size Effects

The interaction between isothermal aging and long-term reliability of fine-pitch ball grid array (BGA) packages with two different die sizes was investigated. Both 5 mm × 5 mm and 10.05 mm × 10.05 mm die-attached packages with Sn-3.0Ag-0.5Cu (wt.%) solder balls were used to compare the correlation between the internal strain difference and isothermal aging on microstructural evolution during thermal cycling. To determine their long-term reliability, the samples were isothermally aged and thermally cycled from 0°C to 100°C with 10-min dwell time. Based on Weibull plots for each aging condition, the packages with large dies attached showed shorter characteristic lifetimes, because of the relatively higher stress, but showed less lifetime degradation with isothermal aging compared with the smaller die-attached samples. Microstructure analysis using orientation imaging microscopy (OIM) revealed the evolution of the microstructure during thermal cycling, showing a higher degree of recrystallization inside the bulk solder for joints that were isothermally aged and experienced higher stress. The possible mechanisms giving rise to these observations are discussed.     

Reliability of CSP Interconnections Under Mechanical Shock Loading Conditions

Failure modes and mechanisms under mechanical shock loading were studied by employing the statistical and fractographic research methods, and the finite element (FE) analysis. The SnAgCu-bumped components were reflow-soldered with the SnAgCu solder paste on Ni(P)|Au-coated and organic solderability preservative-coated multilayer printed wiring boards with and without micro-via structure in the soldering pads. The component boards were designed, fabricated, assembled, and drop tested according to the JESD22-B111 standard for portable electronic products. The test data were analyzed by utilizing the Weibull statistics, and the characteristic lifetimes (eta) and shape parameters (beta) were calculated. Statistically significant differences in the reliability were found between the different coating materials and pad structures. The results on the failed assemblies showed good correlation between the failure modes and the FE calculations. Under high deformation rates the solder material undergoes strong strain-rate hardening, which increases the stresses in the interconnections as compared to those in the thermal cycling tests. Therefore, the failure mechanisms under high deformation rates differed essentially from those observed in thermal cycling tests