Fatigue Performance Of Solder Joints Research Articles

Effect of temperature on the low cycle fatigue properties of BGA solder joints

Since the restriction of hazardous substances (RoHS) directive, lead-free soldering has been widely broad adopted. In many applications, lead-free alloys have been substituted for conventional SnPb (Tin-Lead) solder. Among lead-free alloys, SAC305(Sn-3.0Ag-0.5Cu) has gained the highest acceptance as a solder alloy. The fatigue performance of solder joints has become an essential reliability assessment criterion due to the transition to more reliable lead-free alloys from highly predictable lead-based alloys. This study examines the shear fatigue characteristics of sandwich test vehicles for SnPb and SAC305 at different temperatures using both Organic Solderability Preservative (OSP) and Electroless Nickel-Immersion Silver (ENIG) surface finishes. The fatigue experiments were performed using an Instron micromechanical tester at a constant strain rate, and microstructural analysis was carried out using Scanning Electron Microscopy (SEM) to identify the Intermetallic Compound (IMC) morphology and failure mode. The stress-strain (hysteresis) loops of SnPb and SAC305 were measured, and the fatigue life of the specimens was estimated using the strain-life equation at various temperatures. SAC305 was observed to have a better fatigue life than SnPb, particularly at higher strain levels or testing temperatures. The OSP surface finish demonstrated superior fatigue properties compared to the ENIG surface finish. Additionally, elevated testing temperatures were found to accelerate fatigue failure in solder joints. The Arrhenius model was utilized to develop a general empirical model that predicts the fatigue life of SAC305 and SnPb solder alloys with OSP and ENIG surface finishes as a function of the strain level and testing temperature.

Evaluation of Fatigue Behavior of Lead-Free Solder Joints in Four-Point Bending Test by Finite-Element Modeling

Fatigue performance of solder joints in four-point bending test is evaluated by finite-element modeling. A simplified modeling technique is implemented in order to focus on the behavior of the critical solder joint. The fatigue life is estimated by taking into account the creep behavior of the solder joints and the utilization of Morrow’s fatigue model. Fatigue life curves are generated for room temperature and two elevated temperatures with the critical orientation of chip scale packages on the test board. The results are compared with the real four-point bending test conducted for two different temperatures. In addition, some parametric studies are conducted including the significance of test temperature, copper wiring structure, and the dwell time in the cyclic loading profile. Besides these, the structural integrity of the test board is also checked.

Effect of Solder-Joint Geometry on the Low-Cycle Fatigue Behavior of Sn-xAg-0.7Cu

Low-cycle fatigue tests of Sn-Ag-Cu (SAC) Pb-free solder joints under fixed displacement were performed to evaluate the influence of Ag content (0–3 wt.%) and solder-joint geometry (barrel and hourglass types) on solder-joint fatigue behavior and reliability. The solder joints were composed of fine particles of Ag3Sn and Cu6Sn5, which aggregated as an eutectic constituent at grain boundaries of the primary β-Sn phase and formed a dense network structure. A decrease in the Ag content resulted in coarsening of the β-Sn and eutectic phases, which, in turn, decreased the strength of the joint and caused earlier failure. Solder joints in the hourglass form exhibited better fatigue performance with longer life than barrel-type joints. The sharp contact angle formed between the solder and the Cu substrate by the barrel-type joints concentrated stress, which compromised fatigue reliability. The addition of Ag to the solder, however, enhanced fatigue performance because of strengthening caused by Ag3Sn formation. The cracks of the barrel-type SAC solder joints originated mostly at the contact corner and propagated along the interfacial layer between the interfacial intermetallic compound (IMC) and solder matrix. Hourglass-type solder joints, however, demonstrated both crack initiation and propagation in the solder matrix (solder mode). The addition of 1.5–2.0 wt.% Ag to SAC solder appears to enhance the fatigue performance of solder joints while maintaining sufficient strength.

Impact of Cooling Rate-Induced Recrystallization on High G Mechanical Shock and Thermal Cycling in Sn-Ag-Cu Solder Interconnects

The mechanical stability and thermo-mechanical fatigue performance of solder joints with low silver content Sn-1.0Ag-0.5Cu (wt.%) (SAC105) alloy based on different cooling rates are investigated in high G level shock environment and thermal cycling conditions. The cooling rate-controlled samples ranging from 1°C/min to 75°C/min cooling rate, not only show differences in microstructure, where a fine poly-granular microstructure develops in the case of fast cooling versus normal cooling, but also show various shock performances based on the microstructure changes. The fast cooling rate improves the high G shock performance by over 90% compared to the normal cooled SAC105 alloy air-cooling environment commonly used after assembly reflow. The microstructure effect on thermal cycling performance is also discussed, which is analyzed based on the Sn grain orientation, interconnect stability, and solder joint bulk microstructure.

On Failure Mechanisms in Flip Chip Assembly—Part 2: Optimal Underfill and Interconnecting Materials

The physics explored in this investigation enables short-time scale dynamic phenomenon to be correlated with package failure modes such as solder ball cracking and interlayer debond. It is found that although epoxy-based underfills with nanofillers are shown to be effective in alleviating thermal stresses and improving solder joint fatigue performance in thermal cycling tests of long-time scale, underfill material viscoelasticity is ineffective in attenuating short-time scale propagating shock waves. In addition, the inclusion of Cu interconnecting layers in flip chip area arrays is found to perform significantly better than Al layers in suppressing short-time scale effects. Results reported herein suggest that, if improved flip chip reliability is to be achieved, the compositions of all packaging constituent materials need be formulated to have well-defined short-time scale and long-time scale properties. Chip level circuit design layout also needs be optimized to either discourage or negate short-time wave propagation. The knowledge base established is generally applicable to high performance package configurations of small footprint and high clock speed. The approach along with the numerical procedures developed for the investigation can be a practical tool for realizing better device reliability and thus high manufacturing yield.

Flip chip solder joint reliability analysis using viscoplastic and elastic-plastic-creep constitutive models

Both elastic-plastic-creep and viscoplastic constitutive models may be used for inelastic deformation analysis of solder joints. In this paper, a phenomenological approach using elastic-plastic-creep analysis and an Anand viscoplastic model is reported for solder joint reliability. Flip chip soldered assemblies with 63Sn-37Pb solder joints were subjected to a thermal cyclic loading condition of -40 to +125degC to assess the solder joint fatigue performance. In the finite-element modeling, the viscoplastic strain energy density per cycle obtained from the viscoplastic analysis is compared with the inelastic (plastic and creep) strain energy density per cycle calculated from the elastic-plastic-creep analysis. The inelastic (plastic+creep and viscoplastic) strain energy density extracted from the finite-element analysis results, at the critical solder joint location, were used as a failure parameter for solder fatigue models employed. It was found that the predicted solder joint fatigue life has a better correlation to the first failure or first-time-to-failure result

Mechanical deflection system (MDS) test and methodology for PBGA solder joint reliability

A mechanical deflection system (MDS) was developed for highly accelerated tests to evaluate the solder joint fatigue performance in printed circuit board assemblies. The MDS test system can be used for design verification and qualification tests for solder joint reliability. Cyclic twisting deformation is imposed on an assembled printed circuit board (PCB) at isothermal conditions. The MDS test technique makes a significant contribution to reducing solder joint reliability testing cycle time. Fatigue performance of the PBGA solder joints subjected to the MDS test was investigated by three-dimensional finite element modeling. The solder joint fatigue lives were computed for several different MDS test conditions.

HIGHLY ACCELERATED SOLDER JOINT RELIABILITY TEST USING A THERMO-MECHANICAL DEFLECTION SYSTEM (TMDS)

A Thermo-Mechanical Deflection System (TMDS) test method has been developed for evaluating the fatigue performance of solder joints in printed circuit board (PCB) assemblies. The TMDS test imparts cyclic twisting deflections on an assembled PCB test vehicle which is tested under controlled iso-thermal conditions in a thermal chamber. Tests were conducted at various angles of twist and isothermal conditions at 100°C and 25°C. Failure analysis using SEM showed that the solder joint failure for the TMDS test is comparable to the failure mechanisms for ATC test failures for solder joints. Weibull failure distribution plots for TMDS and ATC tests were compared and a Scale Factor (SF) was used to correlate the test results.

Reliability figures of merit for surface-soldered leadless chip carriers compared to lead packages

Solder attachment reliability figures of merit (FMs) have been developed that account for the primary factors determining long-term interconnection integrity. The FMs provide relative measures of the reliability impact of a component or device/substrate assembly and serve as a GO/NO-GO decision tool for package, physical, and circuit designers. The FMs are an integral part of the design-for-reliability process at key product development stages. These design phases include component design and/or selection, component/substrate assembly design, and system design, including thermal environment, product service life, and attachment reliability target. The use of FMs for attachment reliability evaluation is discussed through examples based on the solder-joint fatigue performance of thermally cycled leaded and leadless surface-mounted devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>