E.H. Wong

Creep fatigue models of solder joints: A critical review

Abstract The goal of creep fatigue modelling is the compounding of the damage caused by creep and fatigue mechanisms. The different approaches for compounding these damage mechanisms have led to several different creep fatigue models: (i) ignore fatigue damage — the creep ductility (energy density) exhaustion models; (ii) lumping plastic and creep strain (energy) into inelastic strain (energy) — the model of Dauvearxs crack initiation and propagation; (iii) linearly sum fatigue and creep damage — the model of linear damage summation; (iv) model creep and fatigue damage using a common parameter — the models of fracture mechanics; (v) partition damage into fatigue, cyclic creep, and cyclic creep-fatigue interaction — strain range/energy partitioning models; (vi) model creep and fatigue damage using a common parameter at rates that are dependent on the current state of damage — the model unified damage; (vii) model creep and fatigue damage using separate damage parameters — the mechanism based model; and (viii) integrate creep damage into the fatigue equation — creep modified strain-life equations. The rigour of the approaches increases from (i) to (vii). The creep modified strain-life equation requires no evaluation of creep strain and facilitates design analysis and evaluation of acceleration factors; however, its rigour depends on the choice of the creep functions. The unified equation is capable of covering the full spectrum of creep-fatigue from pure fatigue to pure creep rupture.

A study of crack propagation in Pb-free solder joints under drop impact

The higher stiffness of Pb-free SAC solders makes Pb-free assemblies more sensitive to drop impact. In order to be able to optimise the drop test performance, it is necessary to have better insight into the crack propagation in the Pb-free solder joints. This study combines on-line resistance measurements of a solder joint during drop testing and high speed bend testing, failure analysis of the assembly with dye-and-pry method and with cross-sections , and electrical FE simulation. The result is a fingerprint of the crack propagation during consecutive loading cycles. The carrier in the study is BGAs, a critical component family for drop impact. Combinations of solder alloys and pad finishes, SnPb on OSP, SAC305 on ENIG, SAC101 on OSP and SAC101(d) on ENIG are studied regarding the failure mode and crack propagation. This study demonstrates that for the large majority of Pb-free solder joints, there is a negligible initiation period; cracks start forming at the first PCB bending cycle. The presence of large cracks, especially at both sides can increase the compliance of the joint and slow down crack growth. Even if large cracks are present, the resistance increase is less than 1 mOmega per interconnect, which is far from the 100Omega that is often taken as a failure criterion. Brittle joints as found with SAC305 on ENIG have erratic propagation rates while ductile joints are much more predictable. Therefore, the way to optimise the drop test performance of a Pb-free BGA assembly, is to prolong the crack propagation within the ductile solder material.

Recent advances in drop-impact reliability

This manuscript presents some of the research works performed by a consortium of companies for the studies of the drop-impact reliability of electronic components. The extensive works covers test methodologies and failure model. Under the test methodologies, the board level high speed cyclic bend test and the component level ball impact shear test were correlated with the board level drop/shock test using 23 groups of diverse solder joints. Under the failure model, the stress-strain characteristics of 4 solder alloys were generated for the medium strain rate regime (up to 300 s-1); the S-N characteristics of 6 solder joints between the PCB fibre strains of 1x10-3 to 3x10-3 at bending frequency of 100 Hz were generated; the S-N characteristics of the solder joints as a function of bending frequency (from 30 Hz to 150 Hz) and test temperatures (-10degC and 25degC) were also investigated. The characteristics of crack propagation were correlated with the observed crack path in the solder joints. In addition, a robust solder joint design was proposed and validated.

A Study of Crack Propagation in Pb-Free Solder Joints

The higher stiffness of Pb-free SAC solders makes Pb-free assemblies more sensitive to drop impact. In order to be able to optimize the drop test performance, it is necessary to have better insight into the crack propagation in the Pb-free solder joints. This study combines crack-front mapping using the dye and pry method and electrical FE simulation to establish a relation between DC electrical resistance and cracked area, and hence monitor the initiation and propagation of cracks in individual solder joints as the PCB assemblies are subjected to JEDEC type mechanical shock and high speed cyclic bending. The carrier in the study is a ball grid array (BGA), a critical component family for drop impact. Combinations of solder alloys and pad finishes, SnPb on organic solderable preservative (OSP), SAC305 on electroless nickel/immersion gold (ENIG), SAC101 on OSP, and SAC101(d) on ENIG are studied regarding the failure mode and crack propagation. This paper demonstrates that, for the large majority of Pb-free solder joints, there is a negligible initiation period; cracks can start forming at the first PCB bending cycle. The presence of large cracks, especially at both sides can increase the compliance of the joint and slow down crack growth. Even if large cracks are present, the resistance increase is less than 1 m¿ per interconnect, which is far from the 100 ¿ that is often taken as a failure criterion. Brittle joints as found with SAC305 on ENIG have erratic propagation rates while ductile joints are much more predictable. Therefore, the way to optimize the drop test performance of a Pb-free BGA assembly is to prolong the crack propagation within the ductile bulk solder material.

Moisture diffusion modeling - A critical review

Abstract Techniques for enforcing the continuity of solute field in heterogeneous solvent under the conditions of steady temperature-humidity, steady temperature but dynamic humidity, and dynamic temperature are reviewed. The continuity of the wetness technique is justified on the principle of equality of chemical potential. The partial pressure technique is one of the many possible forms of pseudo techniques that can be derived from the wetness technique. The direct concentration technique is fundamentally flaw. The peridynamic technique in its original form is restricted to homogeneous solvent. The saturated concentration of solute in solvents decreases with increasing temperature; the rate of change with temperature differs between solvents and this leads to discontinuity of wetness along the interface of solvents. Continuity of wetness at the interface may be enforced using the intervention technique, the internal source technique, or the explicit finite difference scheme. These three techniques have been mutually validated in a reported study.

A Comprehensive Test Method for Bridging the Gap between Product and Board Level Drop Tests

Due to rapid and continual changes in the components and designs of portable electronic devices, it is essential to have the ability to perform quick, cost-effective and accurate assessments of the drop impact performance of component interconnections for portable applications. In order to understand the phenomena of solder joint fatigue under impact-induced PCB bending, a tester was developed specifically for applying high bending strains at high strain rates to a PCB assembly. Fatigue characteristics of solder joints were determined for various PCB strains, allowing the construction of fatigue E-N (strain vs. cycles-to-failure) curves. Fatigue data was also generated for several bending frequencies in a study of the effect of strain rate on drop impact performance. Higher bending frequencies were found to reduce fatigue life. A comprehensive test method would also have to be capable of applying variable loading levels in order to replicate the complex loading spectrums seen in actual product drops. A variable amplitude study is presented, where crack propagation tracking shows the effect of changes in load levels on the crack growth rate. Finally, a study on the effect of low temperatures on drop impact reliability is presented, where fatigue characteristics were obtained using the bend tester fitted with a cold temperature chamber.

Constitutive Properties of Bulk Solder at `Drop-Impact' Strain Rates

Medium strain rate (0.1/s to 300/s) constitutive properties are needed for accurate modeling of drop impact conditions. This study presents an experimental procedure for obtaining the material properties of solder alloys at these medium strain rates. The effect of grain size on the medium strain rate behavior of solder alloys is also studied, using SnPb alloy as an example. Grain size variation is achieved through different aging conditions. For the lower strain rates tested, yield stress generally increases with aging. Interestingly, for the higher strain rates, the yield stress decreases with less severe aging but increases with more severe aging. In addition, aging causes a decrease in the strain rate sensitivity of the solder.

Interface and interconnection stresses in electronic assemblies – A critical review of analytical solutions

The closed-form solutions for the interfacial stresses in assemblies constituting of two relatively stiff adherends sandwiching a relatively compliant adhesive layer are reviewed. The closed-form solutions are categorised into the “non-free edge solutions” that do not satisfy the nil-shear stress condition at the free edge of the adhesive and the “free edge solutions” that do. Being strength of material solutions, the non-free edge solutions are significantly simpler in form. On the other hand, the solutions tend to grossly underestimate the magnitude of the peeling stress at the free edge. Almost all classical “non-free edge solutions” suffer from two setbacks: (i) assumed ?a = 0, thus severely underestimating the magnitude of the peeling stress; and (ii) neglected the thickness of the adhesive in their formulation of the x-compliance of assemblies and the evaluation of the effective bending strain on adherends; the former leads to overestimation while the latter leads to gross underestimation of the shear stress (and hence, ?a(l)). These are demonstrated in a numerical exercise in which two widely followed “non-free edge solutions” and a simplified “free edge solutions” are benchmarked against the finite element analysis.