S.K.W. Seah

Advances in the drop-impact reliability of solder joints for mobile applications

This manuscript presents the research studies in the drop-impact reliability of solder joints in the PCB assemblies intended for mobile applications. The works cover stress–strain characterisation of solders, evaluation of test methods at component and board levels, and investigation of the fatigue characteristics of solder joints. The stress–strain characteristics of four solder alloys were generated for the low and medium strain rate regimes up to the strain rate of 300 s � 1 . In the study on test methods, 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. The high speed cyclic bend test was found to be able to replicate the failure mode and the performance ranking of the solder joints in the board level drop-shock test, while the ball impact shear test was found to have poor correlation with the board level drop-shock test. The S–N characteristics of solder joints between the PCB fibre strains of 1 � 10 � 3 to 3 � 10 � 3 , as a function of bending frequency (from 30 to 150 Hz) and test temperatures (�10 C and 25 C), were generated. The propagation of cracks in the solder joints were monitored through electrical resistance measurement, and the rates of propagation were correlated with the observed crack paths in the solder joints. The effects of load history on crack propagation were investigated using two-step load tests, and the use of Miner’s rule was found to be non-conservative. Design rules were formulated based on analytical solutions, and a robust solder joint design was proposed and validated.

Correlation studies for component level ball impact shear test and board level drop test

This paper presents a comprehensive study of the resistance of solder joints to failure when subjected to strain rates that simulate the conditions of drop-impact on a portable electronic product. Two test methods are used in this study: the board level drop/shock test (BLDT) and the component level ball impact shear test (BIST). The performance of (i) 12 material combinations consisting of six solder alloys and two pad finishes; and (ii) 11 manufacturing variations covering three vendors, two finishes, three immersion gold thicknesses and three thermal aged conditions, were investigated using these two test methods, and analysis of correlations between the methods was performed. Quantitative correlation and sensitivity coefficients for the failure modes and the measured characteristic parameters – number of drops to failure for BLDT and peak load, total fracture energy, and energy-to-peak load for BIST – were evaluated. The lack of universal correlations between the two test methods has ruled out the use of BIST for evaluating solder joint materials, but BIST is recommended as a test method for quality assurance in view of the strong correlation between the measured parameters and the failure mode. The total fracture energy parameter is preferred over the peak load and energy-to-peak load due to its higher sensitivity and reduced susceptibility to measurement error.

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.

A Study of Component-Level Measure of Board-Level Drop Impact Reliability by Ball Impact Test

This paper presents a study of the resistance of solder joints to failure when subjected to strain rates that simulate the conditions of drop impact on a portable electronic product. Two test methods are used in this study: board-level drop/shock test (BLDT) and component-level ball impact test (BIT). The performance of 12 material combinations consisting of 6 solder alloys and 2 pad finishes were investigated using these two test methods, and analysis of correlations between the methods was performed. Quantitative correlation and sensitivity coefficients for the failure modes and the measured characteristics, namely, number of drops to failure for BLDT and peak load, total fracture energy, and energy to peak load for BIT, were evaluated. Analysis of the test results indicates that there is a lack of universal correlation between BLDT and BIT. Nevertheless, BIT can still serve as a test methodology for quality assurance in view of the strong correlation between the measured BIT characteristics and the failure mode. The total fracture energy parameter is preferred over the peak load and energy to peak load due to its higher sensitivity and reduced susceptibility to measurement error.

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.