J.F.J.M. Caers

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.

Low-cost flip-chip on board

For hand-held communication products, like the DECT telephone system, flip-chip on board offers minimization of both the package size and the occupied area on the boards. As a result of the reduction in interconnection lengths, the self-inductance is reduced. For high volume production, the compatibility of the flip-chip mounting technique with standard surface mount technology (SMT) reflow is essential. During reflow, the eutectic PbSn bump wets along the copper track, so the stand-off between integrated circuits (ICs) and the board is accurately defined by the layout of the board and the dimensions of the bump. The eutectic PbSn flip-chip processing is evaluated by impedance and cross-talk measurements, and in several reliability tests. For the electrical measurements, a zero-IF front-end IC is used. Wide-band measurements of the input impedance showed that the residual parasitics associated with the eutectic PbSn bumps are negligible compared with the parameters of the internal IC components. To accommodate the residual stresses from differences in coefficient of thermal expansion (CTE), the gap between the IC and the substrate is underfilled. This underfill material marginally affects the electrical behavior of the IC at frequencies up to a few GHz. As expected, a slight increase in the residual capacitance is observed. The effect of the underfill is studied by both temperature cycle and shock tests; cumulative failure distributions have been plotted. Results show that the adhesion properties and flow characteristics of the underfill material are the dominating factors for the number of cycles to failure. By selecting the proper underfill and curing conditions, the eutectic PbSn flip-chip construction can meet the test requirements for consumer communication products.

A Component Level Test Method for Evaluating the Resistance of Pb-free BGA Solder Joints to Brittle Fracture under Shock Impact

A high-speed shear tester has been used to evaluate the interface strength of Pb-free solder balls with varying Ag and Cu content to a BGA laminate interposer with different finishes. The shear rate used is 0.45 m/s. Components with SAC405 and SnPb solder balls have been used as a reference. Results are compared with SAC305, SAC305 with NiGe addition, Castin 258 (2.5Ag-0.8Cu-0.5Sb), SAC105 (1.0Ag-0.5Ag), SAC101 (1.0Ag-0.1Cu-0.02Ni-xIn), SACX (0.3Ag-0.7Cu-0.1Bi), LF35 (1.2Ag-0.5Cu) and Sn3.5Ag. Electroplated NiAu finishes on the BGA laminate interposer studied come from different suppliers. Three failure modes are observed in the high speed shear test: brittle fracture in the intermetallic layer at the ball/interposer interface, ductile fracture in the solder bump and peel off of the solder pad from the interposer. Analysis of the fracture interface shows that almost all Pb-free solder alloys fail either in the intermetallic layer or by pad peel off. Only SAC101 and Sn3.5Ag show ductile fracture in the bump or pad peel off, but no fracture in the intermetallic layer. This is comparable with results for eutectic SnPb under these test conditions. A JEDEC board level drop test on same packaging with SAC101, SAC305 and SnPbAg shows that the number of drops to failure for packages with SAC 101 is also higher than those with SAC 405 and even slightly higher than with eutectic SnPb solder balls. Failure analysis is done to understand the difference in shock resistance with different solder bumps. From the observations in both of the tests, it is evidenced that the high speed shear test, when combined with proper analysis of the failure mode, is a simple but very powerful tool to evaluate the resistance of Pb-free BGA solder joints to brittle fracture under shock impact.

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.

Towards a predictive behavior of non-conductive adhesive interconnects in moisture environment

Flip chip on flex interconnections with non-conductive adhesive paste are taken as test vehicles to study the behavior of adhesive interconnects in a moisture environment. A dedicated test structure is used to enable a proper four-wire resistance measurement of single interconnects. A new and very powerful approach is used, focusing on the response of the contact resistance of the interconnect to humidity. Two types of accelerated tests have been performed with on-line measurement of the contact resistance with high measuring resolution. The first test aims at the durability of the interconnects and the second is a step stress-type test which shows that the relative response of the contact resistance to humidity is much larger at high relative humidity levels, compared to the response at low relative humidity levels. Combining the results of both types of experiments allows the definition of a simple transform for non-conductive adhesive interconnects in a changing humidity environment. Failure analysis is performed to understand the physical behavior of the interconnects during the accelerated tests.

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.

Copper trace fatigue models for mechanical cycling, vibration and shock/drop of high-density PWAs

When assessing the mechanical durability of electronic assemblies, the focus is generally on the solder interconnect. However, in many package styles, such as BGAs (ball grid arrays), LGAs (land grid arrays), MLFs (micro lead frame) and QFNs (quad flat no-lead), the Cu trace emanating from the solder pad may be the weakest failure site, especially if the solder joint is copper-defined rather than mask-defined. This is particularly true in situations with cyclic mechanical loading, such as cyclic quasi-static bending, vibration and repetitive drop/shock.

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.

Mechanics of Adhesively Bonded Flip-Chip-on-Flex Assemblies. Part II: Effect of Bump Coplanarity on Manufacturability and Durability of Non-Conducting Adhesive Assemblies

This is Part II of a two-part paper on the mechanics of adhesively-bonded Flip-Chip-on-Flex (FCOF) assemblies. Part I dealt with the use of anisotropic conductive adhesives (ACAs) while this paper deals with the use of non-conducting adhesives (NCAs). The central concern here is the influence of bump coplanarity on the manufacturability and durability of the assembly. The assembly is first analyzed with global/local nonlinear finite element models to assess the effect of bump coplanarity on the maximum force needed to achieve reliable bonding. The number of bumps at the low end of the manufacturing height tolerance is parametrically increased to quantify the effect on the bonding force. Results confirm that the bonding force required is inversely proportional to the number of ‘short’ bumps in the assembly. Next, the bonding and adhesive curing process is simulated in detail with finite element models, in order to assess the residual pre-stress between matching interconnect bumps, since this compressive contact stress may be important to the long-term performance of the FCOF assembly. The nonlinearities addressed in the model include elastic–plastic properties of gold, viscoplastic properties of the NCA and evolution of contact area between the mating bumps. Results show that it is necessary to model the viscoplasticity of the NCA to obtain realistic predictions, and that the residual compressive pre-stress between the mating bumps increases as the percentage of ‘short’ bumps increases in the assembly.