Daniel T. Rooney

Materials characterization of the effect of mechanical bending on area array package interconnects

Abstract The mechanical integrity of solder joint interconnects in PWB assemblies with micro-BGA, chip scale, and land grid array packages is being questioned as the size and pitch decrease. Some consumer products manufacturers have mechanically reinforced fine pitch package interconnects with an adhesive underfill, and others are evaluating the need for underfill on a case-by-case basis. Three-point cyclic bend testing provides a useful tool for characterizing the expected mechanical cycling fatigue reliability of PWB assemblies. Cyclic bend testing is useful for characterizing bending issues in electronic assemblies such as repetitive keypad actuation in cell phone products. This paper presents the results of three-point bend testing of PWB assemblies with fine pitch packages. The solder joints on ceramic components performed better than a laminate interposer component in bend testing, because of the stiffening effect of the ceramic packaging materials. The methodology of materials analyses of the metallurgy of solder interconnects following mechanical bending and thermal cycle testing is described. The microstructure and fracture surfaces of solder joint failures in bend test samples differed significantly from thermal cycle test samples.

Evaluation of wire bonding performance, process conditions, and metallurgical integrity of chip on board wire bonds

Abstract Chip on board wire bonding presents challenges to modern wire bonding technology which include smaller, closely spaced wire bond pads; bonding to soft substrates without special processing and pad construction; and diverse first bond and second bond metallurgies. These challenges are addressed by extensive bonding accuracy tests, a design of experiments approach for optimizing wire bond process parameters, reliability testing, and detailed materials characterization of the metallurgical integrity of the wire bonds. The thermo-mechanical integrity of the wire bond interconnects was evaluated by wire pull and hot storage tests. Hot storage testing allowed for detection of samples with an electrolytic gold surface finish that was too thin, and exhibited a contamination-corrosion condition of the nickel under-plating. Other samples with an excessively thick, rough textured nickel under-plating layer exhibited poor wire bond-ability. The methodology of materials analyses of the metallurgy of the wire bond interconnects is described. The paper illustrates a wire bond lift technique that is used to inspect for cratering damage and the “area-uniformity” of gold aluminum intermetallics. An improved understanding of the wire bonding process was achieved by showing the dependence of the visual appearance of the wire bonds on wire bond process parameters.

Characterization of Fine Pitch CSP Solder Joints Under Board-Level Free Fall Drop (BFFD)

This paper presented a board-level free fall drop (BFFD) to simulate product free fall drop (PFFD). In BFFD, the board structure is very close to the actual cellular phone boards with similar size. The board edges were reinforced by metal frame and screws similar to the cellular phone housing. The drop test was performed to characterize the solder joints of 0.4mm pitch chip scale package (CSP) packages. To charaterize the stress level, the acceleration was measured and the free drop test was performed for more than 140 boards with different CSP structures. The plastic strain of solder joints in different location was calculated using finite element analysis (FEA). It was found that the strain level is both location and component dependent. This strain level will determine the probability of drop test failure in terms of number of drops to failure. The impact of component type, body size and component location was investigated by both experimentally and FEA. Through drop test, the number of drop before failure (MDBF) were recorded and compared across various CSPs. Failure analysis was also performed to confirm the failure modes.Copyright

Solder Joint Characteristics and Reliability of Lead-Free Area Array Packages Assembled at Various Tin–Lead Soldering Process Conditions

This paper presents a comprehensive study on the effects of critical process conditions on solder joint metallurgy and reliability of mixed-alloy solder joints. The solder joint metallurgy of mixed alloys was characterized and the lead distribution through the solder joint was analyzed, for different package types and under various process conditions. The results showed that the solder paste amount (ultimately the tin percentage (Sn%) in the alloy) and the reflow temperature play critical roles in the mixed-alloy assembly, both in terms of compositional homogeneity and voiding. The reliability of mixed-alloy solder joints was then studied at various process conditions, under different thermal and mechanical stress environments. The study revealed that the sensitivity of the reliability of the mixed-alloy solder joints to the process condition depends on the type of environmental loading.

Metallurgical analysis and hot storage testing of lead-free solder interconnects: SAC versus SACC

Lead free solder alloys have been developed which contain small concentrations of rare earth additives such as cerium and lanthanum. It is believed that rare earth element additives refine the microstructure of the solder and improve the mechanical durability and thermomechanical integrity of lead free solder joints. This paper presents the results of a comparative analysis of solder joints with the commonly used SAC 305 alloy (Sn 3.0% Ag 0.5% Cu) and a SACC alloy (Sn 3.0% Ag 0.5% Cu 0.019% Ce), which contains a small concentration of a Cerium additive. The influence of cerium on the microstructural refinement of the bulk solder; the metallurgy of the intermetallic compounds; and the damage evolution of the solder during thermal aging tests are investigated. Mechanical die shear tests and lead pull strength tests have been conducted to compare the mechanical strength and the morphology of the fracture surfaces of SAC and SACC solder joints. Lead pull testing of samples with the cerium doped, SACC alloy exhibit more ductile fracture surfaces and PCB pad lifts, rather than in SAC alloy samples, which exhibit brittle interfacial failures. This change in the fracture surface morphology suggests that cerium additives improve the mechanical integrity of the bulk solder, and correlates with previous materials testing on bulk solder samples of SACC and SAC, which show that SACC solder has a higher Youngs modulus (higher stiffness), higher yield stress, and higher strength over a wide range of strain rates and temperature conditions. On the other hand, in the hot storage tests, the SACC samples exhibit thicker intermetallic formations at the bulk solder to component interface than in SAC samples, which suggests that SACC alloy solder joints could be less mechanically robust than SAC solder joints under dynamic loading conditions such as drop testing.

A comparative study on drop test performance of fine pitch BGA assemblies using Pb-free and tin-lead solders

This paper described a new board-level free fall drop (BFFD) and its application in the evaluation of solder joints in fine pitch ball grid array (BGA) or chip-scale packaging (CSP) assemblies. This drop test method simulates the product-level free fall or phone level drop test (PFFD), which was widely used in the industry. A comparison study between solder alloys using Sn-Ag-Cu (SAC) and SnPb were performed experimentally using BFFD. The test vehicle was built from different alloys (SAC vs. SnPb) with various surface finishes and structures. The packages include 0.4mm pitch ultraCSP (chip scale package), uSMD (micro surface mount device) and micro leadless package and stacked die packages. In order to understand the differences between the two alloys (SAC and SnPb), an FEA (finite element analysis) was employed to calculate the stress and strain in the free fall drop for various packages. From the experimental work, it shows there is a clear difference between those two alloys but a mixed trend across all packages. In general, SAC is not as good as SnPb for larger packages. When combining with the strain-stress analysis, it was found from this study that the strain plays an important role