Assembly Materials

“Alternatives to Solder in Interconnect, Packaging and Assembly”

Authors: Herbert J. Neuhaus, Ph.D., and Charles E. Bauer, Ph.D.; This email address is being protected from spambots. You need JavaScript enabled to view it..
Abstract: The intense search for attractive lead-free solders reveals the preeminent importance of solder to the industry. In fact, solder consumes so much attention that solder-less alternatives are often overlooked. Material-based alternatives to solder include conductive adhesives and transient-phase compounds. Developments in nanotechnology spawned a virtual renaissance in conductive adhesives and other solder-less joining materials. As a complement to the solder-less materials developments, embedded assemblies use conventional materials in novel ways to improve performance by cutting interconnect parasitics and increase reliability gains by eliminating wire-bonds and solder-bumps. Freescale, Imbera, GE, Verdant and many others develop and employ diverse approaches to embedding active devices. Particle Interconnect represents another solder alternative. While originally developed for automated test, particle interconnect holds considerable promise in a variety of applications, including LED assembly and printed electronics. This presentation surveys the landscape of alternatives to solder in interconnect, packaging, and assembly. It treats practical implementation challenges such as yield management strategies and supply-chain restructuring. Finally, it concludes with a discussion of scenarios in which solder alternatives offer highly compelling business and technical benefits. (SMTA Journal of SMT, vol. 26, no. 1)

Component Test

“Three-Dimensional Thermomechanical Simulation of Fine-Pitch High-Count Ball Grid Array Flip-Chip Assemblies”

Authors: W. Kpobie, N. Bonfoh, C. Dreistadt, M. Fendler, and P. Lipinski; This email address is being protected from spambots. You need JavaScript enabled to view it..
Abstract: Flip-chip technology is increasingly prevalent in electronics assembly 3D system-in-package and is mainly used at fine pitch for manufacture of megapixel large focal-plane detector arrays. To estimate the reliability of these assemblies, numerical simulations based on finite-element methods appear to be the cheapest approach. However, very large assemblies contain more than one million solder bumps, and the optimization process of such structures through numerical simulations turns out to be a very time-consuming task. In many applications, the interconnection layer of such flip-chip assemblies consists of solder bumps embedded in epoxy filler. For such configurations, we propose an alternative approach, which consists of replacing this heterogeneous interconnection layer by a homogeneous equivalent material (HEM). A micromechanical model for the estimation of its equivalent thermoelastic properties has been developed. The obtained constitutive law of the HEM was then implemented in finite-element software (Abaqus). Thermomechanical responses of tested assemblies submitted to loads corresponding to manufacturing conditions have been analyzed. The homogenization–localization process allowed estimation of the mean values of stresses and strains in each phase of the interconnection layer. To access more precisely the stress and strain fields in these phases, two models of structural zoom, taking into account the real solder bump geometry, have been tested. The obtained local stress and strain fields corroborate the experimentally observed damage initiation of the solder bumps. (Journal of Electronic Materials, July 2013)

Reliability

“SnAgCu Lead-Free Electronics Reliability under Combined Temperature and Vibration Environments”

Authors: Pradeep Lall and Geeta Limaye; This email address is being protected from spambots. You need JavaScript enabled to view it..
Abstract: Electronics installed in automotive systems are subjected simultaneously to mechanical vibrations and thermal loads in underhood applications. Typical failure modes include solder joint failure, pad cratering, chip-cracking, copper trace fracture, and underfill fillet failures. The solder interconnects accrue damage much faster when vibrated at elevated temperatures. Presently, the literature on mechanical behavior of lead-free alloys under simultaneous harsh environment of high-temperature vibration is sparse. In this paper, a test vehicle with a variety of lead-free SAC 305 daisy-chain components, including BGA, QFP, SOP, TSOP, has been tested to failure by subjecting it to two elevated temperatures and harmonic vibrations at its first natural frequency. The test matrix includes variation in the amplitude of vibration from 10G to 14G, as well as variation in temperature. Full field strain on the PCB has been extracted using high-speed cameras operating at 100,000 fps in conjunction with digital image correlation. The vibration simulation using global-local finite element models is correlated with the system characteristics such as modal shapes and natural frequencies. The vibration simulation provides a fatigue life prediction that has been validated with the experimentally obtained cycles to failure. In addition, the packages have been cross-sectioned to study the failure modes. A comparison of failure modes for different surface mount packages at elevated test temperatures and vibration is also presented. (SMTA Journal of SMT, vol. 26, no. 1)

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