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Reliability of Doped Lead-Free Solder Joints Under Isothermal Aging



Reliability of Doped Lead-Free Solder Joints Under Isothermal Aging
This paper presents harsh environment reliability test results for lead-free solder pastes with and without the presence of a variety of dopant elements.
Materials Tech

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Authored By:


Cong Zhao, Thomas Sanders, Chaobo Shen, Zhou Hai, John L. Evans, Ph.D., M.J. Bozack, Ph.D., and Jeffrey Suhling
Center for Advanced Vehicle and Extreme Environment Electronics (CAVE3)
Auburn University
Auburn, AL, USA

Summary


This paper presents harsh environment reliability test results for lead-free, principally Tin-Silver-Copper (SAC) based, solder pastes with and without the presence of a variety of dopant elements. SAC105 and SAC305 solder alloy spheres are mixed with 12 different doped solder pastes and subsequently tested under long-term isothermal aging and thermal cycling condition in order to evaluate the ability of solder doping to mitigate the effect of aging and to enhance the solder joint reliability. In addition to ball grid arrays (BGAs) with package size ranging from 15mm (0.8mm pitch) to 6mm (0.8mm pitch), the test vehicle also incorporates no-lead packages such as QFNs and 2512 Surface Mount Resistors (SMRs). Three surface platings are tested: Organic Solderability Preservative (OSP), Immersion Silver (ImAg), and Electroless Nickel Immersion Gold (ENIG).

All test vehicles were subjected to isothermal aging at temperatures of 125 Celsius with aging time of either 0 or 12 months, followed by accelerated thermal cycle (ATC) testing from -40 Celsius to 125 Celsius. Previous studies shows reliability of BGA packages can degrade up to 70% after aging at elevated temperature under similar conditions. This paper presents results for the first 1500 thermal cycles. Current reliability data indicates that 8 doped solder pastes demonstrate superior reliability, which can help improving the reliability of lead-free solder under aging, while 4 pastes have somewhat lower performance. Failure analysis shows continuous growth of intermetallic compounds (IMC) at solder joint-pad interface and within solder bulk. Crack propagation is observed in the component-side, nearinterfacial region in some materials and through the bulk solder in other materials. 2512 Surface Mount Resistors sampled thus far demonstrate typical fatigue crack propagation mechanics from the inside out through the main fillet.

Conclusions


In this experiment we have investigated 12 different doped solder pastes in combination with a variety of different components and surface finishes. Test vehicles were subjected to long-term isothermal aging and thermal cycling conditions in order to evaluate the viability of solder doping for enhancing solder joint reliability. Based on data analysis at 1500 thermal cycles, 8 doped solder pastes demonstrate superior reliability, while 4 pastes have somewhat lower performance. Current failure percentages indicate that, Paste A is the least reliable of all solder pastes, following paste I. Paste F and G also have poor reliability performance when used with 2515 Surface Mount Resistors. Failure analysis shows thermal-mechanical fatigue failures. BGA joints exhibit crack propagation either along the near-IMC interfacial regions or within the solder bulk. Pastes that have bad reliability performance have significant IMC thickening, with copper diffusion from the copper pads apparently undeterred by doping. For 2512 Resistor, cracks
were found to initiate underneath the component, and then propagate out through the primary fillet until an open circuit occurred.

Initially Published in the SMTA Proceedings

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