NEMI Lead-Free Assembly Project: Comparison Between PbSn and SnAgCu Reliability and Microstructures

The National Electronics Manufacturing Initiative (NEMI) Lead-Free Assembly Project had the following priorities: • to down-select an alloy from the SnAgCu families to be recommended as the main standard lead-free solder; • to demonstrate production-ready parts, materials and processes for lead-free soldering printed wiring board (PWB) assemblies, with an eye to total lead elimination by 2004; • to cooperate with component, board and equipment manufacturers to allow for the smooth transition to components with higher exposure temperature limits of around 260°C; and • to develop criteria for the industry to evaluate leadfree processes and reliability in order to assist industry with timely implementation of lead-free assemblies. This article presents qualitative thermal cycling results for all the components and alloy combinations tested and provides a quantitative comparison of the failure data and microstructures for the CSP169 components for NEMI tinsilver-copper alloy, eutectic tin-lead, and a mixed tin-silvercopper paste with Pb-containing surface finish. INTRODUCTION The National Electronics Manufacturing Initiative (NEMI) formed its Lead-Free Task Force in 1999 with the goal of helping the North American electronics industry develop the capability to produce lead-free products. Realizing that choosing a single solder alloy would significantly benefit the microelectronics industry, the Task Force put substantial effort into reviewing the research that had been done with lead-free solders in the U.S., Europe and Japan. In 2000 NEMI announced i ts recommendations for the “standardized” lead-free solder that would allow the microelectronics industry to implement a replacement sooner, avoid multiple manufacturing processes and enhance basic understanding of the material while assuring its reliability. The NEMI group recommended an alloy of Sn3.9Ag0.6Cu (+/0.2) as the best available option for surface mount reflow solder applications. (All compositions are expressed in terms of mass fraction*100.) Following the selection, the NEMI Task Force began fullscale manufacturing trials and reliability testing. Reliability testing involved defining the reliability test requirements, designing and fabricating the test vehicles, performing a comprehensive matrix of tests, completing failure analyses to determine root cause of failures, performing statistical analyses of the failure data to provide for comparison between lead-free solder joints and those containing lead, documenting results, and developing databases of materials and board properties needed for finite element modeling of the solder joints. Since the dominant failure mechanism expected for the solder joint was thermal fatigue, the reliability plan focused on thermal cycle testing. The thermal cycling behavior of SnPb eutectic, SnAgCu alloy, and SnAgCu with SnPb component surface finishes were compared for a wide range of components and for two thermal cycling conditions. This article presents a comparison of the microstructures and the reliability of the solder joint, for the NEMI tin-silver-copper alloy and eutectic tin-lead RELIABILITY TEST PLAN Component-paste-board finish combinations The six component types tested featured lead-free and leadcontaining termination finishes/ ball compositions, based on the team’s assumption that not all components would be available with lead-free finishes during start-up of a lead-free assembly line. The components used were: • TSOP: type 1, 48 pin, NiPd and SnPb finishes • R2512: zero ohm chip resistor, pure Sn and SnPb finishes • 169 I/O CSP: 0.8mm pitch, SnAgCu and SnPb balls • 208 I/O CSP: 0.8mm pitch, SnAgCu and SnPb balls • 256 I/O PBGA: 1.27mm pitch, SnAgCu and SnPb balls