SMTA International Conference Proceedings


Author: Indraneel Chatterji
Company: Solectron, Inc
Date Published: 9/24/2006   Conference: SMTA International

Abstract: As the material supply base converts from leaded to leadfree components, there will be instances where a combination of leaded solder paste and lead-free solders are being used on the same assembly. Maintaining solder joint reliability is one of the key challenges as the industry complies with new lead-free norms. The complexity of leadfree implementation has caused the industry to evaluate the feasibility of soldering lead-free area array packages using tin-lead solder paste, also known as Backward Compatible OR Mixed assemblies. This endeavor was carried out due to exclusion of one of Solectron’s customer from RoHS compliance thus facing a mixed assembly situation. Area array packages are one component that has caused a lot of concern with respect to mixed technology assemblies.

This paper presents the outcome of a project to assess solder joint reliability for three possible techniques to attach leadfree 0.8mm area array components using tin-lead solder paste; 1) Elevated reflow profile 2) Component reballing and 3) Post assembly rework. Control assemblies using 0.8mm pitch tin-lead area array components and tin-lead paste were used for comparison. A 22 layer production board of 125 mils thick with OSP surface finish having 24 components was selected to conduct this experiment. Each set of experimental design was passed through top and bottom side reflow to simulate the actual production condition. A populated board was thermally profiled to understand the temperature effect on the area array package and also on the adjacent components during reflow and rework process. Some of the lead-free area array packages were reworked at a peak of 230deg C. A flux dip method was used for the rework process. The flux was methodically selected on the basis of flux activity and spreading over the pad area at high temperature.

The packages were put through 2000 ATC cycles in the temperature range of 0 to 100 deg C with a ramp rate of 10deg/min and a dwell time of 10 minutes as per IPC-9701. After every 200-500 cycles, the boards were cross-sectioned to study the microstructure and intermetallic formation of the Mixed (1st pass and rework) assemblies, reballed assemblies and tin-lead controlled assemblies. The packages after each set of thermal cycle were subjected to dye and pry to understand the crack propagation within the ball. The microstructure analysis and dye-pry results are presented in this paper.

Key Words: Mixed Assembly, reliability, area array packages, rework, solder alloy and backward compatible.

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