THE EFFECT OF AGING ON THE SHEAR STRENGTH OF LEAD-FREE AND LEAD-BEARING BGA SOLDER SPHERESAuthors: Indraneel Chatterji, Daryl Santos, & Frank Andros
Company: SUNY Binghamton
Date Published: 12/3/2002 Conference: NEPCON West - Fiberoptic Expo
Although the recent interest in lead-free electronics is not a U.S. legislative initiative at this time, it is being driven by many causes. Probably the most significant of these is the proposed ban on lead in electronics in foreign markets (i.e., Japan and Europe); so manufacturers that wish to remain in those markets will likely be forced to utilize Pb-free alloys. Many lead-free alloys have been developed, but ideally, the industry is looking for a drop-in replacement for the standard Sn/Pb eutectic solder and that allows the use of existing process strategies.
Certainly, higher process temperatures are expected due to the higher melting points of the proposed lead-free alloys, so a direct drop-in replacement will be difficult. But assuming substrates and components can withstand these higher thermal processing conditions, the assembly processes may not be altered significantly. The main need for continued Pb-free study, however, revolves around the fact that there is still a lack of reliability/performance data for assemblies that will use the proposed replacement alloys.
Ball Grid Array packages have gained popularity as a packaging solution for application where performance and reliability is of importance. To improve the reliability, it is necessary to manufacture packages with robust solder ball attachment. The quality of solder ball attachment significantly depends on the pad surface finish and metallization processes used in substrate construction. This paper investigates the ball attach integrity of five different alloys, three lead-free alloys and two lead-bearing alloys for BGA application. The performances of the different alloys are evaluated for two different pad finishes, namely, Ni-Au and OSP.
A shear test approach has been adopted to correlate the shear strength with the intermetallic thickness of the alloys. The test vehicles were initially subjected to accelerated thermal aging. They were then kept at room temperature. Subsequently, evaluations were made at 9 months and 21 months, to find the effect of aging on the shear strength and the microstructure of the alloys. The shear strength and the intermetallic thickness were monitored as a function of time after solder reflow. Cross sectioning and scanning electron microscope are used to identify the microctructural changes in the solder balls with room temperature aging.
Key words: shear strength, surface finish, failure modes, intermetallics, Pb-free, area array.
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