Pan Pacific Symposium Conference Proceedings


Author: Swaminath Prasad & Flynn Carson
Company: ChipPAC
Date Published: 2/13/2001   Conference: Pan Pacific Symposium

Abstract: In the era of the Ball Grid Arrays (BGA), Chip Scale Packages (CSPs) and flip chips, the packaging foundries worldwide still receive huge demands for their capacity to produce leadframe packages. This translates to a need to have a lead-free solution for the traditional packages. Also, since the plating operation involves effluents that are discharged into the environment, the value of this conversion has far reaching benefits.

There are two issues that need to be addressed; the first being the ability of the leadframe surface mount packages to withstand the higher reflow temperature and secondly a viable alternative to the 90Sn/10Pb component lead finish.

Based on the twin objectives, two studies were executed in parallel. The component level reliability at the 260C reflow was achieved after a comprehensive study based on the design of experiment approach. However, since this program has been recent, the extent of data generated in terms of reliability is limited only to the Moisture Sensitivity Levels (MSL) achieved by the packages at the higher temperatures. This study involved the evaluation of the existing silver spotted leadframes and the pre-plated Nickel-Palladium-Gold (Ni/Pd/Au) leadframes in addition to different die attach epoxies and bromine free molding compounds.

There are a number of alternate component lead finish options available, which are being pursued worldwide. Pure tin, tin-copper, and tin-bismuth, are the three options that are currently available. But, the packaging foundries have in their own interests the need to arrive at one plating chemical choice. This is so because it is practically infeasible and expensive to have plating lines operating with two different solutions with the associated loss in cycle time induced due to the conversion. As the transition to lead free is imminent, unless there is only one lead-free solution determined, a plating division will have to support a scenario of multiple lines with multiple chemical solutions, which is not economically viable.

Given the above listed constraints, this study focused on the comparisons of the available lead-free chemical solutions based on processability and reliability. Whisker growth tests were also conducted on the plated leadframes. The primary focus was on two package types: the metal quad flat pack (MQFP) and the Thin Small Outline Package (TSOP). This choice was based on the vast physical differences between the two package types and also on customer demand.

As a result of the two parallel studies, a viable lead-free leadframe package suitable for production in high volume was made available but a long term whisker-free plating solution still needs to be determined.

Key words: Lead-free, leadframe packages, plating chemicals, reliability.

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