ICSR (Soldering and Reliability) Conference Proceedings


RELIABILITY ASSESSMENT OF ALTERNATIVE LEAD-FREE ALLOYS USED DURING WAVE AND REWORK

Authors: Craig Hamilton, et al.
Company: Celestica and IBM Corporation
Date Published: 5/22/2009   Conference: ICSR (Soldering and Reliability)


Abstract: Selecting an appropriate lead-free alloy for use during wave solder and pin through hole (PTH) rework processes is not a trivial task. Although the electronics industry has been preparing for the conversion from a leaded to a lead-free alloy for almost a decade now, it is apparent that the industry is still within its infancy with respect to selecting a common alloy for these PTH processes. Significant effort has been placed on studying the benefits and drawbacks of a variety of lead-free alloy options available over the past few years. However, with the proliferation of next generation alloys being offered, the task of selecting a common alloy through wave solder and rework processes is being made even more difficult. Industry trends continue to indicate that there is a strengthening movement away from using SAC305 or SAC405 alloys within PTH primary attach/rework soldering operations and a movement towards the use of other alternative lead-free alloys [1]. The main reasons for this shift can be linked to lower procurement costs and lower copper (Cu) dissolution rates. Since many of these alternative lead-free alloys are now starting to gain market share, there is a new issue that has emerged relating to the limited availability of reliability data collected for these new alloy systems. The majority of PTH reliability testing data in existence today relates to SAC305/405 alloys. With the EU RoHS exemption reviews and negotiations continuing, high complexity, high reliability applications including telecom, aviation, storage, and server products may soon fuel demand for alternate lead-free alloys. As a result, it is expected that the need for thermal and mechanical reliability testing of these alternate alloys will increase.

This paper, an evolution of earlier work on this topic, discusses results obtained from an internally designed PTH test vehicle constructed to evaluate three commercially available alternative lead-free wave alloys: Sn-Cu-Ni, Sn-Ag-Cu-Bi and Sn-Cu-X by comparing their performance to SAC405 and SnPb alloys. The process performance of each alloy measured during earlier work will be summarized, including profile optimization and process yield analysis. The main intent of this paper is to validate the thermal reliability of a selection of alternative lead-free alloys compared to a SAC405 baseline. The thermal cycling profile used was 0-100°C at 1 cycle per hour (CPH) for 6,000 cycles. Detailed failure analysis is presented to identify and explain observed failure mechanisms. In addition, a comparison of the alloy behavior when subjected to mechanical load conditions vs. non-loaded thermal cycling will be briefly summarized from previous work. The objective of this work is to provide both thermal and mechanical reliability data which can be used to select a suitable wave alloy replacement for SAC305/405. The requirement being that the alternate alloy solution provides similar (or better) overall solder joint quality and reliability, under the conditions tested.

Key words: Wave Solder, Rework, Lead Free, Reliability



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