Electronics Assembly Knowledge, Vision & Wisdom
Low-Silver BGA Assembly
Low-Silver BGA Assembly
This study characterizes the influence of alloy type and reflow parameters on low-silver SAC spheres for BGA assembly.
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Authored By:
Gregory Henshall, Michael Fehrenbach - Hewlett-Packard Co.
Chrys Shea - Shea Engineering Services
Quyen Chu, Girish Wable - Jabil
Ranjit Pandher - Cookson Electronics
Ken Hubbard, Gnyaneshwar Ramakrishna - Cisco Systems
Ahmer Syed - Amkor
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Summary
Some ball grid array suppliers are migrating their sphere alloys from SAC305 (3% Ag) or SAC405 (4% Ag) to alloys with lower silver contents. There are numerous perceived reliability benefits to this move, but process compatibility and thermal fatigue reliability have yet to be fully demonstrated.

The current study has been undertaken to characterize the influence of alloy type and reflow parameters on low-silver SAC spheres assembled with backward and forward compatible pastes and reflow profiles. This study combines low-silver sphere materials with tin-lead and lead-free SAC305 solder pastes under varied reflow conditions. Solder joint formation and reliability are assessed to provide a basis for developing practical reflow processing guidelines and to assist in solder joint reliability assessments.

This is the fifth report in a series being published as results become available, and presents
the preliminary results of the thermal cycling portion of the test program. Thermal cycling conditions include both 0 to 100 C and -40 to 125 C, with 10 minute dwell times.
Conclusions
The investigation reported here continues as the test boards continue to cycle under both profiles. The goal is to reach complete failure, or at least enough failures to reach the characteristic life, for all combinations of solder joint composition, assembly conditions, and package type. Results will be published as they become available.

It is risky to make firm conclusions at this point in the experiment, the but following tentative conclusions can be put forward at this time.

1. 100% Sn-Pb joints are less reliable under the 0/100 C test conditions than either the mixed SnPb/Pb-free joints or 100% Pb-free joints. For mixed SnPb/Pb-free joints, this conclusion is limited to the reflow conditions used in this study, which produce joints with relatively good mixing of Pb throughout. The only exception to this finding is for SnPb-SACX joints, which have a low projected 1% failure life and lowβ, possibly due to problems with proper solder joint formation.

2. Under 0/100 C test conditions, low Ag BGAs soldered with SAC305 paste are more reliable than corresponding 100% SnPb joints. This finding suggests that the risk of using low Ag BGAs in environments that induce solder joint thermal fatigue may be manageable in many applications.
Initially Published in the IPC Proceedings
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