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Acceptance Testing Of Low-Ag Reflow Solder Alloys
Acceptance Testing Of Low-Ag Reflow Solder Alloys
This paper describes initial test results for low-silver alloys using solder paste alloy assessment protocols for BGAs and leaded components.
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Authored By:
Kris Troxel
Hewlett-Packard Company
Boise, ID, USA

Aileen Allen
Hewlett-Packard Company
Palo Alto, CA, USA

Elizabeth Elias Benedetto, Rahul Joshi
Hewlett-Packard Company
Houston, TX, USA

Summary
Since the implementation of the European Union RoHS directive in 2006, the electronics industry has seen an expansion of available low-silver lead (Pb)-free1 alloys for wave soldering, miniwave rework, BGA and CSP solder balls, and, more recently, solder pastes for mass reflow.

The risks associated with the higher processing temperatures of these low-silver (Ag between 0-3 wt%) solder alloys, such as potential laminate or component damage, increased copper dissolution, and reduced thermal process windows may present manufacturing challenges and possible field reliability risks for original equipment manufacturers (OEMs). In order to take advantage of potential cost reduction opportunities afforded by these new alloys, while mitigating manufacturing and reliability risks, the company has defined test protocols [1-4] that can be used for assessing new Sn-Ag-Cu(SAC), Sn-Ag, and Sn-Cu alloys for general use in electronics.

This paper describes initial test results for low-silver alloys using these solder paste alloy assessment protocols for BGAs and leaded components, and the impact of the alloys on printed circuit assembly process windows. Specific pass/fail criteria for acceptance of an alloy are not included, however, as they may vary across industry segments. The assessment evaluates wetting behavior, solder joint thermal fatigue and mechanical shock reliability, intermetallic formation, general physical joint acceptability, and copper dissolution. The variables include multiple component types: two BGA components with the same paste/ball alloy combinations, and numerous leaded components that include common component platings.

Surface mount (SMT) process temperature windows are typically constrained on the low end by the ability to melt solder and form acceptable joints, and on the high end by the maximum process temperatures of other materials, such as components. These two constraints have led to a process window of approximately 25 degrees C when soldering with more conventional, Sn- 3.0Ag-0.5Cu paste. Low-silver SMT alloys have been found to reduce the thermal process window even further.

Conclusions
Prior studies of low-Ag alloys primarily investigated BGA ball alloys and had not fully explored the implications of increased liquidus temperature on reflow paste alloys. This work found the peak reflow temperature has to be increased by 10-15 degrees C over the liquidus temperature when using low-Ag paste alloys.

For the alloys studied, this implies that a minimum reflow peak temperature of 240 degrees C is required. When combined with the maximum package temperature of 245 decgrees C, this results in an effective process window of 0-5 degrees C, when accounting for temperature deltas across the board. However, if solder joints are properly formed, reliability (thermal fatigue, mechanical shock) of low-Ag alloys is comparable to SAC305. The drivers for whether a low-Ag reflow alloy is acceptable are board complexity and thermal mass.

The challenge for assemblers is in developing reflow programs that minimize the temperature delta between the coolest and hottest locations on the board. Indeed, it is unlikely that an acceptable process window is feasible for general-use company PCAs using low-Ag SMT alloys. Low-Ag alloys with liquidus temperatures closer to SAC305 (<220 degrees C), and those with near-eutectic melting characteristics, appear more suitable candidates for general use; however, this will require either an increase in Ag content or in dopant selection.

Initially Published in the IPC Proceedings

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