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Assessment of Low-Ag Solder Alloys for General Lead-Free Assembly
Tuesday, February 21, 2012 | Jasbir Bath, Christopher Associates Inc./Koki Company Ltd.

Editor's Note: This article originally appeared in the February 2012 issue of SMT Magazine.

The electronics industry has been transitioning from Sn37Pb to lead-free Sn3Ag0.5Cu over the last 10 years. One challenge with this transition is the use of 3% silver in lead-free SnAgCu solder causing rapid price increases in lead-free solder alloys due to the limited global availability of silver metal. Based on this, there has been activity and interest in transitioning to low-silver (≤1.2wt%) lead-free solder alloys in BGA/CSP component spheres, wave solder and rework alloys and, now, solder paste.

In terms of the global solder market per year, approximately 180,000 tonnes of solder is used with approximately 20,000 tonnes in solder paste and 160,000 tonnes for wave soldering. If we consider the approximate global silver metal usage per year as 20,000 tonnes, with a world capacity of 21,500 tonnes, there is only a spare capacity of 1,500 tonnes of silver per year which could be used for the solder market. This would be enough silver for a lead-free solder alloy up to approximately 1wt% Ag.

With Sn3Ag0.5Cu (SAC305), due to the limited availability of silver, and with precious metals such as silver and gold being an easy target for investment/speculative buying, the cost of silver has increased considerably in recent years from around $250,000 per tonne in 2005 to $1,000,000 per tonne in 2011. Based on this, there have been numerous requests from customers for a low-silver solution.

Koki developed a low-silver alloy (Koki S01X7C) which contains only 0.1Ag in a lead-free alloy formulation Sn0.1Ag0.7Cu0.03Co. This helped to reduce the cost considerability compared with Sn3Ag0.5Cu as the final alloy price is based mainly on tin metal rather than silver as shown in Figure 1.

Figure 1: Cost breakdown of Sn3Ag0.5Cu versus low-silver lead-free solder alloy (Sn0.1Ag0.7Cu0.03Co) showing lower overall solder price based on less silver used.

Low-silver lead-free alloys have been developed by many solder manufacturers to address the cost and silver supply issue, but they have been mainly found to have reduced thermal cycling reliability performance versus Sn3Ag0.5Cu. Also temperature differences between low-silver alloys and Sn3Ag0.5Cu need to be considered when transitioning over, as the melting point of the low-silver alloy tends to increase. Sn3-4Ag0.5Cu lead-free alloys have a melting point of approximately 217ºC compared with 225ºC for Sn1Ag0.5Cu and 227ºC for Sn0.7Cu.

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