SMTA International Conference Proceedings

Lead-Free Alloy Development

Author: Karl F. Seelig
Company: AIM
Date Published: 10/14/2012   Conference: SMTA International

Abstract: When lead-free solders were first introduced to the electronics industry in the early 1990’s, the tin-silver alloy composed of Sn96.5/3.5Ag was the first investigated. The most obvious difference from the industry standard was the substantially higher melting temperature of 221°C versus 183°C (Sn63/Pb37).

Although Sn/Ag had been used extensively in hybrid electronics, concerns over silver migration and silver creep drove the industry to investigate Sn/Cu and (S)n/(A)g/(C)u (SAC) alloys. Initial selection criteria deemed SAC to be the more reliable option. In early tests, Sn/Cu exhibited inferior reliability in thermal cycling (-65+125C) and was prone to in-process wetting issues. The three leading commercialized SAC alloys were introduced as SAC305, SAC387, and SAC405. (Note: the first number refers to silver content followed by the last number, which is the copper content. SAC305 is tin, 3% silver and 0.5% copper.) All SAC alloys share the common disadvantage versus the tin/lead they replace:
1) Increased cost due to the silver content
2) Higher melting temperatures
3) Grainy/frosty solder joints
4) Increased copper dissolution
5) Potential damage to existing wave solder equipment

The SAC alloys remained the preferred lead free alternatives until the surge in silver prices. Concurrently, AIM patented and released the CASTIN alloy. This SAC305/Sb alloy had a lower melting point (217°C) than Sn/Ag (221°C) and lowered the silver creep and cost problems by reducing the silver by 0.5-1%. However, the antimony content was a concern due to a misperception that it had a high toxicity profile. It was often confused with the fire retardant used in industry, antimony trioxide, a suspected carcinogen.

In an effort to further reduce costs and the adverse properties of silver, a silver-free alloy, SN100C (melting point 227°C), was released in the late 1990’s.

This alloy is a nickel (Ni)-stabilized Sn/Cu alloy providing improved wetting and solder flow. These characteristics made it possible to achieve excellent results in wave soldering at process temperatures comparable to tin lead solder (500°F). As the Ni-stabilized SnCu alloy gained acceptance in the marketplace, other low silver alloys including Sn/Ag.03/Cu.07 (SAC0307/MP 227°C) and Sn/Ag.08/Cu.07 (SAC0807/MP 216-225°C) emerged. The Ni-stabilized SnCu alloy was unique in that it contained 0.05% nickel. Prior to its release, there were no other solders containing nickel that were used commercially. While the nickel controls the grain structure of the Sn/Cu base, resulting in a shiny alloy with good flow, the nickel and the copper do not aid wettability and can affect the mechanics and structure of the solder. Other advantages to the Ni-stabilized SnCu alloy were not only the complete elimination of silver, thereby offering a dramatic cost reduction, but the aesthetic of the reflowed joint was much closer to the Sn63/Pb37 alloy it was replacing. It also minimized copper dissolution and potential wave solder equipment damage.

The purpose of this paper is to investigate several factors relating to the above-mentioned family of alloys; both low-silver and non-silver. The investigation includes the use of these alloys as replacement alloys in SMT solder paste and the comparison of the paste medium chemistries in combination with these alloys and the effect they have as it relates to SMT assembly performance. This paper places alloys in risk classes for drop and thermal shock as well as assessing the impact on wetting, voiding, and head-in-pillow (HiP) mitigation.

Key Words: 

low-silver, non-silver, lead free

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