Lead-Free Alloys

The NEMI consortium in USA recommends SnAg3.9Cu0.6 for surface mount reflow soldering and SnCu0.7 for wave soldering. The JEITA lead-free roadmap in Japan recommends SnAg3.0Cu0.5 for reflow soldering with SnAg and SnZnBi as secondary alternatives. JEITA also recommends SnAg3.0Cu0.5 for wave soldering with SnCu as a secondary alternative. The IDEALS consortium in Europe preferred SnAg3.8Cu0.7 for reflow soldering and SnAg3.8Cu0.7Sb0.25 for wave soldering. SOLDERTEC lead-free roadmap in Europe recommends alloy range SnAg(3.4-4.1)Cu(0.45-0.9) for reflow and wave soldering.

The SnAgCu family is the alloy of choice for all regions of the world at present. The true eutectic composition has been argued to be within the range SnAg(3.5-3.8)Cu(0.7-1). NIST has defined the true eutectic composition as SnAg3.5Cu0.9 .

In Japan 2/3 of companies use SnAgCu for reflow and wave soldering. For surface mount reflow SnAg, SnZnBi, SnAgCuBi and SnInAgBi are also used to a lesser degree. For wave soldering SnCu and SnAg are also used to a lesser degree. About 3/4 of companies use SnAgCu for hand soldering. The predominant SnAgCu alloy in use in Japan is SnAg3.0Cu0.5 and increasing trend elsewhere as well.

Kester is a licensee of ISURF SnAgCu(Bi) patent 5,527,628, Senju-Matsushita SnAgCu(Bi) patent 3027441 and Oatey SnAgBiCu patent 4,879,096.

Below is a reference listing of Pb-free alloys in order of melting point. It is not meant to be an exhaustive list and is not meant to preclude the potential use of other alloys.

* Note – it is generally agreed that all of these SnAgCu alloys melt at ˜217 0 C, but published melting range for each alloy varies; the indicated melting range is estimated from NIST phase diagram; in any case NIST determined there will be =0.1% solid material in any of these alloys at 220C.

Alloy Material Cost vs Sn63Pb37*

* Note – cost is based on metals market price.

Physical Properties of Lead-free Alloys

SnAgCu(Bi) Alloys

• Higher melt point lead-free alternative. SnAgCu family is electronics industry standard which in most cases has shown equal or greater thermal cycle fatigue resistance than SnPb.
• Higher surface tension and poorer wetting than SnPb.
• Ag provides greater strength but less ductility than Pb.
• Cu reduces the melting point of the solder. Cu improves thermal cycle fatigue resistance. Cu improves wettability. Cu retards the dissolution rate of copper from boards and components into the molten solder during soldering.
• Bi reduces melting point of the solder. Bi improves wettability. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.

SnBi58(Ag) Alloy

• Low melt point lead-free alternative potentially suitable for some consumer electronics. Low melt point precludes its use for applications where operating temperature is close to 138C.
• Large Bi proportion greatly reduces melting point of the solder, but alloy is more brittle. Bi improves wettability, but is somewhat offset by higher oxidation rate. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.
• Small amount of Ag can improve strength and thermal cycle fatigue resistance, assuming the absence of lead.

SnZn(Bi) Alloy

• Moderate melt point lead-free alternative is only slightly higher than SnPb.
• Zn lowers melting point. Zn exhibits high oxidation rate and is susceptible to atmospheric corrosion. High oxidation rate precludes it use for wave soldering. Stencil life or shelf life of solder paste may be reduced due to reactive nature of zinc.
• Bi further lowers melting point. Bi improves wettability and slightly improves corrosion resistance. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.

SnInAgBi Alloy

• Moderate melt point lead-free alternative is lower than SnAgCu.
• Ag provides strength.
• Indium reduces melting point. Indium is a ductile material. In the presence of lead from HASL boards or components indium forms a ternary compound that has a phase change at 114C.
• Bi further lowers melting point and improves wettability. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.