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.
|Melting Point °C
|Plumbing industry standard in USA; good shear strength and thermal fatigue resistance
|Common low cost alternative for wave soldering
|Predominant alloy in Japan
|NIST determined to be the true eutectic
|Mitsui Metal patent
|Matsushita (Panasonic) patent
|Prone to atmospheric corrosion and oxidation
|Prone to atmospheric corrosion and oxidation
*Note – it is generally agreed that all of these SnAgCu alloys melt at ˜217°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 220°C.
Alloy Material Cost vs Sn63Pb37*
|Relative Cost Ratio: Sn63Pb37 = 1
*Note – cost is based on metals market price.
Physical Properties of Lead-free 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.
- 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.
- 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.
- 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.
Lead-Free Reflow Profile Recommended reflow profile for Kester solder paste formulations containing the Sn96.5Ag3.5 and Sn96.5Ag3.0Cu0.5 alloys
K100LD & K100 Case Studies Summary This PowerPoint pictorial examines a successful implementation of K100LD and K100 lead-free alloys at SMT Dynamics and Ayrshire Electronics.
Standard Reflow Profile The recommended reflow profile for Kester solder pastes manufactured with standard tin/lead alloys.
K100 Implementation at SMT Dynamics This paper shows results obtained at a contractor in successful builds using SAC305 for reflow soldering and K100 solder for wave assembly.
Developing a Reliable Lead-free SMT Assembly Process This article offers valuable information to be used during the successful lead-free reflow process implementation.
How do you create a RoHS Compliancy-Lead-free Roadmap? This article describes the various considerations in the transition to lead-free but also RoHS comnpliancy.
How do you ready a solder pot for lead-free solder? This article in designed for those cleaning a leaded solder pot to convert it to lead-free solder. It insures the solder is not contaminated with lead during the switchover.
Implementation of Reliable Lead-free Wave and SMT Processes This article describes the points to consider in transitioning to lead-free wave and reflow soldering.
Lead-free Hand-soldering – Ending the Nightmares Hand-soldering with lead-free solders can be challenging but not if the points in this article are understood.
Lead-free Reliability – Building it right the First Time This article describes the comparison done in reference to lead-free and leaded solder joints. The article also gives insight on the points to improve overall reliability with lead-free solders.
Lead-free SMT Soldering Defects How to Prevent Them This paper shows how avoid soldering defects with lead-free solder pastes. Process control is emphasized to reduce defects.
Lead-free Wave Soldering This article shows the impact to wave soldering with lead-free materials and how to create a reliable defect free process.
Creating Solder Joint Reliability with SnCu Based Solders Tin-copper systems with lead-free wave and selective soldering are a growing option. This paper shows how to implement a reliable process using K100 or K100LD SnCu based solders.
Tin-copper based solder options for lead-free assembly This article details the process changes associated with K100 or K100LD solder systems.
Case Study on the Validation of SAC305 and SnCu Based Solders in SMT Wave and Hand-soldering This paper describes the work done at a contract assembler who implemented a successful process to build over 500,000 assemblies using both Kester K100 and SAC305 solders.