Lead-free solder standards

In 1999, he was named Chief of the Experts Committee for the NEDO R D into Lead-free Solder Standardization national project. He is also the deputy leader of the JEITA Lead-free Solder Application Roadmap 2002 project and the Japanese representative to the World Summit on Lead-free Solder. 

In spite of a tremendous amount of effort for the search of lead-free C4s, no industry standard has evolved as yet. Commonly cited lead-free solders are Sn-rich alloys for which electroplating processes are available. The most popular lead-free solder among them is Sn3.9AgO.6Cu with a melting temperature of 217°C. This solder is recommended by National Electronic Manufacturing Initiative (NEMI) and has been extensively studied and characterized by NIST. However, applicability of this solder as a C4 material in chips with advanced ILD is not known. 

PEEK is a semi-crystalline polymer, insoluble in all common solvents and can be used at temperatures of up to 300 °C. This is a particular benefit when it is used in applications which involve the use of lead-free solder. The polymer has excellent chemical and mechanical stability. Victrex has achieved success in the Asian market with PEEK, which has been selected by Shod, a Japanese capacitor and miniature rechargeable battery manufacturer and distributor, to replace PPS resin in all the resin mould cases of its PetitCap aluminium electrolytic capacitors. PEEK-HT (high temperature polymer), which maintains its physical and mechanical properties at temperatures 30 °C higher than standard grades, has been introduced by Victrex. 

A cursory look at steady-state creep data (Ref 28, 34-39) for Sn-Pb and lead-free solders suggests that over a wide temperature range (— 55 to 125 °C, or -67 to 257 °F) and under high enough stress, many of the common lead-free solders (except perhaps for eutectic Sn-Bi) creep at similar rates or faster than standard Sn-Pb. Figures 12 and 13 are plots of isothermal steady-state creep data in shear and tension, respectively, for near-eutectic Sn-Pb and lead-free solders of various compositions. The shear data in Fig. 12(a), 12(b), and 12(c) are for test temperatures of approximately 25, 75 and 125 " C (77, 167, and 257 °F). The tensile data in Fig. 13(a) through 13(d) are for test temperatures of approximately — 55, 25, 75, and 125 " C (— 67, 77, 167, and 257 °F). 

The lead-free solder whose composition is nominally 95.5Sn-3.9Ag-0.6Cu will be the focus of the analysis. The material property information presented should be appropriate for slight deviations from this composition. For simplicity, the alloy will be referred to as Sn-Ag-Cu. Select comparisons will be made with standard eutectic lead-tin solder with 2%Ag (62Sn-36Pb-2Ag). For convenience, the lead-tin eutectic material with 2%Ag will be designated as Sn-Pb. 

T. Siewert, S. Liu, D.R. Smith, and J.C. Madeni, Database for Solder Properties with Emphasis on New Lead-Free Solders, National Institute of Standards and Technology Colorado School of Mines, 2002... 

The removal of flux residues from soldered assemblies is more difficult than the removal of solder paste from stencils and misprints. In cooperation with a reputable local university, various lead-free solder pastes were printed onto standard test substrates and then soldered in a reflow oven at the specific temperature profiles of each respective solder... 

Reflow equipment utilized for lead-free PCB assemblies should be flexible, especially in the ability to manipulate and control heating and cooling rates, because temperature control is critical. In general, lead-free soldering suffers from a lack of standardization [53]. 

The manufacturing processes for soldering electronic assemblies were developed over decades as electronics technology advanced. The substitution of lead-free solders for the standard tin-lead alloys has a considerable deleterious effect on manufacturing methods, the entire supply chain of materials and parts, and the environment. The main contributing factor is the higher temperature required to melt and use the lead-free solder alloys. Development and performance testing of... 

The National Center for Manufacturing Sciences (NCMS) backed by a consortium of North American industrial corporations, national laboratories, and academic institutions conducted an evaluation of alternatives to eutectic Sn-Pb solder between 1992 and 1996. Corporate participants included ATT-Lucent, Ford Motor Company, GM-Delco Electronics, GM-Hughes Aircraft, Hamilton Standard, National Institute for Standards and Technology (NIST), Rensse-lear Polytechnic Institute, Rockwell International, Sandia National Laboratories, Texas Instruments, and the U.S. Navy s Electronics Manufacturing Productivity Facility. The goal of the project was to determine whether safe, cost-effective, nontoxic, reliable lead solder alternatives could be identified. With over 79 alloys examined, the study results indicated that while a number of possible lead-free solders were functionally possible, there was no universal drop-in replacement for lead-based solders in electronics applications. This study is discussed in detail in Chap. 17. 

Materials Reliability Division in the Materials Science and Engineering Laboratory of the National Institute of Standards and Technology, Database for solder properties with emphasis on new lead-free solders, release 4.0. (http //www.boulder.nist.gov/div853/lead%20free/solders.html). 

As noted earlier, lead-free alloys require high reflow temperatures compared to eutectic Sn-Pb— with thermal profile plateaus in the range of 180-190°C, which are too high for fluxes formulated for eutectic Sn-Pb solder. The oxidation of rosin in air and the increased polymerization at elevated temperatures result in residues that are very difficult to clean with standard solvents. Flux vehicles selected for use in lead-free solder pastes are dramatically different from those used in eutectic Sn-Pb solder pastes. It was determined that cleaning products that preformed very well with eutectic Sn-Pb technology did not remove some residues left as a result of the new flux vehicle formulated for a lead-free solder [11]. 

Standards will require revision to accommodate the potential for a variety of leaded and lead-free solder alloys. Acceptable test measurements including inspection criteria must be established including shear and tensile testing, wetting angle measurements, visual criteria defining the acceptable surface appearance of the solidified alloy and retraining will be required. 

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