Lead-Free Solder

Summary

Solder and solderability are increasingly tenuous links in microelectronics assembly as a consequence of ever shrinking chip and package dimensions, and the movement toward environmentally friendly lead-free solders. We are providing the microelectronics industry with measurement tools and data to address solder issues. A thermodynamic database has been publicly distributed for modeling lead-free solder systems. We also work closely with industry groups on measurement tools needed for development of lead-free solders for use in harsh environments, and provide guidance for adoption of these solders into assembly processes through work with industrial standards organizations.

Description

The U.S. microelectronics industry has clearly articulated the measurement needs for lead-free solders, and for solderability and assembly. For example, the urgency for materials data for lead-free solders has been specified in the 1997 IPC, 1999 International Technology Roadmap for Semiconductors, 2000 and 2002 National Electronics Manufacturing Initiative (NEMI), and 2000 IPC Lead-Free Roadmaps. The pressure from the Japanese consumer product market and from the European Union to produce lead-free microelectronics continues to increase. In addition, the lack of understanding and control of current standard solderability measurements has inhibited the development of improved measurements necessary for new solders and for new packaging schemes. These industrial needs are addressed under various NIST projects, and summaries of the extent of the work can be found below:

Phase Diagrams and Computational Thermodynamics of Solder Systems

A collection of calculated binary and ternary systems that are relevant to solders. The thermodynamic descriptions of these systems are compiled in downloadable database files. Selected papers are available in the Phase Diagram Research Publications section.

Review and Analysis of Lead-Free Solder Material Properties

This report summarizes a review and analysis of material properties and isothermal creep data for precipitate-strengthened lead-free solders. The review focuses on the binary Sn-3.5Ag eutectic alloy and ternary near-eutectic Sn-Ag-Cu alloys of composition close to that of the NEMI-selected Sn-3.9Ag-0.6Cu alloy. The intricacies of the development of constitutive and life prediction models for solder joints of electronic assemblies are also discussed through a review of near-eutectic Sn-Pb properties. This report was prepared by Jean Paul Clech through funding support from the National Institute of Standards and Technology. The need for this critical evaluation of mechanical property data of Pb-free solders was established through the NEMI-NIST-NSF-TMS Workshop on Modeling and Data Needs for Lead-Free Solders.

Collaborations with NEMI (National Electronics Manufacturing Initiative)

NEMI lead-free solder website

NEMI lead-free solder workshop

Database for Solder Properties with Emphasis on New Lead-free
Solders in PDF (1.3 MB)

NIST Recommended Practice Guide on Test Procedures for Developing Solder Data

NIST Special Publication 960-8 documents standardized test procedures that can produce valid and reproducible mechanical-property data for lead-free solders. Such data speeds the application of lead-free solders in high-volume, automated production of electronic assemblies, especially when current production expectations combine high levels of quality with the lowest cost. Use of standardized procedures facilitates the comparison of data between laboratories and permits the combination of data from different sources into a single, comprehensive database.

Mechanical Properties of Intermetallic Compounds Formed Between Tin (Solder) and Copper or Nickel

Three intermetallic compounds (Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄) commonly found in solder joints have been prepared by gas atomization and then consolidated into bulk forms with microstructures similar to those observed in actual joints. Physical and mechanical properties relevant to the performance of joints have been measured for these materials. These data are evaluated in light of previously reported results, appropriate theories, and with regard to their applicability to actual layers in solder joints.