Mechanical behavior of lead-free solder materials for advanced automotive microelectronic packaging

Richard Chromik, Richard P. Vinci and Michael R. Notis

Department of Materials Science and Engineering, Lehigh University

Introduction

Environmental concerns have led to a move toward lead-free solders for electronic packaging. At the same time, as package and board packing density increase to accommodate ever-decreasing size requirements, increased power density and use conditions (such as in automotive under-the-hood applications) call for substrate materials and assembly designs that provide for better thermal management. Both of these issues impose serious design and manufacturing issues upon the production of advanced electronic devices and assemblies. The choice of substrates, metallurgical coatings and solders has major effects on thermal matching and resulting creep-fatigue behavior, stability of microstructures, and the formation of reaction products at metallurgical interfaces. The purpose of this investigation is therefore to examine the compatibility of new lead-free solders, such as Sn-Ag-Cu and SN-Ag-Bi based alloys, with new package and board materials; and consequently, to investigate assembly requirements and reliability concerns.

Sample Images

	Shown on the left is an SEM micrograph of nanoindentations in a lead-free solder alloy. The micron marker in the upper left is 10 micrometers long. The indents were made at room temperature.
	The same indents as imaged directly by the Hysitron Triboscope indenter tip. Material pileup can be clearly seen surrounding each indent.

Goals

• investigate nature and kinetics of reactions between selected solders and chip/substrate metallurgies
• characterize mechanical behavior at the local level (nanoindentation) and the global level (shear testing) to relate microstructure to performance
• study oxide formation at soldering temperatures and the interaction of the fluxes during soldering using surface sensitive techniques such as X-ray Photoelectron Spectroscopy

Findings

• Mechanical properties in Sn-Ag-Cu and other lead-free systems have been characterized using a single, consistent technique: nanoindentation.
  
• Several of the intermetallic compounds usually blamed for brittle failure are actually fairly ductile at small scales.
• An indentation size effect has been identified for certain intermetallic compounds.
• Reaction kinetics for Sn-Ag-Cu, Sn-Cu, and Sn-Ag alloys have been characterized.
  

1. R.R. Chromik, R.P. Vinci, S.L. Allen and M.R. Notis, Nanoindentation Measurements on Cu-Sn and Ag-Sn Intermetallics Formed in Pb-Free Solder Joints, J. Mater. Res., 18 (9), 2003, p. 2251.
2. S.L. Allen, M.R. Notis, R.R. Chromik, R.P. Vinci, Microstructural evolution in lead-free solder alloys. Part I: Cast Sn-Ag-Cu eutectic, J. Mater. Res., 19, 2004, pp. 1417-1424.
3. S.L. Allen, M.R. Notis, R.R. Chromik, R.P. Vinci, D.J. Lewis, R. Schaefer, Microstructural evolution in lead-free solder alloys. Part II: Directionally solidified eutectic Sn-Ag-Cu, Sn-Cu and Sn-Ag alloys, J. Mater. Res., 19, 2004, pp. 1425-1431.
4. R.R. Chromik, R.P. Vinci and M.R. Notis, “Application of the Nanoindentation Technique for Characterization of Intermetallics in Pb-Free Solder Joints”, Proceedings of IMAPS Optoelectronics Conference and Workshop, September 2004, Technical publication available at www.imaps.org.
5. R.R. Chromik, D.-N. Wang, A. Shugar, L. Limata, M.R. Notis, and R.P. Vinci, Mechanical Properties of Intermetallic Compounds in the Au-Sn System, J. Mater. Res., 20 (8), 2005, pp. 2161-2172.

Portions of this work were performed collaboratively by Lehigh University, Penn State University, and the Visteon Corporation. Support for Lehigh has been provided by:

• the Pennsylvania Department of Community and Economic Development, Contract No. 20-906-0015
• the U.S. Army Research Office and U.S. Army Research Laboratory, Cooperative Agreement Number DAAD19-02-2-0030.
  

Last updated: August 2, 2005