Lead Free Solder for Flip Chip
Zhenwei Hou & R. Wayne Johnson
Laboratory for Electronics Assembly &
Packaging – Auburn University
162 Broun Hall, ECE Dept.
Auburn, AL 36489 USA
334-844-1880
johnson@eng.auburn.edu
Erin Yaeger, Mark Konarski & Larry Crane
Loctite Corporation
1001 Trout Brook Crossing
Rocky Hill, CT 06067 USA
860-571-2599
Larry_Crane@loctite.com
Abstract
The electronics industry is moving to replace lead in electronic assemblies. The driving
factors are potential legislation, primarily in Europe, and global market pressures for more
environmentally friendly products. The search for a ‘drop-in’ solder alloy replacement has been
ongoing for more than 10 years and none have been found. The current industry trend is to use
the eutectic (or near eutectic) Sn-Ag-Cu alloy. The melting point of this alloy (~217
o
C) is
significantly higher (34
o
C) than eutectic Sn-Pb (183
o
C). This will impact the assembly process. In
addition, the lead-free alloys have a higher modulus than eutectic Sn-Pb. This may change the
stress distribution during thermal cycle testing, affecting reliability and failure modes. This paper
examines the assembly process for flip chip die with Sn-Ag-Cu solder bumps and initial reliability
testing.
Key Words: Flip Chip, Lead-free, Assembly, Reliability
Introduction
The elimination of lead in electronics
assembly has been discussed since 1990.
Initially, the driving force was a proposed
legislative ban in the U.S. At the time no
solder alloy replacement alloys were
identified and the legislation was dropped
under strong pressure from the electronics
industry. However, increasing restrictions on
hazardous materials in landfills, recycling
requirements and manufacturer
responsibility for products from ‘cradle-to-
grave’ have kept the topic of lead in the
mind of manufacturer’s. Today, with
proposed legislation in Europe and global
competitive market pressures, particularly in
Japan, the elimination of lead in many, if not
all, electronic products appears imminent.
The successful introduction of electronic
products assembled with lead-free solders
belies the arguments that it can not be done.
However, just because some assembly
types and reliability requirements can be
satisfied with lead-free solder, does not
translate to all products and all reliability
requirements. Much work remains to be
done.
One of the first challenges to the
industry was the selection of a replacement
solder alloy. The National Center for
Manufacturing Sciences (NCMS) concluded
in 1997, after a major four year research
effort that there were no ‘drop-in’
replacements for eutectic Sn-Pb [1]. The
International Tin Research Institute (ITRI) [2]
and the National Electronics Manufacturing
Initiative [3] are both recommending the Sn-
Ag-Cu eutectic (or near eutectic) alloy for
reflow solder applications. Momentum does
appear to be building for this alloy selection.
The Sn-Ag-Cu eutectic has a melting point
of ~217
o
C, significantly higher than eutectic
Sn-Pb (m.p. 183
o
C). This will require higher
peak reflow temperatures that may in turn
impact fluxes and flux residue. For flip chip
applications, the interaction between the
underfill and the flux residue may degrade
thermal cycle and thermal shock reliability.
The higher Young’s modulus of the Sn-Ag-
Cu alloy (46 GPa versus 33 GPa for eutectic
Sn-Pb) will also alter the stress distribution
on the assembly, potentially impacting
reliability and failure mechanisms.
This paper examines the assembly
process for flip chip die with Sn-Ag-Cu alloy
bumps and initial thermal shock testing. Die
with eutectic Sn-Pb solder were used for
controls in the experiments.