Understanding hidden reactions and the importance of profile in reflow soldering, part 2
element, is an excellent surface finish on
Material Property Effect of addition of Au
Cu pads preventing pad oxidation. The
general coating of Au may vary between
Tensile Strength Increases with Au content up to 5% and then drops
0.5 and 2 microns in thickness. Au, from
Shear Strength Drops after 2%
surface finish on PCBs, diffuses into the
Hardness Increases gradually up to 4% & then rapidly after
solder during the reflow process and forms
Ductility Decreases with Au content & after 8% Au
precipitate within Sn. Owing to the
elongation is negligible
diminutive amounts of Au present the only
intermetallic formed is AuSn
4
.
Voiding Increases after 4% Au content
The AuSn
4
, like other intermetallic
Table 1. Effect of addition of Au on material properties
phases, is brittle in nature and has been
observed to have deleterious effect on the
solder joint reliability, when present at
to preserve the solderability of Cu a thin into contact with molten Sn in the solder,
the interface between the pad and solder.
plating of Au is provided. However, the it forms a terminal intermetallic Ni
3
Sn
4
.
This phase has been reported to interfere
affinity of Cu to Au is such that it quickly The growth of this IM phase stops when the
with the long-term reliability, especially in
diffuses through Au and reaches the top thickness of the phase reaches 3-4 microns.
assisting crack formation and propagation
of the Au layer. In order to prevent the This forms a stable barrier layer between
under fatigue loading. Therefore, the
diffusion of Cu through Au, an Ni layer of the Cu pad and Sn,not allowing the Cu
thickness of the Au needs to be controlled
approximately 150 min is adopted. This Ni to form intermetallic phase as discussed in
during the coating process.
introduction has an effect on the nature of the Sn-Cu system. In the case of Pb-free
The addition of Au to certain
intermetallic formation/growth and hence, solder paste where Ni and Cu are present
extent has been proved to improve
on the solder joint reliability. along with molten Sn, the formation of IM
certain properties of the material and is
discussed in the individual phase diagrams
summarized in Table 1.
Sn-Ni system for Sn-Ni and Sn-Cu would not apply. In
The coating of Au on the Cu involves
Figure 7 shows the Sn-Ni binary phase this situation, it is important to study the
the introduction of a Ni layer between
equilibria. Once the protective layer of Au ternary system of Sn-Ni-Cu system in order
Cu and Au as a diffusion barrier. Since
forms the AuSn
4
precipitate within the to understand the effects of their presence.
Cu is easily oxidized on exposure to air its
matrix, Ni is exposed to react with the Sn. Ternary systems are discussed in detail in
solderability is quickly degraded. In order
When Ni layer coated on the Cu pad comes part three of the series.
In association with
MEPTECPresents
MicroElectronics Packaging and Test Engineering Council Ira A. Fulton School of Engineering
The 3rd Annual MEPTEC
Medical Electronics Symposium
Technology Concepts Enabling Product Reality
September 25, 2008 • Arizona State University • Tempe, Arizona
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GlobalSMT 08 Medical Ad.indd 1 7/15/08 3:59:29 PM
28 – Global SMT & Packaging - August 2008
www.globalsmt.net
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