Understanding the hidden reactions and the importance of profile—part 3
Understanding the
hidden reactions and the
importance of profile—part 3
Dr. S. Manian Ramkumar, Mr. Anand Kannabiran and Ms. Aarthi Baskaran, CEMA at RIT, Rochester NY,
USA; Dr. Viswam Puligandla; Mr. Bjorn Dahle, KIC, San Diego, CA, USA
sn-Ni-cu system—effect of Ni
between Sn, Ni and Cu. The phase diagram
In part one of this three-part se-
coating in pcB on sn-cu interaction
(Figure 1) illustrates the formation of differ-
ries (GS&P 8.6), the important
The reaction between tin (Sn) in the solder
ent IMCs in the Sn-Ni-Cu system.
reflow parameters to be consid-
and copper (Cu) on the pads resulting in the
In a Sn-Pb system, Ni Sn is the typical
3 4
ered when reflow profiling for a
formation of Cu Sn IM phase causes Cu to
6 5
IMC formed when Ni is used as a sur-
given product and component
deplete from the pads, leading to a situation
face finish material on the copper pads.
mix were identified. In part two
called pad erosion. This phenomenon may
Although the IMC formed is stable, the
of the series (GS&P 8.7), other
result in reduction in the strength of the
reaction kinetics is completely changed by
factors, such as board metalliza-
Cu pad-PWB laminate interface leading to
even trace presence of Cu (>0.3% by wt).
premature failures. When using gold (Au)
tion, component finish metal-
Experimental results show that presence of
surface finish, a layer of nickel (Ni) is intro-
lization, mixing of alloys and
Cu results in the formation of Cu Sn sup-
6 5
duced between the pad and the solder alloy.
their effect on the IMC forma-
pressing the formation of the Ni Sn . Such a
3 4
Ni in the pad-finish reacts with Sn in the sol-
tion, were highlighted in detail.
situation arises when Cu is present as a part
der to form Ni Sn . The reaction resulting
3 4
of the solder alloy, which is common in lead-
In this final part, important
in the formation of this IMC is self-limiting
free SAC systems. The results also suggest
ternary systems and their phase
and also acts as a barrier layer for the
that the kinetics of the reaction mechanism
equilibria are discussed.
formation of Cu Sn . The introduction of
6 5
support the formation of [Cu,Ni] Sn , at ex-
6 5
Ni results in an active three-way interaction
tended exposure to high temperatures when
Keywords: Thermal Profiling,
Reflow Soldering, Reflow
Process Window, Process
Parameters, Intermetallic
Compound
Figure 1. Sn-Ni-Cu ternary system phase diagram.
34 – Global SMT & Packaging – September 2008
www.globalsmt.net
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