Reducing copper dissolution in lead-free assembly
Reducing copper dissolution
in lead-free assembly
by D. Di Maio, C. P. Hunt and B. Willis
What is copper dissolution ?
During a successful soldering
Copper dissolution is not a new phe-
operation to a copper surface,
nomenon; it has always occurred when
a small amount of copper is
a soldering operation is conducted on a
dissolved to form a reliable
clean copper surface. If copper were not
interconnection and is
dissolved to some extent, a reliable connec-
tion would not be produced. The dissolu-
Figure 1. Microsections of typical solder joints
perfectly normal. During the
tion process at the interface of the solder
formed between a copper plated through hole and a
soldering operation, copper is
and copper forms an intermetallic, and this
through hole pin. (a) tin/lead joint after soldering,
dissolved by tin to form a tin/
(b) lead-free tin/silver/copper joint after soldering.
is the bond between the solder and copper.
copper intermetallic and the
The grey boundary layers on both solders are interme-
The intermetallic itself is soluble in solder,
tallic alloys, and the particles in the lead-free solder
amount dissolved is dependent and hence the copper dissolution process is
are also intermetallics.
on the soldering process, solder
a two-stage process with the copper forming
alloy, surfaces to be joined,
an intermetallic, and then the intermetallic
temperature, time and solder
dissolving in the solder. Factors that change
flow rate. Using lead-free
the intermetallic composition and mor-
alloys requires higher soldering
phology will be dependent on composition
temperatures and potentially
of the solder and copper as well as on the
copper microstructure, and probably the
longer contact times, and
soldering process and the heat delivery and
hence the propensity for higher
solder flow mechanisms. Hence copper
dissolution of copper.
dissolution is dependent on many factors.
A typical intermetallic
Figure 1 shows typical through-hole solder
produced with a tin/lead joints with a component pin in the hole.
solder can range between 1-3
Copper dissolution, when discussed
Figure 2. Dissolution rate of copper wires immersed
µm. In the case of lead-free
in a lead-free context, refers to excessive in SnAg, SnCu and SnPb solder fountain at 275°C
soldering process this thickness
copper loss; 10 µm and above would be Copper dissolution into the solder will also depend
considered excessive, hence the concern
on the solubility of copper in the solder, and this
can increase above 5 µm. The
shown by industry. In Figure 2, the results
will change as more copper dissolves into the solder.
intermetallics are themselves
of an experiment with copper wire show
Hence as the number of boards processed increases,
soluble in solder, and hence
the different dissolution rates in three
so will the copper dissolving into the solder decrease
potentially the overall copper
as the solubility limit is reached.
eutectic solders. This data clearly confirms
dissolution rate is greater. the relative increase seen in dissolution
Dissolution during lead-free with these simple lead-free alloys. Current
subsequent processing irrespective of the
soldering does not just impact
commercial alloys are capable of achieving
solder alloy used subsequently.
copper pads on printed boards;
lower dissolution rates than seen in Figure
Copper dissolution into the solder will
it can be a potential issue on
2, achieving rates as good as or better than
also depend on the solubility of copper in
thin copper wire, component
SnPb, see NPL Report MAT26: Measurements
the solder, and this will change as more
of Copper Dissolution in Lead-Free Solder Alloys.
terminations and hybrid
copper dissolves into the solder. Hence as
Improvements in resistance to copper
metallisation. Examples
the number of boards processed increases,
dissolution have been achieved by alloying
so will the copper dissolving into the solder
of some typical problems small fractions of key elements, the most
decrease as the solubility limit is reached.
previously experienced in notable of which is Ni, but Co has also
industry are provided in this
been used and there are examples of others.
What is the impact to the user?
guide.
The mechanism for this improvement is
Excessive copper dissolution can lead to
still the subject of research. Exposing a
intermittent or lack of electrical intercon-
copper surface to such a modified alloy with
nection on copper tracks or through hole
these additions is hypothesised to stabilise
plating; in severe cases this will reduce reli-
Keywords: Soldering, Copper
the intermetallic. Hence a PCB finished
ability. To date most cases experienced in
Dissolution, Lead-Free in a HASL process with an alloy with a
industry have been when excessive loss has
Ni addition can reduce copper erosion in
14 – Global SMT & Packaging – October 2009 www.globalsmt.net
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