Reducing copper dissolution in lead-free assembly
occurred and is very obvious during inspec-
tion; however, other less obvious cases may
have undoubtedly occurred and have not
been noted.
how can pcB specification help?
Printed board specifications exist in the
industry to cover the plated copper and
original copper foil thickness for plated
through hole and single sided boards.
Documents from the IEC and IPC are
available and are often supplemented by
Figure 3. Examples of dissolution could be the failure
customer specific documents. Generally
Figure 5. Reduction of cross-sectional area of thin
of track connections to plated through holes or simply
copper dissolution with lead-free alloy has
copper wires. After changing from tin/lead to lead-
the loss of the copper pad. The printed board surface
free soldering dip soldering there was a noticeable
not been addressed. finish in this case was silver.
reduction in the wire thickness.
Currently minimum copper thickness
in a plated through hole is 20 µm, 18 µm
in a small via or blind via hole. Surface
tracking or plated through holes would be
most affected by the soldering process, via
holes in standard assembly are less likely to
be affected.
It is good practice to independently
check the copper thickness on the PCB,
using microsectioning for example.
Microstructure can be important and this
can be checked with a suitable etched
section.
If the minimum copper thickness Figure 4. Loss of through hole connections where
Figure 6. A related issue is the loss of chip compo-
specified is to be increased then it needs to
the knee/pad of the plated hole has been dissolved
nent plating that could lead to intermittent joints.
be discussed and agreed with the supplier.
during selective soldering with a lead-free alloy. The
During soldering the chip capacitor metallisation
Industry experience suggests that copper
surface pad finish on this board was copper organic
has been leached from the surface of the ceramic.
thickness is not the only circuit board
solder preservative (OSP).
To avoid ensure the correct specification of the
related issue, and engineers need to discuss
components.
the type of copper utilised if different
Monitoring copper dissolution
suppliers are being used to supply printed
during fabrication and assembly
circuit boards.
All soldering processes can potentially
heating for each of these processes rather
Copper dissolution will vary with
reduce the amount of copper on the sur-
than use the solder energy as part of the
different solder finishes like lead-free solder
face of the board or in the plated through
pre-heating process, and hence reduce the
levelling, tin, silver and copper organic
holes. In such areas of thin copper the
solder contact time. Thermal profiling is
solder preservative (OSP). In the case of effects of dissolution will be worse.
recommended to optimise these soldering
nickel gold boards, copper dissolution Copper dissolution rates are affected by
processes.
should not be experienced, since during the following (
The most common assembly process
Figure 7):
steps are listed in order of potential copper
soldering, the joint is formed between • Solder alloy used
dissolution rate, each process requires close
the solder and the nickel layer, and not • Soldering temperature and time
monitoring during production. The first
the copper. Nickel has a much slower • Solder flow rate across copper
four processes normally have a significant
dissolution rate in solder when compared surfaces
volume of solder that moves quickly across
to copper. • Copper type and surface coating
exposed copper surfaces:
Copper can be found in at least three
• Rework solder fountains
different forms on the PCB, electroplated, Based on standard assembly and fabrica-
• Selective soldering processes
reverse plated and electrodeposited. These
tion processes conducted with lead-free
copper types have different dissolution rates.
• Wave soldering
alloys and the typical parameters used in
The work at NPL shows that electroplated
• PCB solder levelling
industry, the processes discussed below are
copper suffers the highest dissolution
• Manual soldering/de-soldering
most likely to have the greatest impact on
rate, followed by reverse treated copper.
• Reflow soldering
copper dissolution rates.
Electrodeposited copper has the lowest
As a general guide every attempt
dissolution rate.
should be made to reduce the soldering
Solder levelled boards using a lead-free
Rework solder fountain
temperature and soldering time. It is more
alloy will have already experienced some
Solder fountains are used for reworking
difficult to reduce the soldering flow rate
copper loss during the levelling operation.
large through hole multi-pin components
in processes like rework, selective soldering,
The process, materials and any potential
like connectors and sockets. A solder
levelling and wave soldering, as this is a
rework of panels during PCB manufacture
fundamental part of the process to improve
fountain is used so that all through hole
should be discussed with the supplier.
drainage and reduce short circuits. Every
terminations can be reflowed simultane-
effort should be made to optimise pre-
ously, when the printed board and
www.globalsmt.net Global SMT & Packaging – October 2009 – 15
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