Copper dissolution—report and process guide
Bob Willis
Copper dissolution—report
and process guide
Which process is most affected? Alloys giving the biggest
concern? Which soldering parameters give concerns?
The National Physical Laboratory NPL not just impact copper pads on printed
team have now published a comprehensive boards; it can be a potential issue on thin
report on the results of their collaborative copper wire, component terminations and
project into how and why copper dissolves hybrid metallisation. Examples of some
so readily during lead-free assembly. The typical problems previously experienced in
project involved the close collaboration of industry during the introduction of lead-
material and equipment suppliers as well free are provided in the guide also along
as many well known users. To complement with examples of past dissolution issues
the detailed report, the team have also experienced on chip components and hy-
produced a guide. brid circuits. Material from the project and
Having working on the project from other industry examples are featured in the
the start, it’s been great to see the results NPL Defect Database, a practical online
unfold and back up may of the other stud- resource for engineers. The online defect
ies and provide new conclusions on some guide helps to provide a free resource 24/7
of the major problems still to be solved. A at http://defectdatabase.npl.co.uk
copy of the Good Practice Guide “Reduce Excessive copper dissolution can lead
Copper Dissolution in Lead-Free Assem- to intermittent or lack of electrical inter-
bly” and the full report can be obtained by connection on copper tracks or through
emailing Chris Hunt at chris-hunt@npl.co.uk. hole plating; in severe cases this will reduce tion processes conducted with lead-free
During a successful soldering operation reliability. To date, most cases experienced alloys and the typical parameters used in
to a copper surface, a small amount of cop- in industry have been when excessive loss industry, the processes discussed below are
per is dissolved to form a reliable intercon- has occurred and is very obvious during most likely to have the greatest impact on
nection and is perfectly normal. During inspection; however, other less obvious copper dissolution rates.
the soldering operation, copper is dissolved cases may have undoubtedly occurred and As a general guide, every attempt
by tin to form a tin/copper intermetallic, have not been noted. should be made to reduce the soldering
and the amount dissolved is dependent on All soldering processes can potentially temperature and soldering time. It is more
the soldering process, solder alloy, surfaces reduce the amount of copper on the sur- difficult to reduce the soldering flow rate
to be joined, temperature, time and solder face of the board or in the plated through in processes like rework, selective soldering,
flow rate. Using lead-free alloys requires holes. In such areas of thin copper, the levelling and wave soldering, as this is a
higher soldering temperatures and poten- effects of dissolution will be worse. fundamental part of the process to improve
tially longer contact times, and hence the Copper dissolution rates are affected by drainage and reduce short circuits. Every
propensity for higher dissolution of copper. the following: effort should be made to optimise pre-
A typical intermetallic produced with a heating for each of these processes rather
tin/lead solder can range between 1-3 µm. • Solder alloy used than use the solder energy as part of the
In the case of lead-free soldering process • Soldering temperature and time pre-heating process, and hence reduce the
this thickness can increase above 5 µm. • Solder flow rate across copper solder contact time. Thermal profiling is
The intermetallics are themselves soluble surfaces recommended to optimise these soldering
in solder, and hence the overall copper • Copper type and surface coating processes.
dissolution rate is potentially greater. Dis- The most common assembly process
solution during lead-free soldering does Based on standard assembly and fabrica- steps are listed in order of potential copper
4 – Global SMT & Packaging – June 2009 www.globalsmt.net
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