Reducing copper dissolution in lead-free assembly
total process time may be less than a single conventional leads, will dissolve copper experienced in manufacture. Evidence of
pass. as with any soldering process. The solder printed board assemblies featuring similar
During solder levelling it is good alloy, surface finish and temperature are examples should be investigated urgently
practice to monitor the copper dissolution
the main factors affecting copper dissolu- and the production process or materials
rates, the alloy contamination levels, alloy
tion rates. reviewed.
being used; users should make sure that this
During reflow the flow rate of solder Further information on copper
is being conducted during PCB evaluation
across the surface is minimal. Experience dissolution is available from NPL in the
or supplier audit.
with lead-free solders already shows that form of a report on project work: NPL
wetting and flow are limited and less than Report MAT26: Measurements of Copper
Manual soldering/de-soldering
with tin/lead solders. A typical surface Dissolution in Lead-Free Solder Alloys.
Manual soldering and de-soldering will mount joint experiencing one reflow cycle,
dissolve copper from the exposed pad and may have a tin/copper intermetallic of 2-3 acknowledgements
through hole plating. The copper is dis- µm, and intrusive reflow joint in a through We would like to thank the Department
solved as the solder flows across the surface hole termination, may have 3-5 µm. for Innovation, Universities and Skills
of the copper during soldering, further There is little experience of significant (DIUS) for funding contributions to this
copper can be removed when the solder is copper dissolution on reflow soldering. research. We would also like to acknowl-
removed from the surface.
Copper intermetallic formation will vary
edge Milos Dusek for his help in setting up
During manual soldering it is not
based on soldering temperatures, time in
some of the experiments.
common industry practice to pre-heat
a liquid state and the cooling time above
We would also like to acknowledge
boards for either soldering or de-soldering,
liquidus. Reflow and multiple rework
the support and funding of the following
hence the soldering temperatures are higher
operations of joints may have a cumulative
project partners (included in alphabetical
and soldering times are longer than with
effect.
order):
other processes. Often manual soldering/ Artetch Circuits Celestica Cookson
de-soldering is used in rework after mass
copper dissolution, what do you
Electronic Assembly DKL Metals Dolby
soldering and adds to any dissolution that
look for?
Laboratories European Sapce Agency
has already occurred.
The photographic examples are provided
Falcon Metals Henkel Technologies
for reference to aid engineers and produc-
Hewlett-Packard Co Indium Corporation
Reflow soldering tion staff to identify evidence of copper of Europe MBDA (UK) Photomechanical
Reflowing solder paste on pads for surface dissolution. Copper dissolution to the Services (Essex) Ltd Rockwell Collins Seho
mount terminations or inside plated degree shown on the examples would Vitronics Soltec BV.
through holes for intrusive reflow of be considered extreme but are being
www.globalsmt.net Global SMT & Packaging – October 2009 – 17
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