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October 7, 2016

Copper Dissolution Rate

Does anyone have data on the rate of copper dissolution in solder? Have any studies been done on time/temperature and amount of copper. It's easy to claim that SnPb dissolves Cu but everybody has a different opinion on the rate and amount.

B.W.

Experts Comments

You're right... everyone seems to have a different number for the rate. This is because the test methods used vary so widely. There are multiple factors that affect the dissolution rate:
  • The solder alloy (we'll ignore this here since we are talking specifically about SnPb)
  • The solder temperature
  • The solder contamination levels (how much copper is already dissolved?)
  • The solder flow, including flow velocity and turbulence
  • The copper foil type (microstructure of the copper has a measureable effect)
Several different test methods have been presented in the past decade, including surface trace dissolution, wire dissolution, and pad dissolution. All are workable, though they may lead to systematic differences in results. As an example, Hunt, et. Al (APEX 2009) reported a figure of 1.2um/s (0.047 mils/s) for SnPb solder at 255C, whereas my work with Denis Jean and Dale Lee reported at SMTAI 2006 found a rate of 0.89um/s (0.035 mils/s) for a temperature of 250C.

Both rates were determined using foil dissolution from a PWB coupon, ours using a trace, and Hunt's using a pad. The rate differences found were most likely due mostly to flow, but also may have been influenced by foil type.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
The phenomenon of copper dissolution has been present with conventional tin-lead (SnPb) solder but to a much lesser degree since the lead within the eutectic alloy functions as an inhibitor. The erosion of copper (Cu) from a printed circuit board (PCB) with lead-free through-hole (TH) soldering can be a major quality concern and has undergone major investigation with notable studies conducted by Celestica and Intel as well as others.

There is a much greater tendency for copper erosion with lead-free alloys since these alloys dissolve as much as two to four times the amount of copper as tin-lead solder.  The rate of dissolution for SAC305 is roughly 4X SnPb and 2X Sn100. An additional reason comes down to the sluggish wetting behavior of lead-free solder alloys and the elongated time and temperature process window that is required for lead-free solder alloys because of the difference in their wetting and flow characteristics.

The presence of copper erosion can be seen across the entire exposed copper surfaces of a printed circuit board (PCB) but the effects of copper dissolution are most severe directly adjacent to the knee of the plated through-hole (PTH) and copper pad. 

It should be noted that the effects of copper erosion are prevalent with all forms of lead-free flow soldering including wave soldering and selective soldering, but are much more pronounced during the post-wave or post-selective rework and repair process especially when performed with a rework machine that has turbulent solder flow.
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Alan Cable
President
ACE Production Technologies
Alan Cable, Past President-Retired, ACE Production Technologies has over 40 years experience in the electronics manufacturing arena. Alan's expertise is high production manufacturing automation, equipment design and process engineering. For the past 25 years Alan has focused specifically on soldering issues relating to component solderability, lead tinning and selective soldering, owning several companies with this focus.

The High Density Packaging Users Group(HDPug) has just completed a copper dissolution study. The final report is still being written but all data is collected. Potentially some of this data may be available to the industry.
Jack Fisher
Chairman of the IPC Technology Roadmap Committee
Interconnect. Technology Analysis Inc.
Jack Fisher is a retired IBM executive, chairman of the roadmap since 1994, and member of IPC Hall of Fame.
The testing is done by dipping a metal cylinder into an infinite amount of solder bath (no significant composition change). The radius reduction rate is used to indicate the dissolution of various elements in the solder bath. In real reflow process, the interfacial IMC growth and the solder joint composition variation due to surface material leaching may lead to a different behavior.
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Eric Bastow
Senior Technical Support Engineer
Indium Corporation
Eric is an SMTA-certified process engineer (CSMTPE) and has earned his Six Sigma Green Belt from the Thayer School of Engineering at Dartmouth College. He is also a certified IPC-A-600 and 610D Specialist. He has an associate's degree in Engineering Science from the State University of New York and has authored several technical papers and articles.
Great question on the dissolution rate: I've listed a few places where information is available for you to research. The rate of dissolution is based upon a variety of parameters of which, is the Lead-Free alloy used, the temperature of the wave and the type of copper being dissolved be it either copper foil or electroplated copper for the plated through holes and such.

See more info at:
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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
There are many factors which effect the rate of Cu dissolution such as solder flow rate, alloy type, copper type and of course soldering temperature and contact or dwell time. 

As stated it was known that SnPb did dissolve copper, although at a low rate, and with the introduction of SAC alloys to address RoHS it was found these alloys had a much higher rate of copper dissolution than SnPb.  There has been several studies performed on copper dissolution especially in rework mini-wave applications. 

Hamilton and Snugovsky of Celestica and Kelly of IBM  published a paper titled "A Study of Copper Dissolution During Lead Free PTH Rework Using a Thermally Massive Test Vehicle", SMTAi 2006 which shows the comparison between copper dissolution rates of SnPb, approximately  0.007mils/sec and SAC, approximately 0.035mils/sec for rework process soldering temperatures.

It was also found for lead-free alloys, the SnCu nickel modified alloy, SN100C has a copper dissolution rate similar to SnPb. The paper can be found on the SMTA website,www.smta.org.
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Mike Scimeca
President
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.
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