Ask the Experts
December 19, 2022 - Updated
August 22, 2019 - Originally Posted

Solder Contamination Sources

What are the most common sources of lead-free solder bath contamination?


Expert Panel Responses

The most likely sources of contamination in a solder bath (wave or selective) are metals from the circuit boards and/or components.

For example, if the boards are coated with tin/lead HASL finish and they are being run in a lead free wave solder system, then lead will dissolve from the HASL coating into the solder bath.

If the boards are coated with OSP over copper, then copper will dissolve from the board pads into the solder bath. Component leads are often coated with tin or nickel. Nickel from component leads can dissolve into the solder bath, increasing the overall nickel content.

In some cases the incorrect solder bar may be added to a solder pot by mistake. For example, if tin/lead solder bar is added to a lead free solder bath, then the lead contamination will likely rise over the RoHS limit (0.1% wt).

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

The most common sources of solder pot contamination will typically be material included in the assemblies that you are running across the wave. Copper can be a contaminate at certain levels and minute amounts strip off of each board that goes across the solder pot.

The same goes for gold or silver if you are using it as a surface finish. Cadmium, Zinc, and Aluminum can come of off solder fixtures, holding fixtures, terminals, lugs, etc. Almost anything you run across the wave will strip some material off, although it's normally very small quantities.

Most solder suppliers will provide solder pot analysis as a service to their customers. You take a sample from the solder pot, send it to them, and they do a complete analysis.

In most cases if there is an issue the supplier can recommend custom additions to the pot that will alloy with contaminates and take them off as dross, but in some cases replacement of the solder may be warranted.

Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

It is generally recommended that solder baths containing lead-free alloys be analyzed once every 3-6 months or every 8,000 boards whichever occurs first. Although a complete analysis is typically preformed, a solder analysis is primarily focused on three main metallic contaminates: copper (Cu), lead (Pb) and iron (Fe) as well as other possible contaminants.

The primary source of copper contamination is the dissolution of copper from the circuit board surface finish which forms a copper-tin (CuSn) intermetallic. Copper contamination is tolerable up 1.0% by weight with contamination levels beyond 1.0% making the solder flow sluggish and adversely affect machine performance.

The upper level for lead contamination is 0.1% to ensure RoHS compliance. Despite operating with a lead-free solder bath, lead from tin-lead (SnPb) board surface finishes can leach into the solder pot resulting in lead rich low melting point segments within solder joints resulting in potential cracking and other hidden defects.

Iron contamination is tolerable up to a maximum amount is 0.02% and results primarily from the leaching of machine elements in contact with tin-rich lead-free solder alloys which forms an iron-tin (FeSn2) intermetallic. Levels of iron contamination in excess of 0.02% can potentially result in brittle solder joints and should be avoided.

Carlos Bouras
General Manager
Nordson SELECT
Carlos Bouras is the General Manager of Nordson SELECT and has over 30 years of experience in the electronics manufacturing industry. Carlos's expertise is in process engineering, product development and manufacturing operations. For the past 15 years Carlos has focused specifically on automated assembly issues and is the holder of several US patents for non-contact dispensing and precision dispensing of adhesives for the packaging of microprocessor devices.

We see generally the following while analysing solder
  1. Copper from HASL Solder process running too high will push copper levels up.
  2. Hand Tacking of components with the wrong Solder Wire (often Leaded).
  3. Gold from ENIG process or hard gold process building up in solder.
  4. Copper from OSP finish increasing copper content and decreasing Tin percentages
  5. Leaded Solder put into Lead Free Baths as both machines are sat next to
    each other (happens more often than you would think)

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

You have two major sources. First, your PCB. Due to the different surface finishes available you could add different types trace metals to your bath. For example, boards that have OSP as a surface finish could add copper contamination once the OSP dissolves. Second, the human factor. You could mistakenly add the wrong alloy to the solder pot therefore contaminating the process with lead.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

Dross management can alter the ratio of tin/silver in a lead free alloy, and the Tin/Lead ratio. Attempting to recycle dross by separating metal from oxide is also a major source of contamination.

Lead free dross contains a higher ratio of tin (tin is less noble than silver). Tin lead dross contains a higher ratio of lead. Both effects can change the alloy content of the solder bath.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions
I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

Reader Comment
For Printed circuit board manufacturing, Is the copper contamination of lead-free solder per weight percentage limit is 1%. Is it same for all types of lead-free solder. Example initial copper percentage in SAC305 is 0.5% whereas SN100C is 0.6-0.7%/ Will this affect the weight percentage limit?
Jeyakrishna Raman, Matric Limited

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