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Contaminated Joints Cause an Assembly to Fail RoHS Compliance
Can a few contaminated solder joints cause an entire assembly to fail? Some of the joints were cross-contaminated with leaded solder.
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Contaminated Joints Cause an Assembly to Fail RoHS Compliance
Can a few contaminated solder joints cause an entire assembly to fail RoHS compliance? Let's say I have 400 lead-free components on a board. All were processed with lead-free paste, but some of the joints were cross-contaminated with leaded solder during touch-up.

All of the reworked component joints were tested with an XRF gun. Five joint locations failed. A few were in the yellow zone (around 950ppm), the remaining were in the acceptable range. If I could calculate that the combined solder weight that I added to this assembly was below 1000ppm of lead, would it pass?
Expert's Panel Responses
The maximum ppm value for Lead is 0.1% by weight or 1000 ppm.  If you calculated that the added Lead gives you a value below 1000 ppm then it is RoHS compliant.
Edithel Marietti
Senior Manufacturing Engineer
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.
The answer to this question depends upon the RoHS definition of a homogeneous material.

"Homogeneous material" means one material of uniform composition throughout or a material consisting of a combination of materials, that cannot be disjointed or separated into different materials;  by mechanical actions such as unscrewing, cutting, crushing, grinding and abrasive processes.

It is my understanding that the RoHS restriction applies to each solder joint separately. Theoretically each solder joint could be removed and tested individually. If the lead concentration of even one solder joint violates the limit, then the entire circuit assembly fails RoHS compliance.
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.
From my understanding, the weight of the lead is based upon the weight of the solder on each individual component not the entire board. Unless the rules have changed, one component with leaded solder is enough to fail the lead free requirements of the Directives. If this was an acceptable alternative then we could add non metallic weights to the product so the lead weight is less than .1 % of the whole. Remember, dilution is not the cure for pollution.

This is why the boards have to be identified and the people working on the boards have to be trained to use the proper material when conducting work on lead free boards. This also has to be documented on the material declaration sheets which highlight the leaded materials.
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.
1000 ppm is only for unintentional contamination.

If the joints were deliberately retouched with SnPb, the acceptable level is 0 ppm.
Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
If you regard all of the solder as one separable component of the assembly, I guess you could conclude that the assembly passes, since the level of Pb in that component is below the limit. The problem that could arise is that someone tests in the area that was repaired, and sees much higher concentrations.

So there is some risk.One mitigation to look at is re-reworking the areas that have been contaminated. By wicking most of the contaminated solder off and re-soldering with the proper alloy, you could reduce the contamination by a factor of 10 or so.

Also some words of caution on the XRF "gun" results. These guns "see" a very wide window, and they are averaging what they see within that window. The levels of Pb in some joints within the field of view will almost certainly be much higher than what you are reading.
Fritz Byle
Process Engineer
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.
For better or worse it does not work that way. Every solder joint on a given assembly must be RoHS compliant in order for the whole assembly to be considered compliant.
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.
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