Ask the Experts
January 13, 2023 - Updated
October 22, 2012 - Originally Posted

Solder Paste Alloy Check

We have a production line that runs mainly leaded paste. We accidentally assembled boards using lead-free paste and they went through a leaded profile during reflow. The problem was not discovered in time and I now have mixed product.

Is there any easy method to check which paste has been used?


Expert Panel Responses

If the lead-free solder paste was processed through the leaded thermal profile you may end up with unreflowed solder paste as the temperature may not have been hot enough to reflow all the paste. You could also see some partial wetting of the solder on the terminations.

The difference in melting temperature between the lead and lead-free is 32°C, from 183 to 215°C and since the leaded reflow process is about 220 225°C, this is where the partial reflow may exists.

The easiest method to prevent this from occurring is training and separate control areas where the materials are stored. Don't store the material in the same fridge, buy a lead-free fridge. This forces people to go to a different storage area to get the lead free material.

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.

The industry recommends XRF spectroscopy as a test for lead and other metal constituents in solder joints.Again it is important to insure the XRF is properly calibrated to give meaningful results about the lead percentage or any other element.

If lead-free paste was used the solder joint cosmetics will also be different from 63/37, the joints will exhibit a rougher surface if examined at 10X. The surface coloration is also slightly different. It does take a little comparative assessment but it can be done.

Peter Biocca
Senior Market Development Engineer
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

The best way to see which ones were run with lead free solder paste is to use an XRF analyzer calibrated for sensing Pb (Lead). Scan each board with the XRF tool and separate those that show a no-lead signature.

Visually, you should also notice duller solder joints on the no-lead paste boards.

James DiBurro
Round Rock Consulting
Following a successful 20 year career within world class OEMs and EMS providers including Honeywell, Bull Electronics, IBM, Celestica and Plexus, James founded Round Rock Consulting a business consultancy focused on supporting OEMs and EMS providers with product realization strategies.

The easiest way to check the approximate alloy composition is through XRF (x-ray fluorescence) technology. This equipment can non-destructively analyze the surface of the metal and give you the information that you need.

If you do not have access to an XRF machine, I would suggest trying to find an outside lab that can run some samples for you to determine the solder paste alloy used.

Brian Smith
General Manager - Electronic Assembly Americas
DEK International
Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

The two biggest problems here are:
  • Even a pretty "warm" SnPb profile is just barely above the melting point of the Pb-free alloy, and can result in inadequate reflow. The effects can include poor wetting, reduced joint strength, and insufficient thermal exposure to inactivate flux residues.
  • For BGA components, you cannot guarantee full mixing of the Pb-free paste and SnPb balls. This will result in the potential for greatly reduced reliability
It's a near-certainty that someone will ask you "then why not just re-reflow them on a Pb-free profile and be done with it?" The problems with this solution are as follows:
  • You would need to determine that all the parts, including the PWB, are compatible with a Pb-free process. They may or may not be.
  • If the unintentionally-Pb-free side was the second side assembled, you'd be subjecting SnPb solder joints on the first side to a third,even hotter reflow. This will undoubtedly affect IMC layer thickness and could have a strong adverse effect on reliability.
In addition to the above, even if you have achieved adequate reflow of all joints, you will have different reliability with the Pb-free alloy than with SnPb. Not necessarily a lot worse, but different and unknown. Rework of these assemblies with the different alloy would have to be done differently, especially for BGAs, so you would have to track them through the facility, and afterward if rework is required after they leave the facility.

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.

There is an instrument called an XRF gun that could detect whether the paste used contains lead or is lead-free. The testing is relatively inexpensive.

Renee Michalkiewicz
General Manager
Trace Laboratories
Renee has been with Trace and an IPC member for 16 years. She has managed all military and commercial PB qualification and conformance testing and training, as well as product qualification and testing in the areas of solder pastes, fluxes, solder masks, and conformal coat. She is the chairman of the IPC Testing and the IPC-J-STD-004 Flux Specification Committees and the Vice Chairman of the Assembly and Joining Committee. She has published more than a dozen papers and presented at numerous electronics conferences.

Use XRF (x-ray fluorescence) to look for Pb content.

Linda Woody
SME Production Technical Excellence Staff
Lockheed Martin
Subject matter expert in the field of electronics assembly and soldering.

Reader Comment
While the XRF test method discussed by the other experts is the gold standard by which other tests are measured, if all you need to know is which boards have leaded alloys applied then there is possibly an easier method (depending on the number of assemblies in question).

I routinely use "Instant Lead Testing Swabs". These swabs consist of a cardboard tube with two glass vials inside which are broken to mix powder and liquid components to generate a liquid that turns bright pink in the presence of lead. They are simple to use and a positive test is easy to present to Management based on the color change. When using a chemical test such as this it is highly recommended that the residues be removed by a capable aqueous wash process.

The test method also doesn't address the root cause of the defect so I agree with the other Experts on having a separate location for the different paste alloys (and possibly flux chemistries).
Jon Ashton. Vergent Products

Reader Comment
With all due respect to the experts, I believe they are over thinking this. XRF is a great tool for detecting minute amounts of an element in a solder joint. In this case, however, you are looking for the presence or absence of a large amount of of an element. Expensive XRF equipment is not needed. As Jon Ashton suggested, lead test strips are perfectly adequate for this job and at a fraction of the cost of purchasing an XRF unit.
David Leeper, Panasonic

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