|Ask the Experts|
January 6, 2021
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.
Vice President, Technical Director
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.
Senior Market Development Engineer
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.
Round Rock Consulting
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.
General Manager - Electronic Assembly Americas
The two biggest problems here are:
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.
Use XRF (x-ray fluorescence) to look for Pb content.
SME Production Technical Excellence Staff
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).Jon Ashton. Vergent Products
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).
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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