|Ask the Experts|
June 11, 2021
Using Solder Paste Beyond the Expiration Date
We assembled a batch of PCBs using a water soluble paste that was beyond the expiration date. After cleaning we are seeing a white powder residue.
Do you think it is because the paste used was beyond the expiration date, or is it likely to be something else?
|Expert Panel Responses|
The residues could be from a variety of sources and is also dependent upon the alloy of solder being used.
Tin/lead solders could be acted upon by the water soluble flux and the residues could be either tin or lead chlorides. If it is a no lead solder then the results could be tin chlorides. Keep in mind the flux in the paste is always in contact with the solder spheres in the powder of the paste and is reacting with the metal surfaces. When those elements reflow, the non metallic element float to the surface and impact the visual appearance of the solder joint.
If this is a low solid no clean flux, then cleaning off the residues will defiantly leave a whitish residue on the surface of the solder joint.
As for using the material past is expiration date, you could also impact the solderability of the paste and its ability to coalesce when it is reflowed, leaving behind solder balls and incomplete solder paste reflow.
My recommendations are to have the residues evaluated to determine exactly what they are, contact the solder paste manufacturer to get their replacement material and to find out what other customers have found using this exact material.
Vice President, Technical Director
Good question. When solder paste ages beyond its useful life it it usually due to a viscosity increase that makes it unprintable . Flux begins dissolving powder. Water soluble paste flux can form a crystalline precipitate after extended aging. These crystals would leave an insoluble residue on a cleaned board.
Director of Reclaim Business
Alpha Assembly Solutions
It is unlikely that the issue is related to the fact that the paste has expired. It is more likely that it is linked to the cleaning process.
Technical Support Engineer
A white powder suggests to me a metal salt. If the powder is on the board (outside the solder joint) then it is certainly possible that the age of the paste was an issue. Over time, the acids in the flux will react with the metal powders.
The more reaction, the more salts will be formed. These salts could potentially be left behind. From available evidence, I don't think we can conclude with certainty that the age of the paste was the main issue, however we certainly cannot rule it out.
White residue on assemblies using water-soluble pate could be the result from many factors. However, you mentioned that the solder paste was "beyond the expiration date". There is a possibility that the organic flux in the solder paste oxidized thus generating the white residues.
Senior Manufacturing Engineer
It typically is not considered in the expiration date as to cleanability but the solder wetting and reflow conditions that determine expiration dates. The white residue is a visible sign that this paste flux is a bit more difficult to clean and will require some optimization.
If the solder is wetting and reflowing well for the solderjoint then the flux is doing the job required, but the water soluble elements of the paste may not be as easily soluble as they once were.
President/Senior Technical Consultant
White residue from water-soluble paste is normally a corrosion product from the powder and the flux medium. It is a form of tin salt that is created. Many times these are more cosmetic than reliability concerns but from looking at them you do not know. If the paste had been opened and been on and off the stencil this will generate more white residue. Always think one way with paste. Once it comes out of the jar it never is put back with fresh paste. Use older paste first then add new paste.
Try to avoid refrigeration cycles, especially with water-soluble paste as they will be more apt to absorb moisture from condensation. The moisture will make the activators in the paste more ionic and create more corrosion of the powder. Sometimes it will be more pronounced on one type of component. This indicates that the component has a surface finish oxidation issue as well.
Deck Street Consultants
Solder paste flux and solder powder react slowly over time as the paste is stored. The by-products of those reactions are mostly metal salts and these metal salts are often not soluble in water.
When old solder paste is used then it is very likely that some white residue will be left after washing. The potential for white residue grows as the solder paste gets older. In most cases the white residue can be washed off with a commercial cleaner.
Without an image of the problem it is very hard to say, never a good idea to use out of date No Clean Paste so Water Wash is a definite no no and its generally accepted in most cases to have much stronger activators, so it could well be a reaction and a subsequent insoluble salt left behind.
If the white powder is only located around the solder joints then yes this is a reactionary product but if it is spread around the PCB not near the joints then this could be a red herring and something completely different.
Hope it helps.
Technical Sales Manager
BLT Circuit Services Ltd
It is very difficult to connect the two - did you run the same assembly with fresh paste and you've had the same problem? Is the cleaning process effective? What is beyond the expiration date mean? (1 week, 1 month, 1 year). What is the paste supplier's recommendations for this?
The bottom line is try to use solder paste within the expiration date to eliminate a variable off the process.
Everything else is speculation without more information.
Engineering and Operations Management
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