Question: We all have from time to time had solder, be it in paste or wire form, sitting on shelves for what seems like years. I know there are all types of variables related to its’ longevity, but given this, what’s really going on with the expiration dates and are there tests you can perform to validate or extend this period?
Answer: This is a great question and we hear this all the time, so lets look at a couple of issues starting first with the variables of storage.
For solder paste the issue is the handling and storage of the paste itself, and the manufacturers cannot guarantee the material will be stored and used as directed. They also don’t know the particular environment where the material will be used and things like humidity, size of container and dwell time being exposed on the stencil will affect the functionality of the fluxes, which will impact their solderability characteristics and their viscosity hindering their ability to be screened correctly. The point being they don’t want to be liable for failures in the field due to old material.
As for the cored wire solder, similar storage issues apply, but more to the point, the real concern is the changing activity of the flux in the core due to its corrosive or acidity properties working on the solder wire itself. This reduces the activity levels of the flux and therefore the wire solder may not perform as well as when it was manufactured and is why they have placed expiration dates on the material.
So given all of this where do we end up? Is it cut and dry that once solder has gone past its shelf life it becomes unusable? It is not really cut and dry until you contact your supplier and have the conversation with them. The solder manufacturers are always working to improve their materials and chemistries and they may have guidelines to help you make a determination before you toss the product in the can, i.e. dispose of appropriately of course.
Can I just add new flux as a work around to extend the solder product life? Although you may think so, this is not really a good idea. Adding flux is literally like pouring acid over the product and if the flux is not heated to soldering temperatures it will remain active on the surface of the board and could result in current leakage and create dendritic growth on the laminate material or between component leads which will cause failures.
Ok, so is there a test I can perform to determine if my solders are good enough to use? The best thing to do is return the material to the supplier and have them re-qualify it for you. They always recommend that we should buy solders in smaller quantities and more often to avoid this whole expiration issue, but even the best laid plans will put you in the position of asking this question again.