Question: We have some solder paste that has been in a sealed box at 4 degree Celsius for more than 8 months. The shelf life is 6 months. Can it be used without an issues?
If we do decide to use it, what adverse effects might we expect, if any, using solder paste slightly over the shelf life?
Answer: Solder paste consists of three major elements, solder powder of a give size, flux, and a viscosity agent. All three work together to create the rheology of the solder paste. The size of the powder allows the paste to be used for specific size terminations, the flux is selected to reduce the oxides on the materials being soldered and the viscosity agent being able to maintain suspension of the powder within the mass of solder powder within the solder paste mixture. Without the proper viscosity material the solder powder would separate and sink to the bottom of the jar.
The other issue with solder paste is the function of the flux. In plated through hole soldering the flux is used to prepare the component leads and the circuit board prior to soldering. The same applies to flux within the core of the wire solders being used in the industry, however; the flux in the solder paste has an extra function, preparing the solder powder in addition to preparing the component lead and pad area.
Since the flux is in constant contact with the solder powder, it is continually acting on the oxides on the surfaces of the solder powder or spheres, and this action reduces the activity levels of the fluxes. This is also why it is always recommended to refrigerate the solder paste to reduce this chemical reaction with the solder powder and give a longer shelf life to the material.
Whether or not the solder paste can be used, would depend upon the type of flux being used, be it either a low activity, medium activity, or high activity flux. The adverse affect would be un-reflowed solder paste, insufficient reflow, solder balls, dewetting, etc.
The recommendations would be to perform a solderability test, slump test and flux activity test on the solder paste itself and see if it still performs to the original standards of the material.
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