In order to ensure that the solder paste is suitable for proving the highest performance for PCB assembly it is necessary to ensure that it maintains the required properties. To achieve this it is imperative that the solder paste is stored correctly. It should always be stored in an airtight container to prevent oxidation. The very large surface area of the minute solder spheres, means that oxidation can present a very great problem.
Additionally, the solder must be stored at low temperatures. Not only does this reduce the rate of any oxidation there may be, but it also reduces the rate at which the flux degrades. While a low temperature is imperative, it should not be stored at a temperature below freezing.
In view of the way in which solder paste can degrade, it also has a defined shelf life and it should not be used after its end date. If old solder is used there is a distinct risk of a much higher defect rate, and the cost of any rework incurred would be well beyond the cost of replacing the solder paste.