Shelf life means different things in different situations. Post something on the internet and it will last forever. Leave fresh fruit out on a hot day and it won't make it through the afternoon.
Jim Hisert recently wrote a blog post on Solder Shelf Life where he teams up with Eric Bastow to discuss Solder Shelf Life. As they state, Solder Paste has a very defined shelf life because of the flux component. The shelf life of Solder Preforms, on the other hand, is defined by the solder alloy's propensity to form oxides on the surface of the metal.
How do you minimize Solder Shelf Life issues? There are several ways:
1) Order quantities that are reflective of your usage. It is attractive to get a large-volume price break, but you need to be able to use the product when the time comes.
2) Request that the solder preforms be packaged in quantities that you use them. Getting a year's worth of preforms in one bottle may be cheaper, but the constant opening of the jar will only cause the remaining parts to oxidize and become unusable. If you consume 120,000 preforms per year, consider having your order shipped 10,000 pieces per month to assure a fresh supply of material.
3) Store preforms in their original, unopened containers, in a nitrogen dry box.
4) Once you have opened the jar, keep the lid on while it is at the work station. At the end of the day, return the jar to the nitrogen dry box with the lid off so the nitrogen can purge the oxides that may have begun to form.
5) Consider tape & reel packaging.