This depends on the type of solder paste as well as the manufacturer. For our No Clean pastes we typically get around 4-6 month shelf life at room temperature. On Water Soluble pastes we get a little less shelf life.

What you probably will run into is that paste thickening up over time which is attributed to the flux medium reacting with the solder powder. You might also see some separation of the flux.

The shelf life of the solder paste is established by the Manufacturer based on their internal studies. The expiration date should be either on the label or the paperwork that came with the shipment.

Depending on vendor, the most likely shelf life will be 3 or 6 months at room temperature. However, many vendors will request the syringed solder paste be refrigerated to extend the shelf life.

Classic symptoms of expired paste or paste that has changed significantly from its original properties include: Significant Separation of flux and powder, Increased viscosity (leads to difficult dispensing), and in some cases large air bubble formation in the syringe.

Most of the latest syringe samples that I have encountered have an expiration date stamped next to the manufacturing date. But I don’t if that refers to chilled or room temperature.

Typically when the syringe goes bad, you can see a change in the color uniformity of the solder paste, you might see some discoloration near the open end. If you fail to cap the syringe, the solder paste will start to dry out as the solvents are wicked out thru the needle.

The shelf life of our solder pastes is product specific. Most of our pastes have a shelf life of 4 - 9 months, if stored between 5ºC and 10ºC (41ºF - 50ºF).

When the paste is stored at room temperature and <55% relative humidity, it will have a shelf life of approximately one week. Never open a cold syringe of solder paste as this can introduce moisture into the paste which will have adverse effects on the material.

The syringe should remain sealed until it has had a proper amount of time to thaw (a minimum of 4 hours is highly recommended). You should also make sure the syringe is being stored vertically, tip side down. This will help prevent paste separation.

As a general rule, paste that has been in use for more than 8 hours should be disposed of. Paste which has been on the printer for up to 4 hours can be stored at room temperature for up to 24 hours before being re-used.

Always store used paste in a separate container to prevent contamination of the unused paste. Do not mix fresh paste with used paste unless adding more to the printer itself. 1

The first identifiable defect you will see is more solder balls after reflow, followed by deterioration in dispense quality. After that, there may be possible separation of the flux in the syringe and finally, blocking of the dispense equipment.

A simple solder balling test can quickly determine the condition of solder paste after prolonged use. Simply print a small disk of paste (around 4-5mm dia) onto a non-wettable substrate and re-flow as normal. A single solder ball in a clear pool of residue indicates good coalescing ability. A large number of solder balls remaining in the flux residue pool could be an indication of poor coalescing ability and the paste may be unfit for use. 1

1. Taken from the Henkel Solder Paste Handling Guidelines technical bulletin