Solder paste is composed of metal micro balls and flux. The metal doesn’t change over time much, but the flux does. Any solvent that maintains flux viscosity can ‘dry up’ over time and the total mixture becomes less viscous. This is a problem in a manufacturing environment because it can lead to improper flux distribution along a footprint during the stenciling process (especially if the flux balls up or has an affinity for itself more than what it is being dispensed onto).
This is mostly a problem in large PCB assembly lines.
- If you have any part of the process that doesn’t function well, the total production yield goes down. And you have to spend time hand-working your designs (like with a tombstone SMT part).
- If you were producing quantities above 10000, this could become a problem very fast. So you try to control every aspect of the design to increase the yield. Bad paste isn’t something that assemblers want to deal with. Critical processes, such as medical or aerospace, will also require designers to follow expiration dates on solder.
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