Solder paste has 5 active lives: cold storage life in the fridge - generally 6 months; unopened paste return life - generally 2 hours ~ 2 weeks; active life after opening - generally 7 days; active scratch life on the stencil - generally 8 hours or 12 hours; active retention life after printing ~ before oven - generally 2 hours ~ 4 hours. Whichever life exceeds the control period, the activity of the solder paste will be affected.
Loss of solder paste activity can lead to the appearance of solder beads, cold soldering, dummy soldering, short circuits and other defects.
Factors that reduce the activity of solder paste
Solder paste activity is generally reduced due to two factors: heat and flux loss. Heat means that the solder paste is artificially heated, artificially heated during delivery, or improperly protected during delivery so that the paste is heated.
Flux depletion, i.e. the use of recycled solder paste, prolonged use of the paste on the stencil, long residence time after printing and before reflow, etc. can all lead to excessive flux depletion of the paste.
Insufficient solder paste activity can result in the solder melting without effectively reducing the surface tension of the liquid solder, which can easily encapsulate the gas in the solder joint and form a cavity.
In order to maintain the activity of the solder paste, the following points should be noted:
1. Observe the effective life of the solder paste and do not use expired or out-of-date solder paste.
2. Store and transport the solder paste properly to avoid exposing it to too high or too low temperatures.
3. Control the amount of solder paste used appropriately and do not accumulate too much solder paste on the stencil to avoid rapid flux evaporation.
4. Reduce the use of recycled solder paste and try to use fresh solder paste to avoid the introduction of impurities or oxides.
5. Shorten the retention time after printing and before reflow to avoid oxidation due to exposure of the solder paste to air for too long.
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