The adhesive should be selected to be insensitive to temperature and humidity even when exposed to air for a long time. For example, the printing life of some new - type adhesives can reach more than 5 days. And during the printing process, the remaining adhesive material can be stored in a container and used again.
(6) Adhesives that can achieve appropriate bonding strength in a relatively short time and at a relatively low temperature should be preferred. For good adhesives, the curing time and curing temperature are generally within the range of 30 - 40 seconds and 120 - 130 °C. The strength before and after welding should be sufficient to ensure that the components are firmly bonded and have good heat resistance, with enough adhesion to withstand the shearing effect of the solder wave. The temperature should be lower than the temperature at which the circuit board substrate and components may be damaged. Usually, it should be lower than the glass transition temperature of the substrate, and this temperature is preferably 75 - 95 °C. If the bonding strength is too high, it will be difficult to repair; if it is too low, it will not play a fixing role.
(7) It should be fully cured for the first time as much as possible. There should be no significant shrinkage during curing to reduce the stress on the components. No gas should overflow during curing to prevent pores from absorbing soldering fluxes and other pollutants, which would reduce the reliability of the circuit board.
(8) Comparison of curing methods: For relatively large components, the UV - thermal curing method should be selected to ensure the full curing of the applied adhesive. A typical curing process is UV plus IR radiation curing. The IR curing time of some adhesives can be less than 3 minutes. At the same time, some adhesives cannot be well - cured when heated at a low temperature, so a combined curing process is also required.
(9) After curing, the adhesive no longer plays an active role, but it should not affect the reliability of subsequent processes such as cleaning and maintenance.
(10) After curing, it should have good insulation, moisture resistance, and corrosion resistance, especially moisture resistance in a humid environment. Otherwise, there may be electromigration, leading to a short - circuit.
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