The important properties of adhesives include rheological properties (viscosity, thixotropy, collapse-resistance and tail-dragging, storage life/condition and effective life) and mechanical properties (viscosity, mechanical strength and heat resistance, curing period, electrical stability, etc.)
(1) when choosing adhesives, we must first ensure that they meet the requirements of environmental protection, and then comprehensively consider the properties of adhesives in three aspects: pre-curing performance, curing performance, and curing performance.
(2) because the two-component adhesive needs to be mixed to the appropriate proportion at the appropriate time, increasing the process difficulty, so the single-component system should be preferred.
(3) preferably easy to distinguish with green oil and circuit board materials of non-ferrous adhesives, because you can quickly find whether the missing parts, the amount of glue, whether the pollution of pads/components, empty glue, etc., to facilitate process control; Adhesive colors are usually red, white and yellow.
The adhesive should have enough viscosity and humidity to ensure that the components of the adhesive and the circuit board are firmly bonded before curing. Both typically increase with viscosity, and high viscosity materials prevent components from moving during board mounting and transfer.
(5) For the printing process, the adhesive should have good collapsibility after coating to ensure good contact between components and circuit board, which is particularly important for components with large support height, such as SOIC and chip carrier. Good thixotropic adhesives, the viscosity range is usually 60 ~ 500 Pa· s, high thixotropy helps to ensure good printability and consistent template adhesive quality.
(6) for the printing process, the adhesive should be selected to be exposed to the air for a long time and insensitive to temperature and humidity of the adhesive, such as some new adhesive printing life can reach more than 5 days, and the printing brush process will be the remaining adhesive material stored in the container, can be used again.
(7) those adhesives that can reach the appropriate connection strength in a shorter time and at a lower temperature should be preferred. The better adhesive curing time and curing temperature are generally 30 ~ 40 s, 120 ~ 130 ℃. The strength before and after welding should be enough to ensure the components are firmly bonded and have good heat resistance, and have enough bonding force to withstand the shear action of solder wave. The temperature should be lower than the PCB substrate and components may damage the temperature, usually should be lower than the substrate glass transition temperature, the temperature to 75 ~ 95 ℃ is appropriate. Too strong a connection will cause difficulty in repair, and too small a connection will not play a fixed role.
(8) should be fully cured for the first time as far as possible. There should be no obvious shrinkage during curing to reduce the stress of the components. There should be no gas leakage when curing, so as not to absorb flux and other pollutants, reduce the reliability of the circuit board.
(9) curing mode comparison for wider components should choose UV-thermal curing mode, in order to ensure the full curing of the glue. The typical curing process is UV and IR radiation curing, some adhesive with IR curing time can reach 3 min or less. At the same time, some adhesives can not cure well when heated at low temperatures, so a joint curing process is also needed.
(10) the adhesive will no longer work after curing, but it should not affect the reliability of subsequent processes such as cleaning, maintenance, etc.
(11) After curing, it should have good insulation, moisture resistance, and corrosion resistance, especially moisture resistance in a humid environment, otherwise it may occur electric migration and lead to a short circuit.