The important properties of adhesives include rheological properties (viscosity, thixotropy, anti - sagging property, trailing property, shelf life / conditions and useful life) and mechanical properties (viscosity, mechanical strength, heat resistance, curing cycle, electrical stability, etc.).
(1) When choosing an adhesive, it is necessary to ensure that it meets environmental protection requirements first. Then, comprehensively consider the three aspects of the adhesive's performance: performance before curing, curing performance, and performance after curing.
(2) Since two - component adhesives need to be mixed in the appropriate proportion within a proper time, which increases the process difficulty, single - component systems should be preferred.
(3) It is advisable to give priority to colored adhesives that are easy to distinguish from the green solder mask and circuit board materials. This is because it is easy to quickly detect missing components, the amount of adhesive, whether the pads/components are contaminated, or the presence of air in the adhesive, which is convenient for process control. Adhesives usually come in red, white, and yellow.
(4) The adhesive should have sufficient viscosity and humidity to ensure that the components are firmly bonded to the circuit board before the adhesive cures. Both usually increase with viscosity. High - viscosity materials can prevent components from moving during the circuit board mounting and transportation processes.
(5) For the printing process, the adhesive should have good anti - sagging property after coating to ensure good contact between the components and the circuit board. This is particularly important for components with a large support height, such as SOICs and chip carriers. Adhesives with good thixotropy usually have a viscosity range of 60 - 500 Pa·s. A high thixotropic ratio helps to ensure good printability and consistent stencil printing quality.
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