Practical production uses of SMT adhesives

Adhesives are typically used in surface mount printed circuit board (PCB) assemblies to hold the passive (and sometimes active) components on to the bottom side of the board during wave soldering. The adhesive is used to bond the surface mount device (SMD) to the PCB, thus forming a joint between the SMD body and the solder resist or bare FR-4 board between the solder pads. This is necessary to avoid the displacement of components from their sites during high speed placement of passive components. Dispensing equipment must be able to dispense a precise consistent dot shape with different volumes as appropriate for different device sizes. After the SMD is placed, the wet adhesive must have sufficient "wet" or "green" strength to hold the SMD in position until cured. The hardening process must be fast and simple. The cured adhesive must then have sufficient strength to hold the SMD to the PCB during the solder wave operation. After soldering, the adhesive must not affect circuit operation in any way. Generally, the spacing between the component pads dictates the maximum allowable adhesive dot diameter that can be dispensed between the pads. Typically, the adhesive dot diameter is 2/3 of the pad spacing. Once the allowable adhesive dot diameter has been determined, the appropriate nozzle can be selected. A dual dot nozzle or single dot nozzle should be chosen at this time.