Unlike conventional THT technology, which inserts components onto a board before soldering, SMT technology mounts components onto a board before soldering. Obviously, the reflow soldering process is used for soldering, and the solder paste can be used to paste the components on the circuit board and transferred to the soldering process; however, for the wave soldering process, the double-sided hybrid assembly and the double-sided assembly are separately assembled (second and third). For the circuit board of the assembly method, since the components are located under the circuit board during the soldering process, it is necessary to fix the adhesive with the adhesive. The adhesive used to hold SMT components is called a patch adhesive .
(1) Method of applying patch adhesive
The usual methods of applying the patch glue to the circuit board are a little drop method, an injection method, and a screen printing method.
· Drip method. This method is simple. Use a needle to draw a drop of the patch glue from the container and apply it to the pad of the circuit board or the soldered end of the component. The drip method can only be operated by hand, the efficiency is very low, and the operator is very careful, because the amount of the patch glue is not easy to grasp, and special care must be taken to avoid the soldering on the pads of the components.
· Injection method. This method can be done manually or automatically using the device. Manually injecting the patch glue, the patch glue is loaded into the syringe, and a certain amount of the patch glue is extruded from the needle tube by the thrust of the hand. An experienced operator can accurately grasp the amount of glue injected onto the board and achieve good results.
A computer controlled dispenser used in mass production. According to the position of the components on the circuit board, the syringe array consisting of needle tubes is used to squeeze the patch glue out of the container by compressed air. The amount of glue is determined by the size of the needle tube, the time and pressure of the pressure.
The function of the dispenser can be realized by the SMT automatic placement machine: the placement head of the placement machine is replaced by the dispensing needle of the internal placement glue, and under the control of the computer program, the patch glue is applied to the high speed one by one. On the pad of the printed board.
· SMD adhesive screen printing method. Printed on the circuit board by screen printing, which is a low cost and high efficiency method, especially suitable for the case where the density of the components is not too high and the production batch is relatively large. The key to note is that the circuit substrate must be accurately positioned on the screen printer to ensure that the patch adhesive is applied to the specified location to avoid contamination of the soldered surface.
(2) curing of the patch adhesive
After applying the patch adhesive, the components are mounted. At this time, the patch adhesive needs to be cured to fix the components on the circuit board. There are many ways to cure the patch adhesive. There are three typical methods:
· Use an electric oven or infrared radiation to heat the board on which the components are mounted for a certain period of time;
· Adding a hardener to the adhesive to cure the adhesive attached to the component at room temperature, or accelerate the curing by increasing the ambient temperature;
· Use UV radiation to cure the patch adhesive.
(3) SMT adhesive coating process in the assembly process
In the production process of the circuit board assembly of the component assembly, the application of the patch glue is one of the important processes, and its relationship with the front and rear processes (a) is the solution of inserting the lead components first and then mounting the SMT components. (b) is a solution for first mounting SMT components and then inserting lead components. Comparing these two options, the latter is more suitable for mass production using automated production lines.
(4) Technical requirements for applying patch adhesive
There are two types of patch adhesives that are cured by light or heat. The technical requirements for coating light-curing and thermosetting patch adhesives are also different. (a) The position of the photo-setting patch adhesive, because the patch adhesive should be exposed at least half of the underside of the component to be cured by light; (b) is the position of the thermosetting patch adhesive because of heating The method of curing, so the patch adhesive can be completely covered by the component.
The size and amount of patch glue should be determined according to the size and weight of the components to ensure sufficient bond strength: only a small amount of patch glue is applied under the small components, and the bulky components can be underneath. Apply multiple drops of glue or apply a larger amount of glue drops; the height of the glue drops should ensure that the components can be touched to the bottom of the components after installation; the glue drops should not be too large, so pay special attention to the placement of components. Squeeze the glue onto the soldered end of the component and the pad of the printed board, causing contamination that hinders soldering