▪ Dispensing process is mainly used for the placement and mixing process where the lead component through-hole mounting (THT) and surface mount (SMT) coexist. Throughout the entire production process, we can see that the components on one side of the printed circuit board (PCB) are cured from the beginning of the glue, and then the wave soldering can not be done until the end. This period is longer and other processes are more , The curing of components is particularly important.
Dispensing process is mainly used for the placement and mixing process of THT and SMT.
▪ Process control during the dispensing process. The following process defects are prone to occur in production: unsatisfactory glue dot size, wire drawing, glue dip pads, poor curing strength, easy to fall off, etc. Therefore, the control of various technical process parameters of dispensing is the way to solve the problem.
1. The amount of dispensing
According to work experience, the diameter of the glue dot should be half of the pad spacing, and the diameter of the glue dot after the patch should be 1.5 times the diameter of the glue dot. In this way, you can ensure that there is enough glue to bond the components and avoid too much glue to infiltrate the pad. The amount of dispensing is determined by the length of the dispensing time and the amount of dispensing. In practice, the dispensing parameters should be selected according to production conditions (room temperature, viscosity of glue, etc.).
2. Dispensing pressure
At present, the company's dispenser uses a pressure on the dispensing needle barrel to ensure that enough glue is extruded from the dispensing nozzle. Too much pressure can easily cause too much glue; too little pressure will cause discontinuity in dispensing and leaks, which can cause defects. The pressure should be selected according to the glue of the same quality and the temperature of the working environment. High ambient temperature will reduce the viscosity of the glue and improve the fluidity. At this time, the pressure needs to be reduced to ensure the supply of glue, and vice versa.
3. Size of dispensing nozzle
In practice, the inner diameter of the dispensing nozzle should be 1/2 of the diameter of the dispensing dot. During the dispensing process, the dispensing nozzle should be selected according to the size of the pad on the PCB: for example, the pad size of 0805 and 1206 are not different. Large, you can choose the same kind of needle, but you need to choose different dispensing nozzles for the pads that differ greatly, so that you can not only ensure the quality of the glue point, but also improve production efficiency.
4. The distance between the dispensing nozzle and the PCB
Different dispensing machines use different needles, and the dispensing nozzle has a certain degree of stop. At the beginning of each work, make sure that the stopper of the dispensing nozzle touches the PCB.
5. Glue temperature
Generally, epoxy resin glue should be kept in the refrigerator at 0-50C, and it should be taken out 1/2 hour before use to make the glue fully conform to the working temperature. The use temperature of the glue should be 230C-250C; the ambient temperature has a great influence on the viscosity of the glue. If the temperature is too low, the glue point will become smaller and the wire drawing phenomenon will occur. A 50C difference in ambient temperature will cause a 50% change in dispensing volume. Therefore, the ambient temperature should be controlled. At the same time, the temperature of the environment should also be guaranteed. The small humidity dots tend to dry out and affect the adhesion.
6. The viscosity of the glue
The viscosity of the glue directly affects the quality of the glue. If the viscosity is high, the glue dot will become smaller, or even brushed; if the viscosity is small, the glue dot will become larger, which may infiltrate the pad. During the dispensing process, the glue with different viscosity should be selected with reasonable pressure and dispensing speed.
7. Curing temperature curve
For the curing of glue, the general manufacturer has given a temperature curve. In practice, higher temperatures should be used for curing as much as possible, so that the glue has sufficient strength after curing.
8. Bubble
There must be no bubbles in the glue. A small air bubble will cause many pads to have no glue; the air in the glue bottle should be emptied every time the glue is installed to prevent empty play.
For the adjustment of the above parameters, the changes of any one parameter will affect other aspects in the way of points and faces. At the same time, the occurrence of defects may be caused by multiple aspects, and the possible factors should be checked item by item exclude. In short, each parameter should be adjusted according to the actual situation in production, not only to ensure production quality, but also to improve production efficiency.
