The dispensing process is mainly used in the mixed placement process where the through-hole insertion (THT) and surface mount (SMT) coexist. In the whole production process, we can see that one of the components of printed circuit board (PCB) can be soldered by wave soldering from the beginning of dispensing and curing to the end. During this period, the interval is long, and there are many other processes, so the solidification of components is particularly important.
The dispensing process is mainly used in the mixed placement process where the through-hole insertion (THT) and surface mount (SMT) coexist.
Process control in dispensing process. The following process defects are easy to appear in the production: unqualified glue spot size, wire drawing, glue dip pad, poor curing strength and easy chip dropping. Therefore, it is a solution to control the technical parameters of dispensing.
1. Size of dispensing amount
According to work experience, the size of glue spot diameter should be half of the pad spacing, and the glue spot diameter should be 1.5 times of the glue spot diameter after the patch. This will ensure that there is enough glue to bond the components and avoid excessive glue to contaminate the pad. The dispensing amount is determined by the dispensing time and the dispensing amount. In practice, the dispensing parameters should be selected according to the production conditions (room temperature, glue viscosity, etc.).
2. Dispensing pressure
At present, the company's dispensing machine applies a pressure to the dispensing needle cartridge to ensure enough glue to extrude the dispensing nozzle. If the pressure is too high, it will cause too much glue; if the pressure is too small, it will cause intermittent phenomenon of glue dispensing and leakage, which will cause defects. The pressure should be selected according to the same quality glue and working environment temperature. If the ambient temperature is high, the viscosity of glue will be reduced and the fluidity will be improved. At this time, the supply of glue can be ensured by lowering the pressure, and vice versa.
3. Size of dispensing nozzle
In practice, the inner diameter of the dispensing nozzle should be 1 / 2 of the glue dispensing point diameter. During the dispensing process, the dispensing nozzle should be selected according to the pad size on the PCB: for example, the pad size of 0805 and 1206 is not different, the same kind of needle can be selected, but different dispensing nozzle should be selected for the pad with great difference, which can not only ensure the quality of glue point, but also improve the production efficiency.
4. Distance between dispensing nozzle and PCB board
Different dispensers use different needles, and the dispensing nozzle has a certain degree of stop. At the beginning of each work, make sure that the stop rod of the dispensing nozzle contacts the PCB.
5. Glue temperature
Generally, the epoxy resin glue should be stored in the refrigerator of 0-50c, and it should be taken out 1 / 2 hours in advance to make the glue fully meet the working temperature. The use temperature of glue should be 230c-250c; the ambient temperature has a great influence on the viscosity of glue. If the temperature is too low, the glue point will become smaller and wire drawing will occur. The difference of ambient temperature of 50c will cause 50% change of dispensing amount. Therefore, the ambient temperature should be controlled. At the same time, the environment temperature should also be guaranteed, the humidity is small, the glue point is easy to dry, affecting the adhesion.
6. Viscosity of glue
The viscosity of glue directly affects the quality of dispensing. If the viscosity is high, the glue point will become smaller, even wire drawing; if the viscosity is small, the glue point will become larger, which may dye the pad. In the process of dispensing, 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 the temperature curve. In practice, higher temperature should be used as far as possible to solidify the glue so that it has enough strength after curing.
8. Bubbles
There must be no bubbles in the glue. A small bubble will cause many pads to have no glue; every time glue is filled, the air in the plastic bottle should be emptied to prevent empty hitting.
The adjustment of the above parameters should be carried out from point to surface. The change of any parameter will affect other aspects. At the same time, the defects may be caused by many aspects. The possible factors should be checked one by one and then eliminated. In short, the parameters should be adjusted according to the actual situation in production, which not only ensures the production quality, but also improves the production efficiency.
