SMT dispensing technology analysis
The coexistence and insertion of the through-hole device (THT) and surface mount device (SMT) of the lead element is the most common method of assembly in current electronic products. Throughout the entire production process, we may see that after one side of the printed circuit board (PCB) has cured from the beginning of the dispensing process, wave soldering can only be performed at the end of the process. During this period, the interval time is longer and other processes are performed. The curing of components is particularly important, and therefore it have great significance for the analysis and analysis of the dispensing process.
1) Glue and its technical requirements
The glue used in SMT is mainly used for the peak process of surface mount devices such as chip components, SOTs, and SOICs. The purpose of fixing the surface mount component on the PCB with glue is to avoid the possibility of causing the component to fall off or shift under the impact of high temperature wave crest. Epoxy thermosetting glues are used in general production instead of acrylic glue (UV-curing curing is required).
SMT work on the requirements of the soldering glue:
1. The glue should have good thixotropic properties;
2. No drawing
3. High wet strength;
4. No air bubbles;
5. The curing temperature of the glue is low and the curing time is short;
6. Has sufficient curing strength;
7. Strong hygroscopicity;
8. Has good repair characteristics;
10. Easy to identify color, easy to check the quality of the glue point;
11. Packaging. The package form should be convenient for the use of the device.
2), Process control plays a very important role in the dispensing process.
Prone to the following process defects in the production: the size of the plastic dot is not qualified, drawing, glue to dip the pad, the curing strength is not easy to drop the film and so on. To solve these problems, we should adjust and study the technical parameters in order to find a solution to the problem.
2.1 Dispensing amount
According to work experience, the size of the glue dot diameter should be half of the pad pitch, and the glue dot diameter after the patch should be 1.5 of the dot diameter. This will ensure adequate glue to bond the components and avoid too much glue from dipping the pads. The amount of glue dispensed is determined by the long and short rotation time of the screw pump. In practice, the rotation time of the pump should be selected according to the production conditions (room temperature, viscosity of the glue, etc.).
2.2 Dispensing pressure (back pressure)
At present, the dispensing machine uses a screw pump to supply the dispensing needle hose, and a pressure is applied to ensure that sufficient glue is supplied to the screw pump (taking the axxon/Jet 6000 as an example). Back pressure is too high to cause plastic spillage and excessive amount of glue; if the pressure is too low, there will be discontinuous glue phenomenon, which will cause defects. The pressure should be selected according to the quality of the glue and the working environment temperature. A high ambient temperature will make the viscosity of the glue smaller and the fluidity become better. At this time, it is necessary to reduce the back pressure to ensure the supply of glue, and vice versa.
2.3 Needle size
In actual work, the inner diameter of the needle should be 1/2 of the diameter of the dispensed dot. During the dispensing process, the dispensing tip should be selected according to the size of the pad on the PCB: if the pad sizes of 0805 and 1206 are not much different, You can choose the same kind of pin off, but for the pad with different disparity, you need to select different needles, which can not only guarantee the quality of the glue, but also improve the production efficiency.
2.4 The distance between the needle and the PCB board
Different dispensers use different needles, some of which have a certain degree of stopping (eg axxon/Jet 6000). The calibration of the distance between the needle and the PCB should be done at the beginning of each work, that is, the Z-axis height calibration.
2.5 Glue temperature
General epoxy resin glue should be stored in the refrigerator at 0--5°C, and it should be taken out 1/2 hour before use to make the glue and the working temperature suitable. The use temperature of the glue should be 23°C-25°C; 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 difference of 5°C in ambient temperature will cause a 50% change in the amount of glue dispensed. Therefore, the ambient temperature is controlled. At the same time, the temperature of the environment should also be guaranteed, and the moisture content of the small gel dots can easily dry out, affecting the adhesive force.
2.6 Viscosity of glue
The viscosity of the glue directly affects the quality of the dispensing. If the viscosity is large, the gel dots will become smaller, even drawing; the viscosity is small and the dots will become larger, which may in turn penetrate the pad. During the dispensing process, the glue with different viscosities should be selected, and reasonable back pressure and dispensing speed should be selected.
2.7 Curing temperature curve
For the reason of glue, the general manufacturer has given the temperature curve. In practice, a higher temperature should be used as far as possible to cure, so that the glue has sufficient strength after curing.
The glue must not have bubbles. A small air bubble will cause many pads to have no glue; the air at the connection should be emptied each time the hose is replaced to prevent air rapping.
For the adjustment of the above parameters, the method should be based on the point and surface. Any change in one parameter will affect other aspects. The production of fiscal time defects may be caused by multiple external factors, and the possible factors should be checked one by one. And then ruled out. In short, in the production, the actual tail should be adjusted to adjust the parameters, not only to ensure the quality of production, but also to increase the production efficiency.