Dispensing process in electronic product packaging production

The plug-and-place assembly process where the through-hole device (THT) and the surface mount (SMT) of the lead element coexist is the most commonly used assembly method in current electronic product production. Throughout the entire production process, one component of the printed circuit board (PCB) has been soldered and soldered since the beginning of the dispensing process. During this period, the interval time is long, and other processes are performed, and the components are cured. It is particularly important, so it is of great significance for the analysis of the dispensing process.

1, glue and its technical requirements

The glue used in SMT is mainly used for wave soldering 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. In general production, epoxy thermosetting glue is used instead of acrylic glue (UV curing is required).

SMT work on the requirements of the patch glue:

1. The glue should have good thixotropic properties;

2. No drawing;

3. High wet strength;

4. No air bubbles;

5. The glue curing temperature is low, the curing time is short;

6. Has sufficient curing strength;

7. Low moisture absorption;

8. Has good repair characteristics;

9. Non-toxic;

10. Easy to identify color, easy to check the quality of the glue point;

11. Packaging. The package type should be convenient for the use of the device.

2, in the dispensing process control plays a very important role

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 study the technical parameters of the entire system and find a solution to the problem.

2.1 Dispensing amount

According to work experience, the size of the glue dot should be half the pad pitch, and the glue dot diameter after the patch should be 1.5 times the diameter of the glue dot. This will ensure that there is sufficient glue to bond the components and avoid excessive glue from dipping the pads. The amount of glue dispensed is determined by the length of rotation 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 with a pressure hose to ensure that sufficient glue is supplied to the screw pump (for example, CAMALOT 5000 in the United States). Back pressure too high can easily cause glue overflow and excessive glue volume; when the pressure is too low, there will be discontinuous glue phenomenon and missing points, which will cause defects. The pressure should be selected according to the same quality of glue and 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 lower the back pressure to ensure the supply of glue, and vice versa.

2.3 Needle size

In practice, 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: for example, the pad sizes of 0805 and 1206 are not much different. The same type of needle can be selected, but different needles need to be selected for the pads with different disparity, which can not only ensure 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 CAM/A LOT 5000). 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 0--50C refrigerator, use it should be taken out 1/2 hour in advance, so that the glue fully with 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 difference of 50C in ambient temperature will cause a 50% change in the amount of glue dispensed. Therefore, the ambient temperature should be 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 curing 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.

2.8 Bubbles

The glue must not have bubbles. A small gas will cause many pads to have no glue; the air at the connection should be emptied each time the hose is replaced, preventing air blow.

The adjustment of the above parameters should be done in the form of points and surfaces. The change of any one parameter will affect other aspects. At the same time, the occurrence of defects may be caused by multiple aspects, and the possible factors shall be examined one by one. exclude. In short, in the production should be adjusted according to the actual situation of each parameter, not only to ensure the quality of production, but also improve production efficiency.