1、 Glue and technical requirements

The glue used in SMT is mainly used for wave soldering process of surface mounted devices such as chip components, SOT, SOIC, etc. The purpose of fixing surface mounted components on PCB with glue for laser steel mesh is to avoid components falling off or shifting under the impact of high temperature wave crest. In general production, epoxy resin heat curing glue is used instead of acrylic acid glue (UV radiation curing is required).

SMT work requirements for patch glue:

1. Glue shall have good opportunity thixotropic property; 2. No wire drawing; 3. High wet strength; 4. No bubble; 5. The curing temperature of glue is low and the curing time is short; 6. Sufficient curing strength; 7. Low hygroscopicity; 8. Have good repair characteristics; 9. Non toxicity; 10. The color is easy to identify, so as to check the quality of glue points; 1. Packaging. The packaging type shall be convenient for the use of equipment.

2、 Process control plays an important role in dispensing

The following process defects are easy to occur in production: unqualified glue point size, wire drawing, glue impregnation pad, poor curing strength and easy peeling. To solve these problems, we should study all technical parameters as a whole to find solutions.

2.1 Size of dispensing amount

According to work experience, the diameter of the bonding point should be half of the pad spacing, and the diameter of the bonding point should be 1.5 times of the diameter of the bonding point after mounting. In this way, it can ensure that there is enough glue to bond the components and avoid too much glue from contaminating the pad. The amount of dispensing is determined by the rotation time of the screw pump. In practice, the rotation time of the pump should be selected according to the production situation (room temperature, glue viscosity, etc.).

2.2 Dispensing pressure (back pressure)

At present, the glue dispenser uses a screw pump to supply glue to the rubber tube of the dispensing needle. A pressure is used to ensure that enough glue is supplied to the screw pump (take CAMALOT5000 as an example). Too much back pressure is easy to cause glue overflow and too much glue; If the pressure is too small, intermittent dispensing and leakage will occur, resulting in defects. The pressure shall be selected according to the glue of the same quality and the working ambient temperature. High ambient temperature will make the viscosity of glue smaller and the fluidity 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 tip should be 1/2 of the diameter of the glue dispensing point. During the dispensing process, the glue 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 different, so the same type of tip can be selected, but for the pads with large differences, different tips should be selected, which can ensure the quality of the glue dispensing point and improve the production efficiency.

2.4 Distance between needle and PCB board

Different dispensers use different needles, some of which have absolute stops (such as CAM/A LOT 5000). The distance between the needle head and PCB shall be calibrated each time, that is, the Z-axis height calibration.

2.5 Glue temperature

Generally, the epoxy resin glue should be stored in a 0-50C refrigerator, and should be taken out 1/2 hour in advance when using, so that the glue is fully consistent with the working temperature. The operating temperature of glue shall 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 is 50C, which will cause 50% glue dispensing adjustment. Therefore, the ambient temperature should be controlled. At the same time, the temperature of the environment should also be guaranteed. The glue spots are easy to dry due to low humidity, which affects the adhesion.

2.6 Viscosity of glue

The viscosity of glue directly affects the quality of dispensing. If the viscosity is large, the glue point will become smaller, even wiredrawing; If the viscosity is small, the glue point will become larger, which may dye the pad. During dispensing, select reasonable back pressure and dispensing efficiency for glue with different viscosity.

2.7 Curing temperature curve

For the curing of glue, the general manufacturer has given the temperature curve. In practice, higher temperature shall be used for curing as much as possible to make the glue have enough strength after curing.

2.8 Bubbles

The glue must not have bubbles. A small gas will cause many pads to have no glue; Every time the rubber hose is replaced midway, the air at the connection shall be drained to prevent the occurrence of air strike.