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1. SMT patch processing glue and its technical requirements:
The glue used in SMT is mainly used in the wave soldering process of chip components, SOT, SOIC and other surface mount devices. The purpose of fixing the surface mount components on the PCB with glue is to prevent the components from falling off or shifting under the impact of high-temperature wave crests. Generally, epoxy resin thermal curing glue is used in production, instead of acrylic glue (which requires ultraviolet radiation to cure).
Second, the requirements of SMT work for patch glue:
(1) The glue should have good thixotropic properties.
(2) No wire 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) It has sufficient curing strength.
(7) Low hygroscopicity.
(8) It has good rework characteristics.
(9) Non-toxic.
(10) The color is easy to identify, and it is convenient to check the quality of glue dots.
(11) Packaging. The packaging type should be convenient for the use of the equipment.
3. Process control plays a very important role in the dispensing process.
The following process defects are prone to occur in smt production: unqualified glue dot size, wire drawing, glue-impregnated pads, poor curing strength and easy chipping. To solve these problems, various technical process parameters should be studied as a whole, so as to find a solution to the problem.
(1) The size of the dispensing volume
According to work experience, the size of the glue dot diameter should be half of the pad pitch, and the glue dot diameter after patching should be 1.5 times the glue dot diameter. In this way, it is possible to ensure that there is sufficient glue to bond the components and avoid too much glue to impregnate the pads. The amount of glue to be dispensed is determined by the length of time the screw pump rotates.
In practice, the rotation time of the pump should be selected according to the production situation (room temperature, viscosity of glue, etc.).
(2) Dispensing pressure (back pressure)
The currently used glue dispenser uses a screw pump to supply the glue needle and the hose to take a pressure to ensure that enough glue is supplied to the screw pump. Too much back pressure can easily cause glue overflow and excessive glue volume. If the pressure is too small, there will be intermittent dispensing, leaking points, and defects.
The pressure should be selected according to the same quality glue and working environment temperature. High ambient temperature will reduce the viscosity of the glue and improve its fluidity. At this time, it is necessary to lower the back pressure to ensure the supply of glue, and vice versa.
(3) Glue temperature
Generally, epoxy resin glue should be stored in a refrigerator at 0-50C, and should be taken out 1/2 hour before use, so that the glue is fully in line 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 phenomenon of wire drawing will occur.
A difference of 50C 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, and the small glue dots are easy to dry out and affect the adhesion.
(4) Viscosity of glue
The viscosity of the glue directly affects the quality of the glue. If the viscosity is large, the glue point will become smaller, and even wire drawing. If the viscosity is small, the glue dots will become larger, and the pads may bleed. During the dispensing process, choose a reasonable back pressure and dispensing speed for glues of different viscosities.
For the adjustment of the above parameters, the SMT processing manufacturer should follow the point and surface method. 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 should be dealt with one by one. Check and then rule out.
In the production, the parameters should be adjusted according to the actual situation, not only to ensure the production quality, but also to improve the production efficiency.
