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 mounted devices. The purpose of fixing surface mounted components on PCB with glue is to avoid falling off or displacement of components under the impact of high temperature wave crest. In general, epoxy resin heat curing glue is used instead of acrylic acid glue (UV irradiation is required).
2、 SMT requirements for patch glue:
(1) Glue shall have thixotropic properties of good opportunity.
(2) No wire drawing.
(3) High wet strength.
(4) No bubbles.
(5) The curing temperature of glue is low and the curing time is short.
(6) It has sufficient curing strength.
(7) Low moisture absorption.
(8) It has good repair characteristics.
(9) Non toxic.
(10) The color is easy to identify and check the quality of glue spots.
(11) Packing. The packaging type shall be convenient for the use of the equipment.
3、 In the dispensing process, process control plays a very important role.
The following process defects are easy to occur in production: unqualified glue point size, wire drawing, glue impregnation pad, poor curing strength, easy chip falling, etc. To solve these problems, we should study the technical parameters as a whole, so as to find a solution to the problem.
(1) Size of dispensing amount
According to work experience, the diameter of glue spot shall be half of the pad spacing, and the diameter of glue spot after patch shall be 1 of the diameter of glue spot. Five times. This can ensure that there is enough glue to bond the components and avoid too much glue dipping the pad. The dispensing amount is determined by the rotation time of the screw pump.
In practice, the rotation time of the pump shall be selected according to the production conditions (room temperature, glue viscosity, etc.).
(2) Dispensing pressure (back pressure)
At present, the dispensing machine uses a screw pump to supply the dispensing needle, and the rubber hose adopts a pressure to ensure that enough glue is supplied to the screw pump. 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 reduce the viscosity and improve the fluidity of glue. At this time, it is necessary to reduce the back pressure to ensure the supply of glue, and vice versa.
(3) Glue temperature
Generally, epoxy resin glue shall be stored in a refrigerator at 0-50c. It shall be taken out 1 / 2 hour in advance to make the glue fully consistent with the working temperature. The use temperature of glue shall be 230c-250c. 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.
If the ambient temperature difference is 50c, 50% of the dispensing amount will change. Therefore, the ambient temperature should be controlled. At the same time, the temperature of the environment should also be guaranteed. When the humidity is small, the glue point is easy to dry and affect the adhesion.
(4) 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 drawing. If the viscosity is small, the glue point will become larger, which may infiltrate the pad. During dispensing, select reasonable back pressure and dispensing speed for glue with different viscosity.
For the adjustment of the above parameters, the SMT processing manufacturer shall follow the method of point and surface. The change of any parameter will affect other aspects. At the same time, the defects may be caused by multiple aspects. The possible factors shall be checked one by one and then eliminated.
In production, the parameters should be adjusted according to the actual situation, so as to ensure the production quality and improve the production efficiency.