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The reason why SMT chip processing components stand up or turn over
SMT chip processing component side standing and flipping means that the component is attached to the corresponding pad of the PCB board, but the component is rotated 90° or 180° laterally, then standing sideways, flipping means that the reverse side of the SMT chip component is facing up and the front side is facing. Down.
What are the reasons for the two phenomena of SMT chip processing components standing sideways and flipping?
1. Wrong setting of component thickness during SMT placement, or not touching the PCB pad and being put down can easily cause side stand or flip.
2. Excessive pressure when picking up the SMT placement machine causes the feeder to vibrate, which will overturn the components in the next cavity of the braid.
3. The vacuum of the suction nozzle of the placement machine is opened or closed prematurely, causing the components to stand sideways or turn over.
4. The suction nozzle of the placement machine is worn or partially blocked, which will also cause the components to stand up or turn over.
5. The serious deformation of the PCB board, and the dent exceeds 0.5mm, which will also cause the SMT chip components to stand up and turn over.
The reasons for side standing and overturning are roughly the above. SMT chip processing plants should pay more attention to these problems to ensure the quality of processing.
The technical method of SMT dispensing
1. Needle size
In practice, the inner diameter of the needle should be 1/2 of the diameter of the dispensing dot. During the dispensing process, the dispensing needle should be selected according to the size of the pad on the PCB: for example, the pad size of 0805 and 1206 is not much different. The same type of needle can be selected, but different needles must be selected for pads that are very different, so that the quality of the glue dots can be guaranteed, and the production efficiency can be improved.
2. The distance between the needle and the PCB board
Different dispensers use different needles, and some needles have a certain degree of stop (such as CAM/ALOT5000). The distance between the needle and the PCB should be calibrated at the beginning of each work, that is, the Z-axis height calibration.
3. The size of the dispensing volume
According to work experience, the size of the glue dot diameter should be half of the pad spacing, 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 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.).
4. 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. If the back pressure is too high, it will cause the glue to overflow and the amount of glue will be too much; if the pressure is too low, the glue will be intermittent and leaking, which will cause 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, the back pressure needs to be lowered to ensure the supply of glue, and vice versa.
5. 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.
6. Viscosity of glue
The viscosity of the glue directly affects the quality of the glue. If the viscosity is high, the glue dots will become smaller, or even wire drawing; if the viscosity is low, the glue dots will become larger, which may stain the pad. During the glue dispensing process, choose a reasonable back pressure and glue speed for glues of different viscosities.
7. Curing temperature curve
For the curing of glue, the general SMT manufacturer has given a temperature curve. In practice, it should be possible to use a higher temperature to cure, so that the glue has sufficient strength after curing.
8. Bubbles
The glue must not have bubbles. A little stingy will cause many PCB pads to have no glue; every time the hose is replaced in the middle, the air at the joint should be evacuated to prevent the phenomenon of runaway.
