Common patch glue dispensing process
The coating technology of patch adhesive is varied, and the commonly used methods are offset printing and spot coating:
Point coating process. The so-called dispensing process is to apply the glue to the designated area of ​​the PCB through the dispensing machine. Pressure and time are important parameters for spot coating. They control the size and trailing of the dots. The tailing also changes with the viscosity of the patch glue, and changing the pressure can change the size of the glue dot.
Hanging wires or tailing makes the "tail" of the patch adhesive extend beyond the surface of the base of the component and drags to the next part. The patch adhesive covers the circuit board pad, which will cause poor soldering. Tailing can be reduced by making some adjustments to the dispensing system.
For example: reducing the distance between the circuit board and the nozzle, using a larger diameter nozzle opening and lower air pressure, helps reduce wire hanging. If the dispensing method is pressurized (this is a common situation), any changes in viscosity and limiting flow rate will cause the pressure to drop, resulting in a decrease in flow rate, which will change the size of the dot.
The viscosity of the patch adhesive also plays a role in the formation of hanging wires. For example, patch adhesives with higher viscosity are easier to hang up than patch adhesives with lower viscosity. However, if the viscosity is too low, the amount of glue may be too large. Since the viscosity changes with temperature, the change in ambient temperature may have a significant effect on the amount of glue.
According to data reports: when the ambient temperature meter changes by 5 ° C (from 15 ° C to 20 ° C), the amount of glue dispensed changes by almost 50% (from 0.13 to 0.19 g). All other dispensing variables such as nozzle size, pressure, and time have the same effect. In order to prevent changes in the glue point caused by changes in the ambient temperature, a constant temperature enclosure should be used.
Glue leakage is another common problem in the application of patch glue. The possible causes of glue leakage are blocked nozzles, worn nozzle tips and uneven circuit boards. If the patch glue is left unused for a long time (from hours to days, depending on the patch glue).
The nozzle will usually be blocked. To avoid clogging the nozzle, it should be cleaned after each use, using a wire through the nozzle tip. In addition, higher viscosity may also cause glue leakage.
Offset printing process. The so-called offset printing is to offset the patch to the designated area of ​​the PCB through the screen printing process. Although the offset printing process has similarities with the dispensing process, it belongs to two different production processes. Compared with the latter, the offset printing process has these characteristics:
①It can control the glue volume very stably. For PCB boards with a pad pitch as small as 127 to 254 μm, the offset printing process can easily and very stably control the thickness of the printed offset in the range of 50 μm ± 0.2 μm.
â‘¡Offset printing with different sizes and different shapes can be realized by one printing stroke on the same PCB. The time required to offset a PCB board is only related to the PCB board width and offset printing speed and not to the number of PCB pads. The dispenser is to place glue on the PCB one by one, and the time required for dispensing varies with the number of glue points. The more glue points, the longer it takes to dispense.
Most customers who use offset printing technology are often very experienced in solder paste printing technology. The process parameters of the offset printing technology can be determined by using the process parameters of the solder paste printing technology as a reference point. Next, discuss how the printing process parameters affect the offset printing process.
â‘ Stencil. Compared with solder paste printing, the metal screen used for offset printing technology is relatively thick, generally about 0.2 to 1 mm. Considering that the glue does not have the characteristics of solder paste automatically converging toward the PCB board pads during reflow soldering, the size of the mesh leakage hole should also be smaller, but it is best not to be smaller than the component pin size.
Too much glue will cause short circuit between component pins, especially when the placement machine is difficult to achieve 100% perfect placement accuracy. For PCB boards with small pitch chips, special attention should be paid to the short circuit of chip pins.
â‘¡ Printing gap / scraper. During offset printing, the printing gap of the machine is usually set to a small value (rather than zero) to ensure that the peeling between the stencil and the PCB board follows the printing process of the doctor blade. If zero-gap (contact) printing is used, a smaller separation speed (0.1 to 0.5 mm / s) should be used.
Squeegee hardness is a relatively sensitive process parameter, it is recommended to use a higher hardness blade or metal blade, because the low hardness blade will "hollow out" the offset printing in the screen hole. With a thin template, only when there is a certain printing gap between the template and the PCB can you achieve very high glue points.
During printing, the glue is pressed into the gap between the bottom surface of the template and the PCB. Through the slow separation between the template and the PCB (such as 0.5 mm / s), the glue is pulled out and dropped, depending on the rheology of the glue, resulting in a more or less conical shape.
When using contact printing, the height of the dot is limited due to the relatively small thickness of the template. The scraper will cut off the glue with a large glue point (such as 1.8 mm), so the height is about the same as the thickness of the template. For medium-sized dots (such as 0.8mm), irregular dot shapes may occur because the adhesion to the template and the PCB is almost equal.
During the separation of the template from the PCB, the template lengthens the glue, so the height of the glue dot is greater than the thickness of the template. For the size of 0.3 ~ 0.6mm, due to the better adhesion of the adhesive to the template than the PCB, part of the glue remains in the template. The height of these dots is low and the consistency is very good.
â‘¢Printing pressure / printing speed. The rheology of the glue is better than the solder paste, and the offset printing speed can be relatively higher, but it must not be so high that the glue can not roll at the front of the blade. Generally speaking, the offset printing pressure is 9.8-98.1kPa. The offset pressure should be based on just scraping off the glue on the surface of the screen.
Electroplating letter or plating letter sign has a much stronger coating than normal frabicated letters, both in terms of corrosion resistance and limited degradation. Besides, it can be used in environments where temperatures can change drastically, that is because the metal expands and contracts at the same rate as the coating.
Compared with painted letter sign, electroplating letter is coated evenly both for unique or oddly shaped designs. It is also easier to ensure a relatively even layer of a secondary metal, which is critical when the component has to fit into a system with tight tolerances.
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