The coating technology of the patch glue is various, and the commonly used methods are offset printing and spot coating:
Spot coating process. The so-called spot coating process is to dispense the patch glue to the designated area of ​​the PCB through the dispenser. Pressure and time are important parameters for dot painting. They control the size and tailing of the dots. Trailing also varies with the viscosity of the adhesive, and changing the pressure can change the size of the dots. Hanging or tailing causes the "tail" of the patch to extend beyond the surface of the substrate of the component and drag it to the next area. The patch glue covers the pad of the circuit board and will cause poor soldering. Trailing 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 a lower air pressure, will help reduce the hanging wire. If pressure is applied to the dispensing (this is a common case), any change in viscosity and limiting the flow rate will cause the pressure to drop, resulting in a decrease in the flow rate, which will change the dot size.
The viscosity of the patch glue also plays a role in forming the hanging thread. For example, stickies with larger viscosity are easier to hang than smaller stickies. 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 instrument changes 5 °C (15 °C to 20 °C), the amount of dispensing 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 the change of the glue point caused by the change of the ambient temperature, a constant temperature enclosure should be used.
Leakage is another common problem in the application of patch glue. The possible cause of leakage is that the nozzle is blocked, the tip of the nozzle is worn, and the circuit board is uneven. If the glue is left unused for a long time (from hours to days, depending on the glue). The nozzle will usually be blocked. To avoid clogging the nozzle, clean it after each use and use a wire to connect the tip of the nozzle. In addition, a large viscosity may cause leakage of glue.
Offset printing process. The so-called offset printing is through the screen printing process to offset the patch to the designated area of ​​the PCB. Although offset printing process and dispensing process have similarities, they belong to two different production processes. Compared with the latter, the offset printing process has such features as:
1 It can control the amount of printed rubber very stably. For PCB boards with pad pitches as small as 127 to 254 μm, the offset process can easily and stably control the thickness of the printed paste in the range of 50 μm ± 0.2 μm.
2 It can realize different sizes and different shapes of offset printing on the same PCB through one printing stroke. The time needed to offset a PCB board is only related to the width of the PCB board and the speed of offset printing, and has nothing to do with the number of PCB pads. Dispensers place the glue on the PCB in order, little by little, and the time required for dispensing varies with the number of spots. 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 we discuss how the printing process parameters affect the offset printing process.
1 stencil. Relative to solder paste printing, metal screens used for offset printing are relatively thick, typically about 0.2 to 1 mm. Considering that the glue does not have the property of self-converging to the PCB board pad during solder reflow, the size of the drain hole of the board should also be smaller, but it is better not to be smaller than the component pin size. Excessive amounts of glue will cause short circuits between component pins, especially when the shooter is difficult to achieve 100% perfect placement accuracy. For PCB boards with small pitch chips, special attention should be paid to chip pin short circuit.
2 printing gap / scraper. The print gap of the machine during offset printing is usually set to a small value (instead of zero) to ensure that the peel between the screen plate and the PCB plate occurs after the doctor blade printing process. 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 harder scraper or metal scraper, because the low hardness scraper blade will "hollow out" the offset in the screen's leaky hole. With thin stencils, very high deposits can only be achieved when there is a certain print gap between the stencil and the PCB. The glue is pressed in the gap between the bottom surface of the template and the PCB during printing. Through the slow separation between the template and the PCB (eg, 0.5 mm/s), the glue is pulled out and falls, depending on the rheology of the glue, resulting in a more or less conical shape.
With contact printing, the dot height is limited due to the relatively small thickness of the template. The squeegee cuts off the glue with a large spot (for example, 1.8 mm) so that the height is almost the same as the thickness of the stencil. For medium-sized glue spots (eg, 0.8 mm), irregular dot shapes may occur because the glue adhesion to the template and to the PCB is almost equal. During the separation of the template from the PCB, the template stretches the glue so that the glue dot height is greater than the template thickness. For the size of 0.3-0.6mm, part of the glue remains in the template because the adhesion of the adhesive to the template is better than that of the PCB. The height of these glue spots is low and the consistency is very good.
3 printing pressure / printing speed. The rheology of the glue is better than that of the solder paste. The offset speed can be relatively high, but it must not be so high that the glue cannot roll on the front edge of the blade. In general, the offset pressure is 9.8 to 98.1 kPa. Offset pressure should be just to scratch the surface of the stencil surface glue.
Spot coating process. The so-called spot coating process is to dispense the patch glue to the designated area of ​​the PCB through the dispenser. Pressure and time are important parameters for dot painting. They control the size and tailing of the dots. Trailing also varies with the viscosity of the adhesive, and changing the pressure can change the size of the dots. Hanging or tailing causes the "tail" of the patch to extend beyond the surface of the substrate of the component and drag it to the next area. The patch glue covers the pad of the circuit board and will cause poor soldering. Trailing 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 a lower air pressure, will help reduce the hanging wire. If pressure is applied to the dispensing (this is a common case), any change in viscosity and limiting the flow rate will cause the pressure to drop, resulting in a decrease in the flow rate, which will change the dot size.
The viscosity of the patch glue also plays a role in forming the hanging thread. For example, stickies with larger viscosity are easier to hang than smaller stickies. 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 instrument changes 5 °C (15 °C to 20 °C), the amount of dispensing 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 the change of the glue point caused by the change of the ambient temperature, a constant temperature enclosure should be used.
Leakage is another common problem in the application of patch glue. The possible cause of leakage is that the nozzle is blocked, the tip of the nozzle is worn, and the circuit board is uneven. If the glue is left unused for a long time (from hours to days, depending on the glue). The nozzle will usually be blocked. To avoid clogging the nozzle, clean it after each use and use a wire to connect the tip of the nozzle. In addition, a large viscosity may cause leakage of glue.
Offset printing process. The so-called offset printing is through the screen printing process to offset the patch to the designated area of ​​the PCB. Although offset printing process and dispensing process have similarities, they belong to two different production processes. Compared with the latter, the offset printing process has such features as:
1 It can control the amount of printed rubber very stably. For PCB boards with pad pitches as small as 127 to 254 μm, the offset process can easily and stably control the thickness of the printed paste in the range of 50 μm ± 0.2 μm.
2 It can realize different sizes and different shapes of offset printing on the same PCB through one printing stroke. The time needed to offset a PCB board is only related to the width of the PCB board and the speed of offset printing, and has nothing to do with the number of PCB pads. Dispensers place the glue on the PCB in order, little by little, and the time required for dispensing varies with the number of spots. 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 we discuss how the printing process parameters affect the offset printing process.
1 stencil. Relative to solder paste printing, metal screens used for offset printing are relatively thick, typically about 0.2 to 1 mm. Considering that the glue does not have the property of self-converging to the PCB board pad during solder reflow, the size of the drain hole of the board should also be smaller, but it is better not to be smaller than the component pin size. Excessive amounts of glue will cause short circuits between component pins, especially when the shooter is difficult to achieve 100% perfect placement accuracy. For PCB boards with small pitch chips, special attention should be paid to chip pin short circuit.
2 printing gap / scraper. The print gap of the machine during offset printing is usually set to a small value (instead of zero) to ensure that the peel between the screen plate and the PCB plate occurs after the doctor blade printing process. 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 harder scraper or metal scraper, because the low hardness scraper blade will "hollow out" the offset in the screen's leaky hole. With thin stencils, very high deposits can only be achieved when there is a certain print gap between the stencil and the PCB. The glue is pressed in the gap between the bottom surface of the template and the PCB during printing. Through the slow separation between the template and the PCB (eg, 0.5 mm/s), the glue is pulled out and falls, depending on the rheology of the glue, resulting in a more or less conical shape.
With contact printing, the dot height is limited due to the relatively small thickness of the template. The squeegee cuts off the glue with a large spot (for example, 1.8 mm) so that the height is almost the same as the thickness of the stencil. For medium-sized glue spots (eg, 0.8 mm), irregular dot shapes may occur because the glue adhesion to the template and to the PCB is almost equal. During the separation of the template from the PCB, the template stretches the glue so that the glue dot height is greater than the template thickness. For the size of 0.3-0.6mm, part of the glue remains in the template because the adhesion of the adhesive to the template is better than that of the PCB. The height of these glue spots is low and the consistency is very good.
3 printing pressure / printing speed. The rheology of the glue is better than that of the solder paste. The offset speed can be relatively high, but it must not be so high that the glue cannot roll on the front edge of the blade. In general, the offset pressure is 9.8 to 98.1 kPa. Offset pressure should be just to scratch the surface of the stencil surface glue.
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