PCB Tech - Discussion on PCB Half Plug Hole Method

This article introduces two different PCB half-plug methods, compares the effects and advantages and disadvantages of these two methods, and provides references for PCB manufacturers' half-plug production processes.

1 Introduction

In PCB board production, sometimes we encounter some customers who require part of the hole to be plugged, but it cannot be fully plugged. The back of the plug hole has a solder mask and has a depth require ment. It is popularly called "PCB half plug hole". It is understood that such customers want to perform tests in these holes and will punch test probes into the holes. If there is too much ink in the hole, or the hole wall is contaminated by ink, it is easy to cause false open circuit, which will affect the test result; if the amount of plugging ink is too small, or the hole is not plugged, the plugging requirement cannot be met.

Therefore, the depth of the plug hole must be controlled during the production process, and the plug hole is made according to the depth required by the customer. From the experience of conventional green oil plug holes, it is known that the difficulty of controlling the depth of the plug hole is not insufficient plug, but a relatively small plug hole depth and the accuracy of the specified plug hole depth. At present, there are two main methods. One is to fill the hole plug to a certain depth, and then the back of the plug hole is not exposed, and part of the ink is washed away by developing to achieve the effect of a certain plug hole depth; the other is to strictly control the depth when plugging the hole, and then Both sides of the plug hole are exposed. The following is to test these two methods.

2 Experimental method

The thickness of the test plate is 2.4mm, and the hole diameter is 0.25, 0.30, 0.40, 0.50mm. Use flat screen printer to plug holes. After the test is completed, a metallographic section is made for the plug hole, and the effect of the plug hole is observed through a metallurgical microscope and the copper exposure depth is measured.

3 Results and discussion

3.1 Development parameters control plug hole depth

Test process: pre-treatment - CS surface plug hole (full) - pre-baking - silk screen double-sided - pre-baking - exposure (level 11, SS open window CS cover oil) - development - curing - detection

In the case that all the holes are filled, after the developer, the ink on the hole plugged in the solder mask window will be washed away by the developer and reduced. The development control parameter mainly controls the plug hole depth through the development time. Under the normal development time of 80s, 0.25 and 0.3mm holes can wash out ink with a depth of 0.5~0.6mm, that is, revealing the depth of copper, while 0.4 and 0.5mm holes can wash out ink with a depth of 0.6-0.8mm. Therefore, the developer can wash away part of the ink in the plug hole. Extending the development time and increasing the number of developments can flush out more ink in the hole, but the same development time or number of times of development can wash out the ink in the hole with a small hole diameter, because the developer with a small hole diameter is not easy to interact with the ink, so the hole The ink depth is large, and the exposed copper part is less.

However, this method has the following problems: Because the ink in the hole is not completely dried, there are a lot of solvents, and these solvents are not dissolved by the developer, it is easy to cause the ink to contaminate the board surface, and these inks are not easy to clean and find. Bring great inconvenience to production. In actual production, the development time is controlled between 80-120s. In this time, different apertures can wash out different inks, and small apertures can wash out inks are limited. If the customer requires the plug hole depth to be 50% (using a 2.4mm plate) Thick as an example) is difficult to achieve. Furthermore, accuracy and uniformity are difficult to control.

3.2 Plug hole parameter control depth

Test process: grinding plate - CS plug hole - pre-baking - SS exposure (level 17, aluminum sheet with plug hole for film) - silk screen double-sided - pre-baking - exposure (level 11, SS window opening CS oil cover) - development - heat Curing.

The plane plugging machine can control the depth through many plugging parameters, including the number of plugging knives, cutting speed, and plugging pressure. The main rules are: the fewer the number of plugging knives, the less the amount of plugging; the faster the plugging speed, the less ink in the hole; the higher the blade height, the smaller the pressure, the less ink in the hole; and the same parameters Bottom, there is less ink in small pores. In this test, it is often necessary to adjust multiple parameters to make the plug hole depth meet the required requirements. Table 1 lists the plug hole effects of different cutting speeds when the plug hole pressure is relatively small.

Table 1 Depth of exposed copper at different cutting speeds (unit: mm)

Cutting speed m/min 0.25mm 0.3mm 0.4mm

10 1.34 1.16 0.89

30 1.51 1.34 1.18

50 1.65 1.49 1.32

The test result is that since the inside of the hole has been exposed before development, the surface of the board is clean during development, and the depth of the plug hole can also be well controlled. As can be seen from the above table, for the 0.25mm hole, the exposed copper depth can reach 1.65mm; while the 0.3mm hole is about 1.49mm; the 0.4mm hole is in the range of 1.32mm. Figure 4 is a metallographic photo of the PCB half plug hole controlled by the plug hole parameter of 0.3mm. If you want to make less ink in the hole, you can make the knife faster. In addition, the pressure of the air source, the aperture size of the aluminum sheet, and the angle of the scraper can be adjusted.

4 Conclusion

The first method relies on developing and flushing away part of the ink in the hole to achieve plugging depth control. The advantage is that the process is simple and the operation is simple. The disadvantage is that the ink often contaminates the board surface during development; the depth is affected by the development parameters, which must be considered in PCB production. To control the depth, it is difficult to achieve the optimal value at the same time because it is difficult to achieve the optimal value at the same time, and the uniformity is relatively poor. The second method has a long process and relatively complicated production. It is often necessary to make the first board before the batch is born to confirm whether the depth is appropriate, but there is no need to worry about the board surface being contaminated after development. Two methods are used to make the ink shape map in the plug hole. It can be seen that the direct plug hole control has more exposed copper under the same depth, which is conducive to customer testing.