In production, sometimes encounter some customers, require part of the hole plug hole, but can not be fully plugged, the back of the plug hole solder window, and depth requirements, commonly known as "PCB half plug hole". It is understood that such customers want to test in these holes, and will drive the test probe into the hole. If there is too much ink in the hole, or the hole wall is polluted by ink, it is easy to cause false open circuit and affect the test result; If the amount of ink is too little, or do not plug the hole, and can not meet the requirements of the plug hole.
Therefore, it is necessary to control the depth of the plug hole in the production process and make the plug hole according to the depth required by the customer. From the conventional green oil jack experience, the difficulty of controlling the depth of the jack hole is not that there is not enough plug, but a relatively small plug hole depth and the accuracy of the specified plug hole depth. At present, there are mainly two kinds of methods, one is to fill the hole or a certain depth, and then the back of the hole is not exposed, and part of the ink is washed off through development to achieve a certain depth of the hole; The second is to strictly control the depth of the plug hole, and then expose both sides of the plug hole. The following two methods are tested.
2. Experimental methods
The thickness of the test plate is 2.4mm, and the aperture is 0.25, 0.30, 0.40, 0.50mm. A screen printing machine is used to plug holes. After the test was completed, metallographic sections were made at the hole, and the effect of the hole was observed and the copper exposure depth was measured by metallographic microscope.
3. Results and discussion
Development parameters control the jack depth
Test procedure: pre-treatment →CS side jack (full) → Pre-drying → double screen printing → Pre-drying → exposure (11 grade, SS window opening CS cover oil) → development → curing → detection
In the case that all holes are filled, after the developer, the solder resistance window face plug ink will be washed away by the developer and reduced. The development control parameters mainly control the plug hole depth through the development time. The experimental results are shown in Figure 1. At the conventional development time of 80s, 0.25mm and 0.3mm holes can wash away 0.5-0.6mm depth of ink, that is, the depth of copper is exposed, and 0.4mm and 0.5mm holes can wash away 0.6-0.8mm depth of ink. Therefore, the development can wash away part of the ink in the jack hole. The extension of development time and the increase of development times can wash away more ink in the hole, but the same development time or development times of small aperture plug hole ink is difficult to wash away, because the developer is not easy to interact with the ink with small aperture, so the ink depth in the hole is large, and the 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 pollute the ink surface, and these inks are not easy to clean and find, which brings great inconvenience to the production. In actual production, the development time is controlled between 80-120s, in this time, different apertuses can wash off the ink is different, small apertuses can wash off the ink is limited, if the customer requires the plug hole depth of 50% (to 2.4mm plate thickness as an example) is difficult to achieve. Moreover, accuracy and uniformity are difficult to control.
Parameter control depth
Test procedure: grinding plate →CS plug hole → pre-drying →SS exposure (17 grade, plug hole aluminum sheet as film) → double screen printing → Pre-drying → exposure (11 grade, SS window opening CS cover oil) → development → thermal curing.
The depth can be controlled by a number of plug parameters, including the number of plug cutters, cutting speed, plug pressure, etc. The main rules are as follows: the fewer the number of holes, the less the number of holes; The faster the cutting speed of the jack hole, the less ink in the hole; The higher the height of the scraper, the lower the pressure, the less ink in the hole; In addition, under the same parameters, the ink in the small aperture hole is less. In this test, it is often necessary to adjust several parameters to make the jack depth reach the required requirements. Table 1 lists the hole effect at different cutting speeds when the hole pressure is relatively small.
Table 1 Copper exposure depth 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 results show that because the hole is exposed before development, the surface of the board is clean and the depth of the plug hole can be well controlled. As can be seen from the above table, for the aperture of 0.25mm, the copper exposure depth can reach 1.65mm; The 0.3mm hole is about 1.49mm; The 0.4mm hole is in the 1.32mm range. Figure 4 shows a 0.3mm PCB semi-jack metallographic photo controlled by jack parameters. In order to make the hole less ink, you can make the knife faster. In addition, it can adjust the air source pressure, aluminum sheet aperture size, scraper Angle, etc.
The first method relies on development to wash off part of the ink in the hole to achieve the plug hole depth control, the advantage is simple process, simple operation, the disadvantage is that there is often ink pollution during development; The depth is affected by the development parameters, and it is difficult to reach the best value at the same time because the control depth and other conditions of the development of the board should be considered in production. In addition, the uniformity is relatively poor. The second method, the process is long, the production is relatively complex, and the first plate is often needed to confirm the appropriate depth before the batch, but there is no need to worry about the surface being contaminated after development. Two methods were used to make ink shape diagram in plug hole. It can be seen that direct jack control exposes more copper at the same depth, which is conducive to customer testing. In the production, the author thinks that two production methods should be selected according to the production output and adjusted appropriately to save costs and improve production efficiency.