In production, sometimes some customers will come across some customers who require some holes to be plugged, but they cannot be completely filled. The back of the plug hole has a solder mask window, and there is a depth requirement, which is commonly known as "PCB half plug hole". It is understood that such customers want to test these holes, and will drive test probes into the holes. If there is too much ink in the hole, or the hole wall is polluted by ink, it is easy to cause a false open circuit and affect the test result; if the amount of ink in the plug hole is too small, or the hole is not plugged, it cannot meet the requirements of the plug hole.
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, the difficulty in controlling the depth of the plug hole is not that the plug is not enough, but a relatively small plug hole depth and the accuracy of the specified plug hole depth.
At present, there are mainly two types of methods, one is to fill the hole plug with a certain depth, and then not expose the back of the plug hole, and wash off part of the ink through development to achieve a certain plug hole depth; the other is to strictly control the depth of the plug hole, and then Both sides of the plug hole are exposed. These two methods are tested below.
The thickness of the test plate is 2.4mm, and the hole diameter is 0.25, 0.30, 0.40, 0.50mm. Use a flat screen printing machine to plug holes. After the test is completed, make a metallographic section at the plug hole, observe the effect of the plug hole and measure the copper exposure depth through a metallographic microscope.
1. Development parameters control plug hole depth
Test process: pre-treatment→CS surface plugging (full)→pre-baking→silk printing double-sided→pre-baking→exposure (level 11, SS window CS cover oil)→developing→curing→detection
In the case that all the holes are filled, after passing through the developing solution, the plugging ink on the window opening surface of the solder mask will be washed away by the developing solution and reduced. The development control parameters mainly control the plug hole depth through the development time. Under the normal development time of 80s, the 0.25 and 0.3mm holes can wash away the ink at a depth of 0.5~0.6mm, that is, the depth of exposed copper, while the 0.4 and 0.5mm holes can wash away the ink at a depth of 0.6-0.8mm.
Therefore, the development can wash away part of the ink in the plug hole. Prolonging the development time and increasing the number of times of development can wash away more ink in the hole, but it is difficult to wash out the ink in the plug hole with the same development time or number of times of development, because the developer with a small hole diameter is not easy to interact with the ink, so the hole in the hole The ink depth is large, and the exposed copper part is less.
But 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 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. During this time, different apertures can flush out different inks, and the amount of ink that can be flushed off by small apertures is limited. If the customer requires a plug hole depth of 50% (based on 2.4mm plate thick as an example) is difficult to achieve. Furthermore, accuracy and uniformity are difficult to control.
2. Plug hole parameter control depth
Test process: grinding plate → CS plug hole → pre-baking → SS exposure (level 17, aluminum sheet for plug hole as film) → silk screen double-sided → pre-baking → exposure (level 11, SS window opening CS cover oil) → development → heating solidify.
The plane plug hole machine can control the depth through many plug hole parameters, including the number of plug hole knives, cutting speed, plug hole pressure, etc. The main rules are: the fewer the number of plugging knives, the less the plugging volume; the faster the plugging speed, 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, the same parameters Next, there is less ink in the smaller 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 effect of plug hole with different cutting speed when the plug hole pressure is relatively small.
The test result is that since the inside of the hole has been exposed before developing, the surface of the plate is clean when developing, and the depth of the plugged hole can also be well controlled. It can be seen from the above table that for a hole diameter of 0.25mm, the exposed copper depth can reach 1.65mm; while the 0.3mm hole is around 1.49mm; the 0.4mm hole is in the range of 1.32mm. Figure 4 is a metallographic photo of a PCB half plug hole controlled by plug hole parameters at 0.3mm. To get less ink in the hole, you can make the feed faster. In addition, the pressure of the air source, the size of the aperture of the aluminum sheet, the angle of the scraper, etc. can be adjusted.
The first method relies on development to wash away part of the ink in the hole to achieve the depth control of the plug hole. 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, and control should be considered during production. Depth, and other conditions of the development of the plate surface must be considered, it is difficult to achieve the optimal value at the same time; in addition, the uniformity is relatively poor.
The second method has a long process and is relatively complicated to produce. Before batch production, it is often necessary to make the first plate to confirm whether the depth is appropriate, but there is no need to worry about the surface of the plate being contaminated after development. Two methods are used to produce the shape diagram of the ink in the plug hole. It can be seen that the direct plug hole control has more exposed copper parts at the same depth, which is beneficial to customers' testing. In the production, the author thinks that the two production methods should be selected according to the production output and adjusted appropriately to save costs and improve production efficiency.
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