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What is PCB via hole? Why do we have to plug it?


What is PCB via hole?

In most cases, we also call pcb via hole as the conductive hole. It is composed of two corresponding pads on different layers. Usually, we will electroplate it. That's pcb through-hole plating.

Why do we have to plug PCB via hole?

Vias can connect lines to each other and have higher requirements for surface mount technology. Therefore, the PCB industry has derived the Via Plugging process. To meet customer requirements, we must plug the PCB via hole. The traditional approach uses an aluminum sheet to fill the hole and white mesh for the solder mask. Time has proved that this method is stable and reliable.

But PCB Plug Via must meet the following conditions.

1. If there is copper in the via hole, the solder mask can be plugged or not.
2. The tin-lead thickness in the via hole should reach 4 microns. Be careful not to let the solder mask ink enter the hole, otherwise tin beads will appear.
3. Use solder mask ink to plug the hole and not transmit light.

Closer to home, PCB Plug Via has five functions.

1. Prevent tin from flowing into the component surface from the via hole during wave soldering and causing a short circuit. Especially for BGA pads,        plugholes must be made first to facilitate soldering.
2. Avoid the solder resist residue in the pcb via hole.
3. The pcb, which helps to complete the assembly of the components, passes through the testing machine to form a negative pressure.
4. Prevent the virtual soldering caused by the inflow of solder paste from affecting the placement.
5. Avoid the PCB short circuit caused by the ejection of tin beads during wave soldering.

Two kinds of PCB Plug Via Process

The surface mount technology of BGA and IC has stringent requirements for vias. For example, it must be flat, the difference between bumps and protrusions must not exceed one mil, and the edges must not be red. To achieve these requirements, various Via Plugging processes have appeared on the market. Now we have organized and summarized these Via Plugging processes. You can clearly see their advantages and disadvantages.

1. Hole plugging process after hot air leveling.

Its process is board surface solder mask→HAL→plug hole→curing. They use hot air to screed the flat surface and then use an aluminum sheet or ink screen. As for plugging ink, it is usually photosensitive ink or thermosetting ink. In addition, the plugging ink is preferably the same as the ink on the board surface. The advantage of this process is to ensure that the via hole does not drop oil. But the disadvantage is that the plug hole ink easily contaminates the board surface and makes it uneven. In addition, customers may experience false soldering during surface mounting. Therefore, many customers disapprove of this method.

2. Hole plugging process after hot air leveling.

There are four methods:

  1. 1. Plug the hole with an aluminum sheet. Then it is cured, polished, and graphics are transferred.

Use a CNC drill to make a screen and plug holes in the aluminum sheet. The advantage is that the hole plugging of the via hole is complete. We can
choose thermosetting inks because it has the advantages of high hardness, a slight change in resin shrinkage, and good adhesion to the hole wall.

The process is: pre-processing → plug hole → grinding plate → pattern transfer → etching → board surface solder mask.

In this way, the hole plugging will be very smooth, and there will be no quality problems such as oil explosions and oil drops. But this method also has disadvantages: It only supports one-time thickening so that the copper thickness of the hole wall can reach the standard. Therefore, there are very high requirements for the performance of the whole plate copper plating and the plate grinding machine. We must ensure that the resin on the copper surface is completely removed during the operation without contamination. However, many PCB factories do not have a one-time thickening copper process, or the performance of the equipment does not meet the requirements. Therefore, it is not used much in PCB factories.

  1. 2. Plug the hole with an aluminum sheet first. Then directly solder the board surface.

As above, first, use a drill to drill holes in the aluminum sheet. Then plug it on the screen printing machine. The point is that the residence time after completing the plug hole should not exceed 30 minutes. At this time, 36T silkscreen is used directly for the screen-printed board surface solder mask. The process is: pre-treatment-plug hole-silk screen-pre-baking-exposure-development-curing.

Then its advantage is that the oil cover of the via hole is good, and the plug hole is flat. More importantly, the color after the wet film is consistent. Because the via hole is not easy to be tinned after the hot air is leveled, and no tin beads appear in the hole. However, ink may appear in the hole on the pad after curing, resulting in poor solderability. In addition, the hot air leveling will cause bubbling and oil loss on the edge of the through-hole. Using this method to control products will be more difficult. Therefore, we recommend that engineers adopt specific processes and parameters to ensure the quality of plug holes.

  1. 3. Plug the hole with an aluminum sheet and develop. Afterward, it is pre-cured and polished. Then carry out the solder mask on the board surface.

Similarly, use a CNC drilling machine to drill out the plug holes of the aluminum sheet and then make a screen. Next, plug the hole. Usually, the plughole must be full, preferably with protrusions on both sides. After curing, grind the board again and perform the surface treatment. The process is pre-treatment-plug hole-pre-baking-development-pre-curing-board surface solder mask. The advantage is that the solidification plug hole can ensure that no oil will drop or explode. But HAL can also cause tin beads to be hidden in the plug hole. Moreover, it isn't easy to solve the situation that tin appears on the via hole. So many customers still do not accept this method.

  1. 4. Perform solder mask and plug holes on the board at the same time

In this process, we need to use a pad or a bed of nails to install the 36T screen on the screen printing machine. In other words, when the solder mask is completed, all the vias are plugged. Process: pre-treatment-screen printing-pre-baking-exposure-development-curing. Its process is concise, which can save us a lot of time. In addition, the through-holes after the hot air leveling can not lose oil or tin. We all know that a large amount of air will be left in the via hole if we use the silkscreen plug. In the subsequent process, curing will cause the air to expand and break through the solder mask. The pcb board surface will be uneven, there will be voids, and there will be a small amount of tin hidden in the via holes.

Nevertheless, PCBASIC company has basically solved these problems after a lot of adjustment work. The method is to choose inks of different viscosities and types and adjust the pressure of the silkscreen. Now our company uses this process for mass production. If you are looking for a online pcb manufacturer, please contact us. We will surely satisfy you.





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