PCB Blog - How are PCB through holes plated?

PCB vias refer to drilling the required holes on the copper-clad plate, which carries the conduction between layers and is used for electrical connections and fixing devices. Vias is a crucial and indispensable part of PCB production.

In PCB production, common PCB via processes include through-hole cover oil, through-hole plug oil, through-hole window opening, resin plug hole, electroplating hole filling, etc. Each process has its own characteristics and corresponding application scenarios.

1、Via cover oil

The "oil" of  PCB via cover oil refers to solder mask oil, which covers the hole ring of the through-hole with solder mask ink. The purpose of the hole cover oil is insulation, so it is necessary to ensure that the ink cover of the hole ring is complete and thick enough, so that it will not stick tin during the patch and DIP process in the later stage.

2、Via window opening

Compared to the "via cover oil" treatment method, both the through hole and the hole ring are not covered with solder mask oil.

Opening a window through a hole will increase the heat dissipation area, which is beneficial for heat dissipation. Therefore, if there are high requirements for board heat dissipation, you can choose to open a window through a hole. In addition, if you need to use a multimeter to do some measurement work on the via, then make it a via with a window. However, opening a window through a hole carries a risk, which can easily lead to a short circuit between solder pads.

3、Via plug oil

PCB via sealing oil refers to the process of using aluminum sheets to fill the solder mask ink into the through-hole during PCB production, and then printing the solder mask oil on the entire board. All PCB vias will not be transparent. The purpose is to block the through-hole to prevent solder beads from being hidden in the hole, as the solder beads will dissolve at high temperatures and flow onto the solder pad, causing a short circuit, especially on BGA.

If the ink is not properly plugged into the via, the edge of the via will turn red, causing "false copper exposure". In addition, if the oil for the through-hole plug is not properly prepared, it can also affect the appearance.

4、Resin plug via

Resin plug hole, in simple terms, refers to the process of coating the via wall with copper, filling the hole with epoxy resin, and then coating the surface with copper.

The prerequisite for resin plugging holes is that there must be copper plating inside the holes first. This is because the use of resin plug holes in PCBs is often for BGA parts: traditional BGA may route wires from the back of the PAD to the back, but if the BGA is too dense and cannot go out, holes can be drilled from the PAD to route wires to other layers.

Printed circuit boards using resin plug-hole technology have no dents on the surface, and the holes can be conductive without affecting welding. Therefore, they are highly favored in some products with high layers and large board thicknesses.

In terms of process, these vias are generally divided into three categories, namely blind holes, buried holes, and through holes.

Blind hole: Located on the top and bottom surfaces of printed circuit boards, with a certain depth, used for connecting surface and inner circuits.

Buried hole: A connection hole in the inner layer of a circuit board.

Through hole: It passes through the entire circuit board and is generally used for component positioning and installation.

PCB via copper plating is an important step in the PCB manufacturing process, which can bring various high-quality characteristics to the board and is an important foundation for manufacturing PCBs.

PCB boards require copper plating to increase their strength and durability and reduce thermal deformation resistance, and insulation impedance. Therefore, PCB via copper plating is a very important process, which can enhance the current dissipation ability of PCB connection points, improve power supply performance and signal integrity, and improve signal integrity.

The attention steps for PCB via copper plating are to first place the PCB in a copper plating bath, with a mixed sulfuric acid and copper sulfate solution as the electrolyte, and then add electrodes to allow for the triggering of electrolytic reactions during the process. The electrolysis process is carried out, and the current flows through the PCB board to evenly coat the through-hole surface of the PCB with copper. The thickness of copper on the surface of PCB can be controlled by changing the current intensity, as well as by changing the surface cleanliness, temperature, hardness, etc. before plating.

In PCB, copper plating can provide a thicker and more delicate layer of tin on the surface of the PCB, thereby adding additional features. Via copper plating can also bring other advantages, such as connecting the board to the power supply, increasing the stability of the power supply, transmitting signals to wireless electrical components, improving signal transmission performance, and improving power efficiency.

The function of PCB via

A visa is the smallest metal conductor in PCB design, typically used to connect two or more components and allow current to pass through.

1. Simplify the circuit: The via allows current to pass through without the need for additional conductors or contact points, thereby reducing the complexity and workload of the circuit.

2. Improve signal transmission: vias, signals can be transmitted from one place to another, reducing signal interference and distortion.

3. Enhance packaging density: vias can reduce the number of components on a PCB, thereby increasing the density of the circuit and reducing the size of the PCB.

4. Reduce manufacturing costs: vias do not require additional manufacturing costs as they are a natural component of PCB design.

5. Improve flexibility: vias can be used to connect different component types and positions, providing more design flexibility.

6. Improving reliability: vias can prevent short circuits and open circuits between components, thereby improving the reliability of the circuit.

In PCB design, vias are usually a very important component that can be used to connect various components, such as power supplies, sensors, motors, etc., thereby simplifying circuits, improving signal transmission, reducing manufacturing costs, and improving design flexibility and reliability.