Conductive Hole Via Process: Advantages and Disadvantages
Conductive holes, also known as vias, must be plugged to meet customer requirements. Based on extensive practical experience, we have modified the traditional aluminum board plugging process; now, solder mask plugging on the board surface is accomplished using a white mesh, ensuring stable production and reliable quality.
Vias serve to connect and conduct electrical signals. The development of the electronics industry has also driven the advancement of PCBs, placing higher demands on PCB manufacturing processes and surface-mount technology. Via plugging technology has emerged in response to these needs and must meet the following requirements:
Requirements for Through-Hole Plugging:
- Through-holes can be made of copper., Plug-type electrodes can be used for resistance welding;
- There must be tin-lead in the through-hole., and there are specific thickness requirements (4 micrometers). Solder mask ink must not enter the holes, as this can cause solder balls to form inside them;
- Through-holes must be presentSolder Mask Plugging Holes, light-tight; requirements for Wuxi rings, tin balls, and flatness, etc. One side must be oiled, and the other side must be coated with tin/lead. Tin balls and tin rings are permitted.
As electronic products continue to evolve toward being “lighter, thinner, shorter, and smaller,” PCBs are also becoming more complex and high-density, leading to the widespread adoption of SMT and BGA PCBs. Customers need to fill vias when mounting components; via filling serves five main purposes:
Function of the plug hole:
- To prevent solder from seeping through through-holes onto component surfaces during PCB wave soldering, which could cause short circuits;
- Avoid leaving flux residue inside through-holes;
- After surface-mount and component assembly are completed at an electronics factory, the PCB must be placed in a vacuum on the test machine to create negative pressure;
- Prevent solder paste on the surface from flowing into the holes, which could cause cold solder joints and affect the placement process;
- Prevent solder balls from being ejected during wave soldering, which can cause short circuits.
The Process of Plugging Holes in Printed Circuit Boards and Its Advantages and Disadvantages:
The following is a comparison and explanation of several different PCB via-filling processes, along with their advantages and disadvantages:
Hot-Air Leveling and Hole-Filling Process
The process flow is as follows: surface resistance welding → HAL → via plugging → curing. Production is carried out using a via-free process. After hot-air leveling, all through-holes in the “fortress” areas specified by the customer are plugged using an aluminum mesh or ink screen. The plugging ink can be either photosensitive or thermosetting. This process ensures that the through-holes do not lose ink after hot-air leveling; however, it can easily cause the plugging ink to contaminate the board surface, resulting in unevenness. This can lead to cold solder joints during customer assembly (especially with BGAs). Therefore, many customers do not accept this method.
Pre-Hot-Air-Smoothing Hole-Filling Process
Use a CNC drilling machine to drill holes in the aluminum sheet where plugs are required, create a screen plate, and plug the holes, ensuring the through-holes are fully filled. The hole-plugging ink can also be a thermosetting ink. This method ensures that the plugs in the through-holes are smooth, and prevents quality issues such as ink bursting or leaking around the hole edges during hot-air leveling. However, this process requires a single-pass copper plating to increase the copper thickness on the hole walls to meet customer specifications. Consequently, it places high demands on the copper plating of the entire board, as well as on the performance of the grinding machine, to ensure that the resin on the copper surface is completely removed and the copper surface remains clean and free of contamination. Many PCB manufacturers lack the capability for single-pass copper thickening, and their equipment does not meet the necessary performance standards, resulting in this process being rarely adopted by PCB manufacturers.
Resist-welding the aluminum sheet surface after plugging holes, developing, precuring, and polishing
Use a CNC drilling machine to drill holes in the aluminum plate where plugs are needed, create a screen, and install it on a screen printing machine with a plug-positioning mechanism. The plugs must be filled completely, ideally protruding slightly on both sides. After curing, sand the sheet for surface finishing. This method uses plug-filling and curing to ensure no oil dripping or spattering after HAL treatment; however, it is difficult to completely resolve issues such as solder balls and tin plating in through-holes after HAL, which is why many customers do not accept it.
Resistance welding of the panel surface and hole plugging should be performed simultaneously.
Use a 36T (43T) screen, mounted on the screen
On the printing press, use a pad or a stencil holder. While completing the board surface, plug all through-holes. Using this process for mass production ensures that the through-holes do not lose ink or require tin plating after hot-air leveling. However, because screen printing is used to plug the holes, a large amount of air remains inside the through-holes. Currently, issues with via defects and surface unevenness have been largely resolved by selecting different ink types and viscosities and adjusting screen printing pressure.
The above provides an overview of the role, requirements, manufacturing process, and advantages and disadvantages of via holes in PCB design.
