The production process for printed circuit boards (PCBs) and surface mount technology has evolved to meet higher demands. As a result, via hole plugging technology has emerged to address these needs, which must satisfy the following requirements:

(1) The via holes in the PCB must contain copper, and the solder mask can either be plugged or left unpluggd.

(2) The via holes must include a layer of tin and lead, with a minimum thickness requirement of 4 microns. Additionally, no solder mask ink should enter the holes, as this could lead to hidden tin beads inside the holes.

(3) The through holes must be plugged with solder mask ink, and the mask should be opaque. Furthermore, no tin rings or tin beads should be present, and the flatness must meet specific standards.

As electronic products trend towards being “light, thin, short, and small,” PCBs have followed suit, becoming more densely packed and complex. This has led to the rise of surface mount technology (SMT) and ball grid array (BGA) PCBs. Consequently, customers now require via hole plugging during component mounting, driven by five main functions:

(1) To prevent tin from flowing through the component surface via the through hole and causing a short circuit during wave soldering. This is especially important when a via is placed on a BGA pad, as the hole must first be plugged and then gold-plated to facilitate proper BGA soldering.

(2) To prevent flux residue from remaining in the via holes;

(3) After the surface mounting of components at the electronics factory is completed, and the components have been assembled, the PCB must undergo a vacuum process to create a negative pressure on the testing machine for final completion.

(4) This process helps to prevent the surface solder paste from flowing into the holes, which could result in false soldering and affect component placement.

(5) It also prevents solder balls from popping up during wave soldering, which could cause short circuits.

**Implementation of the Conductive Hole Plugging Process**

For surface mount boards, especially those with BGA and IC components, via hole plugging must be uniform, with no convex or concave deviations greater than ±1 mil. Additionally, there should be no red solder visible on the edges of the via holes, and any tin balls inside the via holes must be properly contained. To meet these requirements, the via hole plugging process can vary significantly. The process flow is lengthy, and process control is challenging, often leading to issues such as ink drops during hot air leveling or green solder mask testing, as well as ink explosions after curing. Based on actual production conditions, various PCB plugging processes are summarized below, with comparisons and explanations of their advantages and disadvantages.

**Note**: The principle of hot air leveling involves using hot air to remove excess solder from the surface and vias of the printed circuit board. The remaining solder forms an even coating on the pads, non-resistive solder traces, and surface mount pads. This is one of the common surface treatment methods for PCBs.

**1. Hole Plugging Process After Hot Air Leveling**

The process flow is: board surface solder mask → HAL → plug hole → curing. The non-plugging process is used for production. After hot air leveling, an aluminum sheet or ink-blocking screen is used to complete the required via hole plugging. The plugging ink can be either photosensitive ink or thermosetting ink. To maintain consistent color across the wet film, it’s ideal to use the same ink for the via hole plugging as for the board surface. This method ensures the via holes retain their solder mask after hot air leveling, preventing solder loss. However, this process can easily lead to contamination of the board surface by the plugging ink, causing unevenness and potential false soldering, particularly in BGA placements. As a result, many customers do not accept this method.

**2. Hot Air Leveling and Plug Hole Technology**

**2.1 Using Aluminum Sheets for Hole Plugging, Curing, and Grinding**

In this process, a CNC drilling machine creates an aluminum sheet to form a screen that ensures the via holes are completely filled. The plugging ink used can also be thermosetting ink. This ink must be highly durable, have low resin shrinkage, and provide strong adhesion to the hole wall. The process flow is as follows: pre-treatment → plug hole → grinding → pattern transfer → etching → board surface solder mask.

This method ensures that the via hole plugging is flat and free from quality issues such as solder loss or excessive solder at the hole edges after hot air leveling. However, this process requires a one-time copper thickening step to ensure the copper wall thickness of the via meets customer specifications. This increases the demands on both the copper plating process and the performance of the plate grinding machine to ensure the copper surface is clean and free of contaminants. Many PCB factories lack the equipment to perform one-time copper thickening, which limits the use of this method.

**2.2 Aluminum Sheet Plugging, Screen Printing, and Solder Mask Application**

This process uses a CNC drilling machine to create an aluminum sheet screen, which is then installed on a screen printer for hole plugging. After plugging, the board should not sit for more than 30 minutes. The process flow is: pre-treatment → plug hole → screen printing → pre-baking → exposure → development → curing.

This method ensures good coverage of the via holes with solder mask, resulting in flat plugging and consistent wet film color. After hot air leveling, it prevents tinning or hidden solder balls in the via holes. However, post-curing can cause ink residue to affect solderability on the pads, and it may lead to bubbling or solder mask loss at the edges of the vias. Controlling this process is challenging and requires special process parameters to ensure high-quality plugging.

**2.3 Aluminum Sheet Plugging, Development, Pre-Curing, and Soldering After Grinding**

In this variation, a CNC drilling machine creates the necessary aluminum sheet for plugging. After installation on the screen printing machine, the vias are plugged, ensuring the plug is complete and protrudes on both sides. The process flow is: pre-treatment → plug hole → pre-baking → development → pre-curing → board surface solder mask.

This approach ensures that the via holes maintain their solder mask coverage and do not lose oil or explode after hot air leveling. However, like the previous method, it doesn’t fully resolve the issue of tin balls in the via holes, which is a common customer concern.

**2.4 Simultaneous Board Surface Solder Mask and Plug Hole Application**

This method uses a 36T (43T) screen installed on a screen printer, along with a pad or bed of nails, to complete both the board surface and via hole plugging in one pass. The process flow is: pre-treatment → screen printing → pre-baking → exposure → development → curing.

This process offers a shorter PCB production time and higher equipment utilization. It ensures that via holes retain their solder mask and do not become tinned after hot air leveling. However, during curing, air expansion can cause the solder mask to break and result in cavities and unevenness. A small number of vias may still be hidden during hot air leveling.

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