The conductive via, also known as a through-hole, is an essential element in printed circuit boards (PCBs) that enables electrical connections between different layers. To meet customer demands, it is crucial to plug these vias properly. After extensive experimentation, the traditional aluminum plug method has been replaced with a more efficient white plate process, which allows for both surface soldering and via plugging. This new approach ensures stable production and consistent quality.
ViAs serve as the connection points between various circuit lines. As the electronics industry continues to evolve, so do the requirements for PCB manufacturing and surface mount technology. The via plugging process was developed to address these growing demands, and several key criteria must be met:
1. The via should contain copper, and the solder resist can either be filled or left open.
2. There must be a tin-lead layer inside the via with a minimum thickness of 4 micrometers. No solder resist should enter the hole, as this could trap tin beads inside.
3. The via must be sealed with opaque solder resist, free from tin rings, solder balls, and meeting specific leveling standards.
With the trend in electronic products moving toward being "light, thin, short, and small," PCBs are now designed with higher density and complexity. This leads to an increase in SMT and BGA PCBs, where customers often require vias to be plugged during component mounting for the following reasons:
1. Preventing solder shorts during wave soldering, especially when vias are located on BGA pads. In such cases, the via must be plugged first and then gold-plated to ensure proper BGA soldering.
2. Avoiding flux residue within the via holes.
3. Ensuring vacuum testing after surface mounting and assembly by creating a negative pressure.
4. Preventing solder paste from flowing into the via and causing poor solder joints.
5. Avoiding solder ball formation during wave soldering, which could lead to short circuits.
**Implementation of the Conductive Via Plugging Process**
For surface mount boards, particularly those with BGA and IC components, the via must be flat, with a tolerance of ±1 mil, and no red tin on the edges. Additionally, there should be no tin beads inside the via. Meeting these requirements involves a complex and lengthy process, with challenges during hot air leveling and green oil solder mask testing. Common issues include oil drops and oil explosions after curing.
Several methods have been developed to address these challenges, each with its own advantages and limitations:
**1. Hot Air Leveling (HAL) and Plug Hole Process**
This method involves applying the solder mask first, followed by HAL, then plugging the vias. A non-plugging process is used, and the vias are filled using an aluminum mesh screen or ink-blocking net. The plug ink can be photosensitive or thermosetting. While this method prevents oil drops after HAL, it may cause surface contamination and unevenness, leading to potential soldering issues, especially under BGA areas. Many customers avoid this method due to these risks.
**2. Pre-HAL Plug Hole Process**
**2.1 Aluminum Sheet Plug, Cure, and Board Grinding for Pattern Transfer**
In this method, a CNC drilling machine is used to create an aluminum sheet for the plug. The plug ink is applied, ensuring full coverage, and the board is then ground for pattern transfer. The process includes pre-treatment, plugging, grinding, etching, and applying the solder mask. This method ensures flat vias and avoids oil explosion or drop issues during HAL. However, it requires thick copper plating, which increases the demand on the entire board plating process and the performance of the grinding equipment.
**2.2 Direct Screen Printing After Aluminum Plug**
Here, an aluminum sheet is drilled and used as a screen to fill the vias. After plugging, the board is directly screen-printed within 30 minutes. The process includes pre-treatment, plugging, screen printing, pre-baking, exposure, development, and curing. This method ensures good cover oil and flat vias, but it may lead to poor solderability if ink remains on the pad after curing. It also presents challenges in controlling the process, requiring special procedures and parameters to maintain quality.
**2.3 Aluminum Plug, Development, Pre-Cure, and Surface Treatment**
This method uses an aluminum sheet to plug the vias, followed by development and pre-curing before surface treatment. While it helps prevent oil loss during HAL, it may leave tin in the via, making it unsuitable for many customers.
**2.4 Simultaneous Surface Soldering and Plug Hole Process**
This method uses a 36T (or 43T) screen to plug all vias while completing the board's surface. The process includes pre-treatment, screen printing, pre-baking, exposure, development, and curing. Although it reduces process time and improves equipment utilization, it can lead to air bubbles in the vias, causing voids and unevenness. Through extensive testing, adjustments in ink types, viscosity, and screen pressure have helped resolve these issues, allowing for mass production using this method.
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