Conductive vias, also known as through holes, play a crucial role in ensuring proper electrical connectivity between different layers of a printed circuit board (PCB). To meet customer requirements, it is essential to plug these vias during the manufacturing process. After extensive experimentation, the traditional aluminum plug method has been replaced with a more efficient approach using white plates to complete both surface soldering and via plugging. This new technique ensures stable production and consistent quality.
The primary function of vias is to connect various conductive lines on the PCB. As the electronics industry continues to evolve, the demand for advanced PCBs increases, placing higher standards on manufacturing processes and surface mount technology. In response, the via plugging process was developed, which must satisfy several key requirements:
1. The via should contain copper, and the solder resist may or may not be plugged.
2. A tin-lead layer must be present in the via, with a minimum thickness of 4 micrometers. Solder resist should not enter the hole, as this could lead to the formation of tin beads inside.
3. The via must be sealed with an opaque solder mask, free from tin rings, solder balls, and any leveling issues.
With the growing trend toward smaller, lighter, and more compact electronic devices, PCBs are becoming increasingly dense and complex. This has led to a rise in the use of Surface Mount Technology (SMT) and Ball Grid Array (BGA) boards, where customers often require via plugging during component mounting. The main reasons for this include:
1. Preventing solder shorts during wave soldering, especially when vias are located on BGA pads. In such cases, the via must be filled and gold-plated before BGA assembly.
2. Avoiding flux residue within the via, which can cause long-term reliability issues.
3. Ensuring that the PCB can be vacuum-tested after surface mount assembly to maintain a negative pressure environment.
4. Preventing solder paste from flowing into the via, which could result in poor solder joints and affect component placement.
5. Avoiding solder ball formation during wave soldering, which could cause short circuits.
The implementation of the via plugging process involves multiple steps and requires careful control. For surface mount boards, especially those with BGA and IC components, the vias must be flat, with a tolerance of ±1 mil, and no red tin or tin beads at the edges. Meeting these stringent requirements makes the process quite lengthy and challenging, particularly when dealing with hot air leveling and solder resist testing.
Several methods are commonly used for via plugging, each with its own advantages and limitations. One approach is the hot air leveling (HAL) and plugging process, where the solder mask is applied first, followed by HAL and then via plugging. While this method prevents oil droplets from forming after HAL, it can lead to surface contamination and unevenness, which many customers find unacceptable.
Another method involves plugging the via before HAL using aluminum sheets. This process includes pre-treatment, plugging, grinding, and pattern transfer. It ensures a flat via and avoids issues like oil explosion or oil drop during HAL. However, it requires thick copper plating, which demands high-quality equipment and precise control.
Screen printing after aluminum plug is another option, where the via is filled and then screen-printed directly. This method helps achieve good coverage and color consistency but can sometimes result in poor solderability due to ink residues. Additionally, it is difficult to control and requires special parameters to maintain quality.
In summary, the choice of via plugging method depends on the specific requirements of the PCB and the capabilities of the manufacturing facility. Through continuous improvement and optimization, many companies have successfully implemented efficient and reliable via plugging processes that meet the highest standards of performance and quality.
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