How to Prevent Copper-Free Holes in High Aspect Ratio PCBs: Key Insights for PCB Manufacturing

Understanding the Importance of Hole Size to Thickness Ratio in PCB Manufacturing

In PCB manufacturing, the hole size to thickness ratio, also known as the aspect ratio, is a critical factor that influences the quality of hole plating. This ratio is calculated by dividing the PCB’s thickness by the hole diameter, often referred to as the length-to-diameter ratio (L/D).

  For instance, if a PCB’s thickness is 1.60 mm and the hole diameter is 0.2 mm, the aspect ratio is 8:1. Lower aspect ratios indicate thinner boards with larger holes, which typically result in better plating results due to more even ion distribution during the electroplating process.

However, high aspect ratio PCBs—those with thin boards and small holes—pose significant challenges during the electroplating process, leading to defects such as copper-free holes (also known as "blind vias" or "voids") and poor quality plating.

What Causes Copper-Free Holes in High Aspect Ratio PCBs?

1. Micro-Bubbles and Blockage
   In the traditional direct current (DC) electroplating process, the electroplating solution may trap micro-bubbles in the hole, preventing the copper from evenly plating the hole walls. These bubbles can block the flow of copper, resulting in areas with insufficient copper deposition.
2. Poor Cleaning During Pre-Treatment
   If the PCB isn't properly cleaned before plating, contaminants or residues may remain inside the holes. This prevents the copper from adhering to the hole walls, creating voids and weak spots.
3. Drill Defects
   When drilling high aspect ratio holes, if the drill bit isn't sharp enough, it can cause issues with the hole's internal surface. Instead of smoothly removing the material, the drill bit might pull and tear resin or fiberglass, leaving rough surfaces. This can hinder the plating process and lead to poor copper adhesion.

How High Aspect Ratio Affects Plating Quality

For high aspect ratio PCBs (e.g., 14:1, 16:1, or even 20:1), the plating process becomes more challenging. The electroplating solution struggles to deposit copper uniformly, especially in the inner regions of the holes, which results in a dog-bone effect. This means the hole's top becomes wider while the lower part remains underplated.

Additionally, the hole's electrical current density varies across the hole’s surface. At the entrance of the hole, the current density is higher, while in the deeper parts, it's lower. This uneven distribution leads to poor plating in the lower parts of the hole, contributing to the formation of copper-free holes.

Advanced Solutions to Prevent Copper-Free Holes in High Aspect Ratio PCBs

To avoid these issues, modern PCB manufacturing factories are turning to pulse plating technology, which overcomes many of the limitations of traditional DC electroplating.

Pulse Plating for Uniform Copper Deposition

Pulse plating uses alternating current with controlled pulses to increase copper deposition efficiency. Unlike conventional DC plating, pulse plating enhances the ion movement inside the hole, allowing for more uniform copper deposition across the hole, both at the entrance and deeper regions. This results in better plating quality and avoids the formation of copper-free holes.

Additionally, pulse plating ensures that copper is evenly deposited without significantly increasing the copper thickness on the board surface, maintaining the integrity of high-density circuits, fine lines, and precision impedance.

Other Strategies

1. Improved Drilling Techniques
   Using high-precision drills can ensure smoother and cleaner holes, improving the surface quality and ensuring that the plating process is more effective.
2. Optimized Pre-Treatment
   Thorough cleaning and treatment of PCB holes can ensure better copper adhesion. Using chemical etching or plasma cleaning techniques can eliminate any contaminants and improve the quality of the hole wall, leading to better plating results.
3. Use of Advanced Plating Equipment
   Modern PCB factories utilize equipment that is specifically designed to handle high aspect ratio PCBs. This includes improved electroplating tanks, advanced solution filtration systems, and better current control mechanisms.

    

    

    

   How to make the hole in the picture below?

    

   ✅Solution: Richfulljoy's pulsed VCP can avoid the frequent occurrence of excessive pits and holes in DC plating. High current density plating efficiency is high, and under the pulse plating system, it can make the inside and outside of the hole very good. The coating distribution effect can be achieved, and the surface copper does not increase, and the surface copper does not increase, and the thin circuit and high-precision impedance can be guaranteed.

                                                                                                             Improving Quality in High Aspect Ratio PCB Manufacturing

   In high aspect ratio PCB manufacturing, preventing copper-free holes is crucial for maintaining product integrity and performance. By understanding the challenges associated with plating in high aspect ratio designs and adopting advanced technologies like pulse plating, PCB manufacturers can ensure consistent and reliable plating quality.

   If you're involved in PCB fabrication or working with PCB manufacturing factories, it’s important to be aware of these techniques and work with manufacturers who have the expertise to handle high aspect ratio PCBs with the latest technology.