Drilling Printed Circuit Boards

For the Printed Circuit Board manufacturing processes, drilling is one of the most expensive and time-consuming. As it is also irreversible, at Rush PCB UK, we implement the PCB drilling process very carefully—the smallest error may lead to the entire board becoming scrap. Therefore, we consider the drilling process as the most critical, and this is also a bottleneck in the entire PCB manufacturing process.

Contrary to popular belief, drilling is not the prerogative of through-hole components alone. Even boards using surface mount components may require drilling. In reality, the drilling process forms the foundation for creation of vias acting as the connectivity between different layers in a multi-layered board. Understanding the methods used for drilling PCBs on the shop floor can reduce the cost per PCB, while delivering the boards with a minimum turnaround time.

There are two popular techniques for drilling holes in printed circuit boards:

• Mechanical Drilling
• Laser Drilling

A mechanical drill is the older technology. Using drill bits of different sizes, a computer guided drill can accurately drill holes of diameter as small as 6 mils. On the other hand, a laser drill is the latest technology for drilling PCBs. A laser drill is more accurate than a mechanical drill, and it can drill holes down to a diameter of 4 mils.

Unlike mechanical drilling, laser drilling is a non-contact process. That means, the tool does not touch the PCB at any time during the drilling process. However, controlling the drill depth is simple. Using laser technology, it is possible to drill blind and buried holes easily. Latest laser drilling machines can drill holes with 1 mil hole placement tolerance.

Unlike other manufacturing process like etching and plating, the time taken for drilling holes in PCBs depends on two factors:

• Total number of holes to be drilled
• Number of holes of different diameter to be drilled

While the time taken to drill all holes varies directly as the total number of holes in the board, it increases even further if the holes are with different diameters. Extra time is necessary for changing the drill bit.

Two significant factors affect the drilling process—Aspect ratio and Drill-to-Copper clearance.

The aspect ratio decides the reliability of a PCB. For through-hole boards, drilled with a mechanical drill, the ratio between the board thickness and the diameter of the drilled hole is known as the aspect ratio. For laser drilled micro-vias, aspect ratio signifies the ratio between the hole depth to its diameter.

The aspect ratio effectively defines the ability of the plating process to deposit copper within the hole or via. Depositing copper becomes increasingly tedious as the diameter of the hole decreases, while the depth of the hole increases. As the diameter of the hole decreases, the copper plating bath must increase its throwing power proportionately so that it can push the liquid through the tiny holes while depositing copper. Ideally, the aspect ratio for through holes is 10:1, while for micro-vias, the ideal aspect ratio is 0.75:1.

When the aspect ratio is smaller than the ideal, the copper plating within the hole can be non-uniform and hence unsatisfactory. As the aspect ratio increases beyond the ideal, achieving a reliable copper plating on the inside of the hole or via becomes more challenging. Therefore, for a good reliability, PCB holes or vias must have a proper aspect ratio.

Drill-to-Copper Clearance

The land present between the edge of a drilled hole and the nearest copper feature is known as the drill-to-copper clearance. The nearest copper clearance can be any active copper region, like a copper trace or copper pour. A deviation from the stated clearance can easily disrupt a copper trace. That makes the drill-to-copper clearance such a predominant factor.

Conclusion

Drilling is an important process during the manufacturing of a printed circuit board. Rush PCB UK uses the latest drilling machines to offer the most reliable and high quality boards to our customers.