From Mechanical Drilling to Laser Drilling of PCB Microvias

Zachariah Peterson
|  Created: December 20, 2018  |  Updated: September 25, 2020
From Mechanical Drilling to Laser Drilling of Microvias

 

PCB via fabrication relies on a specialized process of drilling and plating, where the drilling step is executed in two possible ways: mechanical drilling and laser drilling. Mechanical drilling is the practice used for standard PCBs where the hole sizes are not too small. HDI (High-Density Interconnect) design practices can be used for mechanical drilling, but only down to a certain limit. Eventually, laser drilling will be needed to fabricate a PCB, which is determined by the diameter of the holes that need to be drilled and the thickness of the layer where holes are being placed.

It is the designer's responsibility to understand the limits of mechanical drilling and laser drilling. If laser drill hole sizes are specified in a laminate thickness or material type that cannot be laser drilled, you will be forced to make changes to the PCB design in order to proceed to production.

If you want to avoid the need to make changes to your design before manufacturing, pay attention to the mechanical drilling and laser drilling limits defined in this article.

PCB Drilling Limits

As drilled holes get smaller in a PCB, they run up against certain fabrication limits, which should be considered in the design. In general, drill size limits will be selected based on the following factors:

  • Total board thickness for through-hole vias
  • Layer or lamination thickness for blind/buried vias
  • Fabrication process being used
  • Desired landing pad size and annular ring requirement
  • Manufacturer's capability limits

Keep these points in mind as we look at the typical size restrictions on drills.

Through-Hole Mechanical Drilling Limits

Mechanical drilling is primarily limited by available drill sizes; you can only drill so small before you have to switch to laser drilling. The mechanically drilled hole diameter and the board thickness define an aspect ratio limit for a particular drill size. Your aspect ratio needs to stay below a certain value for a given hole diameter, and this will limit the total board thickness.

What are the exact aspect ratio limits? This depends on the limits set by your fabrication house. They can only guarantee accurate hole size and positional tolerance for certain maximum aspect ratio values. An example for a few drill diameters is shown in the following table.

Aspect ratio

Typical values:

  • 10 mil drill: AR < 12:1
  • 8 mil drill: AR < 10:1
  • 6 mil drill: AR < 8:1

Drilled hole diameter

6 mil minimum

Positional tolerance

Based on IPC Producibility Level

Pad size

Based on IPC annular ring/product class

Table 1: Typical mechanical drilling limits for vias

Different PCB fabrication houses may have different aspect ratio limits, so check with them before creating your design. If you do not check with them first, you risk being given holes that are too small, and once the design is corrected with a larger hole, there may be clearance problems that need to be resolved.

Laser Drilled Microvia Limits

Laser drilling allows for very fast throughput during manufacturing, but it has limited drilling depth into the PCB material. Due to the limitation on drilling depth, the aspect ratio limitation on laser-drilled microvias will be much lower. Typical values you might find in a standard microvia laser drilling process are shown in the following table.

Aspect ratio

Typical values:

  • AR < 1:1

Drilled hole diameter

2-3 mil minimum

Positional tolerance

Based on IPC Producibility Level

Pad size

Based on IPC annular ring/product class

Table 2: Typical laser drilling limits for microvias

Laser-drilled microvias with an aspect ratio less than one generally require very thin dielectric layers. However, larger microvias can also be fabricated, though they won’t really be classified as microvias despite the fact that they are laser-drilled. For example, the 10 mil maximum laser drill hole diameter limit with a 0.75:1 aspect ratio demands a laminate thickness of 7.5 mils. Typically, if you intend to place drills with this diameter as blind or buried vias, you would use mechanical drilling, which I detail more in the next section.

An important point regarding the fabrication of laser-drilled microvias is that the pre-preg material being used must be laser-drillable. Not all PCB laminate materials are marketed as laser-drillable, although the growth in HDI markets has greatly expanded the range of compatible materials.

When viewed under the microscope, laser-drillable materials that can be used as HDI buildup layers in sequential lamination are not much different from standard materials in terms of structure. You still have the same glass weave styles and similar resin material systems.