PCB DEPARTMENT MANAGER
The Aspect Ratio (Aspect ratio) we see in PCB design and manufacture can be defined as the ratio of the thickness of a PCB and the smallest hole (Plated Hole) drilled on the PCB. The PCB thickness is considered as the bare raw material (i.e. doesn’t include masks such as copper, solder, etc.). The larger this ratio, the more difficult the coating process will be. As the Aspect Ratio increases there will be more coating on the inner parts of the Via, thus the risk of cracking during soldering may be higher. Keeping this ratio low will ensure consistency in Via coatings and offer more durability throughout the service life of the PCB. A high Aspect Ratio also indicates the level of the manufacturer's production capability.
The typical (PTH) ratio is 8:1. Accordingly; The PTH is about 0.2mm for a PCB with a thickness of 1.6mm. The PTH is about 0.25mm for a PCB with a thickness of 2.0mm.
According to the IPC-2221A and IPC-222 design guidelines, the recommended Aspect Ratio is 8:1. For a standard application, use of Vias with a bore of 0.2 mm or larger is recommended, as it will help keep the costs low, improve reliability and alleviate the production related issues.
The aspect ratio of micro Via structures in HDI circuits is limited to 1:1, while 0.7~0.8: 1 is preferred to facilitate the coating process.
Ideally, the tip remaining after Back Drilling should not be less than 0.25 mm.
BACK DRILLING (CDD)
In high-speed PCB designs, the non-functional protruding parts of a path may cause signal integrity problems. Ensuring the signal integrity requires suitable design and applications. For this, the protrusions should be reduced. An intervention can be made by means of Back Drilling application. This is a controlled depth drilling process. (C.D.D. Controlled Depth Drilling) Undesired parts are removed from the PTH hole by using a tip with a larger diameter. PCB manufacturers are expected to have specific precision machining capabilities for this process. Ideally, the tip remaining after Back Drilling should not be less than 0.25 mm. In order to calculate the remaining residual end portion and signal losses as a percentage (%), calculations must be made with the support of specific design tools.
Advantages:
- It has minimal impact on the design and layout.
- Increases channel bandwidth and data speed due to deviations that cause distortion.
- Reduces resonant frequency mode excitation.
- Lower costs can be achieved compared to sequential lamination.
- Reduces the Bit Error Rate per unit time with the application.
- Reduces EMI/EMC radiation as it ensures signal integrity and compatibility.
- Reduces signal integrity distortion problems as it reduces the deterministic vibrations.
- It is easy and cost-effective to manufacture.
Disadvantages - Suitable for high speed (1GHz to 3GHz) circuit boards.
- No blind Via should exist in the design.
- A specific precision technique should be used during the application to avoid damaging the hole perimeter.
