The one place that bigger is not better is in the electronics industry where miniaturization is a growing trend. With size of devices shrinking what needs to keep up, is the size of boards. The challenge clearly is to enhance functionality in a shrinking board size with high component density. One of the things that has made this possible is the use of vias.
Vias are available in different sizes and types and play different functions. Let us look at some of them below:
- Signal or Ground Via– The function of this via is to complete a current path and its size needs to be large enough to carry maximum current.
- Thermal Via– Its function is largely to dissipate heat. In terms of size, it needs to be enough to achieve the desired heat dissipation without damaging the board or any of its components.
- Tented Via– Its function is to prevent current flow as well as to aid in solder flow. In terms of size, 0.25-0.3 mm vias are typically preferred. However, small vias may necessitate laser drilling.
The choice of vias, however, also needs to be made taking into account several other aspects of design and development of the PCB. Here are some of the considerations that need to be kept in mind for your next PCB Assembly project:
Considerations for PCB Via Size Selection
- Classification of the Board
The selection of via size has to do with the kind of board and its component density. On these parameters, the classification of boards can be made as under:
- Level A- These are low density
- Level B- Moderate in terms of density
- Level C- High density
- Fabricator Equipment Capabilities
It is important to go with a contract manufacturer that can produce the size of via that your design demands. Professional CMs will have a range of drill hole sizes that can work as per your requirements.
- Density of the Board
The density of components on the board has a strong role to play when it comes to the clearance requirements, size of stack up, and number of vias. In the absence of taking this into account, your board is likely to suffer in terms of signal integrity. So, this needs to be taken care of while in the PCB Fabrication phase.
- Via Density
Typically, you may land up using a large number of vias. High via density can in turn impact aspects such as impedance and structural integrity.
Clearly then the selection of via size is not a job that can be done in isolation. It needs to take into account a number of design aspects as well as aspects to do with manufacturability.
Optimizing Via Size Selection
The below best practices can go a long way in optimizing via size selection:
- Current carrying Requirement – It is important that the vias should be able to transfer signals with minimum loss and high-fidelity.
- Using effective trace widths– With vias playing a big role in completing circuits between surface components, the more effective your trace routing, the better your via selection needs to be.
- IPC-2222 guidelines– In order to determine the minimum hole size, it is important to comply with IPC-2222 standards as below:
Level A Minimum Hole Size = maximum lead diameter + 0.25 mm
Level B Minimum Hole Size = maximum lead diameter + 0.20 mm
Level C Minimum Hole Size = maximum lead diameter + 0.15 mm
- IPC-2221 guidelines- The pad diameter needs to be determined as per IPC- 2221 guidelines as below:
Level A Pad Diameter = minimum hole size + 0.1 mm + 0.60 mm
Level A Pad Diameter = minimum hole size + 0.1 mm + 0.50 mm
Level A Pad Diameter = minimum hole size + 0.1 mm + 0.40 mm
In addition to the above best practices, it is important to remember that as a thumb rule it is best to try and reduce the number of vias as their overuse can lead to an impact on the board’s electrical properties.
A professional contract Manufacturer can go a long way in offering you a range of via types and sizes. In turn, choosing the right type and size will have an impact on your design. What it will also ensure is that the PCB will meet its performance objectives.
Suresh Patel has worked as a Sales Engineer and other management roles at Mer-Mar Electronics. He brings 25 years of experience in printed-circuit-board sales and technical client service and managing business.