PCBs or Printed Circuit Boards use various types of vias—through types, tented vias, blind vias, buried vias, stacked vias, staggered vias, and via in pad. Most designers know that the via type they use affects its size. In this article, PCB Trace Technologies Inc. discusses the restrictions typically present on PCB via size and the best way to optimize the via size on your board.
Contrary to popular belief, bigger is not always better. This is especially true for PCBA or printed circuit board assembly. Rather, the trend in PCBA is towards smaller circuit boards, as most electronic products are continuously reducing their form factors. The challenge here is improving or enhancing the capability and functionality while simultaneously reducing the size of the board. Several technological advancements are making these changes possible. These include the availability of smaller components, thinner on-board traces, and compact connectors for HDI or high-density interconnect boards.
The demand for smaller boards has led to their use of multiple-layer stack-ups, with Vias providing the interconnections between the layers. Designers use vias in various sizes, with different applications on circuit boards. It is possible to both underuse or overuse them, and designers often have to find a proper balance for their design. As a good design practice, designers must follow a set of PCB via size guidelines to estimate the amount of space the vias should occupy.
Via Types & Size
On a board, both traces and vias have the same objective—to provide electrical connectivity between components or other PCBA components. For this, both must be aptly rated for carrying the necessary current. The only difference between them is traces proceed horizontally across the board/layer, while vias proceed vertically through the board structure. This is true for all types of vias.
Moreover, all via types comprise a copper tube or barrel that originates and terminates in a pad. This feature separates vias from drill holes. Via types include:
Through Vias
This is the most common type of via in a PCB, starting from its topmost layer, traversing through the board, and terminating in its bottom-most layer. On its way through the board, it may or may not connect to circuits in the intermediate layers. Through vias are the cheapest to make, as they can be drilled after stacking the layers.
Tented Vias
This is essentially a through via, but sealed at both ends by tenting them with an epoxy layer. The tenting effectively prevents anything from entering the barrel of the via. The cost of tenting effectively adds to the basic cost of the through via.
Blind Vias
Rather than traversing the thickness of the board like a through via, a blind via starts from either the topmost or the bottom-most layer, but terminates in an intermediate layer, while connecting to circuits in the layers it traverses. Blind vias are more expensive to fabricate as compared to through vias. They also take longer as the fabrication requires drilling through individual layers.
Buried Vias
These vias are similar to bind vias but start from and terminate in intermediate layers while connecting to circuits in the layers its traverses. As such, buried vias are not visible on the topmost or bottom-most layers. Buried vias are more expensive and take longer to fabricate than other types of vias as they require drilling through individual layers.
Stacked Vias
Rather than drill through several layers, fabricators make vias connecting two adjacent layers, taking care they stack up one on top of the other when the various layer pairs are stacked. This method allows them to use the stacked vias to form through, blind, or buried vias as needed. Each via is typically filled with epoxy. Although this method of fabricating vias is more expensive, it is effective for HDI boards, as operators can use lasers to create very small vias.
Staggered Vias
Staggered vias are fabricated in the same manner as stacked vias, but unlike the latter, staggered vias are not positioned one above the other. Rather, they are staggered to facilitate more effective routing. They are more expensive to construct, as staggering requires more pads and traces.
Via-in-Pad
This is an innovative method of using vias, especially useful for routing close-pitch components like BGAs. Unlike all other types of vias, the via-in-pad does not require a separate pad, as it appears within the existing pad of a component. This saves a considerable amount of real estate on the board and makes it available for routing.
The downside of using a via-in-pad is it causes dry solder on the pad, as molten solder can wick down the barrel. To prevent this from happening, the via barrel must be filled with an epoxy filling.
Via Functions
Vias can have different functions on a board. Their size or diameter depends on their function. The various functions and corresponding via sizes are as follows:
Signal or Ground Via
The basic function of this via is to complete a current path. Hence, the size or diameter of this via must be adequate for carrying the projected maximum current.
Thermal Via
The function of this type of via is to transfer heat from a hot component to a metal core or a heat sink. Hence, the size or diameter of this via must be adequate for achieving the desired heat removal, thereby preventing damage to a component, trace, or the board itself.
Tented Via
This type of vias permits current flow but prevents solder and heat flow. Therefore, their size or diameter must be adequate to handle the maximum projected current.
The guidelines above should help designers select the via size and meet the performance requirements of their board. However, there are other considerations for them to address.
Additional Via Size Guidelines
During circuit board design, the designer must also consider the impact of their design on other aspects of the PCB development process. For instance, manufacturability should be a major concern, as a design is useless until someone can build it. Therefore, designers must consider some essential issues when selecting the size of vias in their PCB design.
Board Classification
Before starting the board layout, the designer must classify the board depending on its component density. According to IPC 2222, PCB classification is typically in three levels:
- Level A — Low-density
- Level B — Medium density
- Level C — High-density
Fabricator Capabilities
While almost all manufacturers can drill a wide range of hole sizes, the designer must make sure their chosen fabricator can produce via sizes that their design requires.
Clearance Requirements
As stated earlier, the density of components on the board determines its classification. At the same time, it also directly impacts the clearance requirements. Consequently, this affects the size of the stack and the number of vias necessary. Ill-designed stackups and inadequate clearances can increase the EMI generated by the board, while significantly degrading the signal integrity.
Via Density
If the board has many close-pitch components with a high pin count, it is likely to have a significantly large number of vias. In addition, the board may have high power requirements and thermal vias, which may also lead to an increase in the number of vias, and hence, the via density. Designers must exercise caution, as high via density can affect board parameters like structural integrity and impedance.
PCB Design Package
Designers understand that selecting the via size of a board is not an isolated task. Rather, it has implications for other design parameters, including manufacturability. Using a suitable PCB design package can help designers easily optimize their board layout.
Vias are typically round or circular. However, they may connect to component pads that can take on other shapes. For current carrying pads, their geometry influences the size of the via and the electrical parameters of the signal. Using a PCB design package that includes via management functionality, allows the designer to implement various via sizes, via constraints, and advanced routing capabilities.
Optimizing Via Size
With the right tools, designers can use the right approach and follow PCB via size guidelines like:
Current Carrying Requirements
Vias may have to carry current for ground, power, and signal. It is important the current flow generates minimal amounts of heat and signal attenuation.
Effective Trace Width and Spacing
Routing vias and surface traces are part of the same activity. Essentially, vias must complete the circuit between surface components. Therefore, designers must aim for effective trace routing and spacing.
Follow IPC-2222 Standards
Depending on the component density classification, designers must follow IPC-2222 standards for determining the minimum hole size and pad diameter.
Minimum Number of Vias
Using the minimum number of vias possible is a good rule of thumb. Using more than the minimum necessary may have implications on the board’s electrical and mechanical properties.
Collaborate with Fabricator
Collaborating with the fabricator has the benefit of getting the most effective board. This includes discussing the via drill holes, as the capabilities may vary from one fabricator to another.
Conclusion
According to PCB Trace Technologies Inc., a proper selection of via types and their sizes is important for creating a printed circuit board that optimizes space and meets performance objectives. Additionally, for effective routing, the designer must consider the relationship between routing on the surface, and routing through or to planes.
