When it comes to multilayered PCBs, their interconnection has a strong role to play in their efficacy. It is here that a PCB via comes into play. They are used to electrically as well as thermally connect the traces, pads & polygons on the different layers of the PCB. With the miniaturization of SMD chips & the rise of high-density BGAs, the role of vias is only increasing. Let us look at the different aspects of a via in some detail.
What does a PCB Via consist of?
Broadly there are 3 parts to a via. These are:
1. Barrel
This is the conductive tube that fills the drilled hole.
2. Pad
It connects the end of the barrel to the component, plane or trace.
3. Antipad
This is the clearance hole between the barrel and the no-connect metal layer.
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Different types of Vias in PCB
PCB Vias can take the following different forms:
1. Through-hole PCB via
This is a commonly used via. In such a via, the holes are drilled through the various layers of the board. Such vias are easy to make and they consume a lot of space on the board.
2. Buried PCB via
This via connects internal layers of the board. It is of great use in miniature appliances where component density is high.
3. Blind PCB via
This via starts on either the top or bottom of the board but doesn’t go to the internal layers. Needless to mention that drilling a blind via PCB requires a lot of dexterity.
4. Microvia
These connect the adjacent layers of the board and are under 150 microns. These are put to efficient use in high density interconnect boards. Micro vias in PCB consume a lot less space.
The kind of via to be used depends on several factors including but not limited to:
- Technology used
- Needs of the circuitry
- Cost of fabrication
What is Via in Pad?
Simply put, it is a design practice that involves placing a via in the copper landing pad of a component. The big advantage of via in pad is that it allows high component density and supports miniaturization. While with traditional vias, the signal is routed away from the pad and then to the via, in case of a via in pad, the drill hole for the via is inside the pad. Therefore, it reduces the space taken up by trace routing & helps in miniaturizing the PCB.
PCB Via in pad is particularly convenient when it comes to routing as the signals do not stand the risk of going outside the device’s footprint.
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Creating Precise Vias in Pad
Some guidelines to create precise via in pad include:
- Use small via diameters.
- Ensure that when it comes to component placement as well as via capping or filling, you follow the recommendations laid by the component manufacturer.
- The non-component side needs to be capped with solder mask.
- Vias shouldn’t be kept open as the inner copper tends to oxidize, which can, in turn, impact the lifespan of the PCB.
Advantages of Via in Pad
PCB Via in pads offer a number of advantages such as:
- They promote space saving & lend themselves to high density of components.
- They help in thermal dissipation.
- Offer lower inductance on account of no dog-bone pads with traces.
- Increase voltage capacity of the via.
When to Use Vias?
Via in pad finds extensive use when small pitch components are in use. Additionally, when routing complex BGA & LGA packages capped via in pad can make the process simple. They help in minimizing parasitic inductance as well as EMF emissions in the case of high-frequency designs. Via in thermal pads is also useful in heat management.
Reason behind PCB Via failure
Typically, failure of vias occurs on account of the differential expansion and contraction when it comes to the copper plating & the PCB in the out of plane direction. On account of the differential expansion and contraction, there is cyclic fatigue in the copper plating that can lead to cracks. It is imperative to follow the IPC guidelines for vias. The IPC 4761 defines the various via types from type I to type VII. While type one defines tented vias, type VII defines filled and capped vias which are filled with non-conductive paste and are over plated on both sides.
Guidelines for Vias
Some guidelines to create precise via in pad include:
- Use small via diameters
- Ensure that when it comes to component placement as well as via capping or filling, you follow the recommendations laid by the component manufacturer.
- The non-component side needs to be capped with solder mask.
- Vias shouldn’t be kept open as the inner copper tends to oxidize, which can, in turn, impact the lifespan of the PCB.
To sum up
PCB vias are extremely crucial in multi-layered circuit boards. For board components that bear power or quick signals, you would need several vias to link the layers. The use of multiple vias as opposed to a single large via helps keep inductance in-check and works well in case one of the vias tends to fail.
Mermar Electronics thoroughly evaluate the PCB via requirements and deliver a high-quality product with a quick turnaround time. Mermar Electronics also provides PCB design and PCB assembly services. To request a free quote, fill out the form below. You can also send mail at sales@mermarinc.com.
PCB Via FAQs
1. Why fill PCB vias with epoxy?
Non-conductive epoxy helps in that it ensures that the board expands or contracts in conjunction with the laminate material & so there are reduced chances of any fractures caused on account of stress. Conductive epoxy helps with transferring thermal energy and hence conducts heat away from the board.
2. Do vias increase the cost of a PCB?
While the use of vias, particularly the use of micro vias add to the cost of the PCB, importantly they help in keeping the PCB size small and component density high.
