February 24, 2015
Why are VIAs cool?
VIAs are kind of cool in electronic design… (dare I say sexy)… hidden below the surface of a flex circuit, in many cases, but carrying the power, the signals, the logic, the intelligence – all the secret sauce we can’t see. It’s kind of like elevators that connect floors of a high rise. Or like a highway interchange that connects multiple highways.
VIAs have that Matrix (the movie) kinda feeling.
VIAs allow us to miniaturize things, make them smaller, faster, better.
So today we will take a quick look at the flex circuit VIA. Most of you already know what a VIA is. For those of you who don’t, you might walk away with a new bit of knowledge.
Flex Circuit VIAs
VIAs, to a certain extent, are the core of electronics – interconnecting multilayers of copper to create modules of fast moving electrons. VIAs are used in IC design, PCBs and, of course, flex circuits and rigid flex.
What is a VIA?
VIAs can carry signals or power between layers. The most common form of VIAs use plated through hole (PTH) technology. In most cases, a VIA/PTH is a drilled, electroplated hole through multiple layers of a PCB, flex or rigid flex and create an electronic or power connection between multi-layers of a PCB, flex or rigid-flex circuit. The PTH connects the pins of connectors to inner signal layers.
The parts of a VIA:
• The barrel – the electroplated tube
• The Pad – connects each end of the barrel to a trace or component
Types of VIAs
On the left is a diagram of the different types of VIAs.
1. Standard Through Hole VIA– provides a through hole opening from the top of the flex through to the bottom. Many times used for a component.
2. Standard buried VIA – Inner layer VIA – not visible from the top or bottom of the circuit. Used for High Density Interconnect – space savings
3. Semi Blind/Buried VIA – VIA exposed to only one side of the circuit
Most vias/PTH’s are electroplated plated with copper. At a later date, we will start to talk about VIA fill – where other via materials may be used as well as copper.
In flex circuit and PCB fabrication the aspect ratio is the thickness of a flex circuit divided by the diameter size of a drilled hole. It’s importance relates to the plating process — plating solutions must flow through the hole efficiently in order to achieve acceptable copper plating. Hole sizes that are small compared with the board thickness can result in unsatisfactory plating.
For example, if your flex circuit is 1.0mm thick and you drill a hole through it of 0.5mm, then the Aspect Ratio is 2:1. Common aspect ratios for flex circuits range from 10-1 to 12-1 depending on materials used, adhesives.
VIAs have become more and more important for HDI (High Density Interconnect). HDI is another via technology used to form very small vias where drilling holes, using a conventional drill bit, is impractical. Also known as micro-vias, this technology creates the hole using laser technology before plating.
We will discuss HDI at a later date since this is a new and key technology that PFC will be heavily investing in. Stay TUNED!
Flex circuit VIAs are part of the signal routing. They provide critical vertical connections between layers to simplify trace routing around other components or when there is a high density of interconnections to be made. The flex circuit VIAs also have an associated capacitance, inductance and impedance and must be taken into consideration when designing a flex circuit.
These added capacitance and inductance will have the effect of more time delay, longer rise and fall times of the signal, degrading signal integrity. This maybe not a problem at lower frequencies designs, however, it could become a critical issue in high speed flex circuit designs.
VIAs can also cause a function change in trace impedance. A typical value of the impedance drop on the VIA is about 10%. PFC takes this into consideration when assisting in your flex circuit design.
Flex Circuit VIA Considerations
Since flex circuits are constructed with the idea that they can flex and bend, it is imperative that VIAs are not designed in bend areas. The last thing you want is a cracked VIA barrel. In addition, PFC always recommends Fillet the connection to a pad or VIA hole.
Ok, so that may have not really been too sexy, but hopefully you learned something. There is so much more to this subject, but I need content for another day. We will dive deeper into the VIA in the future!