Vias In Flexible Circuit: The Ultimate Guide

//

Oakley Mae

What’s the role of vias in flexible circuit boards?

But assuming you understand what vias are, what are the common via types, and what should you consider when creating them?

Well, you’re in the right place if such questions are giving you trouble. 

It’s no secret that a circuit board comprises many components and parts that contribute immensely to its operation.

One such important component is the via, which allows transmitting signals between circuit board layers.

And while several via types perform the same function, you must define each accurately in the documentation for a seamless assembly. 

Keep reading for critical information relating to vias in flexible circuit boards. 

Table of Contents

What’s A Via in Flexible Circuit

A close look at a PCB

(A close look at a PCB)

This is simply a metallic hole that offers a mechanism for transmitting signals between circuit board layers.

However, these holes pass through several PCB levels, with designers having the freedom to determine to what extent it goes based on flexible PCB design

Surprisingly, you’ll find some vias only going through the bottom or bottom layers. Also, other vias only penetrate through the inner layers.

All in all, they are critical circuit board components, so you must understand their role with perfection.

Generally, via reduces the PCB size by eliminating the bulky wires for electrical conductivity. 

The Common Via Types

There are several via types, as we’ll see below: 

Blind Via

These holes penetrate the bottom or top circuit board layers and not through the board. As the name suggests, you can’t see through the blind via. 

Generally, you can drill blind people using either a laser or mechanically.

However, it would be best to be careful not to affect unintended parts, especially with the drilling accuracy.

Most manufacturers avoid these vias due to the high accuracy required to drill, thus presenting high chances of damage. 

Surprisingly, you can use a blind via to connect one PCB surface layer to another. 

Buried Via

As the name suggests, these vias connect the inside layers of a circuit board.

Therefore, we highly recommend them for PCBs having dense routing.

However, seeing through buried vias with your naked eye is impossible. 

Through-Hole

Through-hole technology

(Through-hole technology)

These are the most common and popular via types.

The holes go through all the circuit board layers and are larger than buried and blind vias.

And since the vias cut through the circuit board, it’s possible to go through them. 

Additionally, they might be the easiest vias to drill.

All you need to do is drill through the circuit board layer without caring about anything else.

These vias find application in plated through hole technology. 

Microvias

Microvias are the tiniest options and involve all vias below 150 microns.

They find application in high-density interconnects since their small size takes less PCB space.

Surprisingly, layer connection is only possible through copper plating. 

However, they can only go through two neighboring layers and no further.

Therefore, if your board requires a visa through multiple layers, the only option is to stack several microvias. 

Via-in-pad

A photo of macro BGA ball grid

(A photo of macro BGA ball grid)

Finally, on our list is the via-in-pad which employs vias on BGA (ball grid array) pads.

These vias have become popular among manufacturers since they use less space.

Therefore, they’re the perfect option for designing circuit boards for compact devices. 

Benefits of Vias in Flexible Circuits

Generally, vias present the following benefits:

Signal Routing

There is no better signal routing enhancer in circuit boards than Vias.

Surprisingly, denser board designs require buried or blind vias.

Also, you can use microvias in such situations. 

Power Routing

Vias are very helpful for power and ground nets. In most cases, they carry larger currents. 

Trace Density

This one is self-explanatory since adding vias on the circuit board will add to the traces.

When designing vias, you can run them over and under each other in every direction.

Consequently, you’ll connect different traces. 

Signal Transmission

Lastly, we come to the main function of vias, power, and signal transmission.

As we mentioned, blind vias transmit signals between surface layers.

On the other hand, buried vias transmit signals between inner layers. 

Vias in flexible circuit Design Tip

BGA ball grid

(BGA ball grid)

Follow the tips below to design vias successfully: 

  • Buried and blind vias should involve controlled depth.
  • Always check the component leads and copper foil.
  • Confirm whether the blind has any irregularities.
  • For the case of high-density interconnection (HDI) boards, use smaller vias. Surprisingly, it’ll help to limit strain capacitance and inductance.
  • Unless via-in-pads are within the thermal pads, fill them up.
  • Moreover, employ a small aspect ratio to eliminate signal reflections in high-speed vias. Generally, you’ll get higher performance and manage better signal integrity. 
  • Always check the outer layer to create a good via.
  • Finally, always check the fine pitch BGA components and hole diameter for via-in-pad. 

What To Consider When Creating Vias in Flexible Circuit

You don’t just create vias blindly. There are several considerations to abide by:

Signal Integrity

If you fail to drill the via carefully, it might cause signal integrity challenges.

For instance, if you have a through hole connecting the ends of a six-layer circuit board, the board will have four layers of unnecessary metal material.

Such material can result in serious problems if not well taken care of. 

Therefore, you must identify such problems and find quick solutions.

We recommend back-drilling the via to eliminate these unused metal materials. 

Aspect ratio:

The aspect ratio is the relationship between width and height; for vias, it represents the ratio of board thickness to hole diameter.

Therefore, your board thickness will determine the drill size, considering there is a preferable aspect ratio.

Surprisingly, most manufacturers prefer an aspect ratio of 10:1.

However, nothing stops you from using an aspect ratio of 1:1.

Routing Density

We don’t recommend vias when you want to route dense circuit board sections.

Doing so might block the ground plane’s return path.

The hole depth and outer layers can improve the routing density. 

Annular Ring

Even after drilling the via, you should leave behind large annular rings.

A large annular ring eliminates a drill breakout which might compromise the via.

Also, it keeps the via under serious check. 

FAQ

Which vias should you use?

We recommend through hole vias anytime since they are the least expensive.

Most manufacturers prefer using them since they cut through all the circuit board layers.

However, before you use buried or blind vias, have a serious discussion with your circuit board manufacturer. 

When should you use Vias?

The main function of vias is to lay down a path for transmitting signals and thermal and electrical energy.

Therefore, if you expect your circuit board to dissipate more heat energy, you should have more vias.

The same applies to circuit parts that carry signals or power.

Surprisingly, many smaller vias work better than one large one. 

How are vias made?

Generally, vias are developed during the circuit board fabrication process.

First, the manufacturer drills holes through the circuit board’s copper pads.

Next, they apply chemicals to dissolve the internal layer of epoxy.

This will further expose the internal copper pads. Lastly, they insert copper into the holes with electroplating

Conclusion

By now, we’re sure you understand Vias more than before.

But in summary, they are metallic holes that offer a mechanism for transmitting thermal and electrical energy.

Also, they help to maintain the integrity of the circuit board signal.

As we have highlighted, vias are available in different sizes and types, and the circuit board requirements mainly determine this.

Therefore, always keep in touch with your circuit board manufacturer to get the best via design.