rigid-flex pcb board

What is HDI Flex PCB

Flex PCBs offer fantastic flexibility and diversity to circuits. But there are certain limits to how much you can achieve with a simple flex alone. Designers and fabricators are trying to push the boundaries of flex, but it’s still a work in progress.

One of the major milestones in achieving further complexity in a flex circuit is by designing it as an HDI. It combines the features of HDI, making the flex denser in terms of components (within its mechanical limits), which allow more design flexibility and compactness.


Currently, most flex PCBs depend upon plated through holes for connections. For a more complex circuit, rigid-flex is usually preferred over using a plain flex. But an HDI flex can replace the conventional HDI rigid-flex in many cases. Especially if it’s for a dynamic application.

Like a normal flex, and HDI flex allows engineers to design in three dimensions. This alone gives a pure flex a significant advantage over a rigid circuit, in terms of:

1. Reducing the weight of the circuit

2. Lighter circuits are less likely to damage in a mechanically demanding situation

3. Efficient use of the place available for circuitry

All three of these benefits can be enhanced by integrating the HDI technology with the flex. Designing in HDI will allow you much more efficient use of the real estate available. Thanks to thinner traces, microvias, and via-in-pad, an HDI flex circuit can be much smaller in dimensions than the similar circuit in pure flex.

Some of the benefits of using HDI Flex are:

Smaller Form Factor

As an HDI, a flex can have much narrower traces. Designing a flex is as much a mechanical endeavor as it is an electrical one. And having finer traces can allow you a much higher degree of control over the bend radius and dynamic flex capabilities of the circuit. This not only increases the functionality of the circuit but contributes to its small form factor as well. An efficiently designed HDI flex can be significantly smaller than a similar-sized pure flex that would have offered more mechanical restraints thanks to the presence of plated through holes.

More Durable Circuit

HDI flex PCBs are usually covered with PI. Compared to rigid circuits that are covered and protected using solder marks, an HDI flex covered in PI is much more durable and can survive much harsher environments and vibrations. A PI layer handles jerks and vibrations very differently than brittle solder marks. So an HDI Flex might be used in places like waterproof wearable’s, pacemakers, and complex implants.

Better Interconnects

Flex has always been a favorite replacement for traditional cables and connections between two rigid circuits. An HDI flex can take it even further by allowing much denser interconnects, and a much higher number of connections per unit area. The resulting circuit will be smaller, lighter, and have a significantly longer life span.

Enhanced Dynamic Capabilities

This is perhaps the best benefit of an HDI flex. Since an HDI allows for much higher density, it can either be used to reduce the form factor of a circuit or substantially reduce the number of layers. Either way, it offers better flexibility than a pure flex. This makes it much more pliant for dynamic cycles. And HDI flex is known to carry out millions of cycles of flexing without causing fractures in the copper.

Low Layer Count

HDI flex PCBs are much more efficient in employing the surface area of the flex. This means many more components than on a pure flex circuit board. With more components on the surfaces, the need for extra layers is drastically reduced. And low layers result in high flexibility and eliminates the need for complex constructions like book-binding methods. The low layer count also has an impact on the cost. According to an estimate, an HDI-flex can be 40% cheaper than a non-HDI flex. Even in a multilayer HDI flex, the layers in between can be used much more efficiently with the microvias. The use of microvias offers you much more freedom to design per layer.

Enhanced Electrical and Signal Characteristics

The use of microvia instead of plated through holes increases the electrical capabilities and signal integration of the flex. This saves costs by eliminating the need to use complex high-frequency materials or going for a rigid-flex design. Since the paths are much shorter in an HDI flex, the signal integrity increases, and noise and crosstalk problem decreases from the conventional flex PCB.

Thermal Capabilities

The use of microvias in the HDI flex reduces the thermal stress that propagates along the z-axis of a conventional flex, allowing much better heat dissipation and lower layer-to-layer thermal transference. This frees the designer from taking special consideration when designing a high-power circuit.


An HDI flex has significant benefits over a conventional flex or a simple rigid HDI. But these benefits come with their own cost. Drilling microvias in a flex increases the cost of the flex. And while this cost can be reduced by efficiently designing the HDI flex PCB, making good use of the panel space, and decreasing the layer count and form factor of the flex.

An HDI flex has a wide range of applications, from interconnectivity and providing a higher number of connections on a very small circuit, to its use in flat-screen monitors and wearable electronics. This is especially popular in medical applications. And it’s used in pacemakers, imagers, hearing aids, and other medical implants.

Since HDI flex is a still-evolving technology, not every fabricator is proficient in it. The choice and availability of tools, materials, and knowledge of cutting edge fabrication techniques can allow you to design a very cost-friendly HDI flex PCB. For that, you have to make sure that you find the right fabricator.