What is HDI blind via Flex PCB

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Flex PCB 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.

Flex provides several advantages to cables. Flex’s higher upfront cost will dramatically reduce cost in the long run. Cables can take several weeks to go through production, while flex PCBs can be completed in several days. In short, using flex eliminates or completely decreases cable cost, reduces assembly time, and produces a lighter end-product.

How do you control flex pcb drilling depth to drill blind vias?

We have advanced machines in our facility to perform the drilling process. These machines can drill vias with controlled depth as low as 1.5 mils.

Can we implement via-in-pads in flex pcb boards?

Having via-in-pads in a flex board completely depends on your design. For a two-sided board, the via can be drilled from the bottom side without disrupting the pad on the top layer. For a multilayer board, a dog bone architecture can be implemented. Here, the trace will be routed away from the pad towards the via. Via-in-pad is not recommended in the multilayer flex boards as it is required to planarize the copper surface after via filling. If planarization is performed on the flex materials, there is a risk of damaging the substrate.

What is the minimum laser drill for blind or buried vias in the rigid section of a flex PCB?

In the rigid section, the minimum laser drill allowed is 5 mils. Pad size is D + 4 mils. Where D is the finished hole size. Please note that laser drills are not allowed in the flex PCB bending area.

What are the special flex pcb materials used in patient contact medical devices?

We use Dupont material to build circuit boards for medtech applications that come in contact with patients.

What is the most cost-effective type of via for HDI flex PCB designs, stacked microvia or blind/buried vias?

The real cost driver is the number of laminations. Vias should never be in an area that is dynamic. You don’t have to use stiffeners, but we do not suggest it in the flex area. Make sure vias stay out of the bending area as well.

What is a suitable approach for laminating a 6-layer rigid flex board?

We can do sequential lamination, from 1 to 2, 2 to 3, 3 to 4, 5 to 4, and from 6 to 5.

We will use sequential lamination for tightly spaced HDI flex pcb boards.


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 pcb 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 pcb 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 flex pcb 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 flex pcb 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.

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