HDI Rigid Flex

In this guide, you will find all information you’re looking for about HDI rigid flex PCBs.

Whether you want to learn how to choose vias, best material or other HDI rigid PCB fabrication process – you will find the information right here.

So, keep reading to learn more.

What Is HDI Rigid Flex PCB?

HDI is short for high-density interconnect PCB. A HDI Rigid Flex PCB is a board with higher wiring density per unit area furnished on a rigid flex substrate.

A HDI rigid flex PCB utilizes via systems and in-pad techniques, line and space reduction, alongside sequential lamination, as high-density features.

HDI rigid flex PCB

 You find that these features enable the manufacture of compact devices with small size and weight and enhanced electrical performance.

Where Do You Employ HDI Rigid Flex PCBs?

The HDI rigid flex PCB finds use across various industries given its high electrical performance and increased features.

Some of the common applications include:

Consumer Electronics

You find HDI rigid flex PCBs in consumer electronics including, smartphones, curved televisions, laptops, and home appliances.

Their use in these products is due to their tiny nature.

Aerospace

A HDI rigid flex PCB can survive harsh environmental conditions such as space.

You therefore find this PCB type included in missile systems, space aircraft, and satellites.

Healthcare And Medical Use

You find use of HDI rigid flex PCBs in healthcare and medical areas for aiding biological functions and medical examination.

They feature in devices such as transferrable X-Rays, pacemakers and external devices like hearing aids.

Wearable Products

Use of HDI rigid flex PCBs in wearables is down to their compact nature and increased electrical performance.

Smart watches and clothing, VR gear and sports vest all utilize HDI rigid flex PCB.

What Are The Benefits Of HDI Rigid Flex PCB?

Using HDI rigid flex PCB affords you certain benefits in your application including the following:

• You can densely pack components with versatile routing by employing micro-vias and via-in-pad technique.

Consequently, it results in faster signal transfer with greater quality.

• The HDI rigid flex PCB allows you to design and insert additional components on either board sides.

You attribute this to finer routing procedures and a larger wiring density.

• Using stacked vias in the HDI rigid flex PCB boards makes them highly dependable.

These vias support the board against harsh environmental conditions.

• Unlike traditional PCBs where you have many boards, HDI rigid flex PCBs allow you to pack all functionality into one board.

As a result, you reduce the board size and general fabrication costs.

• You employ laser drilling in HDI rigid flex PCBs creating holes with smaller diameters while enhancing the board’s thermal characteristics.

What Via Types Can You Employ IV HDI Rigid Flex PCB?

Vias are small conductive holes in an HDI rigid flex PCB that links numerous layers allowing signals transfer.

You have four via types available for a HDI rigid flex PCB, depending on the functionality as follows:

Through-Hole Vias

Here, you drill a hole through the PCB from top to bottom connecting all the layers of a PCB.

It is the most cost-effective via type since it is simple to produce and can be plated or non-plated.

Blind Vias

With this via type, you connect the external layer of a multi-layer HDI rigid flex PCB to an interior layer.

It is blind since you can only see it from one side of the PCB board making construction complex.

Buried Vias

This type of via is complex and costly connecting two internal HDI rigid flex PCB layers. You use buried vias with an even layer count.

Micro-vias

These are the tiniest vias whose construction is via laser drilling with very small diameters below 150 microns.

Micro vias find common use in HDI rigid flex PCBs connecting successive layers and allowing denser boards.

Vias

How Do you Furnish Blind and Buried Vias in HDI Rigid Flex PCBs?

When making blind and buried vias, you do not employ laser drilling with depth-control.

Instead, you drill individual cores before plating them, stacking and then pressing.

When furnishing a via, you need to ensure it cuts through an even layer count.

Additionally, while it can begin from the top and end at the bottom, the opposite is not true.

Furthermore, you cannot start or end a blind or buried vias from or on another blind or buried via. However, you can enclose a blind or buried via within another though it’s a complex and costly process.

Can You Undertake via Filling in HDI Rigid Flex PCBs?

Via filling involves closing vias by filling them with a non-conductive material.

You can use vacuum lifter in the fixation where your board has significant drill counts.

Sealing a via also controls solder runoff. Additionally, when you have buried vias, you can apply via filling in the inner layers.

You can employ different methods in sealing vias depending on the application.

You can tent, fill, plug both sides or cap them using a metal cover such as in via-in-pad systems.

When filling HDI rigid flex PCBs, you use the IPC 4761 standard. This standard specifies the various types of covering and filling for vias.

Why Do You Use Microvias in HDI Rigid Flex PCB?

You employ micro-vias between layers in a HDI rigid flex PCB with a 1:1 aspect ratio.

Where the aspect ratio is greater than 1:1, you employ a stacked micro-via.

Using a stacked micro-via phases out sequential lamination.

This is due to the commencement of a new blind via on a layer similar where the previous blind via terminates.

The most frequent and cost-effective type of micro-via is the staggered via. Staggered micro-vias need more space because they are not formed on the same core.

The demand for boards with high-speed and density is on the rise and micro-vias help to achieve this. Micro-vias aid in the proper unfolding of internal wires throughout several board layers.

How Do You Manufacture HDI Rigid Flex PCB?

When making a HDI rigid flex PCB, you need to decide on the layer count and board configuration. Additionally, despite the final board size, you make a HDI rigid flex PCB through paneling.

You find the following process essential in the manufacture of HDI rigid flex PCB:

Design Review

After placing an order, you review the design before commencing manufacturing.

Review typically follows a checklist where you examine the drill files, the panelization approach and the output details.

Plotting And Imaging

You use a photo plotter in the creation of photo-tools for the solder mask and legend for every individual layer.

You apply the traces, ground, pads and other primary images using the imaging process.

AOI And Lamination

Where you have a multilayer HDI rigid flex PCB, you inspect the layers by comparing the actual image to the design.

This way, you discover any faults before fusing the inner layers, preventing costly problems.

You employ a hydraulic press in the lamination of inspected layers which include prepreg and conductive core.

The process is under high temperature and pressure which melts the prepreg joining the layers together.

Drilling And Copper Deposition

Drilled holes allow you to attach components, join copper layers, and/or mount the HDI rigid flex PCB in its housing.

You deposit copper following the drilling operation through an electroless plating technique.

Plating And Etching

An electroplating procedure follows, where you plate copper over the conductive outline and the holes to match the circuitry’s design requirements.

You remove the dry film resist remaining on the panel after plating leaving the traces, pads and other copper features.

Solder Mask And Surface Finish

You protect the copper surface of the HDI rigid flex PCB using a solder mask.

A solder mask prevents the solder bridging between components which can cause electrical shorts.

The surface finish encompasses covering the circuitry to prevent oxidation of the exposed copper. You cannot in essence solder oxidized copper.

Electrical Test

In testing a HDI rigid flex PCB, you employ a flying probe tester that transmits electrical signals via probes.

This test allows you to detect opens or shorts, allowing you to intervene before final assembly.

How Can You Stack up a HDI Rigid Flex PCB?

HDI rigid flex PCBs can feature multiple layers tightly routed and employing vias for electrical connection.

You laminate these layers to form a single board preceded by an etching process.

Common stack ups of the HDI rigid flex PCB include:

Stack-ups come in a variety of forms, including:

HDI Rigid Flex PCB (1+N+1)

This is the most basic HDI rigid flex PCB design favorable for ball grid arrays with low input and output counts.

It encompasses line reduction and micro-vias with good mounting stability and dependability.

HDI rigid flex PCB ( 1+N+1)

HDI Rigid Flex PCB (2+N+2)

You find this design with moderate sophistication encompassing two or more HDI layers allowing interconnection using micro-vias with copper.

HDI rigid flex PCB (2+N+2)

 This design finds use in demanding designs necessitating signal transmission performance of high-level.

What Are Some Of The Standards You Employ For HDI Rigid Flex PCBs?

When manufacturing a HDI rigid flex PCB, you find the following standards useful:

• IPC-2226: Encompasses the sectional design standard for HDI rigid flex PCBs.
• IPC-DD-135: Highlights the testing qualifications for organic interlayer dielectric materials deposited for multichip modules.
• IPC-4104: Outlines the materials for use in High Density Interconnect rigid flex PCBs.
• IPC-6016: Dictates the performance specification and board’s qualification for layers in a HDI rigid flex PCB.
• IPC/JPCA-2315: Provides the design guide for micro-vias in HDI rigid flex PCBs.

What Features Define HDI Rigid Flex PCB?

A HDI rigid flex PCB ensures you fit a larger density of components on a board, keeping the board size small.

You find some components require HDI approaches for optimal routing.

In an HDI layout, you’ll find the following features:

• Thinner Traces: In HDI rigid flex PCBs, thinner traces link vias from different layers allowing higher trace density.
• Lower Signal Levels: High voltage or current applications are unsuitable for HDI rigid flex PCBs. High voltages cause electrostatic discharge resulting from the high field strength between adjoining lines.
• Large Layer Count: A HDI rigid flex PCB can support higher layer counts interconnected with vias.

You can increase your component density by using multiple layers in a HDI rigid flex PCB even over 20 layers.

• Smaller Vias: For layer transitions, you employ micro-vias for HDI rigid flex PCBs alongside staggered, blind and buried vias.

The aspect ratios of these vias are less than those of standard through-hole vias.

What Design Rules Do You Consider When Making HDI Rigid Flex PCBs?

There are some basic criteria that apply to any HDI rigid flex PCB configuration including the following:

• Monitor the trace and pad distance.

The solder masks’ clearance parameters are normally similar in size to the trace width and determined by pad size and pitch.

• You don’t have to use staggered vias.

When routing a HDI rigid flex PCB, the goal is to leave enough room on the surface and inner layers.

• Use fewer layers. Boards with a high layer count require more assembly procedures and increase production costs.
• Keep signal integrity in mind. A HDI rigid flex PCB layout should conform to high-speed design principles.

What Factors Guide Your Material Choice For HDI Rigid Flex PCB?

When selecting materials for your HDI rigid flex PCB, you need to consider the following:

• The dielectric constant of the material which refers to its ability to store energy in an electric field.

The material’s direction influences dielectric constant and therefore changes with a shift in the material’s axis.

• The dissipation factor should be low, to prevent the degradation of the transmission signal and hence signal wastage.
• The material’s loss tangent based its molecular structure especially for high-frequency signals. You find low-frequency signals are unaffected.
• Crosstalk and skin effect which are dependent on spacing parameters.

Crosstalk occurs when a HDI rigid flex PCB interacts with itself creating unwanted coupling.

• The material’s peel off resistance which when higher enhances the impact endurance.

It also increases heat resistance and allows for better signal quality.

• The material’s coefficient of thermal expansion which influences processes such as drilling and assembly.

Differences in thermal expansion can cause copper foil separation when exposed to temperature changes.

• The environment in which your gadget operates is a major concern.

As a result, the material should have low water absorption to prevent interruption of dielectric properties.

What Are Some of the Materials You Can Employ For Your HDI Rigid Flex PCB?

When fabricating a HDI rigid flex PCB, you can use FR-4 material which has good dielectric properties while easily available.

FR-4 material composes fibre glass with epoxy resin whose combination can be altered to achieve certain qualities.

Some common materials employing FR-4 include Rogers RO3000 series, Taconic TLX, Isola IS620 and Arlon 85N.

Polyimide is also a useful material in HDI rigid flex PCBs due to its impressive flexibility quality.

How Do You Create Controlled Impedance Transmission Lines For Your HDI Rigid Flex PCB?

It is critical to develop regulated impedance transmission lines to prevent signal loss in your HDI rigid flex PCB.

You find the following two common technologies for creating regulated impedance transmission lines: micro-strip and strip-line.

Micro-Strip

Micro-strip refers to having a trace over the top layer with a ground plane beneath it.

Determining the micro-strip’s impedance is a little complicated.

It is dependent on several elements such as the board material’s relative permittivity, trace thickness and width, and plane height.

You achieve better impedance control by positioning the ground close to the top layer.

Strip-Line

This is nearly identical to micro-strip with the main difference being an extra ground plane over the trace.

Here, you position the trace between a plane pairing allowing you to better contain EMI radiation.

What Features Make Rogers Material Popular in HDI Rigid Flex PCB?

Rogers material are a product range of the Rogers company finding use in HDI rigid flex PCBs.

Rogers material have several distinguishing characteristics allowing their use in various applications.

Rogers material employ glass-reinforced hydrocarbon woven fabric achieving electrical properties similar to PTFE with thin conduction layer.

Some of the reasons for the popularity of Rogers PCBs include:

• Minimal dielectric and electrical signal losses.
• Wide range of dielectric constant values, ranging from 2.55 to 10.
• These materials ensure PCB manufacture cost is low.
• Can easily accommodate controlled impedance.
• High heat management capability.
• Outgassing is minimal in space applications.

What Defines High-Speed Signal In A HDI Rigid Flex PCB?

You find signals with frequencies ranging from 50 MHz to 3 GHz e.g. clock signals, are high-speed signals. While changing the signal from low to high is unlikely, it seems trapezoidal thanks to rise and fall times. The amplitude of the clock signal’s higher frequency harmonics is proportional to its rise and fall time. The magnitude of the harmonics will get less if the rise time is greater.

Greater ground bounce, ringing, reflections, crosstalk, and impedance mismatch difficulties all occur as speed increases.

It has an impact on the system’s digital and also analog qualities.

Is Signal Integrity Important In HDI Rigid Flex PCB?

In essence, a signal should flow unaltered from a source to a load on a HDI rigid flex PCB.

However, this does not occur in practice with some losses resulting from interference such as crosstalk, impedance mismatch, and switching issues.

Signal integrity describes how you measure these high-frequency signal distortions.

By giving realistic solutions, signal integrity aids in the prediction and understanding of these crucial concerns.

When designing a HDI rigid flex PCB, you have to view traces as transmission lines rather than basic wires.

The highest operational frequency in the design aids in identifying handling of traces as transmission lines.

How Do You Overcome Issues Of Signal Integrity In HDI Rigid Flex PCBs?

You can overcome signal integrity issues in your HDI rigid flex PCB by employing the following design techniques:

Signal integrity in PCB

Multiple Routing

You find orthogonal signal routing guides signals to different layers while reducing the linked region. Keep parallel run durations between signals to a minimum.

Impedance Matching

The substrate material, trace width, and trace height from the ground/power plane consist the three major parameters that influence impedance control.

You can employ appropriate termination strategies to control impedance mismatch depending on the area of application.

EMI Mitigation

You can minimize electromagnetic interference in your HDI rigid flex PCB by employing components with low inductance.

Using solid ground planes close to signal planes is also effective.

Attenuation

Dielectric absorption and skin effect can cause transmission losses in your HDI rigid flex PCB.

With dielectric absorption the board’s dielectric material absorbs the signal energy reducing its intensity.

You can counter skin effect by increasing the width of the trace.

Right-Angle Traces and Vias

Having a right-angled trace can create greater radiation by increasing the capacitance value in the bend altering characteristic impedance.

Replacing the perpendicular bend with a 45-degree corners pairing or round bend reduces reflections.

While vias are necessary for routing, they increase a board’s inductance and capacitance.

Avoid putting vias in different traces since they extend trace length.

Crosstalk

Signal transfer through a wire creates surrounding magnetic fields which may interact in case of adjacent wires creating crosstalk.

You can combat crosstalk by employing material with reduced dielectric properties and having ground planes between layers.

Why Should You Carry Out Design Simulations And Checks For Your HDI Rigid Flex PCB?

Maintaining design transparency necessitates signal integrity checks. You cannot eliminate upon board production unless conducted during the design phase.

A signal integrity check through design simulations allows you to highlight errors prior to production. These simulations allows you to detect and eliminate issues that cause a signal’s quality to deteriorate during travel.

Signal rise durations are short when you operate your HDI rigid flex PCB at high frequency, making signal integrity important.

Implementing appropriate termination schemes, ensures your PCB can operate seamlessly at high frequencies.

For all your HDI rigid flex PCBs, contact Venture Electronics now.