HDI Rigid-flex PCB

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Oakley Mae

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If you have been careful enough, you have noted that electronic devices have reduced size over time.

Surprisingly, this has only been possible thanks to new technologies like HDI rigid-flex PCB. 

An HDI (high-density interconnect) PCB is a board with higher wiring density per unit area.

This wiring is furnished on a rigid-flex substrate and employs line and space reduction, sequential lamination via systems, and in-pad techniques.

Such features guarantee the design and fabrication of compact devices with improved electrical performance. 

However, many questions remain unanswered on the applications, benefits, and how to manufacture an HDI rigid-flex PCB. Read on for answers. 

Table of Contents

What’s An HDI Rigid-flex PCB?

This circuit board is sometimes called a multilayer rigid-flex circuit board.

Therefore, expect it to provide the benefits of both flexible and rigid circuit boards.

Moreover, it has the added advantage of having higher-density interconnects, something you’ll never find in normal circuit boards.

It employs micro-vias, smaller vias, and advanced packaging technologies to achieve this. 

Surprisingly, you develop HDI rigid-flex PCBs by bonding multiple thin, flexible circuit material layers.

You’ll then laminate the layer to create a flexible circuit board.

Consequently, you bond the layers with an adhesive. Afterward, you subject the resulting laminate to high temperature and pressure to guarantee stronger bonds. 

Moreover, we create HDI rigid-flex PCBs using a thin conductor metal layer on the external surface.

In most, we use copper, which connects the PCB circuitry to external components like resistors, transistors, and inductors. 

Generally, HDI rigid-flex PCBs can contain about 4 to 16 layers. And since they’re highly flexible, you can employ them in applications requiring limited space.

Also, we recommend them in high-density applications. 

Applications of HDI Rigid-flex PCB

Application of HDI rigid-flex PCBs in smartphones

(Application of HDI rigid-flex PCBs in smartphones)

Check out the HDI rigid-flex applications below:

Aerospace

You’ll find these circuit boards in aerospace applications thanks to their ability to survive in harsh conditions.

Some common applications here include space aircraft, satellites, and missile systems. 

Medical Use and Healthcare

HDI rigid-flex circuit boards are finding increasing application in the medical field and healthcare for assisting medical examination and biological functions.

Therefore, you’ll find them in pacemakers, hearing aids, and transferrable X-Rays. 

Wearable Products

A wearable watch

(A wearable watch)

Generally, HDI rigid-flex circuit boards provide increased performance and work well in compact spaces.

Therefore, they find common applications in wearables like VR gear, smart watches, sports vests, and smart clothing. 

Consumer Electronics

Thanks to their tiny nature, most electronics like smartphones, home appliances, laptops, and curved television use HDI rigid-flex PCBs.

Moreover, these PCBs have a higher electrical performance than the traditional options. 

Advantages of HDI Rigid-flex PCB

You’ll get the following benefits from using HDI rigid-flex PCBs:

  • They allow you to densely pack your components with versatile routing using via-in-pad and micro-vias techniques. Consequently, you’ll get great quality and faster signal processing. 
  • Thanks to a larger wiring density and finer routing procedures, the boards let you design and place additional components on both sides of the PCB. 
  • These circuit boards work well in harsh conditions making them highly dependable. 
  • HDI rigid-flex PCBs are smaller in size and have lower fabrication costs. This is because you can pack all functionalities on a single board. 
  • The circuit boards are thinner and lightweight. Surprisingly, they’re about 0.004 inches thin, at least half thinner than traditional options. Moreover, they’re about 50% lighter than the traditional options. 
  • You employ laser drilling in HDI rigid-flex PCBs, creating holes with smaller diameters while enhancing the board’s thermal characteristics.

Via That You Employ In HDI Rigid-flex PCB

Young lady checking PCB

(Young lady checking PCB)

A Via is a small conductive hole in your PCB that connects several layers, thus allowing for signal transfer.

Generally, we have four via types, as we’ll highlight below: 

Through-Hole Vias

This involves drilling holes through the circuit board to connect all PCB layers.

They can either be non-plated or plated and are the most cost-effective Via option. 

Blind Vias

These vias connect the multilayer HDI rigid-flex PCB’s external layer to the internal one. Therefore, you can only visualize this Via from one PCB side. 

Buried Vias

This visa type connects two internal circuit board layers.

Therefore, it’s complex, costly, and applicable with an even layer count. 

Micro-Vias

These are the smallest option and are constructed using laser drilling.

They have very small diameters of less than 150 microns. You can use them to connect successive layers and allow sensor boards. 

Manufacturing HDI Rigid-flex PCB

PCB manufacturing

(PCB manufacturing)

When manufacturing an HDI rigid-flex PCB, you must decide on the board configuration and layer count.

Surprisingly, you’ll have to create your circuit board through the paneling. 

Follow the steps below to manufacture your PCB:

Design Review

We consider this the first stage, which involves reviewing your design before manufacturing begins.

This happens after placing your order.

Generally, it involves following a checklist where you’ll examine the output details, the penalization approach, and the drill files. 

Plotting and Imaging

The second step involves employing a photo plotter to create a legend for every layer and a photo tool for the solder mask.

Also, you use the imaging process to apply the ground, pads, traces, and other primary images. 

AOI and Lamination

If you have several layers on your PCB design, inspect them by comparing them with the actual image.

Therefore, you’ll identify any problems before going further into your manufacturing process.

Surprisingly, this process happens under high pressure and temperature. 

Drilling and Copper Deposition

Next, you drill the circuit board to join copper layers, mount the PCB in its housing, and attach components.

Afterward, you deposit copper using an electroless plating technique. 

Plating and Etching

This step involves electroplating copper over the holes and conductive outline to match your circuitry requirements.

Afterward, you’ll remove the dry film resits on your panel to leave pads, traces, and other copper features. 

Solder Mask and Finish

Next, you protect the PCB surface with a solder mask.

This eliminates electrical shorts by preventing the solder bridge between components. 

Electrical Test

Finally, you can use flying probes to test your HDI rigid-flex circuit board.

This way, you can identify shorts and open and intervene before the final assembly process. 

Features Designing HDI Rigid-flex PCB

Generally, this circuit board guarantees a large density of components to keep it compact.

Therefore, it has the following features:

  • It has thinner traces to guarantee a higher trace density.
  • The circuit board has lower signal levels. Surprisingly, high voltage or current will lead to electrostatic discharge.
  • It has a large layer count that could go as high as 20.
  • Finally, small vias allow for layer transitions.

Standards for Designing HDI Rigid-flex PCB

Keep the following standards in mind while designing HDI rigid-flex PCBs:

  • Firstly, we have the IPC-2226, which involves the sectional design standards for the circuit boards. 
  • Secondly, there is IPC-DD-135 which defines the organic interlayer dielectric materials testing qualifications.
  • Thirdly, IPC-4104 defines the materials you should use for manufacturing the circuit board. 
  • Moreover, we have the IPC-6016, which highlights the board’s layer qualification and performance specification. 
  • Finally, the IPC/JPCA-2315 defines the micro-vias design guide.

Overcoming Signal Integrity Issues in HDI Rigid-flex PCBs

Consider the below techniques to overcome signal integrity challenges:

  • Multiple routing: Ensure the parallel run durations between signals remain at minimum levels.
  • Impedance matching: Generally, the trace height, trace width, and substrate material are the parameters that affect impedance control. Therefore, use the right strategies to regulate impedance mismatch. 
  • EMI mitigation: use components having low inductance to reduce electromagnetic interference
  • Attenuation: Increase the trace width to eliminate the skin effect and dielectric absorption. 
  • Crosstalk: To control crosstalk, use material with less dielectric properties. 
  • Right-angle traces and vias: Surprisingly, right-angle traces increases capacitance, consequently increasing radiation. Therefore, replace these right-angle bends with 45-degree ones. Also, don’t place vias in different traces. 

FAQ

Is HDI Rigid-flex PCB worth it?

Yes! HDI rigid-flex circuit boards offer higher-density interconnects, something you’ll never find in normal circuit boards.

Additionally, they’re reliable, high-quality, and durable. However, you’ll find them more costly than the traditional options. 

What’s HDI Rigid-flex lifespan?

We guarantee that HDI rigid-flex PCB is a highly durable board. In most cases, it lasts for at least 50 years.

Surprisingly, the time could exceed 70 years with proper maintenance practices. 

Conclusion

As highlighted, HDI rigid-flex PCB, which we sometimes call a multilayer rigid-flex circuit board, contains a higher wiring density per unit area.

Surprisingly, it contains the features and benefits of rigid and flexible boards. Moreover, it uses micro-vias, smaller vias, and advanced packaging technologies. 

Generally, these circuit boards are reliable, high-quality, and durable. However, be ready to pay more than you’d for the traditional options.