HDI PCB Design Guide for a Safe, Manufacturable Board

Want to get smaller, lighter, or more powerful? (Who doesn’t?) 

In electronics, customers are demanding all three – at the same time.

It might seem like a near-impossible balancing act, but HDI printed circuit boards are making it happen today. HDI – or high-density interconnect – PCB boards are essential to modern electronics. HDI technology supports the necessary evil of design miniaturization, performing big feats in a small package for consumer, defense, aerospace, and automotive electronics.

Unfortunately, packing all this functionality into a tiny space takes away room from another area – your margin for error. To keep your HDI PCBs just as successful as your standard designs, follow these guidelines.

6 Considerations for Meeting HDI PCB Design Guidelines

There are six main considerations for HDI design, with many of them hinting at IPC’s guidelines for high-density interconnect PCBs:

1. Layer stacking & structure
2. Material choice
3. Design rules & limitations
4. Signal integrity & EMI
5. Managing the environment
6. Design for manufacturability

Let’s start with the foundation!

1. Layer Stacking & Structure

High-density interconnect layout is the technique of arranging PCB components when trace width is less than 8 mils or so. The primary goal of HDI PCB layout is to fit more components on a single board.

The must-haves of HDI board design are:

Microvias

Occasionally, industry veterans refer to HDI boards as microvia PCBs. That’s because high-density circuit boards need smaller connections called microvias to make layer transitions. They slope inward during a transition and terminate at a pad in the next layer.

This limits their useful depth. Avoid making a microvia span multiple layers, as that might reduce reliability.

There are two types of microvias you should know:

Note that buried microvias make the PCB trickier to manufacture, and possibly more expensive.

Stacked vs. Staggered Vias

Stacked vias go directly on top of each other through multiple layers, while staggered vias are offset. Understanding the difference will help you meet your design goals.

There’s no one-size-fits-all answer to whether stacked or staggered vias are better. The choice often comes down to a trade-off between density and cost vs. reliability and manufacturability. Some designers use a combination of both, depending on the section of the PCB and its respective needs. A less-common HDI structure, the ELIC (every layer interconnect) is another option when you have unusually high connectivity requirements.

Sequential Lamination

HDI PCBs typically require multiple lamination cycles, which allows for the construction of complex layer structures. This process involves bonding layers of the board together in stages.

Excited about all that space you’re freed up to work with? Use it to minimize your layer count, rather than as an excuse for a sprawling layout.

Each PCB layer adds another assembly step and manufacturing expense to your project. By keeping your design compact – which was the point in the first place – you can lower your cost per board. You’ll also avoid potentially compressing the board and causing it to lose its intended thickness.

2. Material Choice

The materials that make up HDI boards influence their manufacturability, field performance, and service life. In particular, the dielectric and copper foil significantly impact final results. They each come with special considerations in HDI PCB construction.

Dielectric Materials

As the “glue” that holds the PCB’s layers together, dielectric materials serve as a buffer preventing signals from crossing. There are several factors in choosing the right dielectric for an HDI PCB:

• Compatibility: Incompatibility with the core substrate can lead to delamination or other manufacturing defects.
• Adhesion: A strong bond between the dielectric and the plated copper is essential for the PCB’s long-term mechanical stability. Many OEMs require that the dielectric provides an adhesion strength of at least 6 lb./in. for copper.
• Spacing: The material must provide adequate spacing between layers in high-density designs to prevent shorting and maintain law and order among signals.
• Thermal performance: Will your HDI PCBs reside in areas that experience major hot/cold fluctuation? The dielectric must hold up accordingly and possess a high glass transition temperature (Tg) to withstand wire bonding and rework.
• Thermal shock resistance: HDI PCBs may need multiple soldering and reflow cycles during assembly. The dielectric material should be able to survive thermal shocks without degrading, ensuring the reliability of multiple sequential build-up layers.
• Microvia reliability: The dielectric must support the formation of plated microvias to maintain electrical connectivity between layers.
  
  

Copper Foils

The quality of the copper foils in an HDI PCB is another vital factor in an assembly’s success.

The copper itself must be high-quality in any design with fine, dense traces. This minimizes the risk of defects and improves the consistency of electrical performance.

Foil thickness affects your ability to replicate a compact design in the real world. Thinner copper layers are usually preferable in HDI PCBs because they allow for more precise etching of fine traces. Proper use can reduce the overall thickness of your PCB.

3. Design Rules & Limitations

Here’s where your lower margin of error comes into play.

To do more with less, HDI PCBs use finer lines (traces) and spaces between them. These lines and spaces are often as tiny as 50 microns. Because the lines and gaps are so small, manufacturing HDI boards is more challenging. 

Even slight deviations during manufacturing can cause a major setback. For example, if a trace is too thin, it may not be able to carry the required current, leading to overheating or failure.

Designers need to follow strict design rules to ensure that the PCB is reliably manufacturable. If the design is messy or too small, you could face several issues.

Aspect ratio is one of the guidelines that affects via performance. This measurement is the ratio of the hole depth to the hole diameter. For microvias, an aspect ratio of 0.75:1 or lower is ideal for reliable plating and strong electrical connections.

4. Signal Integrity & EMI

The methodology to signal integrity in HDI boards is basically a scaled-up version of the guidelines for regular PCBs.

While narrower traces are a godsend for size reduction, they still must be capable of transmitting a signal effectively. There are several ways you can improve signal integrity:

• Use the shortest trace possible
• Keep impedance paths consistent
• Isolate digital, analog, & power signals
• Include sufficient ground planes

When your HDI board needs impedance-controlled routing, you'll need to carefully trace and layer the design.

Top Tip: Some CAD tools are especially useful for building HDI models. Find one that offers features like impedance calculating, automated microvia placement, and 3D visualization.

The denser your design gets, the more it’s at risk for electromagnetic interference (EMI). High-power components or signals are usually what sets it off. Parasitic capacitance or inductance from nearby pins or pads can also harm signal integrity.

To avoid this, perform EMI modeling during the design phase.

Of course, there are still multiple steps you can take to limit the threat of EMI:

• Ground planes – provide stable reference voltage; reduce loop area
• Shielding layers – block external magnetic fields; isolate crosstalk risks
• Decoupling capacitors – filter out high-frequency noise; stabilize voltage levels
  
  

5. Managing the Environment

Think about the environment your HDI PCB design will call home. Depending on the application, your product may need to resist some pretty serious stuff.

Extreme temperatures are a common HDI board issue because they’re often used in high-power applications. For sufficient heat management, use:

• Thermal vias
• Heatsinks
• Materials with high thermal conductivity

Conduct a thermal analysis during the design phase to identify potential hotspots and validate that the PCB will remain a cool customer under real-world conditions.

Humidity is an unfriendly reminder that electricity and moisture don’t mix. To protect your sensitive components from water, use:

• Moisture-resistant board materials
• Conformal coating
• Proper sealing techniques

Mechanical stress is a threat to PCBs, too. Fortunately, there are design tricks you can use to improve the board’s resistance to vibration and other sources of fatigue:

• Reinforce the PCB with a thicker or more rigid substrate
• Strategically place components to minimize stress concentration
• Use a strong mounting technique
  
  

6. Prototyping & Testing

HDI PCBs are more complex than traditional PCBs due to their higher wiring density, use of microvias, and multilayered build. This complexity increases the risk of design flaws that might not be apparent until the board is actually built and tested.

What’s a circuit board designer to do?

Prototyping an HDI PCB

Prototyping allows electronics engineers to explore a real-world scenario before committing to full-scale production. Catching and fixing issues in the prototype stage means you won’t have to scrap or rework an entire production run because of performance or manufacturability issues.

Issues you can spot during prototyping include:

• Poor signal integrity
• Overheating
• Physical design errors

Rapid prototyping techniques, like PCB additive manufacturing (3D printing) or quick-turn HCI PCB fabrication services, can help you mock up a design efficiently. From there you can adjust and retest the design with little impact on the project timeline.

Testing Methods for HDI PCBs

Given the high density and complexity of HDI PCBs, testing is not a step to skip. 

There are many ways to verify that your final product meets performance and reliability standards:

The HDI PCB Manufacturing Process: Help Is Out There

HDI printed circuit boards are a no-brainer for complex, compact, and high-performance designs. But they require you to walk a finer tightrope to avoid a host of PCB layout pitfalls.

Collaborate with your manufacturer early in the design process to keep small problems from becoming big ones. Not all manufacturers have the same engineering capabilities, so first make sure that your potential partner can fabricate a high-density PCB layout.

For more tips on validating your PCB design, check out our free e-book: