High-Density PCB Layout Tips for Designers

Key Takeaways

• High-density PCB layouts offer benefits such as reduced real estate requirements, increased circuitry on the board, and improved signal integrity through compression, but they also require precise planning of the layer stackup to achieve optimal signal integrity levels.

• When working on high-density designs, designers should consider factors such as pin-pitch, line widths, spaces, and various via types, adapting their routing strategies accordingly.

• The transition to high-density designs may involve using new manufacturing processes and materials, which should be carefully balanced against overall production costs to ensure cost-effectiveness.

An enlarged view of vias in a high-density PCB layout

High-density PCB layout has become the standard for many circuit applications. The reduced space required for the layout allows designers to miniaturize the design, increase the product's functionality, or achieve both of these goals. However, as expected, there are layout details that designers must be aware of when using high-density interconnect technology.

High-Density PCB Layout Benefits and Concerns

Printed circuit boards are shrinking in size to build the next generation of compact electronic devices while increasing the amount of functionality they can support. These changes are accomplished by using high pin-count components with pin pitches of 0.5 mm (or less), necessitating trace widths and spaces of 2 mils or 50 microns for their connections. Multiple via technologies, along with advanced fabrication materials and manufacturing processes, define these designs as high-density PCB layouts.

There are many benefits to using high-density interconnect, or HDI, in circuit board design, along with some expected concerns:

Benefits:

• Miniaturization: Advanced components with smaller package sizes and pin pitches reduce the real estate necessary for circuit layout.
• High-density circuitry: Component-to-component spacing is less than standard manufacturing requirements allowing for more circuitry on the board.
• Narrow trace width: Thinner trace width and spacings allow for more routing per square inch than is possible with standard PCB layouts.
• Diverse via types: Multiple via types, including thru-hole, blind & buried, and microvias, also help increase the amount of routing space.
• Thinner dielectric materials:  Decrease board widths and improve PCB reliability.
• Performance enhancement: This compression helps circuit performance with reduced inductance, less susceptibility to EMI, better thermal management, and improved signal integrity.

The improvements in quality and reliability in HDI designs effectively lower their overall costs making a high-density PCB layout a desirable option. However, there are some concerns that designers need to consider first before jumping into the deep end of the HDI pool.

Concerns:

• Layer stackup planning: For the best performance, precise planning of the board layer stackup is essential for achieving the highest levels of signal integrity.
• Adaptive routing strategies: Routing strategies will change due to the pin-pitch, line widths, and spaces, and the vias used, and designers must be ready to incorporate these new technologies into their thinking.
• New manufacturing processes and materials: The manufacturing of HDI design will require new manufacturing processes and materials, and their associated costs must be balanced against the overall production costs. In other words, HDI technology may not be warranted if it results in an unnecessary escalation in price.
• Cost analysis and tradeoffs: Standard manufacturers may be unable to handle the increased complexity of HDI boards, and designers may have to find new vendors if necessary.

Considering the potential changes in manufacturing and the resultant new design requirements, designers must research before starting an HDI design. Next, we will look at some basics that layout designers need to know when working with high-density PCBs.

PCB Layout Tips for High-Density Designs

As a seasoned PCB layout designer, you already have a solid knowledge foundation for laying out an HDI design. These include your experience working with PCB layout fundamentals, the electrical requirements of your designs, general manufacturing processes and how to design for them, and your CAD tools. 

However, there are some new design requirements with high-density PCB layouts that you must consider:

As we started with, learning HDI design is an extension of what you already know, and that’s a good thing, too, considering that the next level of HDI is now before us.

What Comes Next in High-Density PCB Layout?

The next level of HDI design is ultra-high-density PCB layout. These designs will see smaller trace widths and spaces that go below 50 microns and microvias that are less than 75 microns. Smart technology, such as cell phones and medical equipment, has driven the need for HDI, with some designs built using 25-micron trace widths and spaces. Many products today are already being manufactured with ultra-high-density PCB layouts.

The problem designers are having is finding manufacturers set up to build this technology. Many are not yet ready to fabricate PCBs this small as it requires a significant change in equipment and manufacturing processes. Direct imaging and the ability to handle the ultra-thin foils used in fabrication are essential. Even new automated optical inspection equipment is required to verify the tighter manufacturing specifications. And those are just some of the considerations that board builders will face as they move into manufacturing ultra-high-density PCB layouts. But this technology is coming, and designers must be ready and armed with the tools they need to lay out HDI design successfully.

Constraint management systems like this are essential in high-density PCB layouts

HDI Layout in Cadence Design Tools

Designing the latest PCB technologies requires CAD tools with advanced technologies already incorporated within them. Managing multiple design constraints is one of the capabilities of a design system that is an absolute must in high-density PCB layout. Designers must be able to control the spacing of their components, as you see above, as well as connection types, trace widths & spaces, differential pairs, vias, and the list goes on and on.

As a designer, you will need the advanced capabilities of OrCAD PCB Designer for HDI designs. And in addition to its constraint system, you will have a wealth of design and analysis software at your fingertips.

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