HDI PCB Designs and the CAD Tools Behind Them

Although it is said that the only guarantees in life are death and taxes, I would submit that the third guarantee in life is that electronic devices will continue to get smaller and smaller. I’m pretty sure that no one wants their cell phone to grow back to the size of a brick again, so to continue this path of ever decreasing sized electronics means that circuit boards need to shrink in size as well. The best way to do this is to use high density interconnect (HDI) technology in the creation of smaller circuit boards.

HDI PCB designs allow for a number of advantages starting with being able to put more circuitry into a smaller space. Along with using smaller components, an HDI design will allow those components to be placed closer together on both sides of the board. The smaller components will require smaller component pads, and the use of compacted routing as well as buried vias and microvias will all help to reduce the size of the board. As far as performance goes, the smaller board will have better signal integrity which lends itself very well to medical, cell phone, IoT, and other such devices that use them.

Microvias, via in pads, laser drilling, sequential build up, and new and unique board materials are all part of what goes into the manufacturing technology for HDI printed circuit boards. On the design side there are some PCB design tool advancements needed as well. Design rules need to accommodate the precision needed for tighter trace width, spacing and via tolerances. The tools also need to be able to work with all ranges of vias from standard thru-hole vias to microvias, and they need advanced routing features for the compact routing of an HDI PCB design.

With HDI technology becoming more and more prevalent, a lot of PCB design tools are venturing into the arena of HDI design, and fortunately, Altium ^® is one of the best. PCB designers are able to utilize a strong PCB layout tool to coincide with their design process and develop anything from flex PCBs to multilayer PCB to IoT devices into PCB manufacturing. Here are some of the areas that Altium Designer can help you with your HDI PCB designs.

Board Layer Stackups, Rules, and Design Constraints

One of the most important requirements of designing a board with high speed circuitry is to determine the board layer stackup first so that you can correctly calculate the impedance. With a high density interconnect board (HDI), the need for an accurate and complete board layer stackup becomes even more important. It is critical to work with your board manufacturer at this stage of the design because they are the ones who will be building your boards, and they have the experience and know how to make your design work. Once you’ve decided on the appropriate layer stack for your HDI design, it’s time to create that layer stack in Altium Designer.

To create your board stackup, go to the “Design” pulldown menu and select “Layer Stack Manager.” From here you will have the option to configure the layers as needed. In the picture below you can see that we’ve created a 10 layer board. The names of the layers are on the left, and you can see where the signal and plane layers are sandwiched between layers of dielectric material. The third column dictates the material, and you can see which dielectric layers are core and which are prepreg. There are also columns for type, thickness, dielectric material, dielectric constant, and others.

The Layer Stack Manager in Altium Designer

In the picture above you can also see some of the other options that the layer stack manager gives you. For instance, there is an impedance calculator, and you can see how we have opened the drill-pair manager. In order to configure buried vias and micro vias, you will have to designate drill pairs, but we’ll talk more about vias a little later.

With all of the tighter design constraints necessary for an HDI design, you also need to have precision control over your design rules to make sure that your line widths and spacings are the way you want them to be. In the picture below you can see that Altium Designer provides a complete and comprehensive set of design rules and constraints to work with.

Design rules in Altium Designer

To access the design rules, go back to the “Design” pulldown menu, and select “Rules.” You can see in this example that we’ve added some rules to a net class. In this case, the rules are for trace widths. Net classes become really important with any design, especially HDI designs, where there are multiple nets that need to have the same rules applied to them.

Vias; Microvias, Buried Vias, and Thru Vias

Another important part of designing a high density interconnect PCB is the vias that you will use. HDI designs routinely use a full spread of vias from standard thru-hole vias, to blind and buried vias, to microvias (laser drilled). To give you the best routing performance, your PCB design tools need to be able to accommodate all of these vias without a problem, and once again Altium Designer is ready to deliver.

Creating a via in Altium Designer

In the picture above you can see that we’ve created a microvia in Altium Designer. We’ve configured it to have a 6 mil hole size, with a 20 mil pad size. Remember how we mentioned drill pairs above? Here we have set the drill pair on this via to go between our inner layer 1 and inner layer 2. Altium Designer gives us full control over pad sizes on all layers if we choose to do that, as well as options to set solder mask expansion or allow the via to be used as a test point.

Altium Designer also gives you pad and via templates to work from. You can access these templates in the Pad and Via Templates editor in the PCB panel.

Being able to choose what templates and constraints to work from within your PCB edtior is phenomenal.

Another helpful advantage in Altium Designer is the ability to set up design rules for the vias you create so that those vias will be used where you need them. In the picture above we’ve created a rule for our microvia so that it will be used whenever we have a connection that goes between layers Inner Layer 1, and Inner Layer 2.

Routing Your HDI PCB Design

Last but certainly not least, you need some powerful routing tools when working on your HDI designs. As the name would suggest, high density designs are bound to have some high density routing. Again Altium Designer is prepared to help you with any type of routing challenges that you may come across with state of the art interactive routing tools and auto-interactive routing tools.

One of the key attributes of Altium Designer is its unified design environment. To put it simply, all the tools within the Altium Designer suite of tools are designed to work smoothly with each other. Whether you are using the interactive routing tools or the auto-interactive routing tools, all of them will use the design rules from the same location; the master set of design rules that you have been working with all along. This will save from the possibility of errors as well as the annoyance of relying on a system of exporting design data to a third party router with design rules that may or may not fully translate along the way.

A group of nets that need to be routed

In the picture above you can see a very simple example of an area with an obstacle that we want to route around. Altium Designer’s Active Route technology will allow you to direct the path of your auto routing. You can select all of the nets in the group that you want to route, as shown below, and then draw a guide to show the router what direction you want the routing to take.

Designating the path that Altium Designer’s Active Route will take

Once your path, or guide, has been drawn, ^® will do the rest. As you can see in the picture below, the routing has been created the way we directed it to.

The final routed results

Whether working through placing blind vias, or routing through a number of layers, all of the routing tools within Altium use the master design rules as their guide. There are different functions for interactive routing, auto-routing, and auto-interactive routing with Active Route. Altium also gives you differential pair routing, trace length matching, and trace tuning features as well.

With Altium’s ^®, you can specify high-speed design signal path rules to control your routing topologies. Along with Altium’s schematic editor, simulation tools, and real-time cloud connections to part vendors for up-to-date pricing and part specifications, you have everything you need for your HDI PCB designs. Don’t settle for a PCB layout tool that won’t utilize your PCB designers as effectively and will leave you at PCB manufacturing without satisfaction.

Altium Designer is full-featured PCB design software that has been created to complement your engineering processes instead of getting in your way. No matter what type of design work that you are doing, you will find that Altium Designer is the advanced PCB design software that you need to get the job done.

Would you like to find out more about how Altium can help you with your next HDI PCB design? Talk to an expert at Altium.