Just as Moore's Law has brought transistor gate sizes to ever smaller dimensions, PCBs have followed suit. Although HDI layout is not a new technique, it is critical in many systems that have high component count and high net count. These boards have enabled many new products, such as new smartphones, powerful custom computers (such as those we build for our customers), networking equipment, and much more.
Designers don't normally plan to create an HDI layout unless they are working with specific components that require such layout and routing techniques. On the contrary, they are often forced to work in the HDI regime for a number of reasons. If you suspect you'll need to work with component densities that require HDI layout and routing, here's what you need to know about your next board and how to plan your design.
High density interconnect (HDI) layout refers to a set of techniques used to layout a PCB when traces widths generally drop below 8 mils (0.2 mm). These techniques are designed to ensure you can fit a higher density of components onto a single board, allowing you to keep your board size small while increasing component count. Not all boards need HDI layout techniques for a number of reasons, while some components require HDI techniques for proper routing.
An HDI board requires smaller vias to make layer transitions, particularly in fine-pitch BGA components and more traces per sq. mm. In order to accommodate fine-pitch components, you'll find the following typical features in an HDI layout:
If you look at the above points and the IPC standards, you can figure out when a board makes the transition to the HDI regime, thus requiring HDI layout and routing techniques. According to the IPC-2221A/IPC-2222 standards, the maximum recommended aspect ratio for through-hole vias is 8:1 (aspect ratio = via depth/via diameter). This means, for a standard thickness 1.57 mm PCB, the minimum drill diameter for a through-hole via is 0.196 mm, or ~8 mils. If your vias need to be smaller to accommodate fine-pitch components, then you'll need to use some HDI-specific via designs and thinner traces.
Clearly, the need accommodate fine pitch components or simply a higher density of components is what motivates the need for HDI designs. Traces are fabricated in the same manner as traces on any other PCB, although care is taken to prevent over-etching. Vias are a different beast and may require laser drilling (for low aspect ratios) through a single layer, known as blind or buried vias.
If you need to span multiple layers in an HDI board, you can't use a through-hole via anymore and still comply with IPC standards. For larger diameter microvias, some manufacturers has experience fabricating blind vias that span multiple layers, but this is not common. Furthermore, placing this in your design will likely violate your manufacturer's DFM rules and will cause your board to receive no-bid status. Instead, vias can be placed in individual layers and stacked on top of each other (e.g., stacked microvias or blind-buried vias). The various microvia configurations are shown below.
Microvia configurations in a typical HDI layout.
These different via configurations are primarily targeted at BGA components with different pitch. Once your BGA pitch gets to 0.8 mm, you'll likely be able to use a dog bone fanout with in-pad microvias. At 0.75 mm and less, you're better off using plated in-pad microvias (VIPPO) to reach inner signal layers, although you can also route signals between BGA pads to reach the outer pad section during fanout. At much lower pitch (e.g., 0.5 mm), it is not recommended to route between pads unless your manufacturer can reliably fabricate traces below 4 mils.
When working with fine-pitch BGA, and the traces and vias that connect to it, there are some basic guidelines that apply to any HDI layout.
Two HDI layout examples with BGAs. Top: Dog bone fanout with microvias. Bottom: Microvia in-pad.
HDI layout and design routing requires working with the right design tools and an experienced design firm. The design team at NWES has experience designing HDI boards for custom computers and wearables, and we're here to help electronics companies design modern PCBs and create cutting-edge technology. We've also partnered with a number of advanced PCB manufacturers, and we'll make sure your next HDI layout is fully manufacturable. Contact NWES for a consultation.