Tips for Designing HDI PCBs

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For some designs, using HDI can save you time and cost. Make sure your HDI board is being correctly designed. HDI expert Happy Holden was kind enough to send over a few tips and suggestions. Here are a few things to keep in mind when using HDI in your next PCB design.

Consider your goal when designing PCBs. Are they:
Low cost
Quick turntime
Board size
Electrical performance

Designing Using HDI
HDI is a necessity when the component pitch requires it or the board size demands it. Examples of this include smartphones, wearable technology, and robotics. A properly designed HDI board enables a smaller board (board size goes down), or enables two through-hole (TH) boards being combined into one board (pins per area goes up). A properly designed HDI board has 2X to 3X more average trace lengths per layer that a through-hole board. Some very good HDI designers report 4X performance. So know what your current through-hole design density performance is, and you’ll be able to determine good HDI design performance. HDI boards are much harder to plan then traditional multilayer boards because, with multilayer, you only have two degrees of freedom, boards size, and signal layers.

HDI has a myriad of constructions and variables that all affect density. To understand how blind vias, skip vias, staggered vias, stacked vias and buried vias contribute to density, you have to do a “Routing Test” and play with all these different stackups and constructions. By measuring the metrics (inches/square inch and pins/square inch), you can create a table that relates these density measures to the corresponding construction.

Understanding HDI Stackup and Via Construction
It took us 10 years designing HDI boards at HP before we realized that these “were more than small through-holes.” We kept designing TH multilayers and using the small-laser-drilled blind vias in BGA breakouts in place of the larger drill TH vias. Then we realized that at a rate of >600 vias/second, 20,000 laser-drilled holes (or 200,000) costs the same as 200 laser-drilled holes. We then started to figure out ways to replace a, any mechanically drilled hole with a laser-drilled one.

We also discovered that there was room to put 2 laser-drilled blind vias between the lands of a BGA where we had space for only one TH before. This let us to discover ‘channels’ and ‘boulevards’ created by the placement of these blind vias on the inner layers. Channels and Boulevards are the ‘secret’ to obtaining 2X to 4X routing density on inner layers, which means you can reduce the number of signal layers which also leads to the reduction of the number of reference planes.

Go back and look at past HDI boards you have built and measure their ‘pins/sq in’ and ‘in / sq inch’ wiring on each layer, as well as future builds and create a ‘Table of your own’ to give to people.

HDI Cost
You can pay for HDI processes if you can make a board smaller—or you can delete signal layers (with the 2X — 4X density increase) or if you can combine a number of boards into one HDI board. Otherwise, if you add laser drilling to a conventional multilayer board the price has to go up.

In Conclusion
Use HDI PCB Design techniques when considering that need to be mounted on a particular size of board—or the average connections per square inch of board is extremely high. If the connections of all the parts and test points divided by the size of the board is less than 120 — 130 pins per square inch, there is no need to be using HDI.

One last tip from Happy? You can add laser drilling to a conventional multilayer, but always expect to pay more for this solution.

For more information on HDI, download our HDI Design Guide.

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