Power Integrity: 5 Common Myths
Power integrity is nothing new, but it is becoming more and more of a concern and will continue to be on the forefront of people's minds moving forward. With the continuing trend of faster and smaller products, we no longer have the luxury of space. Each millimeter is precious, and we need to design that way. If you’ve been in the industry for more than a few years, you may have heard many of the following PI myths.
Throw Copper at It
You may have been told that the more copper, the better. And by simply doing a copper pour you can solve most if not all of your problems as they relate to power integrity. This, of course, is not the case; you may be able to solve some heat issues, but you also invite other problems such as the creation of islands and peninsulas. Although they seem harmless, islands and peninsulas have specific resonant frequencies, which could cause failures when certain conditions exist. These failures may appear random and are thus extremely difficult to pinpoint and fix. So before you blame it on black magic or some sort of voodoo, make sure to check if your copper pours are creating islands or peninsulas, because you don’t want to toss your design away and redo your layout.
Something else to think about, which as engineers we might not always consider, is cost. Copper isn’t cheap, and we can’t just add extra plane layers willy-nilly, especially with today’s budget constraints. Overdesign is expensive.
Stick to IPC-2152
This one might come as a surprise to you as it does with many others. Yes, IPC-2152 is important and provides guidance on how to avoid issues by means of minimum trace widths for acceptable temperature rises. Applying IPC-2152 in this manner forces the designer to allocate more space than necessary to your PDN, taking up valuable real estate or even creating more layers for a design.
IPC- 2152 is a great tool to have at your disposal and should be well understood if you want to make the most efficient power delivery designs, but it shouldn’t be blindly applied. Engineers who use IPC-2152 in a more thoughtful way alongside a power integrity tool can reduce their power distribution network area while still ensuring a safe design goes out for production.
You Can Never Have Too Many Vias
If you are well versed with IPC-2152, you may have noticed that they are quite loose when it comes to vias. Like with trace width, IPC-2152 is very conservative and may leave your board with larger and more vias than necessary. This can be a problem when your copper shapes are now perforated with large holes. What this does is decrease the area available for the current, which increases the current density and consequently, the temperature. Not only that but it also takes away real estate from the rest of your design, making it especially challenging and time-consuming to finish routing that last 10% of your board. As with the other IPC-2152 rules, take them into account and understand them but don’t just blindly follow.
Copper is Copper
Not all copper is created equal, and we often overlook this. A small and commonly neglected factor when analyzing a power distribution network is the conductivity of copper. As stated before, not all copper is equal. The conductivity of PCB copper is different than the conductivity of pure copper and therefore analyzing your design with a different conductivity can give you drastically different results. We’ve found that on average the conductivity of PCB is 4.7e7 S/m while pure copper 5.88e7 S/m. That's a 22.3% difference! You should always check with your manufacturer for clarification on this important variable.
You Need to Be an Expert
It used to be true that you needed to be (or know) an expert to perform power integrity analysis. Many times you would have to export your design to a power integrity or simulation expert because the software used is too intricate and loaded with different options and parameters. This is not an ideal solution, not even close. It’s expensive, takes a lot of time and you’re going to have to export and import many times until your board is finally done.
With today’s designs so small, and consumers expecting so much, we can’t avoid power integrity analysis any longer; it's a must-have for modern products. Remember, copper doesn’t solve all your problems, vias aren’t as friendly as we thought, and IPC=2152 isn’t absolute truth but more of a jumping off point.
What you really need is an easy to use tool that's integrated with your current design software, which visually shows you the issues and allows you to make changes during design time. That way you don’t need to constantly go back and forth with your simulation guru.