Ensuring your initial rules are properly applied the first time is critical to eliminating the need for redesign. By creating conditional limitations and implementing embedded design rules, you can circumvent the potential for error to avoid respins and ensure precise power ratings in your PCB layout. Read on to learn more.
I remember my first circuit design that failed during qualification. It was a proven design that had already passed a previous qualification, so I was really surprised to hear the news from my manager that the test failed. “It caught on fire”, the rumor went. Seldom does an electronic circuit actually produce a flame - they’re more likely to smoke a bit. Either way, the scorched mark on my PCB told the story: a transistor overheated to the point it thermally ran away and smoked. But how? This was a proven design. What had changed?
After a brief investigation, the circuit proved to be the same. The same components, the same inputs and outputs, even the same lot and manufacturer. The one thing that did change was the layout. When I laid out this board, the form factor had to be adjusted to fit around part of the mechanical assembly. The copper surrounding the power transistor was originally about a square inch. In this design, it was cut to about a third of that. Due to space constraints, other power producing components were positioned on the PCB closer than would be ideal. Both factors created too much heat for this small amount of copper to dissipate, contributing to the untimely demise of my transistor.
Maintain a List of Design Rules to Quantify Critical Components
Transfer of knowledge is almost always the largest bottleneck and time waster in the workplace. In a perfect world, all designers will diligently take notes when they are constructing their original designs so that their thought process is preserved; however, this is rarely the case since a) it’s tedious and b) notes get lost. The catch is, on paper it’s more efficient to reuse vetted designs, but if many of the critical design parameters have been forgotten, either due to poor note taking or an incomplete transfer of knowledge, then you’re going to waste time guessing and checking design parameters. Embedding a few design rules will help to keep small changes from devastating a design and save time on team designs. Here’s how to do it right from the start:
Create and Embed Design Rules so that they Last Forever
I love rules that make sense, which is why it is important that you define the constraints of your power dissipation before assigning design rules to manage them. The best way to do this it to Make a Datasheet. A well-written datasheet will typically have at least three classifications of power ratings that range from the best to worst case scenarios.
The first rating will be the power that the component alone will dissipate in free space. This can be considered the worst-case power dissipation.
The second rating should be power dissipation of the component when soldered to a FR-4 copper PCB in a “typical” fashion, “typical” being the subjective term. This power value can be considered average, but should be verified empirically when possible.
The third rating is the best-case power dissipation. This is achieved when the component is perfectly mounted to one square inch of two ounce copper. This value should be considered a theoretical value that is unlikely to be practically achieved.
The power dissipation value used in the calculation should be a judgment call somewhere between the typical and best cases.
Now that you know how much power you need to dissipate, you can Create a Rule. This way, when someone uses your design they may not even need to check your original data sheet. That’s not to say you should throw caution to wind and poorly document your datasheets, but let them serve as backup or further clarification to your designs. Modern design software with component libraries can help save time evaluating components, and allow for users to attach rules to specific components. I know I’d prefer to “debug” my design if the layout doesn’t provide an adequate heatsink when it is still virtual and can’t catch on fire.
Other Thermal Tricks
Embedding design rules is the obvious solution for transferring knowledge of component power dissipation, but there are a couple more tricks to keep in mind:
Use Stitching Vias – Stitching vias are thermal conduits to the other side of the PCB. Often if components are too crowded on one side of the board, stitching vias can be used quite effectively to conduct the heat to the other side of the board where, perhaps, more surface area is available to use as a heat sink.
External Heat Sources – Consider the likelihood of heat conducting to your PCB from external sources. A circuit might work fine by itself on a bench, however, if that same circuit is bolted to a metal surface that is heated from another source, flame on. Even passive heating from direct sunlight on a car dash can be enough to damage marginal designs. It’s usually the unanticipated action that damages.
Eliminate the Need for Redesign With Embedded Design Rules
Considering these situations when laying out a PCB can trigger small design adjustments now, that can save unanticipated future revisions. Keeping notes is always a must, but depending how these notes are shared within the team, they could get lost. Embedding rules in the design keeps relevant information right where it is needed – in the design. Using Altium Designer’s design rule feature, every designer can “see around the corner,” and avoid the pitfalls that make redesigns necessary.
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