by Carrie Sundra
SOT-523. Is that one I can hand-solder, or is that one of those ridiculously tiny packages? SC-89. Is that basically the same as an SC-75, just one has bent pins and one has flat pins? And wait, are those both actually the same as a SOT-523?
If you're been around the block with PCB design, you'll know exactly the frustrating mess of nomenclature I'm talking about. After finally having had enough of it, I spent 3 weeks reasearching standards, poring over countless datasheets, and using Digi-Key's priceless parametric search to bring you...drumroll please....the WHAT THE SOT?! and FOR THE LOVE OF SOD! printed circuit board footprint rulers.
Not only did I want a tool that would help me identify the right size part to put into a design, but I also wanted a tool that would help me identify a drop-in replacement when the inevitable stock issue or obsolescence issue came up. Because there is NOTHING worse than having to re-spin a PCB just to change a footprint!
Did someone say stock issue? With the worldwide parts shortage the electronics industry is currently experiencing, my email has been bombarded by panicked current and past clients who are trying to order boards only to find that 1/3 of the parts are out of stock. Having these handy references on my desk has allowed me to quickly and easily identify alternates and save some manufacturing schedules. Warning: side effects may include your clients or boss thinking you have superpowers.
Here's my process for identifying an alternate. Let's say a trusty N-channel MOSFET in a SOT-523 package that you designed into a board is out of stock for 52 weeks. Your client might be sweating bullets, thinking their manufacturing schedule and income for the next year is completely shot. But you're cool, calm, and collected because you have the WHAT THE SOT?! ruler and Digi-Key's parametric search.
First, you type "transistor" into Digi-Key's search bar, and select "FETs, MOSFETs - Single".
Now to narrow it down from 41k results. Of course we want only parts that are "In Stock" and they should be "Active" so that we can (hopefully) continue to source them for several years to come.
That gets us down to a mere 10k results. Ok, now to the specifics. We want an N-channel FET, and we want it in a SOT-523 or footprint-compatible package. Looking at our trusty WHAT THE SOT?! ruler, we see that there are flat-lead versions that will very likely fit on our board that used a gull-wing part. So we scroll the Package/Case category, using a CNTL-click to select all compatible parts. Check out how the Suppliers have even more names for the packages!
This finally brings us down to a very manageable 25 parts. Now you can go onto other features, like Vgs, Rds(on), etc, to find the most compatible one.
I hope these ruler footprint references help you too! WHAT THE SOT?! has transistor patterns from 3 to 8 pins, from as small as the insanity-sized SOT-883 to the big whopping D3PAK. FOR THE LOVE OF SOD! has diode and passive 2-pin patterns from the finely-ground-salt-sized SOT-962 to the mighty SMC. Each footprint has the more colloquial "small outline transistor" SOT or "small outline diode" SOD size names (with alternates), as well as the equivalent JEDEC "transistor outline" TO or "diode outline" DO names, as well as the Electronics Industry Association of Japan SC names.
You can buy these handy footprint rulers from our Tindie Store, or from Digi-Key. We're a Marketplace seller, we ship fast and have a $5 shipping option.