First, we need a scientific taxonomy of form factors to classify them for better understanding and to make the clandestine origins of many of them pop out. Taking this chore into my own hands, since no one else has done it with any clarity, I see basically three categories of board form factors: 1. Rack mounted (the PCBs are easily inserted or removed from a rack, without tools, in a matter of seconds, and without divine intervention); 2. Canned (the PCBs are enclosed in some kind of container and not easily inserted or removed without the use of tools, a lot of time, and divine intervention); and 3. Snap-ons (mezzanine cards that snap onto one of the form factors in 1 or 2 above). Of course, PMC/XMC/AMC mezzanine cards fall into this category. I am sure that I could concoct a species-genus-family-order-class-phylum-kingdom-domain-life taxonomy here, but I am only allotted a finite amount of space by my evil editorial masters. So, this will have to do for now.
In the rack-mounted arena, we are bound by the 19-inch rack (or in telecom, the strange, disproportional, and comical 21-inch rack). Here, all PCBs are defined in height (a unit of measure called a U, which is 1.75 inches), depth (typically in 60 mm increments), and width (the horizontal distance between boards in a rack, referred to as the pitch, measured in inches or millimeters, depending upon which side of the Atlantic you reside). So, there is some order in the chaos of proliferating form factors, but only in the 19-inch rack kingdom. However, we still have 3U, 6U, 8U, 9U, and 12U cards available, with depths of 160 mm, 220 mm, 280 mm, 340 mm, and 400 mm. We also have pitches of .6, .8, and 1.0 inches, from my empirical observation. While somewhat prolific, at least there are internationally accepted standards for these form factor derivations. This fact exonerates the rack-mounted form factors from the conspiracy theory mentioned previously. VME and VXS boards are all 6U x 160 mm (with pitches of either 0.6 or 0.8 inches for conduction- or air-cooled boards, respectively). VPX boards can be 3U or 6U tall, but both are 160 mm deep. And, the pitch of these boards can be 0.6, 0.8, or 1.0 inches, depending on how they are cooled (conduction, air, or liquid).
Now, we must enter the mushy morass of canned form factors. They have names such as EBX, ITX, ATX, ETX, AT, and NLX, just for starters. If that wasn’t confusing enough, there are totally random variations of these form factors, designated by such adjectives as Baby, Nano, Pico, and Micro preceding the acronym. And, these preceding adjectives lack, in any conceivable way, any direct bearing on the PCB’s size. Surely there is a "femto" in our future as this craziness continues. Maybe even a "Groucho," a "Chico," and a "Harpo." These are mostly motherboard form factors that came down from the commodity desktop computer marketplace with the same subtlety as the Huns invading Europe. But wait. There’s more bad news.
There is a whole load of smaller board sizes in this category with names like PC/104-Plus and COM Express. They all differ by only a few millimeters in dimensions, the connectors they use, and the company that makes them. If we want to get into deeper taxonomy here, this would be called the small form factor phylum. This particular group of board sizes exposes the most glaring, repulsive, and grotesque mutations in the entire history of PCB fabrication.
Finally, we have the snap-on category. These are mezzanine cards that snap onto one of the other form factors in the 19-inch rack segment, or onto the larger canned form factors. They enable easy and low-cost customization of the parent PCB (the form factor they snap onto) to meet the application or customer requirements. Their dimensions are consequently dictated by the size of the parent PCB, but that limitation has not delayed designers from their appointed proliferation objectives. They carry names such as PMC, XMC, FMC, and AMC, to list a few. However, I see mezzanine form factor derivations as a necessary evil. They are one of the few form factors derived from common sense and need, and are acquitted of conspiracy charges.
So, why do we have so many form factors? Because certain companies want to make the markets for boards heterogeneous by fragmenting them into ever-smaller niche segments. Then the big companies can’t play in such small segments, or play in all of them, so certain companies have a nice protected business environment. That is the conspiracy underlying the proliferation of form factors. This proliferation has nothing to do with application needs or customer requirements in most cases. It has everything to do with companies trying to create protected markets.
With the concurrence of my evil editorial masters, we have created a list of more than 100 different form factors, including the owner of the specification, the dimensions of the PCBs (even the mutant board sizes), and other revealing information. This list is available at www.smallformfactors.com/list. Make sure you check out this exclusive, exhaustive, and ever-expanding list. It will be a memorable experience for you, just as writing this column was for me.
For more information, contact Ray at [email protected].