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Powerful Prototypes: Why Datasheets Matter

Some parts just look cooler than others. One of my favorites is the edge mount high-speed RF connector. I'm not sure why, but I just like the look of them. Unfortunately, "I like the look" doesn't necessarily translate to "It is easy to build." The edge mount connector requires a proper footprint and a match with the PCB thickness, and this is where the datasheet comes in (Figure 1).

Figure 1: Edge mount connector.

The center pin needs to be flush with the board surface, so if the board is too thin, the assembler will have to use extra caution to ensure that the air gap is on the back side of your board. Doing so will reduce the mechanical strength of the connection. To prevent that from happening, these parts come in multiple varieties to accommodate different board thicknesses. Whenever possible, match the opening of the connector labeled "board thick" in Figure 2. Without the datasheet, you may have a perfect footprint, but still not be able to mount the connector because of the board thickness.

Figure 2: Table from a Cinch Connectivity datasheet.

If the PCB is too thick for the particular variant of connector, you probably won't be able to put the connector on. If the board is thinner, whoever is assembling it will have to take extra care to ensure a solid and straight connection. Any air gap on the solder side will need to be filled with solder, which can change the signal propagation characteristics of the connection.

Some of these connectors will only have one or two board thickness options, and some more, so you may not be able to match your board thickness exactly. But, if you can, you will end up with a more reliable and robust installation. Just make sure you also follow the electrical and footprint guidance in the component datasheet.

The order of pin numbering is another reason to never assume but to dig up the datasheet. Our all-things-about-electronics-manufacturing standards body, IPC (Association Connecting Electronics Industries), specifies the proper numbering order for most components. That's a pretty nice thing that they do there, but it's not always enough to prevent layout mishaps without the datasheet—case in point, a line of small PCB mount switches.

IPC calls out pin numbering for dual inline components, with pin one on the upper left (at zero degrees rotation), counting down, then over to the bottom right, and counting back up (Figure 3).

Figure 3: Pin numbering for dual inline components.

Given that, it would be logical to assume that all dual inline components follow the same pattern—logical, yes, but accurate, no. Multi-color LEDs, connectors, and switches are some of the more common offenders.

In this particular switch, it's not just a case of the numbering not following convention; it's also different from one variant to another. I understand why. The switch isn't changed between through-hole, top mount surface mount, and side mount surface mount, but the leads have to be accessible from different parts of the package.

The following two footprints are from the same switch: one mount on its side, and the other, standing up (Figure 4).

Figure 4: Two footprints from the same switch.

The pin one numbering doesn't follow convention, nor does the numbering of pins 4–6. And, you may have also noticed that the two are top-to-bottom mirror images of each other. Ugh.

This is why my mantra is: Always check the datasheet. Always.

Duane Benson is marketing manager and CTO at Screaming Circuits.