Screaming Circuits: SMT Connectors


SMT Connectors

When we last left Mark Rules, he had made most of the component decisions for his miniature microcontroller and motor driver board. He still hadn't made a final call on the exact PIC processor. All of the options fit the same 6x6mm footprint, so there won't be any size changes regardless. Connectors looked to be the real problem.

The best he's found so far is a Digi-Key part number 609-1847-1-ND (x3), 609-1851-1-ND and 609-1854-1-ND. [Screaming Circuits will gladly accept Digi-Key part numbers in a Bill of Materials]. These are all larger then desired, totaling just over 200 square mm of board space, but it may not be possible to go smaller. That'll probably force all the connectors on one side and most of the passives on the other side with the IC's. We'll look at a couple of other suppliers just for kicks, but most likely, that will be it.

This job is mostly a layout challenge, since it's based on an existing design. There will be a few changes but not many. Still, there really isn't any safety in "just a layout." With big thru-hole parts, layout tends to not be all that important. However, when you start moving into higher speed and smaller geometries, layout becomes very important.

This isn't a high-speed design, but there are critical layout considerations. QFNs require special care. Check out our QFN layout guide. The guide will take you through the techniques required to create a solid, reliable design with QFN packaged parts. We also have our LED markation guide to promote more accurate assembly.

Speaking of LEDs, Mark ran into something else he hadn't thought of. In his prior design, he just used 1/8 Watt thru-hole resistors for the LEDs without giving thought to power requirements. His first thought here was to just use 0201 parts everywhere. That was until someone suggested he actually calculate out the power dissipation. Doing so brought the LED current-limit resistors up to 0603 size 1/10 Watt. Looking at the other resistors, he even had to move a few up to 0805 1/8 Watt.

Duane Benson
Watt are you looking at?


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