Custom CAD parts libraries

Back to Mark Rules and the mini control board again. He's using Eagle CAD which comes with good and not so good. The good is that Eagle is pretty powerful, yet inexpensive and easy to learn and use. The no so good is that the parts libraries tend to not have the newest parts or packages. The means before getting started with the schematic, Mark had to create the libraries.

He found a PIC18 with a QFN28 package variant, but the package didn't quite match the manufacture recommendations or Screaming Circuits' guidelines. It was a decent start but needed adjustments on the solder paste layer and the solder mask layer.

He started with a similar DFN8 packaged regulator, but in the end found it easier to start from a blank slate. The DFN10 motor driver used the regulator part as a base but required a lot of work to4qfns_paste_llayer_1  get the copper aligned, the mask in the right place and the solder paste set up. The center pad now has a segmented solder paste patter with room for two rows of three thermal vias. He needs to find a board shop that can drill and fill 8 mil vias. The FlipChip package was fairly easy, but again, will require a board house that can really register the solder mask.

The image shows copper, slik outline and solder paster layers.

Duane Benson

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?

Meet Mark Rules, designer

Mark works for a small company that designs and markets microcontroller development boards and motor drivers. The products tend to be used in small robotics projects, mostly in the education and hobby markets. They are starting to see more and more commercial business though, which is driving changes in product requirements.

Mark's new project requires that he take a current design controller board and make it as small as is possible with as few circuit changes as possible. The current board is 2” x 2.5” and uses a PIC 8-bit processor. It’s a pretty simple design that can control a pair of motor drivers and accept a variety of I/O connections.

The new version will combine the dual motor drivers on the controller board and still have almost the same I/O capabilities. The PIC18F2321, in a 6x6mm QFN package, looks to be the smallest Microchip processor that supports dual hardware PWM. He considered a 4x4mm QFN PIC16F690 using software PWM, but didn't like the performance hit.

For 5V regulation, he found the Microchip MCP1726 in a 3x3mm DFN package. The RS232 driver comes from ST micro, ST3243, in a 2.4x4mm FlipChip BGA. The dual motor driver is a pretty impressive 3x3mm A3901 from Allegro. That’s 64 square millimeters of chip space using QFN and BGA packages instead of 266 square millimeters for the processor alone in the old design.

The passive components come out to 4x 0201 parts, 4x 0402, 3x 0603, 3x 0805 and 2x 3216-18 capacitors. All of those should fit on the back of the board with enough room left over for a small TVS that may or may not be needed to suppress EMI from the motors. A few LED status indicators should also fit nicely on the top of the board.

Next comes the real challenge – super small connectors. Stay tuned.