I had some Shrinky Dinks when I was a kid. Amazingly, you can still buy them. You can also use that concept in your prototyping. I did that recently. I have a robot board design that I’d like to shrink about in half and add in a LiPoly charger chip. Most of the design came from something I had built previously, but the charger chip was new to me, as was the compression needed to meet my size goals. Sadly, you can’t just put your PCB in the oven and have it shrink like a Shrinky Dink. Maybe if you could put stretchy copper traces on it so they wouldn’t peel off while the substrate shrinks…

The charger comes in both DFN-10 and MSOP-10 packages, and the MCU comes in SOIC and QFN packages. The QFN is the 44 pin version, while the SOIC is the 28 pin version of the chip. Same core. Just more I/O.

Rather than test my ability to shrink and use the LiPoly charger at the same time, I added it into the original design without changing the size. There’s much more room for probing or even for adding test points if I needed them. Once that design checked out okay (which it did), I just went into the schematic editor, changed the SOIC to the QFN package, the MSOP to the DFN and most of the passives to 0402 packages. I really didn’t have to make any changes to the schematic.

That almost worked perfectly. The 28 pin MCU doesn’t come in a variant with a QFN package, so I couldn’t just change the package type in the schematic editor. I had to delete the SOIC version, place and wire in the 44 pin QFN variant. I made a few other changes too. I added in a QFN packaged RS232 driver and a hard power switch. In the original, I had envisioned a soft power switch, but I changed my mind. I also had to modify the library parts to make sure that the solder paste layer on the QFN and DFN parts fit our guidelines. Lastly, I removed some LEDs that I only had on the board for debugging purposes.

The most important two considerations were watching out for physical part interference and getting the paste layer correct on the QFN/DFN parts.

Duane Benson
It’s the size of a small walnut