So, I've been selling 6-axis IMUs
and USB bus interface boards
since August of 2007. I've built a lot of very small surface mount boards in that time, and I've learned a couple hard lessons that have cost me a lot of time and money. I've also built a bunch of foot pressure sensor boards, almost none of which worked, and haven't managed to sell more than a handful of them.Lesson 1
- Don't use MLF packages. The pin spacing (0.5 mm) is way too small, and its far too easy to get bridges between pins, and also between pins and the ground pad. Since all the pins are tucked under the board, its almost impossible to fix these kinds of problems, and wastes large amounts of time in the process (not to mention money in wasted boards and components).Lesson 2
- Don't put components on the bottom of your boards. If you're just building one, then fine. But when you're trying to do small scale production, having components on the bottom doubles the amount of time it takes to make a single board. Having them all on top means you only need one stencil, you only apply solder paste once, you only place components once, and you only need to put the board in the oven once.
So I've now redesigned both my IMU and my foot pressure sensor boards to take heed of these lessons learned. I'm using the same microcontroller, the ATmega168, but now its in a slightly larger (and more importantly, all pins visible) TQFP 32 format. All the components are on the top of the board. This required making a few changes, including using a non-standard programming pin layout, but the space savings are worth any extra trouble that causes.
With the foot pressure sensor boards especially, building the wires that connect the sensors to the board is as time consuming as building the boards themselves. I did a lot of looking, and found out Digikey sells small pieces of 28-gauge wire with Hirose 0.050" crimp terminals already attached to one end, in various colors and lengths. This greatly simplifies the amount of work I have to do to build these wire connections, and thus the total amount of time spent building the entire package. Time spent is a critically important metric to consider when you're building boards to sell in a low profit margin area like hobby robotics.
I don't make much money off these boards - not even enough to cover my time in building them. But, since you can't buy these boards anywhere else, I consider it a worthwhile Bioloid-community service.
Here's a picture of my new foot pressure sensor prototype board, next to one of the old (green) boards. You can see the larger chip on the new board, as well as the connectors with all the wires leading out to the sensors.
What this all means is that within two or three weeks, I should be able to start selling foot pressure sensor boards finally...