The Design Process: How to Get from Nothing to Something - Building Open Source Hardware - page 64

At this point, you should also recognize why some form of formal testing is a gate-
keeping item in iterating through the design process. Whether it's a manufacturing test, a
circuit performance test, a unit stress test, an interoperability test, or a user experience
beta test, the only way to discover the shortcomings and limitations of your design is to
see how well it works in all the ways you intend for people to use it. It should also be
clear that it's not just users you have to worry about—you also need to beta test your pro-
duction line. You'll never know if you have the right PCB footprint for your power supply
capacitor until you run a manufacturing test, make a board, and solder that capacitor into
place. Then you need to ensure that your contract manufacturer can solder 10,000 capacit-
ors to 10,000 boards and have the process work every time.
Mistakes, bad design, sourcing scarcity, and manufacturing challenges are issues you
can never escape. But by taking a methodical approach to design, they become the kinds
of problems you can tackle while they're small—before they become showstoppers.
Setting up Your Workflow
No one ever makes a perfect PCB from scratch. Most projects start with bread-boarding
bits and pieces together to make a proof of concept. The hardware doesn't have to be small,
perfect, or manufacturable at this point: You just want to prove your circuit works and to
determine which parts you need to build it.
Once you have that first proof of concept made, the next step is to move your design
from the bench-top into CAD software so that you can make a PCB. This workflow is the
same no matter which program you're using (e.g., Eagle, Cadence, Altium).
First: select your parts—and I mean, really select your parts. For anything critical (like
microcontrollers or any nongeneric component), you should have both a manufacturer
part number and a vendor part number for the specific part you're using. For generic
parts you can buy anywhere, you can cheat and just specify the part as “MLCC Capacitor,
Footprint: 0603, Value: 10 nF.” Next, make sure you have each of those parts correctly
entered into your part library . For extra credit, have a physical sample of the part on
hand and use it to confirm that both the schematic symbol and the part PCB footprint
are correct. One trick I use to check part footprints is to print out a 1:1 copy of the PCB on
a paper; I then set the parts down on it to visually check the dimensions and placement.
Once your have a part in your library, you can place it in your schematic . In the
schematic capture process, focus on reproducing the logical design of your bread-board
circuit. While you're adding components to your parts library and schematic, you should
also be adding them to a separate document called your bill of materials ( BOM ). Once
the schematic is complete and you've passed the design review , you can start the PCB
layout (in the same CAD program). All of these files should be shared as your source files