I received these files to begin with; they were already in existence along with the original board. My first duty would be to clean these up--for example, making sure connections are complete, assigning the right properties to each part, etc. The biggest concern here is actually what is called the PCB (printed circuit board, as these are all called) footprint. Example: A capacitor is represented as two parallel lines on a circuit diagram; its footprint on the actual board would be two nontouching conducting squares.
Anyway, that's the back of the TDC board (the current one). All those little silvery shapes are footprints for the different elements in the schematic; their different sizes and shapes allow for the placement of the actual variable parts that will go on them later on. In OrCAD, I basically assign the right footprint to each part in the schematic and make sure they match up (I wouldn't want an inductor to take the place of a transistor, for instance).
Once I finish that and debug the schematics until they are error-free, I can then make a netlist using the program--it's nothing more than a simple text file that PADS will be able to read. This netlist contains all the information necessary about the different parts and connections that are going on in the PCB. In PADS, I start by making the board outline (just draw a shape, simple enough), and then I import the netlist file. This is the result:
So the board outline is there, and the parts are all neatly dispersed next to it. Also, I've defined the different layers (this board has six), with each having its own properties and whatnot. Guess what happens next? Yup, I would have to (manually) move every single part to its appropriate spot on the board. Best fun ever, considering how many parts there are to move. To assist me, I consult the actual physical board seen above, as well as the CAM files that I have. With respect to actual positions, it's a lot of guess-timating, as I can't really put anything at exact locations. Either way, my eyes get tired pretty often, with so many little things crunched together in such small areas.
From Monday to Thursday, I slaved away trying to get everything in their approximate positions. The red parts are on top, while the blue ones are on bottom (that's just my personal preference of color). It is not perfect, of course, but at the end of the day I was extremely satisfied that this second stage was (more or less) completed. On Friday I started on the next step--routing.
In some ways this is even more frustrating than placing parts. I have to make the connections from one part to another; even though there are guides to assist me, the fact that they are so small and that there are so many of them doesn't make it any easier. And this is where the physical board is limited--I can't see the inner layers of course, so I have to rely on the CAM files to get information on all the different through holes (there are so MANY of them too).
The other major problem is that the imported netlist is not perfect. There are instances where the schematic really doesn't match the board at all, so PADS never read the differences in. To overcome that, I have to invoke the ECO (Engineering Change Order) mode, which basically allows me to create and override anything I want. This is of course not normally permitted to prevent the user from making any unintentional mistakes; ECO is only used when one is consciously aware of its necessity. It is probably something that all aspiring engineers will have to learn to deal with in their careers. http://en.wikipedia.org/wiki/Engineering_Change_Order
This is after Friday's work. I basically routed a fourth of the bottom of the board so far. The reason this first go took so long was because I had to manually make sure everything was correct, invoking ECO quite often. Luckily, the right side is virtually identical to the left, and shouldn't take nearly as long. As for the top... more on that later, I guess.
No comments:
Post a Comment