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Build your PCB out of Paper...before you fabricate it.

posted Jan 31, 2012, 4:23 AM by Kevin Fodor   [ updated Jan 31, 2012, 6:58 PM ]

As anyone who was made their own PCBs before knows, there is a lot of work that goes into making even the simplest of boards. We painstakingly go through the part shuffling process placing components onto a bare board as if we are clairvoyant or have some insight into the eventual placement of these parts once the PCB design is done. The reality is the parts invariably never end up anywhere near where you initially thought they should be at the time you started. It's a long process of jockeying around ICs, resistors, LEDs and all kinds of components to find what you hope to be the optimal placement. If that weren't enough, you then begin with trying to resolve that rats nest of wires and begin the process of routing dozens, possibly hundreds of traces.

Starring at the board until we are nearly blind, tilting our heads seeking out new routes, we seek optimal pathways for the electron journey while slowly realizing our long thought out design. Placing a trace here, ripping another there. Finding clever routes which are short, direct and wont get you get locked into a corner is the goal. Finding a successful route is a kin to an ancient navigator succeeding to find the shortest trade route amongst hundreds available, basking in the glory of a clever and insightful path. The DRC rule check is run early and often, checking our way through our long journey. Then as an absolute last resort, we create a PCB equivalent of a "get out of jail free card", known as the via. You avoid them at all costs, but sometimes they just need to exist; there is no other way. Begrudgedly (and perhaps regretting) we add them when needed.

Finally the routing is done, but it's not over yet! We now carefully and methodically layout the silkscreen, placing all of the details we hope we will need to assemble the boards when they finally return from the board house. With all our work behind us, we run a final DRC check This after running it more times than one can count, finally arrive at aboard with no DRC errors. Well at least only DRC errors you are willing to live with. Finally, we are done! It's been a long process, but now its over. We sigh in relief.

Well almost. We now need to package up our design and have it fabricated. This part is probably the easiest part for us all. As long as we follow our board houses rules; everything goes smooth. We zip up the Gerber, perhaps checking it just once more and them email them off. There, it's done, off to China it goes. Into the electronic abyss where the PCB fairy works to produce our very own PCB. Like it or not, it is done. There is no turning back.

The problem with this part of the process is it takes weeks (at least if your a budget conscious hobbyist like my self) to get the boards back. So we wait. It's torture! The weeks you wait for those precious PCBs is one riddled with excitement and anticipation not unlike the kid on "A Christmas Story" waiting in the mail for his "Little Orphan Annie Secret Decoder Ring". Are they here today? Did they have a manufacturing problem? Did I remember to connect that one pin? Did I have enough clearance in my traces? Doubt riddles our mind. We pull up the design file and check, perhaps double check, and check again. All of this doesn't matter much however, because our PCB is submitted, off being produced in a far away land.

Then at last that day comes. An unusually heavy, but small blister pack of shiny new PCBs shows up in the mailbox. You rip them open like a child at Christmas. We do a quick inspection. Traces look good. No cut traces, no cracks, nothing looks shorted, silk screen looks legible. Great! Ready to build.

You rush to our workbench to begin populating the first few parts. Like a blacksmith stoking the coals in a furnace you fire up the soldering iron. Should we start with the power supply, to see if it smokes? Or maybe the section which was the most difficult to design? No, we start with the smallest parts first, yeah that'll do it. And then it happens...uhg! The same kind of feeling you get when you get a new toy and someone forgot to buy the batteries. One of the parts doesn't fit!

One of the most disappointing things to happen, after all this work is complete, when finally the waiting is over, we find out we made a mistake. A part wont fit, or interferes with another. Maybe we laid out the footprint backwards? Whatever the reason, either the Dremel tool comes out or the PCBs go into the trash. Bummer!

This happened to me once with one of my first designs. I figured there has to be a better way. That's when I thought, "Why don't I just print out a scale drawing of the PCB layout when I am finished?".

I can paste it onto a piece of Styrofoam and place all my components, BEFORE I have the PCB fabricated. That is exactly what I did. It has been an incredibly helpful step in the process ever since. Granted it's more useful on through-hole parts (although super glue works well for SMTs) and it's not guaranteed to find all the problems. But, at least any mechanical and placement issues can be easily seen before you commit to the design and wait weeks for the results.

Paper Printed PCB
Here are some examples of how I used this technique on some of my own designs. In the pictures you can see how I simply printed the design to scale, cut it out with scissors and tapped it to a chunk of Styrofoam. The nice thing about this is that you can take all your actual parts and stuff the board as if it were your actual PCB. This gives you an opportunity to verify the layout, check for part clearances and overall physical make up of what will be your PCB design.


Oops! Bridge Rectifier is Too Big.
In this example on the right, I show how a very simple design went wrong. The rectifier footprint I chose did not match the parts I had on hand. The one I had was twice the size. I assumed during the design they had the same footprint.They had the same pin-out, just not nearly the same size. Bad move. Well in the end I decided I liked the physical size of the board so I simply just bought the part which has the footprint I used in the design. Easy enough, problem solved. At least I didn't have to find this out after the boards arrived.


Wrong lead variation.
On the left is another example which shows how the MOSFET package I had on hand for use during the prototype had the wrong lead configuration. I had parts with "short" leads, I designed in "long" leads. No matter, just be sure to order the right variation before the boards come. Plenty of time to do that.

Wow! Perfect.

To the right is another example of a design I tried out on paper first. This one actually worked out OK.

 


I highly recommend trying this technique out.It has really saved me a few times wasting time and money making boards which would have been otherwise thrown out.

One note of caution. Some ESD sensitive parts might not like so well being inserted into a Styrofoam block. So be careful. You can also sometimes use dead chips or similar package parts which are throw aways just to get the physical layout of the PCB. After all it's not like this Styrofoam board is actually going to work!

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