Electronics

Pushing the Envelope

Sun, 28 Feb 2010 05:07:04 GMT

The thing that I most like about dabbling in electronics is that I’ve been able to create circuit boards at a resolution that should be very difficult to do at home. In fact, I’ve managed to do so with relative consistency. Despite my success, I’m not entirely thrilled with the lithographic process. It requires a lot of discipline (which I have not got) and chemicals. If a board doesn’t turn out, there are a myriad of possible reasons. There may have been a fingerprint on the mask, the etchant may not have been up to temperature, the developer could be nearing the end of its useful life, I might forget to set a timer and over-expose or develop the board, etc. The chemical process is also a bit time consuming, and it doesn’t lend itself toward the rapid development of small boards.

The “holy grail” of home board production, has managed to elude me. I’ve got a small CNC mill which I use for drilling the boards before I etch them. Once the board has been drilled, I use the drill holes to line up the lithographic mask for exposure. For quite some time, I’ve thought that it would be really nice to be able to isloation mill small boards instead of relying entirely upon the lithographic process. My efforts have been largely in vain, though.

Consider for a moment the dimensions of a small surface mount board. The Atmel chips that I’ve been working with generally have a pitch of about .7 or .8 mm. That means that the pins are .7 mm apart on center. My nephew recently dropped off a Microchip PIC that is .65mm pitch, and I offered to make a DIP adapter for him. At .65MM pitch, the distance between pins is in the neighborhood of .3mm. Finding a cutter that size is no small achievement. A stepper motor has about 200 positions, so once you combine that with a 10 TPI screw, your maximum resolution is 1/2000 of an inch. The good news is that resolution isn’t the big problem, however if you have any backlash at all you’ll have a difficult time cutting at this size.

M y first attempt resulted in the near instant destruction of $25 worth of end mills. After some stewing I purchased a set of engraving bits from E-Bay. The aren’t precise, and the plunge might be a problem…but the price was right. This afternoon I dropped a bit into my mill and destroyed it seconds later. The feed speed was 20 inches per minute and the depth of the cut was .01”. With a V-shaped engraving bit, a .01” cut is really deep (and wide). Combine that with a fast feed speed and you can imagine what I did to my cutter.

I reset the program to feed at only 3 inches per minute, which is terribly slow except that I’m only feeding a total of about 10 inches anyway. Also, I set the depth to only .005” and then I loaded a new cutter.

The results were, well, surprisingly good. I think all but two of the traces will test out just fine. I also think I can solder the chip onto the board. In the picture to the left, you can see that the traces are pretty clean. What you cannot get a sense of in this picture is the actual size of the circuit board. Take a moment to visualize it in your mind, then click here to see another picture of it.

New and Improved

Thu, 01 Oct 2009 02:54:33 GMT

I’ve been spending a lot of time on CNC lately. Quite a while ago I wrote about mounting my Olimex MT-128 circuit board in a plastic box to make it easy to take with me places. That was a good idea, and it certainly kept the cat away from my projects. Unfortunately, it also made it quite difficult for me to work on a project. The problem was in getting to the back of the circuit board to plug in programming headers, connect to pins, etc. For some reason it took a while for me to realize the neatest, cleanest solution: mount the experiment board to the top of the pencil box. Genius!

Not so fast, though. I tried that once before and it was mostly a failure. Trying to hack up a flimsy plastic box with any precision is no easy task. This time, though, I was armed with a lot of recent CNC experience. I opened up TurboCAD and drew up the dimensions (or so I thought) of the faceplate. Before machining, I printed it out. It’s a good thing, too. My dimensions were way off. Have I mentioned that I can’t measure very well?

Fortunately Olimex has a drawing with dimensions for the board on their website. Using those, I was able to create a perfect drawing.

Next, I exported to a DXF and pulled the file into CamBam to define cutter path and create the CNC G-Code.

When I loaded the code onto my mill and cut the shape, the results were…terrible! I didn’t support the plastic from the bottom, and it flexed so much that the dimensions were all over the place. This evening I cut a 2x4 to fit under the box lid. To mount the board to the mill I recessed two holes in the board, then I clamped the pencil box lid from the top.

I think the results the second time around speak for themselves (despite some fisheye from the camera).

Toast!

Sun, 16 Aug 2009 17:34:28 GMT

Recently I decided that it was high time I tried to reflow a circuit board. Previously I had been waiting to create a circuit to control the toaster oven, but I thought “Hey, what if I just control the oven by hand. That should work out,right?”

Unfortunately, the heating profile was pretty lousy. The heating elements scorched the phenolic (paper) circuit board, and the solder reflowed at a much different temperature than it should have. My guess is that the direct heat from the elements creates high localized temperatures, even though the oven temperature might still be quite low. In order to improve the heating profile, I’m going to add metal covers around the heating elements in order to diffuse the heat. A fan will be used to circulate the heat around the oven, and I will probably break down and build the controller for the oven.

Back in the Lab

Wed, 10 Jun 2009 04:28:50 GMT

This week I decided that it was high time I got back into some projects. My next goal is to create another robot. This one will be quite a bit more intelligent, though. The robot will be controlled by a PC (probably a Windows laptop, although I have several mini-ITX options as well). Most laptops no longer have serial ports, so the robot will need to be controlled via USB. Although it won’t suffice for later revisions AT90USB647 seems like a good choice for right now. It has six 16 bit PWM channels, and four 8 bit channels. The device supports USB, and serial interfaces. Perhaps best of all, it supports enough USB endpoints that it could conceivably show up as two serial ports. One could accept commands locally and the other could pass them on to another chip that doesn’t support USB…say ATMega2560 (25 PWM channels).

There are a couple of hiccups. My most recent circuit board was an AT90USB647 board, and it was also a failure. After trying to revive the existing prototype I decided it would be better to update the design and try again. Unfortunately my PC died in the interim, as did my wife’s printer.

You can easily see why my PC failure is a problem. Getting a well-tuned environment running again is a pain. The failure of my wife’s printer might not be so obvious though. No printer, no PCB printing mask. She’s buying another one tomorrow, so hopefully that will solve the printing issue.

Also, I have not used my mill in months…but I really don’t want to hand drill a bunch of holes. My supplies are running low, and I also want to try using a toaster oven to reflow a board. So, here is the plan. I’ll cut two boards and chuck one into the mill and pre-drill it. The other will be hand drilled (ick). The existing board will donate its parts, thanks to some help from ChipQic. Whichever board looks less successful will get paired with the used chip, and then it will be baked and reflowed. The other board will get the usual hand treatment.

So, barring any major supply shortages (which wouldn’t shock me) I should come up with a working prototype by this time next week. Keep your fingers crossed for me! By the way, the new printer is an HP OfficeJet. It looks like a fantastic machine. Hopefully it will work as well as our super cheap HP inkjet did!

Good news, and an apology

Tue, 26 May 2009 03:17:00 GMT

I have good news, and an apology. The good news is that since DotNetNuke (my hosting software) now supports the MetaWebLog API, I can dump WordPress (my old blogging software). More good news is that I was able to bring over most of my posts, with one caveat.

The bad news is that the formatting is messed up...so you might not see bolding and paragraph breaks. Really, a pretty small price to pay!

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CNC PCB Drilling

Sun, 21 Dec 2008 00:11:15 GMT

I've been playing with an ATNGW100 lately. The idea is to use the NGW100 to communicate with a peripheral board remotely using an XBee wireless module. Of course, the XBee headers are a non-standard header size. I've got adapter boards, but they have an Octal buffer\driver on them. If I were using a Max232 chip, it would dump low voltage out one side and high voltage out the other. I didn't want to risk the same from the Octal buffer\driver, so I whipped up an XBee adapter board. Naturally, it had 40 tiny holes. What a great chance to play with my mill for PCB drilling. At first, I tried drilling before etching. The idea was that I would lay the mask over the holes. Unfortunately, I filled the pads when I printed the mask...so the alignment wasn't so good. On the next two boards, I decided to drill them after exposing, developing, and etching them. Getting them lined up wasn't as bad as I expected. The alignment was off a bit, but one of the boards turned out nearly perfect. Since the drill bit is tiny and relatively expensive, I used a piece of plastic cutting board under the circuit board. And, of course, there is a video too: http://www.youtube.com/watch?v=GbFfrd--yWI

Satisfaction

Sun, 30 Nov 2008 23:48:59 GMT

The drill pattern in the above piece of copper clad board is less than two inches on a side. Probably about 1.75 inches. You will notice that the holes are perfectly aligned, but there is no visible pattern on the board. As I wrote previously, I intend to begin using my CNC mill to drill my circuit boards. This is the first example that was drilled using my mill, and they were drilled without any manual intervention. The mask from which the drill pattern came is shown below. If you're very detail oriented, you noticed that three holes are missing from the copper board. This is because I was using a demo computer aided manufacturing program that only processes the first 250 lines of G-Code. The very, very good news is that the holes lined up precisely when I laid the etching mask over them. It really is a shame that I didn't get off my bum and set my mill up for this a long time ago. Like the time I drilled 144 holes in circuit board that turned out to be a failure anyway. Just an example... This evening I got most of the backlash out of the mill. The X and Y axes both seem very tight, so my next stunt will be an attempt to do isolation routing in this board using an endmill. I expect that will work, as the chip is at a 1mm pitch. The next board I try will be at .5mm pitch, and I expect it to fail. Any backlash would be magnified so much that getting it to work properly seems unlikely. There is only .5mm between traces on the chip, which means that the general spacing on the rest of the board is also just as fine. Still, I'm more than ecstatic that the second most labor intensive part of prototyping a board is now automated.

Warming Up with CNC

Sat, 22 Nov 2008 17:32:29 GMT

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Lot's of More Goodness

Tue, 11 Nov 2008 23:59:04 GMT

Wow! What an exciting couple of weeks. Sure, most folks wont care about all the fantastic things I'm about to reveal, but at least I can count on Mom to read what I write, smile, and nod. The first thing I want to share is a pencil box that I modified to hold some of my projects. I made holes in the side for plugging in the cables. Now, projects can sit on my desk safely without fear of being molested by the cats when I'm not looking. The MT128 board is attached using plastic concrete anchors from the hardware instead of "real" standoffs. The other three boards are attached with double stick tape. You'll notice that the board in the upper right hand corner is one of my home-brew AT90USB162 boards, being used as a UART bridge. By the way, yes, I know that putting my boards in a plastic box is like running around in a thunderstorm holding up a lightning rod...especially in the winter. I'm considering some strategies for ESD dissipation, so if you have good advice, I'm listening! Speaking of AT90, I was also able to use an AT90 to program another board via ICSP, thanks to the code available from AVRopendous. Their circuit boards are reasonably priced too, and you might consider purchasing one. They are about $20. That code is made possible by a fellow who is on my short list of AVR heroes. In fact, that list consists only of Smiley from Smiley Micros and Dean Camera from the MyUSB project. Aside from being absurdly nice, Dean is also frighteningly smart. His new library includes a dual CDC demo. Now, think about that...a $4 AT90USB162 chip that can simultaneously act as a programmer and a UART-USB bridge via two COM ports. It's a near perfect solution. Unfortunately, I've been able to enumerate two ports...but haven't gotten them to really do much of anything. All in good time, though. In other news, the receiver for my Futaba 6ex came in over the weekend! Now, to see if it will interfere with the XBee modules. Following that, I'll be using the receiver along with the mega 2560 board I'm working on.

The Results Are In!

Mon, 13 Oct 2008 23:17:30 GMT

This evening, as soon as Evelyn was supposed to go to bed, I snuck off to the basement. The 2560 circuit board has a lot of holes in it, so I cheated and drilled out only the holes I would need to get the board to power up for me. There were seven vias and ten holes for the programming header. There were also, of course, 100 very tiny pins to solder. As soon as I finished soldering everything together, I raced upstairs to plug the board into my computer. As these things go, instant success is rare, so I wasn't surprised when the programmer failed to detect the board. After rooting around with a multimeter, I found the problem. This is a problem I noticed the last time I printed the board, but somehow I failed to remedy it. The reset pin was not connected to the programming header. A friend called this afternoon to chastise me for not simply shipping the design off to a board house to have a professional board made. You will notice my temporary "reset" wire, connected to the programmer. This would also be the reason I have not shipped the board out to manufacturer yet. Once I finished the design I'll have several professional boards printed. It took three tries to get this board printed properly. It took two tries to populate the board with the bare minimum number of components. Two times I had to improvise a reset wire. Finally, after working on this thing for two months, I was ready to plug it in and see if it would run for me. After carefully attaching the board to the programmer, I gingerly plugged in the programmer's USB cable. My computer made the "USB Device Connected" noise, so I opened up the AVR Studio programming application. I gingerly pressed "read signature" and waited for the failure notice to pop up. Instead, I got...a valid signature! I almost cried.