Dr. Scott M. Baker

PCB Fab Tips

PCB Fabrication Tips:

If you’re going to be doing work with microcontrollers, or complex digital circuits, being able to fabricate your own PC Boards is a very useful ability. I’ve learned a bit while doing this myself, and I can come up with some suggestions:

Schematic Capture and Layout:

    In all of the schematic/layout software you’ll find two phases to creating your board. In the first phase you draw up the schematic. There are usually a variety of tools to help you, such as a library of available parts and the ability to create your own custom parts. Once the parts are down, draw lines between them to connect the various pins.In the second phase, you take the ‘netlist’ from the schematic and use it to generate a pcboard. You place the components on the pcboard where you want them to be located. You can then route your traces either manually or automatically. Auto-routing is a fantastic process to watch as the software attempts to find the most efficient means of connecting your devices. It’ll try to automatically minimize the length of the traces and other important criteria. Most software allows you to create single sided, double sided, or even multilayer boards. Often times there’s an option to create single sided boards with jumpers for those wishing to make simple prototype boards at home.Orcad. The first program I used was called ‘Orcad’, which has schematic capture and layout capabilities. Orcad was fine, but it was also very expensive and these days feels outdated to compared to some of the other programs that I’ve been playing with.

    Eagle. The next program I tried is called “Eagle”. Eagle has several versions available including ‘free’ and ‘noncommercial’ versions. Eagle is a great program and is used by many hobbyists. It includes a pretty good components library and other people (such as sparkfun) sometimes offer add-on libraries for less common components that they sell. The one drawback that I find with eagle is the limitations of the free version, which is limited to 160×80mm boards. There’s been the occasional times that I’ve needed to create a very large but very simple board (for example to put large 4-inch tall seven segment LED displays on) and the free version of eagle won’t do it. I think it would have required the most expensive version to handle such a large board even though it only had 8 parts on it.

    This tends to cause people to try to cram a lot of parts together onto a small board using Eagle, using smaller traces than they’d like. I understand Cadsoft’s desire to separate the hobbyist from the commercial user, but I wish a more flexible licensing process would have been used with Eagle.

    Diptrace. The third program that I’ve used it diptrace. The advantage of Diptrace is that it’s ‘free’ version limits on number of pins instead of board size. This allowed me to generate that one very large board (with very few components) that Eagle did not. Diptrace feels like a good capable program. I’ve considered switching to it from Eagle, but there seems to be a lot more Eagle users out there.

Sending your design off to be fabricated

Now that you’ve made the design, you need to get it realized onto an actual board. There’s several ways to do this, and the first one that I’ll talk about is having a PCB fab house make the boards for you.

This has several advantages:

  1. Double-layer boards can be produced more reliably than at home
  2. Trace widths and spacings down to 8 mils (0.008″) are often allowed
  3. Solder-masks and silkscreen can be applied

The disadvantages are of course time and money. Since someone else is doing the work, you have to wait for them to schedule and complete the job, and you’re going to have to pay them for their work!

The two places that I’ve been using are APCircuits and BatchPCB. Each has its advantages and disadvantages.

  • APCircuits. The advantage with this shop is the quick turnaround. Give them a deviceĀ  Monday morning, and it’ll be shipped on tuesday and arrive in at your doorstep Wednesday. This basic prototype service does not include solder mask or silkscreen, but does support dual-layer boards. You can get two eagle-size boards (3.95″x3.20″) for $66 plus $28 shipping. APCircuits also offers solder mask and silkscreen but that extends the processing time by an additional few business days and increases the price by about 50%.
  • BatchPCB is another option. They send boards off to China to be created. Their basic fee is $2.50 per square inch plus a $10 setup fee and that include silkscreen and solder mask. The final cost for two eagle-size boards lies somewhere inbetween the APCircuits options at $73 for a pair of 3.95″x3.20″ boards with silkscreen and solder mask. This means it’s cheaper (and much faster) to go with APCircuits for non-soldermasked boards and cheaper to go with BatchPCB if you need the soldermask. The other advantage of BatchPCB is the website, which makes it easy to upload, DRC-check, and preview your designs. You can even use their site to sell your designs to other users. The main disadvantage with BatchPCB is the excruciating long processing time. Since the boards get panelized together with other boards and have to ship all the way from china before being reshipped to you, processing time can take up to a month.

In the next section, I’ll talk about some of the ways that you can make your own circuit boards at home.

Methods to create an etch resist on a pcb:

  1. Use a material called “press and peel”. This is a transparency sheet coated with a blue powder. You run it through your laser printer and then iron it onto the pc board. Boards that I’ve made this way usually have only a few imperfections that need touch-up. I bought my press-n-peel from www.elexp.com. Other users have also reported success using direct toner transfer methods with transparency sheets in the laser printer. Google around for lots of DIY options.
  2. Alternatively, if you can find an old flatbed pen plotter (i bought a graphtec MP2300 on ebay for about $30), you can possibly use the plotter to draw on the circuit board directly. If you want to try this, be prepared to spend a lot of time tinkering around figuring out how to get it to work. I tried this for a while and then went back to the press-n-peel method because I just couldn’t get enough precision for fine traces and spaces using the plotter.
  3. The last and most tedious alternative is to draw the schematic manually using an etch resist pen. This works for simple schematics, but for anything more complex, I’d highly recomment press-n-peel.
  4. Touch-up: Look for any imperfections and touch them up with an etch resist pen (a fine tip scripto marker will do. My favorite Staedtler Lumocolor 313 superfine). Correcting problems before you etch the board is a lot easier than correcting it after.

Etching the board:

    There are two chemicals that are popular for etching: Ferric Chloride and Ammonium Persulphate. FeCl is a nasty substance to work with. It’s highly poisonous and will stain anything that gets near it. In my opinion, Ammonium Persulphate is the far better choice. It’s not as dangerous and does not stain. The drawback is that Ammonium Persulphate needs to be heated to be used (FeCl does not).I purchased my Ammonium Persulphate and an etching tank/heater from www.webtronics.com. The heater looks is a typical aquarium heater, and you might be able to find one cheaper.

Tin Plating:

    I’ve tried electroless tin plating and have never had satisfactory results. In my experience, tin plating the board is entirely unnecessary.

Drilling:

    I use a dremel and the dremel drill press (which is a miniature drill press that you clamp your dremel into). You’ll need a lot of small drill bits, which are not so easy to come by. I usually pilot-drill the holes with a 0.025 bit and then enlarge them to 0.034 as necessary.

Soldering:

    Save yourself a lot of grief and buy a good temperature controlled soldering iron with a thin tip. I use an XYTronic iron that I bought from Jameco about 10 years ago and it’s still running strong. Buy some nice thin solder (Radio Shack has something that’ll work).The technique that I use is to hold the iron such that the tip touches both the pin of the component and the copper pad. Count about three seconds and then touch the solder to it. Make sure the solder flows around both the pad and pin, and then remove the solder and iron.

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