Building Tips

Building more sensor nodes is straightforward, but might be a daunting task for someone who's never fabricated circuit boards before. The board layouts provided should work for most printed circuit board houses. You'll want to find someone who will do double-sided, with solder mask on both sides, silkscreen on at least one side. Line widths should go down to 8 mils (0.008 in). I fabricated mine using Advanced Circuits, mostly because they made my prototype boards for $33 each. 30 boards delivered in 4 days cost $16 each, including all the setup fees. Ordering more boards and waiting longer for them both decrease the unit cost.

In addition, the ECE department recommends the following board houses:

• AP Circuits
• Metro Circuits
• Milplex Circuits, Inc.
• Eddy Electronics

Some board houses will also mount surface-mount components for you. Pad2Pad will do this even for small prototype quantities.

Generation of Gerber and Excellon drill files from the provided Eagle files is covered in the Eagle manual. You'll need top and bottom copper, top and bottom soldermask, and top silkscreen. You'll also need an NC drill file. The files I used for my prototypes are in the archive gerber.zip, but these have problems in the ground plane layer and should not be used to produce new boards.

Once you have boards, you will need to mount components on them. With a fine-tipped soldering iron and a pair of tweezers, even the smallest surface mount components can be mounted easily. Use the finest solder you can get (for example, Kester 63/37 no-clean solder, part no. 23-6337-8806).

To solder a passive (resistor, capacitor, inductor) component on, first melt a small amount of solder onto one of the pads. Then, holding the component with tweezers near the pad, melt the solder bump with your iron and slide the component's contact into it. Make sure the component's other contact is above the other contact before letting the solder solidify. This joint is a cold solder joint, and it'll have to be redone once the other contact is soldered.

Next, place your soldering iron so that it touches the unsoldered pad and the contact on the component. Then, touch some solder to the pad so that it doesn't touch your iron. Eventually, a small amount will melt and solder the component in place. Finally, go back and resolder the first joint, being careful to heat both the pad and the component's contact.

Here is a page with some tips on soldering surface mount components by hand: http://www.robotroom.com/PCB3.html.

Now, the passive components are easy enough, and even the tiny pins on the PIC18LF4320 can be tackled with a keen eye and steady hand, but what about those darn TR1000 radio modules? They don't have any pins to solder–just pads on the bottom. This is where solder paste comes in handy.

Before you read the rest of this, realize that if you're going to assemble large numbers of boards, you'll be better off contracting assembly out. They will have the equipment to ensure that the correct reflow profile is used and that components aren't damaged during soldering. With that said, you can obtain decent enough results with a toaster oven.

First, find yourself some solder paste with either no-clean or water soluble flux. You can buy it in a 35 gm syringe for about $15 if you look around (buy it from a soldering supply company like Wassco--not Digikey). Kester R276 no-clean solder paste should work well. The company you order this from may force you to ship it overnight, so keep that in mind. Once you get it, it's supposed to be refrigerated until use. I haven't had problems with leaving it at room temperature for a few days. You'll also need tips for dispensing, which you should be able to buy from the same place you bought the solder paste from. You'll want something around 26 or 28 gauge for easiest dispensing on small pads.

Passives are easy--just lay a glob of paste down on each pad, then put the component in place with tweezers. ICs are also fairly easy. The easiest way we found was to lay a uniform line of solder paste down perpendicular to all of the pads. It looks like it should bridge between them, but as long as you didn't apply too much solder paste, you won't have a problem. When the solder paste is heated, it automatically flows only to the pins and the pads.

The radio module can be done the same way. Just lay solder paste down in a thin line perpendicular to the pads on each side, line the module up (it helps to mark where the pins are on the side of the can), and let it stick.

Finally, take your board and stick it in a toaster oven at 350 degrees for about two minutes. This may vary with your model of toaster oven. Watch the board carefully and remove it a few seconds after you see the solder melt onto the components.

With some practice, this method produces nearly perfect results every time, and is great for making a small prototype run. No matter how sloppy it looks, as long as you have the right amount of solder near where it needs to be, you should get good results. The soler paste even sticks well enough that you can use this to mount components on both sides of the board, also.