While at the EELive! conference last week I met up with the PCB-POOL folks. I first heard about this PCB fab house from my buddy Wayne Yamaguchi. Despite their being located in Ireland, Wayne said they were getting the prototype boards to him in a week. Best yet, at that time, they did not charge extra for non-rectangular board shapes, and Wayne’s boards were all round, used to convert a Maglite flashlight to an LED flashlight.
What caught my eye at the PCB-POOL booth at EELive! was a card that had a toaster oven picture and the headline: “Create your own solder reflow station.” Now it was Wayne that tipped me off about doing reflow for prototypes in your garage. He too used a toaster oven. He just did a few experiments on when to turn it on, when to put in the PCB and when to turn it off. He said he decent results, but the problem with this is that it is an essentially uncontrolled process.
Enter PCB-POOL. Sure, they sell the toaster ovens. The real deal is they sell the third version of a controller so you can create a profile on your toaster oven. Please don’t use the toaster in your kitchen; flux is not the best butter for your English muffins. So like the picture explains, buy the reflow controller from PCB-POOL for $315, get a brand new toaster oven for 80 bucks, and if you order 5 PCBs from PCB-POOL, they cost 30 dollars each, and PCB-POOL gives you a free solderpaste stencil with the order.
A solder stencil is a thin steel sheet that is laser-cut to have the pads of your circuit board. You carefully position it on top of your bare PCB and then you can squeegee solder paste over it, like doing ink on a silkscreen. Only instead of ink, you are deposing a thin coating of solder paste on all the places where surface-mount parts will mount.
Now that you have the solder on the pads, the surface mount components will just stick to the board and self-align as the solder melts. Sometimes you can even put parts on both sides and use the solder paste to suspend the parts on the bottom. For heavier parts on the bottom you need a dab of hot-wax or silicon to keep the part in place through the reflow process.
The great thing is that your reflow process has a real temperature profile, like it should. I assume the controller has a SCR or maybe it is just a bang-bang controller that cycles power to achieve a given profile and temperature. The more control you have the more repeatable your process. One nice thing about using the stencil at all is that it proves out your CAD padstacks. If you made some part and forgot to put a solder paste element in the pads for that part, you will realize it really quick when you see there is no solder paste on those pads. This lets you fix your CAD file before it goes into production.
The next level would be to send the whole board to an assembly house like Screaming Circuits in Oregon or Advanced Assembly in Colorado (right down the street from Advanced Circuits, but a different company). Indeed, the first outfit I saw giving out free stencils was Sunstone, which is near Screaming Circuits in Oregon. When you send your fabbed boards to these small-lot assembly houses you are doing more than just sparing yourself the headache of soldering up the board yourself. You are proving out the solder-paste file from your CAD program, as well as the “insert” file as OrCAD 9 calls is, what the pick-and-place machine uses to place your components on the board before reflow soldering. Now you might find that the TO-220 parts have an insert location way off to the side and won’t let the machine vacuum pick them up. So when the nice folks at Screaming Circuits explains this to you, you can fix the CAD files before they go into higher volume production. The real job of an engineer is to make a set of comprehensible coherent documentation that lets the manufacturing world make lots of your design. This is every bit as important as getting the chips to work and the firmware to run.
Most all the fab houses can hook you up with short-run assembly. Some can have your prototypes hand-soldered; many need 3-feet of tape and reel parts so it fits in their machine. That is the cool thing about Screaming circuits, they have adapted their machines so you put in 4 or 5 pieces on some DigiKey cut-tape and make just 5 boards with no parts left over. And don’t forget my pals at Sierra Proto Express. It was Ken Bahl who created the whole short-run prototype concept 20 years ago. These days they specialize in high performance boards, down to a few mills or many ounces of copper along with blind and buried vias. Best yet, they have a partner in China, so when you are ready for high-volume, they can guarantee the partner can make any board you had made at Proto Express.