The Doctor's Experiment
Dr. Azzy has been active on the project's comments. He's one of those brave souls who make their own PCB's. He was curious to know what results to expect, and we were happy to oblige.
He send me 2 boards. The black one is 0.012" black FR4, with 2oz copper, double-sided (http://www.ebay.com/itm/310696411367). The other is plain old 0.062" FR4, with 1oz copper, single-sided.
The doctor writes:
They were fabricated by the toner transfer process, using an HP laser printer with Hammermill Color Laser Gloss paper. There are numerous descriptions of this process online - pattern is printed, ironed onto the board, and then the paper + board are soaked in water, so the paper turns to mush and can be easily removed. No "toner reactive film" was used. It was etched in cupric chloride in hydrochloric acid, but any of the common etchants can be used.
LED Demo: The simplest test board I could think of that uses SOIC-14, 5050 RGB LEDs, and SMD pin header, and forms a working circuit. I tried to get the important packages: SOIC because that's a very common package, and probably a decent model of TQFP et al as well. The 5050 RGB LEDs are a popular component in general, and also a good model for WS2812's (which are great for home-made boards, since the arrays are easy to lay out in single layer, and ready made arrays seem to have eyewatering markups) - plus, the package is intensely unpleasant to hand-solder. SMD pin header, because it's a godsend for anyone making boards at home. Not only do you not have to drill holes in it, but you can easily route traces between the pins without making it difficult to successfully make at home.
You should be able to apply power to the first two pins (assuming the LEDs and chip are installed the right way around), and then put a positive voltage onto pins 4-9 of the pin header, and each of those pins should turn on one, and only one, LED. Pin 3 shouldn't do anything (nothing is connected to the output).
In both board designs, I used 0.016 traces for about half of it, and 0.024 for the other half (this was less noticeable looking at the boards than I'd anticipated, partly due to these not being shining examples of home fabsmanship).
I applied paste to the pads. Given that the boards didn't have any solder mask, I expected the solder to flow along the traces so I applied the paste generously (hint: this was a mistake).
And then placed the components
And then the same with the other board
They were then placed in my reflow oven. This oven has produced around 600 boards for me over 18 months. It's been modified many times as I learn new build methods and try different versions of ControLeo. A true workhorse!
And the results! Starting with the black board - not good. The board warped with the high temperatures. I didn't spend much time looking at this board because the warp caused 4 pins of the IC to ride high above the pads.
A few shorts, but these are easily fixed. Serves me right for just slopping down the paste!
Anybody notice the capacitors on the boards before they went into the oven? This is a very simple reflow oven test that tells you if the oven is running too hot. A yellow tantalum capacitor turns brown if exposed to too much heat. This isn't an exhaustive, comprehensive temperature test, but if your yellow capacitor turns brown the oven is too hot - period!
The capacitor experienced some slight browning, which is normal. I've seen these turn so brown that it becomes difficult to determine the polarity.
The other board didn't warp at all, thanks to the 1.6mm FR4. Definitely paying the price for the sloppy application of solder paste though - ouch! The resistor on the right actually works as expected, but doesn't look good. The good news is that the circuit worked after removing the shorts with some solder braid.
And how did the "yellow capacitor" test work out? Pretty well I'd say!
What have we learnt:
- Spend some time putting the paste down. Time spent before the reflow will save time fixing dry joints and shorts later on.
- Stencils are worth it! A shout-out to OSHstencils.com for Polyimide stencils and smart-prototyping.com for stainless steel ones.
- Lead-free reflow on home-made boards can work
A big thanks to Dr. Azzy for pushing me to run this test.
I can't resist including a photo of a ControLeo2 board, made using a stainless steel stencil.