At Home SMD Circuit Board Work
I make electronic things at home. Sometimes from individual components, down to designing and fabricating PCBs from blank boards. Sometimes by connecting groups of purchased modules. Surface Mount Devices sit on top of the board, while through hole components are placed through machined holes that span all layers of the board. SMD pieces have many advantages over through hole components for industry use. They're cheaper, they can be placed easily and effectively by pick-and-place machines, and the final manufacturing steps are easier and cheaper than the steps for through hole parts.
As a very quick overview of the typical fabrication process with SMD:
- The blank printed circuit board will be put on a special-purpose jig to align it.
- Using a stencil that matches the board and the jig, a mixture called solder paste will be applied to all the spots where a surface mount component will need to be soldered to the board.
- A Pick and place machine will put components on the board one by one, rotating them as necessary to fit on the PCB and pushing them onto the paste. Components are still able to move at this state and the boards are fairly fragile.
- Boards will go through an oven which will turn the solder paste into liquid solder. The flux in the paste will ensure that the component and the board are securely soldered together.
- Once the board cools, the components are both electrically connected and mechanically secure.
At home, I have none of the nice things that come with industry use. I don't have the ovens that regulate the temperature. I don't have the pick and place machines, I don't even have racks or clamps to move the board between different processes without components coming off of their pads and paste. I have..... a hot plate, tape, tweezers, and a cheap USB microscope. It turns out that this was sufficient to get me through several boards yesterday.
[ Process Pictures ]
The at home process basically mimics the industry process, but you replace every automated bit with labor.
- Blank PCB is taped to the work surface. [ Board Design ]
- Stencil is manually aligned and taped to the work surface and the board. If you're clever and your board is adaqueately sized, you can create places for alignment pins that go through the stencil and the board. I did not do this for my boards, as I was aiming for very small pieces. [ taped ]
- Solder paste is mixed and put on the board.
- Components are placed one by one. Some components are very small. It's no exageration to say they are the size of a grain of sand. They are put out of place by things that you don't think of until you've done it. The wind a piece of paper makes as you flip through the instructions that came with your kit. The friction of your fingerprints can pick them up. If you put too much pressure on the tweezers they can go flying, careening into an ant like a meterotie. Even taking them out of their packaging is enough to lose them. Typically parts this small cost fractions of pennies. Buy extra. Some components have directionality, even though their packaging is symetrical, so you've got to line up a dot or a line to a spot on the board. Double check that you haven't missed any. They're still the size of a grain of sand once they're on the board, so a checklist can help.
- Reflow with your method of choice. I am using a hotplate. Sparkfun wrote a really long thing and at the end suggested it. Some other source, which I now forget, online mentioned using sand to create a more even heating surface, as hotplates typically have "hot spots" which make it harder to reflow all at once. Unfortunately for me the sand never came to temperature and I moved directly to the plate. The plate unsurprisingly had hot spots, I measured 50 degree differences in some spots. I switched to puting boards directly on the hot plate and things went better, but it is difficult to control the process.
[ Some success ]
- I started off with two "learn to solder" kits from Sparkfun that were designed as SMD tutors. These were very helpful in establishing technique for stenciling and pasting.
- My first board was a total mess. The paste was too dry and never reflowed well. I fixed most of the connections by hand with a fine tipped iron.
- Before the second board, I mixed some flux in with the paste and things went much better.
- For boards three through 5 I was working with my own design, and WS2812 LEDs, which I think would be near impossible to solder by hand, since the leads are under the package and only a small amount is visible. I got the boards manufactured from OSHPark, and I got a stencil from OSHStencils. The components themselves came from DigiKey. Overall it cost me $30 for three boards, including the stencil and PCBs. On every one of my boards, at least one LED was damaged, usually resulting in it missing one, two, or three of its colors. On the first board the first LED was damaged, and the second LED was totally destroyed. By the third board I had only one board, and it is missing Green and Red.
Overall it was a great way to spend a saturday, I'm feeling much better about SMD prototyping. For anything more than a few boards, I'll probably get them manufactured by a small run board house, like http://www.smallbatchassembly....
21st February 2016
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