Most electronic devices are assembled using SMD technology these days. Its kind of a cool process and I figured I should try and build an SMD project someday just to get a feel for the technology.
SMD technology allow automation of the circuit board building process. Machines lay down the paste-like solder, then place the components on the board, and finally heat everything so the paste flows like normal solder.
The components are REALLY TINY. Here is a microprocessor and 6 resistors:
Today’s goal is to assemble a SMD kit of the old Simon Game I purchased from http://www.sparkfun.com some time ago. I admit I’ve been pretty hesitant to do this because it seems like the kind of project that will fail for me because of the precision required.
Sparkfun is a great company and they’ve got all kinds of neat stuff to play with. So far the stuff I’ve seen has been of decent quality and well documented. The Simon game is no exception:
The first step is to position the stencil (dark red in the above pic) exactly on the PCB so only the areas needing solder paste will get it. You then spread paste across the stencil with a business-card sized piece of plastic. Then you come back again picking the paste back up against the stencil. If you do it right, the paste exactly covers the PCB pads where all of the components will sit:
OK, it probably took me 10 tries to get it right. Between each try I had to clean everything off with isopropyl alcohol. However, you can see the grey paste on all of the pads. I did use the included tweezers to make sure the pads for the microcontroller had some clearance. The clearance was so tight it was hard to tell by eye.
The next step was actually the hardest for me: getting all of the components laid out on the paste covered pads. I don’t think my hands shake much, but when trying to lay down components the size of a 1/4 of a grain of rice I’m just not that steady. The microcontroller was the hardest. It took 3 tries to get it. Which means twice I had to wipe paste off the board and reapply it. I was really doubting I was going to get this to work.
On the 3rd try I managed to the the microcontroller very close to perfect but I had to position it a bit with the tweezers which appears to smear the paste. I had to just hope that when the paste got hot it would act like normal solder and not stick to the PCB.
Once all SMD components were laid out, I started using a hair blower to heat the solder. It didn’t seem to be working (and I don’t know what is normal) so I decided to get out my heat gun and run it on low. Within a few minutes I could see the paste turn shiny like normal solder. It was working!
I let the whole assembly cool, then tested each component to make sure it was on solidly. It all looks good:
Looking very carefully with a magnifying glass, it even appeared the solder on the microcontroller’s legs was clean.
That was the hard part. Then I just had to solder the normal through hole components into place:
Now I just had to stick in the battery and turn it on. THE MOMENT OF TRUTH. Let’s face it, with my kind of luck nothing works the first time. BUT WOW IT DID!
This project went fairly well as far as I’m concerned. Most of my problems were with getting the solder paste perfectly applied. Unfortunately that is the kind of thing you just have to play with until you get the feel for it. Since it could be removed and reapplied learning how to do it wasn’t too painful.
My preference would be to not do another SMD PCB. I will continue designing myself to be through-hole components. But if I have to I just might be able to pull it off again.
Here is the Simon game in operation: