What a great time to be an electronics hobbyist! Who could have imagined 10 years ago the cool parts that are available for experimentation and projects today:
- Triple axis accelerometers
- Compact GPS receivers
- 32 bit superscalar microcontrollers at hundreds of MHz
- RGB LEDs
- LCD panels
- touchscreens
- Standard interfaces like I2C, SPI, CAN, etc. to easily hook all of the above together.
- PCBs manufactured for $25 or less.
And more! And they’re cheap! The problem is that many of them are also surface mount parts, including BGA packages, which are seemingly difficult or impossible for the do-it-yourselfer to solder. Well, it turns out that there are ways for the do-it-yourselfer to make professional looking PCBs using surface mount parts, even fine pitch ones like QFPs and inaccessible ones like BGAs.
These packages are attached to PCBs by assembly houses using reflow processes. In this process a bare board has solder paste applied to the IC pads by using a mask, then the components are precisely placed, then the assembly is processed in an oven with a specific temperature profile. Everything is pre-heated initially to drive out moisture and get all components to a common temperature. Then the temperature is raised enough to melt the solder paste. The surface tension of the molten solder is enough to hold the parts in place, and even correct slight misalignments. After sufficient time at the melting temperature (generally a little over 200 C, for about 30 seconds), the board is cooled and inspected for faults. Heat either comes from hot air or from infrared lamps.
DIY reflow
This process isn’t that difficult to replicate and I have found quite a few resources on the internet that can help you out. First up is Bill Shaw’s encouraging WWW page, titled “You, too, can do 0402“. 0402 refers to the smallest SMT package that passive components come in, which is .4x.2 mm. These things are navel lint tiny. Bill’s is a great article with links to PCB manufacturers, solder paste mask suppliers, and good advice on hand placing solder paste without a mask. Bill’s technique is to use a toaster oven, either with a temperature indicating stick or with a controller that uses a temperature sensor to provide the temperature profile described above. The controller is called TempTell.
Instructables
Instructables has a few articles on reflowing. One that has some DIY to it uses a portable range top. This article describes how to add a gadget for temperature instrumentation to a skillet placed on the range top using an infrared sensor connected to a PIC via SMB, and from there to a PC via RS-232. The author was prepared to control the temperature using a relay to switch A/C power, but in the end found that simply setting the range to max, and turning it off when the solder melted, was good enough. There is a Python script that logs the temperature. As a bonus this project also uses an empty yogurt cup filled with sand.
Another Instructables article is about building a GPS logger, with information in the middle about reflowing the board using a pancake griddle. It has good pictures of using an acrylic mask to apply solder paste. It also has some good information about using Li-Ion batteries.
This Instructables article describes a simple toaster oven method. It’s a simple put it in and turn it on technique but has good advice regarding flux for BGA packages (you have to use flux).
Those guys over at Spark Fun
The funny guys over at SparkFun have articles on reflowing using both a toaster oven and an electric skillet. Their conclusion is that the skillet is best, the toaster oven is a close second, and their $2300 Madell commercial reflow oven is a distant third. In fact, they like hand soldering and hot air rework better than their Madell — yikes!
SparkFun, by the way, is a page that you should have bookmarked (if not memorized). They sell lots of cool components for experimenters, plus kits of interesting projects. Many components that would otherwise be difficult to use are available on breakout boards which bring the traces out to traditional .1″ spacing, making them usable on protoboards.
How about the solder paste mask?
If you don’t have a mask to apply the solder paste, then you have to squeeze it from a syringe. This requires steady hands and possibly large amounts of coffee for you caffeine addicts. Like soldering by hand, it’s OK in small amounts but for real fun you need the mask. Fortunately there are lots of ways for the DIY-er to get solder masks. Instructables is always a good place to start, they have two tutorials on mask creation. The first one uses a Cricut paper cutter to cut holes in a sheet of transparency like you’d use in an overhead projector. Remember overhead projectors from that incredibly boring controls class? Neither do I, I was asleep also. The author says, and I agree, that it’s not worth buying a Cricut just for making masks, the quality isn’t that great. However if you have one it can be put to use and you get your mask right away.
The second article from Instructables describes making a mask from etched copper, the same way you’d make a PCB at home. If you’re comfortable with making PCBs at home then making a mask this way makes a lot of sense.
You can also make your mask from acrylic using a printout of your pads and a sharp hobby knife — see comment above about hand soldering!
If you are mail ordering PCBs then you might as well mail order your masks as well. The sites that are recommended the most, as far as I can tell, are Quick Stencil and Pololu. Pololu gets you a stencil shipped for around $30, depending on size.
So there you go, plenty of ways to stuff your next project full of SMT devices and assemble it yourself.
Happy gadgeting,
Steve Ravet
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