In preparation for building an openBMS battery management system, I’ve been researching new ways for a hobbyist to solder and build a surface-mount design. I have previous experience building prototype electronics in an electronics test lab. That lab had deep pockets and an impressive array of electronics prototyping equipment. I enjoyed expressing my “inner craftsman” there.
The real challenge is how to build quality electronics without spending thousands of dollars on equipment.
The first thing you’ll need to do is designate an electronics work area (read: this is gonna get messy). We’re going to be dealing with hot soldering irons and chemicals that could damage the kitchen table, so that’s out of the question. Also, you probably won’t finish work in one evening, so you’ll need a place that can get messy and stay messy for awhile. A corner workbench in the basement should do fine.
A well-lit room is not good enough. The small parts and intricate work we’re going to be doing here will require a bright overhead light. A swing arm desk lamp or two would work great.
You’ll need a soldering iron, wick, solder, and solder flux.
When soldering together wires, you’ll want to use something like a Helping Hands. When working on a PCB you’ll want something more like a circuit board holder. This will help keep you from pushing the board all over your table while soldering, and your neck will thank you because of the added height.
Basically there’s leaded and lead-free solder paste. The lead is added to bring the melting point lower. Lead free-solder has a higher melting point, so it’s a little bit more difficult to melt without destroying components. The only problem with lead is that, once it enters your body, it never leaves. We used leaded solder paste at work, and it was a pain because we had to wear gloves, and we had to carefully dispose of all paste-soiled materials in hazardous materials cans.
The take away here is: when you’re working at home, definitely use lead-free solder paste.
Just about anything can be soldered by hand, but small parts and fine pitched IC’s are a real challenge. Some people have luck with the “solder all the pins toghether and remove excess solder with solder wick” method. I have not. I found it difficult to remove solder bridges and nearly impossible to remove all solder bridges without creating cold solder joints under some of the pins. It’s messy.
Plus, there’s a limit on how much time you can spend applying heat to a circuit board. The action of molten solder and flux will chew up the board and sometimes lift up the copper pad. At that point, you basically have to throw away the board and start over — not fun if you have a lot of work already invested in that board.
Using Solder Paste
There are two ways of using solder paste. You can apply it straight to the PCB pads manually, or you can use a stencil. The method you choose depends on how many boards you intend to make. Sparkfun has a great tutorial on solder paste stenciling.
I plan to apply the paste directly to the board, position the part I’m soldering, and then touch the solder paste with the soldering iron. I’ve found this method to work very well on larger parts.
For smaller parts, or possibly all of the parts, you need to heat up the entire board so that all of the paste melts. Sparkfun has a neat post about using an electric skillet to reflow surface mounted parts. that seems like it would work great for single-sided boards, but not for double-sided boards.
I plan on using an old toaster oven as a reflow oven. All you really need is a thermocouple and a digital multimeter to bake a few boards. The solder paste should come with a temperature profile that you can follow manually.
That’s basically the plan for now. All I have left to do is order the boards from batch pcb and order the parts. That’s if I don’t increase the size of the smallest components used on the openBMS device. Some of the components are sized 0402. I’ve found that these are nearly impossible to place by hand; I wouldn’t even try it without using solder paste.