Hello!
Can you help me understand the process of designing PCB, choosing components and placing them on, etc?
Sure.
Some specific questions I have, but not comprehensive: ( please tell me more than what I ask
)
I'll try
If you're going to reflow solder, how do you make place the solder on the PCB, make it line up with the component's wires, and keep the solder from touching the other patches of solder?
tricky to know exactly what you're asking here.
You design the PCB having a certain layout that is determined by the packages that the components are like.
This pattern is etched leaving you with the copper traces.
You apply solder paste to the pads, place the components on top, and reflow the solder.
The solder has significant surface tension and will draw together when it melts. This acts to both remove solder between traces, and to pull the components onto the pads. As long as you don't have too much solder, you won't get bridges -- well, not often. You may get small balls of solder , but they will generally clean off the board.
How error prone is the process?
Well, the placement can be tricky. It's not rare to place the component down and have to jiggle it a bit to get it in the right position. The worst case is where the gaps between the pads are roughly the size of the pins themselves and where the IC (it happens mostly to DO, SOP, etc and flatpack chips) falls into the holes between the pads -- Yes the thickness of the copper is significant!
Or you place the component around the wrong way.
This sort of thing can case the paste to get smeared around, but it doesn't usually cause problems.
A more significant problem is where uneven heating of a board causes the solder at one end of a small component (a resistor or capacitor typically) to melt before the other and the surface tension can pull the device upright on the board.
The smaller the pitch of the devices the worse problems are (BGA is in a class of its own) and for ICs, a slightly bent lead can cause one or more pins to fail to solder.
You do need to check your work.
Is it going to fail often and make you re-do it? Are you going to short a lot of components?
With large components (1206 and larger, SO-x and larger, it's all easy, and you can often rework with a normal soldering iron. When you get the 0402, you have no option but to use a reflow tool. If you're playing with BGA, you may need to X-Ray boards to find faults.
Shorting is rarely a problem, when it happens, excess solder and visible bridges across IC pins is the worst offender. Solder wick is the fastest way of cleaning this up. It is very rare to have a short under a component.
I assume you put the components on with a tweezer. Is there a faster way, if you're doing multiple of the same project?
Yep. non-magnetic tweezers. I have several pairs, and I also have some anti-static plastic tweezers.
If you're doing *lots* you use pick and place equipment. But that's not cheap. That's what board assemblers use.
How do you make them stay in place while you solder it on?
Several ways.
1) the solder paste is quite sticky and will hold the components in place as long as you don't jostle the board too much.
2) glue. Some pick n place machines can dispense a small drop if glue to hold a component in place. You can also do this by hand. The trick is having a glue that won't hold the component away from the board.
3) manually. If you are soldering manually you can physically hold a component down whilst soldering it.
