Hi,
I’m making my first PCB. Well, second if you count the one I built undergrad. Anyway, I’m not entirely clear on why you would use surface mount over through hole. I know on professional PCB’s, a lot of boards use both surface mount and through-hole components. Is that because there is a limitation on surface mount wattage/voltage? If that’s the case, does it make any, difference besides the board footprint, to use only through-hole components?
I’m planning on using surface mount as much as possible on my board, but I don’t want to make some major design flaw on my first project.
Thanks
Surface mount parts came about mostly to save board space. The smaller the parts the more you can get on per square inch. Just look at the cell phone for an example of miniaturization.
Some parts only come in SMT. This is one of the things that keep companies like Sparkfun in business! They sell the magic breakout boards to use these parts. As an example, I was recently unable to find a single level shifter converter switch in a DIP package. Grrr.
Some parts only come in through hole. Very few and mostly older parts.
Some parts have too many leads to be practicle in a through hole. A 144 pin through hole would be a moster part at 1/10 spacing. It would need 14 inches of circumferance. Think the size of a deck of cards and not a coin.
SMT is often easier for pick and place machines to automate a build.
All in all SMT is the way of the future with the possible exception of power hungry chips like a linear regulator that need a heat sink. But even that is likely to switch over in the future.
I’ve been prototyping with a PDIP PIC, but now it’s looking like a different package type is a better option. I’m looking at QFN. It has a different pin count, but the specs are the same. Are there any advantages to the different package types?
Actually, as someone who made the transition to using SMD years ago, I would ask “why would you use through-hole?”.
Generally, the only through-hole parts I use are connectors, which are far better at resisting damage when force is applied. SMD connectors tend to tear away the pads from PCBs, and if I need to use them, I try to “anchor” the pads by using numerous vias to tie them to pads on the other side of the PCB.
Another example of where I use through-hole is where I need resistors with high surge power requirements (eg when protecting the power supply input of a circuit that could be exposed to large spikes). There really is no substitute for through-hole, “carbon composition” resistors in this application.
tylerwolf:
I’ve been prototyping with a PDIP PIC, but now it’s looking like a different package type is a better option. I’m looking at QFN. It has a different pin count, but the specs are the same. Are there any advantages to the different package types?
If you use solder paste, QFN can be surprisingly easy to solder. It turns out that solder doesn’t like to “bridge” between QFN pins, especially compared to other packages like “QFP”. If they do occur, shorts tend to be on the outside, where you can access with a soldering iron & solder wick (albeit with a bit of difficulty).
After saying that, I still like packages like QFP, since you can see all of the solder contacts, so it’s easier to have confidence when inspecting a board.