On a board that I intend to produce in low volume, I can’t avoid having smt parts on both sides of the PCB. All but a few of them are on the bottom, but the problem is the ones on the top are going to be the hardest to solder (a leadless QFN-16 and two tiny leadless LEDs).
As far as I know, one can’t use the skillet method on both sides, so I have to decide which side to I will be using the stencil/skillet on, and I’ll probably have to do the other side with hot air or a soldering iron. I really want things to look clean, so I’m leaning towards reflowing the bottom (with the majority of components) with the skillet. Still it’s not easy getting a tiny QFN to sit still when there are components on the other side of the board, since they not allow the board to lie flat.
So, I was wondering, how to assembling services do smt on both sides? Is there a better way for me to do it (I’m not going to use an assembly service)?
There are a couple of ways - one is to put a tiny dot of glue beneath each part when placing the parts in solder paste, then reflow the whole works. This really requires some sort of reflow environment though, rather than a hot plate
Another technique is different melting point solders. Run one side at the higher temp, then the other side at the lower temp. This requires a fairly precision oven though, so probably not so useful for you.
So it there a better option than the ones I stated, given I only have access to basic equipment (paste, mylar stencil, skillet, iron, hot air gun, etc)?
If you do the top of a board (in whatever way you please), then put paste on the bottom, place components, and put it in an oven, the solder’s surface tension will hold the already soldered parts in place, even though they’re upside down.
Assuming your board has mounting holes, you could put some standoffs on a blank board with and put that with the standoff pointing upward so you can put the board to be soldered on the standoffs, and they’ll keep it level for soldering. The standoffs just need to be taller than the components that are already mounted.
You could try soldering them by hand. Since the boards already have some solder on them, you can get away with just using a flux pen. I soldered a bunch of SMD chips to a board from batch pcb simply by putting some extra flux on each pin, placing the IC in place, and touching each pin with a soldering iron. No extra solder was used and it looks very clean and they have all worked well.
khearn:
Assuming your board has mounting holes, you could put some standoffs on a blank board with and put that with the standoff pointing upward so you can put the board to be soldered on the standoffs, and they’ll keep it level for soldering. The standoffs just need to be taller than the components that are already mounted.
Keith
That sounds like a good idea. I just have to hope the parts on the bottom don’t fall off, since I’ll have to use hot air on the QFN16. I know surface tension is supposed to hold the parts, but I’ve had some SOT-89s and SOIC-8s drop off before.