Well, just a short update here. I think I will probably be at home for another month or so, so I am going to go ahead and order the dot matrix clock display board. However, I need to decide between a surface mount crystal or a through hole crystal. I think my post on edaboard.com pretty much sums up my dilemma:
I have two possible versions I have been designing for some time now. Since it is looking like I am going to be home long enough to finish it I guess I should get it fabricated. However, one thing needs to be decided before I order it and that is whether or not I should use a Through-Hole or Surface Mount crystal.
Originally I was going to just use a through hole crystal, but after having a friend of mine look over the board as another set of eyes he pointed out that my crystal traces were over 3in long and had several vias in them. So, we identified a spot where a surface mount crystal could go with no vias and 0.5″ traces. The criteria for fitting in this space was that the crystal had to be no more than 7mm long, 3mm wide, and 1.5mm tall. The obvious problem here is that nothing really fit in that space except for a crystal made by Abracon that is 3.2×2.5×0.7mm. Now the problem is size: is the crystal too small?
My resources: 0.015″ solder, 1/32″ spade tip (on a variable temperature soldering iron), solder wick, and a relatively steady hand (successfully soldered a .4mm pitch 64tqfp by hand without overheating it).
Things I don’t have: A magnifier, reflow oven
Things I don’t have but could get: Heat gun, solder pasteI don’t have enough experience with this sort of thing to answer it myself and I really can’t afford to buy two versions of this board. Is using a crystal this small really worth it?
Well, hopefully I will have that problem solved before the end of the month so I can place my order as soon as Olimex comes out of their summer break (for some reason, Europeans take a month long vacation from work in the summer…how odd…).
Often I lay out a PCB in such a way that the “same” component in several different packages will fit.
I’ve seen many PCBs designed by other people that have similar “overlapping” footprints.
Would it be too much work to both put a surface-mount footprint for the small crystal on your PCB, and also extend the traces to the large crystal?
Then when you get the PCB, you can start putting on the small crystal first, and then either
(a) scream in frustration and throw the impossible-to-solder SMT part across the room, then smile as you fit in the through-hole part, or
(b) smile as the SMT part solders right into place, and you’re left wondering why people make such a big deal out of SMT. Then (optional) saw through the unnecessarily-long traces that lead to the now-unused through-hole footprint.
Either way you are left smiling.