IPC footprint sizes - less, nominal or more?

[shadewind]

I'm designing my first SMT board and noticed that A!tium has three versions of all IPC footprints marked L, N and M. I understand that these are different sizes depending on how much you need to pack the components.

How hard is it to solder the high density (L) footprints? I'm thinking mainly for QFP 0.5mm and 0603 components. Since this is the default (default in A!tium for chip caps and what Luminary used in the component library for A!tium which you can get at their website), I had already designed the board when I noticed this. Is it very hard to solder this compared to the nominal or larger sizes or should I simply leave it as it is and give it a try?

Since this is a two-sided board, the decoupling caps and associated routing on top close to the MCU is indeed quite crowded as it is so a change to larger footprints might cause some trouble.

[DrGeoff]

It's not hard to hand solder these components.
I'd recommend using a magnifier to work with the board though. There are several techniques discussed on the forum here for soldering SMT parts and I usually use a syringe of flux paste applied to the pads to be soldered. This helps position the part since the flux paste is sticky, and the solder flows very easily onto the pad and pin without bridges between adjacent pins.

[shadewind]

Right, but do you mean that it's easy regardless of pad size? I'm considering going up to nominal at least to be on the safe side even though I might have to modify the routing a bit.

By flux paste, do you mean solder paste or simply just flux? I do have flux but not solder paste as it is quite expensive, at least the ones I've seen.

[DrGeoff]

I always lay out a board with the large size pad for the component. If you get a component whose tolerance is on the higher side then it's harder to solder on a smaller pad. It's also much easier when hand soldering.

Flux paste is just that. Flux in a syringe.
Like this
http://www.mgchemicals.com/products/8341.html

[shadewind]

I'll try and use the largest size that I can comfortably fit on the board. I don't want to move my decoupling caps too far away, though.

Edit: The largest footprint was quite a bit larger than the smallest I was using. It's proving to be not very easy to reroute this...

[ToBeFrank]

I use nominal, although I don't solder by hand. I use a solder paste and a small infrawave oven to reflow. You can get solder paste from a few places online for $30US or so. A little of it goes a long way. I haven't had a failure yet due to soldering.

[ToBeFrank]

One thing I forgot to mention, I think it was in Tom Hausherr's blog I read that he uses least for 0603 and smaller components to help prevent tombstoning. This also applies to reflow though.

[shadewind]

I finally managed to fit the components anyway so I think I'll go with "Most". Quick question, for decoupling caps, it doesn't really matter whether the ground or the power pin is closer to the pin, it's the total impedance that matters, right?

[allanw]

I use nominal and solder them fine with an iron.

When doing them by hand, I think it helps to have the pad extend past the actual pin, otherwise it's very difficult to get your iron to touch both the pad and pin to solder with. With tall 1206 capacitors sometimes this is hard. If you're reflowing then this is less of a problem.

[shadewind]

I'll go with "Most" since I already converted the board to that and I don't want to have to redo it

When I get the board, I can get a feel for how difficult it is myself so I'll know for next time.