Hi David,

The influence of the surface tension of molten solder on the fluid dynamics of the reflow process are indeed forgiving. However, it depends largly on the dimensions involved and the volume of fluid (solder). What you describe can be taken for granted when soldering parts with a 0.100" pitch (e.g. old DIP and SIP parts). Hence the old wave soldering machines. And as long as you're careful not to put too much solder paste down, it still works with the first generation fine pitch parts (0.050" pitch, e.g. SOIC). For that matter, even if you put too much solder on a 0.050" pitch part, and bridge all the pins together, you can still go over the row with a solder wick and everything will be good to go.

But the CP2102 doesn't have 0.050" pitch. It has 0.009" pads on 0.019" centers. The space between the pads is only 0.010". And that's the actual part pad, not the pads on the board. The pads on the PWB are specified at 0.012" wide pads with 0.008" spaces between them. And the CP2102 has no leads, so if you form a solder bridge, there is no wicking it up. What's more, on that scale, the surface tension effects you're counting on are possible, but far from the certainty they were at 0.050" or 0.100".

And you're quite right that no part of this scale can be done without solder mask.

Joe