Erik's right. You spend a lot of time and potentially waste a lot of parts soldering to a no-mask board.

Silkscreen doesn't matter, but the mask is a must-have, particularly if you use the latest packaging technology.

Be sure you know what to expect in terms of tolerances, too, particularly if you put mask where it counts most, namely between the leads on 0.5mm pin-pitch parts. Many houses can't even manage that. Further, I insist on dry-film solder mask because it can be placed more precisely and can be applied with more thickness, which helps with component alignment. If their tolerances are too generous, however, it prevents the parts from contacting the pads properly, which is BAD. Don't forget that BGA's can't be inspected unless you have XRAY capabilities. That can get expensive, so you should be really conservative with your prototypes, particularly if you use parts with lots of <more than 500> balls. One missed connection and the whole board, parts and all, is rubbish.

You also want to be able to order very thin top and bottom layers, as ultra-thin (~5-mil) outer layers will allow smaller vias, since they plate more easily and reliably than thicker layers.

Around here, many fellows use the Advanced Circuits prototype offers from www.4pcb.com. They're pretty anxious for your business. You'll have to judge for yourself whether their capabilities meet your requirements.

RE