Hi.
Sorry if this is thread necromancy, but you don't seem to have had any kind of reply yet, so I thought I'd make a few suggestions.....
1) Sampling on a not yet proven process (i.e. first runs, prototype batches etc) is a flawed concept - as your SMT line has not yet had the opportunity to be optimized and as such sampling is not appropriate - I'd suggest that for batches of less than 20, you perform 100% inspection
2) for larger batches, you need to determine what your defect rate is... there are a lot of opportunitys for defects on an SMT board - somehting like every joint x10 (if you assume each join has an opportunity for solderballing, life, misalign, insufficient toe fillet, side fillet, heel fillet, outgassing, void, flux contamination) - you said you are using the IPC standard as guidance, so you should be able to determine this yourself. - If you have such a good stable process that you have a six sigma process against these opportunities for defect, then by all means drop down to standard ship lot sampling, otherwise 100% inspect.
3) Another thing to consider of course is the application - is this aerospace, military, medical, high reliability, or is it consumer stuff that's not going to do much mroe than inconvenience the end user when his uninspected board shorts itself out, or those dry joints eventually seperate? - Your contract review process should earth out the customers expectations - if he expects 100% failure free product, then there is a cost to bear - otherwise, if he can accept a few DOAs or ELFs, then you can reduce your inspection regimen, and save him a few $$$s
I hope that this helps a bit
Piston
