Having mounted at least a few million parts recently, stay with 0603's or even 0805's unless you need the space savings. The 0603's have some sort of
3 digit number on them usually to help identify the value, and 0805 and larger have enough room for 4 digits so 1% resistors are not written with something cryptic. Some people prefer to use larger parts, even 1206's to make the product bigger! See below - People often don't like to pay 10k+ for a device that makes a altoids can look huge!Reliability might even be a little better with the larger parts due to the larger solder joints, and apparently more current noise can be produced with
0402 resistors due to the higher current density.
I don't even bother looking at Digikey for production prices. I can buy a typical 5,000 piece reel of 0603 resistors from $5 to $10 CND.
Something's wrong then. We just placed about 110,000 (110k) 0402's and ended up with about 10 tombstones - and most of these were attributed to poor placement accuracy (I need to do / get done a calibration for the placement heads on our main chip shooter if I ever get time, as 1 or 2 (out of 12) of them are out about 2 to 4 hundredths of a mm (0.02 - 0.04 mm)). It's amazing that good pick and place machines can place parts accurately at the speeds they run - ours (turret design) places most part sizes one at a time at 0.15 seconds per part with (when calibrated) a 3 sigma variance of 0.08 mm! On our newer assembly line, the fine pitch placer is accurate to 0.025 mm @ 3 sigma!
We do notice that there seems to be a lot more mispicks with 0402's. We typically assume 1 to 3 percent part wastage due to mispicks for larger parts in a larger production, but I think some 0402's approach 5% or more.
One other thing is that 0402's require fairly new machinery - 0603's can be mounted with much older machinery (there are machines that are over 20 years old that will still mount 0603's very reliably at reasonable speeds). Using parts no smaller then 0603's and SOT 23's allows the use of old equipment, and thus possibly offers a shorter manufacturing lead time or possibly a cheaper production as the machines have been paid for.
0603's and bigger are also much easier to inspect visually.Good choices!
1206's still have there uses too (other then dissipation / voltage / capacity reasons) - they are good for making it bigger! People see things like cell phones getting cheaper as they get smaller and then costing nothing, as long as you sign up for a basic contract, so they start to associate smaller = much cheaper! How can you, in most cases charge 10 - 20K for a product with a surface area of a couple of inches!? Repair / rework / modifications are easier for the people unskilled in micro soldering with large parts.
Yes, they will be around for a long time yet!
The QFN's are not a problem - I just mounted over 900 of them yesterday, and only had a couple of failures, all of which were on the same panel, which was due to that particular board shifting during the solder paste printing process (didn't use the post print inspection, as the lot was small and the process is generally very reliable). Reworking them is also fairly easy. You do have to be careful when you design the footprints though. DFN's also have very little problems. I've never worked with a chipscale package, but they seem to be like miniature BGA's - which are not too bad to mount either, as long as the proper gear is available.
Jeff