Xiaofan Chen wrote: > On Sun, Jul 20, 2008 at 10:00 AM, William Chops Westfield > wrote: >> Tell me about SMT footprints for passive component (resistors, >> capacitors, etc), and how sensitive automatic placement and soldering >> techniques is or isn't to the exact size of pads and whether they >> have vias in the middle or at the edges? >> >> Ideally, I want to create some "universal" footprints for PCBs that >> would allow any of several different sized component to be mounted, >> for flexibility of component choice. I can certainly make SMTs big >> enough for a 1206 part that are close enough together for a 0603 >> part, and I'm pretty sure it will work OK for hand assembly, but I >> don't want to preclude automated assembly... > > Not a good idea for the process and the process engineer will just > reject your board outright. If the pads are too big, the part will > flow. The pads are designed in a way that a "self-center" effect > will make the parts sit right in the desired position. > > I've seen boards (low cost vendors) using pads for many > 0603/0805 resistors. And we will just consider this > supply not acceptable. > > Overlapped pads are bad idea. Quality is never a secondary > effect. What Xiaofan says is all true. We have many problems with customers who send the same design they used for hand-soldering to be made by us. Oversized pads and traces/vias not separated by soldermask from pad are the most common problems. Ideally, the pad should be symmetrical and just a bit bigger than the metal surface touching the pad and surrounded by all four sides by soldermask (and no vias inside the pad). However, there is one trick that works for your purposes: make the pads narrower (say 50%) than the width of the metal on the component like this: MMMXXXXXXXXXMMM +--------MMM|XXXXXXX|MMM--------+ +--------MMM|XXXXXXX|MMM--------+ MMMXXXXXXXXXMMM The results are not perfect but this is much better than a huge "sea of solder" pads. If you want I can take a picture of this layout. Djula -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist