On Feb 2, 2008 11:41 AM, joe tarantino <[EMAIL PROTECTED]> wrote: > > > On Feb 2, 2008 11:23 AM, Jeffrey Baker <[EMAIL PROTECTED]> wrote: > > > Nearly every major part I need for my latest project is packaged in a > > QFN (or LFCSP as they call it over at Analog Devices). I'm worried because > > the parts have round leads and Sunstone, the PCB prototype shop, says they > > can't make radiused SMD pads. Has anyone run into trouble with this, or are > > square pads OK? > > > > Generally speaking the .5mm-pitch QFN seems to be on the edge of or > > beyond the capabilities of a proto shop like Sunstone, but Sunstone has the > > best tolerances I've been able to find. > > >
> > > I have had one of the Sunstone shops build boards with QFN32 and QFN48 > as I recall. Results were pretty good. They had square-ended pads. > DId you draw them that way, or did you draw them rounded? > > -Be careful of solder resist "floaters" due to the little pieces between > the leads. Best bet is to remove all resist between pads. > Interesting. Manufacturers seem to disagree on this point. Freescale says remove solder mask between pins. Intersil says keep it. Sunstone can do .005" solder mask web so it's just barely possible to have it either way. You don't worry about solder bridges without solder mask? > > -You may want to check your gerber files to see how much solder paste will > be applied and check with Sunstone to see if it is sufficient (or too much). > > Does your pad have a large heat sink pad on the back? If so, you must > carefully construct this pad in the footprint (possibly out of multiple > squares). Again, check the paste layer. > Oh yes, the dreaded exposed pad. I appreciate the thermal properties of this package. However I find drawing it to be a huge nuisance. I drew the stencil layer with an array of rectangles of the same size as the SMD pads (.30mmx.65mm) separated by .13mm. It seems like that should give a decent coverage without leaving the part high-centered. Thanks for the tips!
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