> Techies: > Our engineering department wishes to use a MELF package (National > Semiconductor LL-34) for a diode on a new design. Okay, I'm gonna' ask a dumb question, why? Can't they get it in a SOT package? > They intend to mount this part on the bottom side of the PCB. YEE HAW! We're talking fun stuff now! Some more questions though, are ya' planning on wave soldering the bottom? or double sided reflow? A lot of melf diodes I've seen have glass bodies. I'm not sure, but I wouldn't think they would like the thermal shock hittin' the wave too much...I've never waved melfs on the bottomside before. I guess it would be easier to give my opinion of reasons why melfs are a big pain in the you-know-what for assembly: 1. Most of the pick and place machines I've ever worked with can place diodes, but they usually don't do as well as yer' standard passives do. Most machines do have special nozzles that have a groove so that the melf can center itself on the nozzle tip and stay there, but that's not always 100% foolproof. To illustrate my point, take a soda straw and suck on it while trying to pick-up a cylindrical object like a small section of a pencil or something, and keep it fixed in one position and you'll see how "dicey" that is...crude comparison, but you get the idea. The machines can do it, but not real well. 2. Melf diodes can be put on the pads the wrong way, ya' can't do that with a SOT...(well, I take that back, I suppose you could put a SOT on upside down...don't laugh, I actually had someone who didn't have the best vision hand-placing some prototypes for me do that, they looked like dead bugs laying on the PCB with their legs in the air! Ha!) 3. Forget the notch thing with the pads, it don't buy you nuthin', in fact, I've seen that very footprint cause problems. Just lay out some ol' regular square pads spaced apart correctly if yer' gonna' use melfs... 4. Like I said earlier, I've never waved melfs, or even done a double-sided reflow for that matter with em' on the bottom. It would seem to me that there may not be enough surface tension available should you become liquidous on the bottom (and you usually do, a little bit anyway) for the melfs to stay put. If it were up to me, and I had my 'druthers, I'd druther use SOT's than melfs any day! __\/__ . / ^ _ \ . |\| (o)(o) |/| #------.OOOo----oo----oOOO.-----# # Steve Gregory # # [log in to unmask] # # # #________________Oooo.__________# .oooO ( ) ( ) ) / \ ( (_/ \_) *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************