> 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!
     
     
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