In the land of prototypes, sometimes “close enough” is good enough. That can save money on PCBs and assembly when a particular package version of your part is out of stock. But, it’s not universal. Sometimes you can’t go that way.
I’ve got an MCP78338 Li Poly charger chip. It comes in 10-DFN and 10-MSOP packages. I originally used the MSOP version on my first PCB pass. Everything worked just fine, so I re-laid out the board to be about half the area. That meant that wherever possible, passives went from 0603 to 0402 and chips went from whatever to QFN/DFN packages.
Unfortunately, the DFN package Li Poly charger seems to be out of stock with long lead times. That got me looking at my options. Option 1 would of course be to just wait. Option 2 would be to lay out the board for the MSOP part in that space. Option three is to use the “we’ll make it fit” mantra. There are no guarantees at this point, but sometimes it’s worth a try.
But… Twas not to be. If you look at the second image, you can see that the footprint of the MSOP part leads is wider than the land pads for the DFN. I suppose there are still a few really messy and potentially expensive options You could solder a small wire on to the pads, sticking out from the pads, effectively making them big enough to accommodate the chip. Very ugly, but might work. Probably too spendy, though.
Duane Benson
Carpe DFN