日本語
I've found a part which is perfect for a revision to an existing product. Except that it's in a 10mm square QFN package. The good news is that the pitch of the QFN is only 0.8mm, which is coarser than the 0.5mm pitch TQFP's we assemble every day. The bad news is that there are 25 individual pads in the middle of this (1.5mm pitch, 0.6 diameter) - although most of them are going to be NC in this application.
My assembly staff has had standing orders to shoot me if I ever utter the words QFN, BTC or BGA in relation to having us assemble a board in house - since I've obviously lost my mind. I'd like to figure out if I was being too harsh and can retract those orders.
I have always been of the opinion that one shouldn't assemble QFN's without an XRAY. But recently I've heard several people say that QFN's are easier to deal with than a fine pitch TQFP and that inspection is largely unnecessary as long as you make sure that the paste is deposited reasonably correctly.
So I guess I'd like it if those of who deal with similar QFN's with center pads could let me know how bad they really are to assemble. Information about dealing with inspection and/or defects/rework/etc would be greatly helpful. I'm really hoping I'm just letting FUD get in the way here, but I also don't want to go down this path if this is going to be more problem than it's worth (I'd probably just get this product assembled outside since it is going to be fairly low component count and I'd run several hundred a year).
- Comments(1)
A****min
Sep 11.2019, 09:16:26
QFNs really are nicer to work with than fine pitch QFPs. If you are pasting the board and reflowing it, not attempting to do it with a soldering iron you shouldn't have any problems, you can line up .8mm pitch ones with tweezers by eye quite easily (or .5 for that matter), just by gently placing them in place and giving them a nudge and they also self correct quite nicely as unlike a QFN the legs cant drop between the solder deposits. And because they are just square things event the shonkiest vision or alignment system on a p&p should manage them quite well.
Arguably if you do need to rework them they can be a little tricky (but still way easier than a 100pin QFN), if you remove them with hot air there is often enough solder around to put a new chip straight back down, depending on any vias that might be around slurping solder away. Or you could flood the area with flux and re-wet the pads with a soldering iron Louis Rossman style (sorry I think the mans a cowboy but what he does works for him so, meh). Lastly another trick is I think called "bumping" where you paste and reflow your replacement QFN on its own and then rework it to the board with your trusty hot air pen.
