Re: QFN mounting
Mazz said:
Biff
biff44 said:
I weep when I see these sort of packages, because it means the demise of being able to breadboard circuits in my basement lab anymore!
why you say this?
Mazz
Because they stink!
Connections are 100% under the body of the plastic housing means:
1) you can no longer inspect them to see if the pads are actually soldered (no visible fillet)
2) you can no longer rework them yourself with a soldering iron, you need a hot gas reflow unit heating below the substrate (since heating on top will melt the plastic package). This is especially true of most Microwave packages, that have a huge ground slug underneath them that is hard to solder.
3) Logistics cost money. You have a QFN package on a large board you are testing. It is not working. The first thing you suspect is a bad pad solder joint. You screw around with it as much as you can, and still can not get the chip to work. You drive over to your assy house (assuming it is not in Taiwan), and have them reflow it. A long time later you get it back. Wasted time! If there were at least SOME metal going up the side of the plastic package to allow wetting of solder, you could do a lot of repair work by yourself almost instantly.
4) Reliability. unless it is a big production run, you are never sure of the solder joint reliability, since it is not inspectable. What if you start getting field failures? Dendritic growth because of some trapped solder balls or entraped no-clean flux? Momentary contact due to flexing or thermal cycling of a solder pad joint that worked at initial test, but microcracked because the soldering conditions were not 100% ideal?
I guess if you are making 10,000 cell phones a day (with tightly controlled production), it is not an issue. If you are making 20 complex multichip microwave modules a week, you will get to really hate the QFN "revolution".
Rich