So, I happen to have a full low-volume SMD assembly line here... for our own products (although I did have similar thoughts to John about spinning a couple of carrier boards for these type of parts but designed so they are suitable for assembly on our line).
Our take on QFN's is that they're not as bad as one would imagine, especially if one extends the pads outside of the QFN footprint so you can have a chance of reworking them, since most re-work issues are an issue of a bit of flux and heating the solder up to melting point. I still try to avoid them if at all possible, because I hate something you can't easily inspect, and QFN's are pretty much in that category. The problem with inspection being that you have to generally have expensive inspection equipment, the most common being an x-ray machine, in order to really tell if the part is soldered correctly. If you don't have this, you pretty much have to rely on a functional test which can be problematic since there are a lot of solder defects which result in boards which test fine, yet are not truly soldered correctly - which fail in the field. But, everytime that I've relented and used them, they've been remarkably trouble-free, often easier to deal with than an equivalent pitch TQFP since bridges/soldering seem to happen less often than on a leaded part, and usually these issues clear just by applying some flux and reheating the joints. Assuming you find the bridge/defect in the first place. The especially troublesome QFN's are the ones with 'interior pads' since there is no easy way to see how well they soldered, and reworking those joints are a challenge. The single ground mostly-thermal pad ones aren't too bad (such as the part we're talking about), with the caveat that you have to put some thought into how to handle the ground vias so they don't suck the all the solder from the pad into the via holes. This usually means plugged vias (aka small enough that the plating fills 100% of the hole). In addition there is a lot of discussion about how much voiding (unsoldered area) is acceptable on that center pad, and the answer generally is that "it depends". But, when soldering, with a reasonable stencil design, you're going to typically get more than enough fill to not have any problems. The QFN's with multiple interior pads, I've tried successfully to stay away from, since it seems that defects are much more likely on these. Although there's a voltage regulator wih this pad style that I've got my eye on that I'm seriously considering. But that one is unique in that the center pads share vias with perimeter pads, so you can just run a single pad all the way from the edge under the unit, so it would still be possible to reheat the joint. I understand that some people have had luck hand-soldering QFN's with the center pads by adding good sized vias where heat and possibly solder can be applied *through* the board. I'm not sure I would trust this for production, though. As far as doing this at home in a toaster oven, I wouldn't be surprised if it was not only possible but worked well, assuming everything else was fine. With modern pastes and components, the soldering process is remarkably insensitive to variations. One hint: If you do want to experiment, there are 'dummy components' available which could help with the verification process and cost less than the real chips. If you search for "QFN44 dummy component" you'll find topline and maybe another vendor or two. These are definitely less expensive than expensive parts, but in most cases, I've also discovered that I have been able to find some other very low cost 'real' component in the same package. NXP has a app note at https://www.nxp.com/docs/en/application-note/AN1902.pdf which covers the basics of what I described above. One final note to mention: Many/most QFN's are moisture sensitive. This pretty much means that once you open the package you have a limited amount of time to either mount them, or put them in a dry box. (or re-package them in a moisture proof container with an appropriate dessicant pack). If this doesn't happen correctly, then the part absorbs enough moisture from the air that when you bake it, the part cracks as it turns to steam. This is sometimes visible, sometimes not. Either way, is causes reliabilty issues. The data I have access to indicates that the SI5340A is currently rated at MSL2, which means that the 'open time' is 1 Year (assuming normal humidity levels). BUT... you never know until you get the package, and even then you should double check with the manufacturer based on the exact manufacturing date and factory. On Thu, Jan 25, 2018 at 2:41 PM, Hal Murray <hmur...@megapathdsl.net> wrote: > > j...@febo.com said: > > The challenge is that the chip is a 7x7 mm 44-QFN package and really > wants > > to be put on a six-layer circuit board. That's doable, but challenging, > > for home assembly. > > Can anybody comment on the toaster oven approach? > > Is it practical for things like this? How much does a solder mask cost? > How > much other stuff do I need? Does the solder paste need to be refrigerated > and other quirks like that? > > What are the chances of a newbie getting a 44-QFN right on the first try? > > > -- > These are my opinions. I hate spam. > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > -- *Forrest Christian* *CEO**, PacketFlux Technologies, Inc.* Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602 forre...@imach.com | http://www.packetflux.com <http://www.linkedin.com/in/fwchristian> <http://facebook.com/packetflux> <http://twitter.com/@packetflux> _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.