[time-nuts] Slightly OT: interest in a four-output, ultra-low jitter, synthesizer block?

Forrest Christian (List Account) lists at packetflux.com
Thu Jan 25 19:54:53 EST 2018

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 <hmurray at megapathdsl.net> wrote:

> jra at 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.
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*Forrest Christian* *CEO**, PacketFlux Technologies, Inc.*
Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602
forrestc at imach.com | http://www.packetflux.com
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