jra@febo.com said:

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.

Hi

You can mod toaster ovens, they work ok, EEVB has lots of info on that. When you buy
your PCB you can get solder stencils at the same time. Bought as a package they are in the
$10 or so range. Without buying the boards with them, I’m sure the price goes up a bit. The
metal ones are a bit more than plastic ones.

Solder paste should be refrigerated if you want it to last. How long it will do sitting on the bench
is a “that depends” sort of thing. Amazon will send you a  (small) tube for $10 or so.

The whole “can you do it” depends a bit on how good your microscope is and how steady your
hands are.

Again, EEVB is your friend if you really want to get into this.

Bob

I have had very good luck with a converted toaster oven, GC-10 solder
paste, and OSH Stencils metal stencils. Basically, if the temperature
profile is good and you have good solder paste application, decent
placement, good solder mask and correct pad sizes, everything solders
itself. I have built 70 of my boards with zero solder defects. I use a 4
pin castellated part.

https://sites.google.com/site/markstcxo/
https://sites.google.com/site/markscontroleo2build/
https://drive.google.com/open?id=0B2gd5QRdoS7BVTRSNzZFTTB6RlU

In this case though, I would probably opt for the eval board at $150. I
have spent way too much time on my small board project and there are lots
of little details to getting a clean oscillator.

73,

Mark
W7MLG

On Thu, Jan 25, 2018 at 2:41 PM, Hal Murray hmurray@megapathdsl.net wrote:

If you have a toaster that you have modified to retain heat better and
has an actual reflow controller, the toaster approach works /really/
well. The toaster I built follows the JEDEC-standard reflow profile
pretty much to the letter, other than taking longer to cool down than it
should (due to not having any sort of fan to move air). So as far as the
solder and components are concerned, it's basically no different than a
decent commercial reflow oven.

If you (or anyone) goes the toaster route, I really recommend getting a
conversion kit -- I used the Controleo2 (from http://whizoo.com/) --
they have a v3 now with a graphical touchscreen. You want the kit
(rather than just the controller) because a normal toaster oven just
leaks too much heat, and won't heat up fast enough, so you need to do a
bunch of other things to one to actually be able to hit your curves.
(They have their whole build guide at http://whizoo.com/reflowoven, so
you can see what you have to do. I built mine over a weekend. I believe
their current kit comes with everything you need but the toaster (their
previous kit required the addition of sealant, a tray, and the
insulation blanket)).

It's basically fire-and-forget -- populate board, stick in oven, press
'start', come back in a bit...

Stencils are pretty cheap (and fast!). I don't remember the exact price
off the top of my head, but I did a not-small board recently (60x110mm),
and the stencil (via OSHStencils) was less than $10, and got to me in
two days, and they're pretty much the same stencils any assembly house
would use.

The main problem with solder paste is that the flux degrades over time
(and you /really/ want your flux). Refrigeration slows that down, but
there's limits. You can, though, buy your solder paste in small
quantities (I think a 15g syringe, which will do a pretty decent number
of boards, is ~$15), so having it eventually go bad isn't that big a
deal. People have reported getting ok results with years-old paste,
though -- I suspect the results will be at least partially dependent on
the details of your board design (how fine-pitch the footprints are, and
such).

Odds of getting a 44-QFN right on the first try are pretty good. Neither
the chip nor the solder paste need be perfectly aligned for things to
work, at least if you're using a PCB with a proper soldermask. At that
pitch, you could probably even just smear some solder paste over the
pads, place the chip, and have success (though I still suggest a
stencil). The biggest problem source (for me, at least) is getting /too
much/ solder paste down (and ending up with bridge pins because there's
just no other place for the solder to go), thus the use of a stencil...

-j

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@megapathdsl.net wrote:

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