Hi:
I frequently assemble PCBs by hand soldering MLCCs of the 0603, 0805,
1206, and a few larger ones (and other parts too--as those
capacitor-only circuits aren't much fun!).
The whole key is temperature soakings and keeping them and their degree
to a minimum. ANY reflowing of chip caps detaches end terminations and
the chip may read correctly but not perform to spec or even fail in use.
My hand soldering technique is to use 0.015" wire solder,
1. first adding a tiny bit of solder to one pad.
The tinier the better, but you must be able to hit the bead with the
chip in place, *without* hitting the chip with the iron.
2. Then I flux both pads, and tack the part to the pad with the added
solder.
Only reflow the bead long enough to seat the part square and flat and
allow seizure. Reflux that pad, and then solder the OTHER pad first,very
quickly, only adding heat to the PCB pad, not the chip termination 'end
cap' (of whatever variety). Never go back to that pad again. It is done.
Now solder the other pad to the right fillet size to match the first.
Never reflow either pad. The key is to add no further heat OR thermal
stress to the part internals or the solder terminations. That heat
transfers directly into the chip. These failure modes are very easy to
illustrate with HV caps (MLCCs) They read correct and act correct at low
voltage after thermal damage, but at higher operating voltages, they fail
from end termination detachment issues.
3. Sometimes at this point, I put my tweezers on top of the part to
create a downward force,
It must be VERY light force, and the tweezer "noses" must form a flat,
NOT pointed face to press on the chip with, or you can introduce tiny
micro-fractures in the chips.
and reflow the tacked joint to make the part
seat squarely on the pads.
your first bead should be so tiny that you can place it out of the way
of the squarely placed part, so that when you reflow that tiny bead, it
and the flux and the capillary attraction will lock it down. Only add
heat for the 'moment' of that reflow act, and avoid contact with the
actual terminations of the chips themselves. You are making it tack
down, not be integral yet, so add no more heat than needed to perform
that function. Heat kills chips.
4. Then I solder the other pad-to-part joint.
That is the correct process step order.
5. If necessary to make it look nicer, I add flux and reflow the tacked
joint.
if the tack is small enough, it *should* be your second joint anyway,
just to make the proper electrical connection. The 'tacking' process step
should have been minimal in both solder and the heat infused to do it.
The question is this: How much should I worry about thermal stress
cracking or otherwise damaging the MLCC dielectrics?
A lot. That is why you take serious steps to develop and follow a strict
process like that which I iterated to you. You may want to differ from
what I gave you, but that is your choice. The killer is heat. Theprocess
is where it gets introduced. One solder joint construction flow (for
effect) is all these chips should be subjected to (there is a severe
shock). They are meant for automated processes where the temp
differential at reflow time is a mere 20 or 40 degrees and the
terminations do not flex to their breakage point.
So, a hot air reflow (done quickly) with solder paste is an option as
well, but adjacent peripheral components get heated as well.
For hand assembly, short of preheating the board to a couple hundred
degrees and the chips in a hot pot... if you are soldering them cold, as
it were, the shock *will* be severe, so you have to perform the ops very
quickly and only once, if possible.
I have heard that using a pre-heated PCB, and soldering the MLCCs both
joints at once using hot air is the preferred approach, to avoid damage.
Yes, but do NOT try to do "both joints at once" using soldering irons.
You are actually doing the exact thing you do NOT want to do at that
point.
Also, that MLCCs with thinner dielectrics are more susceptible, like
high values in small sizes.
It took them many years to be able to even make them. Pretty likely
that they are easier to break or cause to shift from stated specs.
Yet in practice, I've never noticed a bad part.
ceramic caps always appear fine on metrological inspection.
"At voltage" is when it counts though. HV caps MUST be "good". When a
multiplier cap in an HV circuit fails, one typically replaces all of them
because the time savings is cheaper than trying to find which "good
reading" cap is "really bad".
Then again, since most
of them are bypass caps, it's hard to notice a bad part.
Unless they cause a problem in that location.
Any experiences with soldering causing MLCC damage?
MLCC damage is usually from soldering. If you strike and break one from
physical contact, it has nothing to do with a normal electronic industry
process. It was some dumb dope who should learn some material handling
practices before he (or she) handles delicate components again.
Solder joint physics area different animal. Depends on the pad/fillet
shape, and the type of terminations the part you buy has.
What amount of flex causes trouble?
If your post process assemblies are warped, you have a process issue
other than broken terminations when a stupid tech or manager flexes a
warped, solid board back toward its proper design shape. He needs to
learn some physics. They need to be held FLAT while they are still hot,
all the way until they cool, and if they are still warping, then the PCB
maker has induced it in their MFG process and it needs to be addresses
even further upstream.
If you grab a board and flex the fucking thing, you lack knowledge and
experience about soldering, solder creep, and PCB assembly practices.
If it is warped, it is a failure, if it does not fit form fit and
function *WITHOUT* attempting to "undo" the flexure and warpage.
That is so decidedly NOT "ordinary", by any measure. In some circles,
you would be looking for a new position.
Assembling CPU coolers onto PC motherboards is always a
treacherous experience!
Not really. Could be an aptitude thing.
I wonder how many MLCCs survive that experience.
Do you walk down the aisle of a crowded train car, shoving folks out of
your way with your elbows as you tromp toward the exit door?
You one of those asshole dock workers that/who toss packages full of
electronic devices up on the dock because they are too dumb to understand
what the term FRAGILE means?
You do not assemble electronic assemblies with FORD tools, idiot. In
this case that references your brain. You MUST be a Ford owner! You
sport a Ford owner mentality!
