|
I've completed assembly of my first QMX kit. The instructions are extremely well thought out, and all went well.
However, during the final inspection prior to applying power, the inspection step 2.31.6 showed a very worrying thing on the buck regulator (you can actually see the same situation in the photo in the instructions):?
The power tab for Q103 extends out past the PCB pad, and actually overhangs the tinned pad for Q104. Although the actual Q104 pin is clear of the overhanging power tab, and I don't measure a short there, the tab and PCB pad are closer together than the thickness of a sheet of printer paper. The tiniest bit of conductive debris will short these points and destroy the unit.?
It's impossible to move the power-tab package, of course. I considered shaving off the edge of the Q104 pad to eliminate the overhang and improve spacing, but that would break continuity with other parts of that trace, so that's also a non-starter. I'm extremely concerned about the possibility of catastrophic failure from this almost-short. (In retrospect, I'm glad I didn't go ahead and wash the PCBs to remove the flux from my through-hole soldering, as that could easily have led to a short.)
The only other remedy I can think of is to apply a drop of thin cyanoacrylate glue to the gap as a conformal coating to isolate the tab from the pad. Does anyone know whether CyA glue is suitable or unsuitable for this application?
Going forward, I recommend adjusting the layout of the buck regulator for the next PCB run for the QMX to eliminate this issue. It has probably destroyed more than one apparently-sound QMX already...
Regards,
Steve
|
On second thought, I think the better solution is to go ahead and shave off the part of the Q104 pad that extends under Q103, and externally complete that net with a jumper wire. Less risk of a short that way.
Steve
|
Steve,
Before surgery, I would try to move Q103 a little by heating up the Tab until the solder melts and push Q103 over a little by inserting a small flat blade screwdriver between the bodies of Q103 and Q104. This is DELICATE work but you only have to move the Q103 part slightly (<0.5mm). Don't try to move Q103 until the Tab solder melts. Add a little solder on the iron tip and touch the Tab from the edge of the SMPS board ...do not get near the "short" end of the Q103 Tab. The reason for using a little solder on the iron is to better transfer heat to the Tab.?
73 Kees K5BCQ?
|
My advice is: don't solve a problem you don't have.?
We know Q104 and Q104 are a little close, and a bridge can form across them. All boards are now tested here before shipment and fixed if necessary. Future PCB revisions can move them a little further apart.?
In the mean time if you don't have a short, don't fix what ain't broke... meddling also carries risks. If you don't have a short now you really most probably never will have.? 73 Hans G0UPL
Steve,
Before surgery, I would try to move Q103 a little by heating up the Tab until the solder melts and push Q103 over a little by inserting a small flat blade screwdriver between the bodies of Q103 and Q104. This is DELICATE work but you only have to move the Q103 part slightly (<0.5mm). Don't try to move Q103 until the Tab solder melts. Add a little solder on the iron tip and touch the Tab from the edge of the SMPS board ...do not get near the "short" end of the Q103 Tab. The reason for using a little solder on the iron is to better transfer heat to the Tab.?
73 Kees K5BCQ?
|
Am I missing something here. The problem using manual and auto tuners
could be easily addressed using the venerable noise bridge for adjust the?
tuner in receive mode. Once you tune out the noise as indicated zero to?
little noise, one should be golden.
Regards
Chuck WD4HXG
On Sep 11, 2023, at 1:32 PM, Hans Summers <hans.summers@...> wrote:
My advice is: don't solve a problem you don't have.?
We know Q104 and Q104 are a little close, and a bridge can form across them. All boards are now tested here before shipment and fixed if necessary. Future PCB revisions can move them a little further apart.?
In the mean time if you don't have a short, don't fix what ain't broke... meddling also carries risks. If you don't have a short now you really most probably never will have.?
Steve,
Before surgery, I would try to move Q103 a little by heating up the Tab until the solder melts and push Q103 over a little by inserting a small flat blade screwdriver between the bodies of Q103 and Q104. This is DELICATE work but you only have to move the Q103 part slightly (<0.5mm). Don't try to move Q103 until the Tab solder melts. Add a little solder on the iron tip and touch the Tab from the edge of the SMPS board ...do not get near the "short" end of the Q103 Tab. The reason for using a little solder on the iron is to better transfer heat to the Tab.?
73 Kees K5BCQ?
|
Personally I am all in favor of taking a mallet to the p.c.b..... then "the problem" will (most certainly) go away....
not-so-anonymous
|
Kees, it has been my experience in the past that it's almost impossible, and quite risky, to move such solder-tab packages. They are simply too well thermally coupled to the junction and the PCB.?
Hans, thanks for the advice. I confess to being skeptical about "probably never..." (Murphy hates being provoked), but I take your point about there being risks from intervention as well. The middle ground would certainly seem to be a drop of CyA.
Thanks,
Steve
|
Here is a picture of Q103 moved over just a little. Definitely no short to the Tab now and it was easy to do.?
Just remember to get the Tab hot enough to melt the solder before exerting pressure.
73 Kees K5BCQ
|
How about
A dab or silicone
Sealant
To insulate the pads?
John
|
The only other thing I did was reheat the Source and Gate pins on Q103 to relieve any built up stresses from twisting the part a little. I don't think any pad insulation is required.
73 Kees K5BCQ
|
Apparently, moving Q103 works for some people and not for
others... I actually had a solder bridge, in addition to the
positioning of Q103 over the pad for Q104. In my case, I used an
air gun to lift Q103 and then I resoldered it into the correct
position.
Unfortunately, I accidentally also removed a resistor and diode
but didn't notice. Repairing that, I left a stray clipped lead
that shorted and killed the MCU.
Lesson learned: Don't get so focused on what you're doing that
you lose awareness of unanticipated events! :-(
New QMX on order.? :-\
Paul -- AI7JR
On 9/11/23 13:15, Kees T wrote:
toggle quoted message
Show quoted text
Here is a picture of Q103 moved over just a little. Definitely no
short to the Tab now and it was easy to do.?
Just remember to get the Tab hot enough to melt the solder before
exerting pressure.
73 Kees K5BCQ
|
Hi Paul
Moving Q103 always works. It always clears the short. I've done upward of a hundred. It always works afterward.?
However we have to accept that it's a delicate operation and not everyone will be successful at it, the first time.?
I'm just saying that in principle it does work if done correctly. There's about a 1mm gap between the drain pads. Once the short is cleared there's no further need for action. In all the ones I've seen there's also no marginal situations. The bridge is either clearly there, across the 1mm gap, or it isn't.
Apparently, moving Q103 works for some people and not for
others... I actually had a solder bridge, in addition to the
positioning of Q103 over the pad for Q104. In my case, I used an
air gun to lift Q103 and then I resoldered it into the correct
position.
Unfortunately, I accidentally also removed a resistor and diode
but didn't notice. Repairing that, I left a stray clipped lead
that shorted and killed the MCU.
Lesson learned: Don't get so focused on what you're doing that
you lose awareness of unanticipated events! :-(
New QMX on order.? :-\
Paul -- AI7JR
On 9/11/23 13:15, Kees T wrote:
Here is a picture of Q103 moved over just a little. Definitely no
short to the Tab now and it was easy to do.?
Just remember to get the Tab hot enough to melt the solder before
exerting pressure.
73 Kees K5BCQ
|
I'll bow to your experience. In my case, I don't think the short
would have been fixed by moving Q103--There was a genuine solder
bridge that I could see, independent of Q103's position!
This was my first venture in trying to adjust SMT stuff, so I'm
choosing to chalk it up to experience! ;-)
73, Paul -- AI7JR
On 9/11/23 21:11, Hans Summers wrote:
toggle quoted message
Show quoted text
Hi Paul
Moving Q103 always works. It always clears the
short. I've done upward of a hundred. It always works
afterward.?
However we have to accept that it's a delicate
operation and not everyone will be successful at it, the first
time.?
I'm just saying that in principle it does work
if done correctly. There's about a 1mm gap between the drain
pads. Once the short is cleared there's no further need for
action. In all the ones I've seen there's also no marginal
situations. The bridge is either clearly there, across the 1mm
gap, or it isn't.
Apparently, moving Q103 works for some people and not for
others... I actually had a solder bridge, in addition to
the positioning of Q103 over the pad for Q104. In my case,
I used an air gun to lift Q103 and then I resoldered it
into the correct position.
Unfortunately, I accidentally also removed a resistor and
diode but didn't notice. Repairing that, I left a stray
clipped lead that shorted and killed the MCU.
Lesson learned: Don't get so focused on what you're doing
that you lose awareness of unanticipated events! :-(
New QMX on order.? :-\
Paul -- AI7JR
On 9/11/23 13:15, Kees T wrote:
Here is a picture of Q103 moved over just a little.
Definitely no short to the Tab now and it was easy to do.?
Just remember to get the Tab hot enough to melt the solder
before exerting pressure.
73 Kees K5BCQ
|
I’ve also been able to move Q103 with lots of flux and hot air. But I always get anxious about damaging components with hot air. ?I’m a lot more comfortable with an iron. ?An alternative that I find works well is to replace Q103. ?I remove it with Chip Quik low temperature solder and a conventional soldering iron. ?I use lots of flux, clean the pads with solder wick and solder in a new one properly positioned. ?I find it easier and lower risk. ?
Jeff
W1NC
|
Hello Paul What I found is once you've heated up Q103, if you can actually see the solder bridge when you nudge Q103 away from Q104, my trick is to nudge Q103 intentionally CLOSER to Q104. Then nudge it back further away. Doing that, Q103 picks up and clears?the solder bridge.?
I'll bow to your experience. In my case, I don't think the short
would have been fixed by moving Q103--There was a genuine solder
bridge that I could see, independent of Q103's position!
This was my first venture in trying to adjust SMT stuff, so I'm
choosing to chalk it up to experience! ;-)
73, Paul -- AI7JR
On 9/11/23 21:11, Hans Summers wrote:
Hi Paul
Moving Q103 always works. It always clears the
short. I've done upward of a hundred. It always works
afterward.?
However we have to accept that it's a delicate
operation and not everyone will be successful at it, the first
time.?
I'm just saying that in principle it does work
if done correctly. There's about a 1mm gap between the drain
pads. Once the short is cleared there's no further need for
action. In all the ones I've seen there's also no marginal
situations. The bridge is either clearly there, across the 1mm
gap, or it isn't.
Apparently, moving Q103 works for some people and not for
others... I actually had a solder bridge, in addition to
the positioning of Q103 over the pad for Q104. In my case,
I used an air gun to lift Q103 and then I resoldered it
into the correct position.
Unfortunately, I accidentally also removed a resistor and
diode but didn't notice. Repairing that, I left a stray
clipped lead that shorted and killed the MCU.
Lesson learned: Don't get so focused on what you're doing
that you lose awareness of unanticipated events! :-(
New QMX on order.? :-\
Paul -- AI7JR
On 9/11/23 13:15, Kees T wrote:
Here is a picture of Q103 moved over just a little.
Definitely no short to the Tab now and it was easy to do.?
Just remember to get the Tab hot enough to melt the solder
before exerting pressure.
73 Kees K5BCQ
|
On Tue, Sep 12, 2023 at 01:38 AM, Paul - AI7JR wrote:
There was a genuine solder bridge that I could see, independent of Q103's position!
A solder bridge can be severed by cutting it with a sharp blade; a #15 scalpel blade is my go-to tool for this. 73, Don N2VGY
|
You can also remove the solder bridge by using a dab of flux applied with toothpick if you can get the soldering iron tip in there. The flux will increase the surface tension of the solder and break the bridge. The only "required solder" is between the lead and the pad. Using the smallest Solder Wick and a small solder iron tip also works.
73 Kees K5BCQ
|
Good points, all!
Paul
On 9/12/23 07:41, Kees T wrote:
toggle quoted message
Show quoted text
You can also remove the solder bridge by using a dab of flux
applied with toothpick if you can get the soldering iron tip in
there. The flux will increase the surface tension of the solder
and break the bridge. The only "required solder" is between
the lead and the pad. Using the smallest Solder Wick
and a small solder iron tip also works.
73 Kees K5BCQ
|
Steve H
Paul - AI7JR
Donald S Brant Jr