PCB track fault - infuriating one

[N_Cook]

Intermittant cutting out of an amp , something to do with the uPC1237HA
protection IC. Number of control inputs (o/p DC fault sense etc) that takes
o/p low of main speaker relay which clicks over. In fault mode this o/p goes
to 60V via the relay, all quite normal and in spec but for a small SIL IC
with 3.4V on the supply pin, a bit disconcerting. Incidently what does Vcc
mean wnen an IC is rated for 60 volt Vcc (from the NEC datasheet) but 3.4V
the recommended supply voltage

Next time coming across similar fault I will change the 100uF ,3 second
power-on-delay timer , temporarily, for 1 or 10uF as although the main fault
would operate immediately it would take 3 seconds to reset making tracing
infuriating as not immediate on/off.

Changed the IC and caps around the IC and just about to change Rs around it
although nothing has checked bad - still the problem persists. I decided to
change the 4.7uF electrolytic (power off muting one) for the second time
(most twizzle sensitive to inducing fault near this C) and this time the
solder pad came away with the molten solder.
Of course everything all nearby and elsewhere concerning soldering and pads
looked fine and tested for continuity fine.

Perfect ring of solder with a white pad (like overlay printing)
showing under the solder blob , with the cap wire still in place passing
through. Track , perfectly wide about 2mm, had failed in a neat circle
around the pad. Only ever touching contact between track and solder pad or
resonance vibration effect tearing in use ? bearing in min only a very small
electro. Only 2V or 0V on this cap and less than 20mA , so I assume
manufacturing fault. What led to the fault and what to visibly check for (if
possible) with other pads before returning amp to owner ?

 

[Ross Herbert]

:Intermittant cutting out of an amp , something to do with the uPC1237HA
rotection IC. Number of control inputs (o/p DC fault sense etc) that takes
/p low of main speaker relay which clicks over. In fault mode this o/p goes
:to 60V via the relay, all quite normal and in spec but for a small SIL IC
:with 3.4V on the supply pin, a bit disconcerting. Incidently what does Vcc
:mean wnen an IC is rated for 60 volt Vcc (from the NEC datasheet) but 3.4V
:the recommended supply voltage
:
:Next time coming across similar fault I will change the 100uF ,3 second
ower-on-delay timer , temporarily, for 1 or 10uF as although the main fault
:would operate immediately it would take 3 seconds to reset making tracing
:infuriating as not immediate on/off.
:
:Changed the IC and caps around the IC and just about to change Rs around it
:although nothing has checked bad - still the problem persists. I decided to
:change the 4.7uF electrolytic (power off muting one) for the second time
most twizzle sensitive to inducing fault near this C) and this time the
:solder pad came away with the molten solder.
:Of course everything all nearby and elsewhere concerning soldering and pads
:looked fine and tested for continuity fine.
:
erfect ring of solder with a white pad (like overlay printing)
: showing under the solder blob , with the cap wire still in place passing
:through. Track , perfectly wide about 2mm, had failed in a neat circle
:around the pad. Only ever touching contact between track and solder pad or
:resonance vibration effect tearing in use ? bearing in min only a very small
:electro. Only 2V or 0V on this cap and less than 20mA , so I assume
:manufacturing fault. What led to the fault and what to visibly check for (if
ossible) with other pads before returning amp to owner ?
:

On a particularly badly designed smps in the old Westinghouse Canada W842
terminals I often noted similar intermittents. In the end I resorted to
resoldering every pad and this revealed poor soldering technique or process.

When I touched the soldering iron tip to the pads and melted some new solder
with resin core flux the old solder immediately ran back from the copper pad
into a blob around the tip. The copper pad had a greyish oxide film over it thus
telling me that either insufficient heat and/or flux had resulted in a failure
to wet the copper pad when it was initially soldered. While the pad had
obviously been clean enough to cause the solder to cover the bare copper so that
it looked normal, over the years air had permeated the inadequate bond between
the solder and the copper pad and caused oxidation which then resulted in
intermittent connections.

 

[N_Cook]

:Intermittant cutting out of an amp , something to do with the uPC1237HA
rotection IC. Number of control inputs (o/p DC fault sense etc) that takes
/p low of main speaker relay which clicks over. In fault mode this o/p goes
:to 60V via the relay, all quite normal and in spec but for a small SIL IC
:with 3.4V on the supply pin, a bit disconcerting. Incidently what does Vcc
:mean wnen an IC is rated for 60 volt Vcc (from the NEC datasheet) but 3.4V
:the recommended supply voltage
:
:Next time coming across similar fault I will change the 100uF ,3 second
ower-on-delay timer , temporarily, for 1 or 10uF as although the main fault
:would operate immediately it would take 3 seconds to reset making tracing
:infuriating as not immediate on/off.
:
:Changed the IC and caps around the IC and just about to change Rs around it
:although nothing has checked bad - still the problem persists. I decided to
:change the 4.7uF electrolytic (power off muting one) for the second time
most twizzle sensitive to inducing fault near this C) and this time the
:solder pad came away with the molten solder.
:Of course everything all nearby and elsewhere concerning soldering and pads
:looked fine and tested for continuity fine.
:
erfect ring of solder with a white pad (like overlay printing)
: showing under the solder blob , with the cap wire still in place passing
:through. Track , perfectly wide about 2mm, had failed in a neat circle
:around the pad. Only ever touching contact between track and solder pad or
:resonance vibration effect tearing in use ? bearing in min only a very small
:electro. Only 2V or 0V on this cap and less than 20mA , so I assume
:manufacturing fault. What led to the fault and what to visibly check for (if
ossible) with other pads before returning amp to owner ?
:

On a particularly badly designed smps in the old Westinghouse Canada W842
terminals I often noted similar intermittents. In the end I resorted to
resoldering every pad and this revealed poor soldering technique or process.

When I touched the soldering iron tip to the pads and melted some new solder
with resin core flux the old solder immediately ran back from the copper pad
into a blob around the tip. The copper pad had a greyish oxide film over it thus
telling me that either insufficient heat and/or flux had resulted in a failure
to wet the copper pad when it was initially soldered. While the pad had
obviously been clean enough to cause the solder to cover the bare copper so that
it looked normal, over the years air had permeated the inadequate bond between
the solder and the copper pad and caused oxidation which then resulted in
intermittent connections.

That seems about the likely cause
The components looked hand populated despite 2005 manufacture, including
resistors canted over and touching one another.
A propbably related effect is the solder run reinforcement for current
carrying along the main rails. There are bare sections with what loooks like
oxidised dull brown copper instead and looks like fractured ends to the
remaining solder. I found no loose bits of solder inside so probably had
dropped away at manufacture, so passing final assembly inspection, all in
all a bad job.
I will explore the bare copper rail sections a bit more and the saved white
faced pad on the C lead.

 

[N_Cook]

around decided for

That seems about the likely cause
The components looked hand populated despite 2005 manufacture, including
resistors canted over and touching one another.
A propbably related effect is the solder run reinforcement for current
carrying along the main rails. There are bare sections with what loooks like
oxidised dull brown copper instead and looks like fractured ends to the
remaining solder. I found no loose bits of solder inside so probably had
dropped away at manufacture, so passing final assembly inspection, all in
all a bad job.
I will explore the bare copper rail sections a bit more and the saved white
faced pad on the C lead.

curiouser and curioser
For anyone else coming across similar it is a
Kustom KPS PM300, 300W mixer amp, 2005
Been exploring with a microscope.
Not broken away pieces of beef-up overlain solder runs along the main rail
traces, incomplete soldering in the first place, covered in the overall thin
conformal coating, the bare copper going dark brown underneath in just 4
years.

As for the failed C pad
This pad had survived original soldering, my first desolder and resolder and
came away with second desoldering leaving bare board where the pad was.
I expected to see copper colour after scraping into the white circle but
only silver colour of solder seen - no copper pad there ever? so thin it
disolved ? when I desoldered/resoldered initially there was a normal pad of
solder there

 

[N_Cook]

[snip]

As for the failed C pad
This pad had survived original soldering, my first desolder and resolder and
came away with second desoldering leaving bare board where the pad was.
I expected to see copper colour after scraping into the white circle but
only silver colour of solder seen - no copper pad there ever? so thin it
disolved ? when I desoldered/resoldered initially there was a normal pad of
solder there

Most likely the pad came away with the soldering
iron tip on the last reflow. It's probably
sitting on your tip cleaning sponge with all the
flux residue, or in your vacuum desoldering tool.
It's very easy to do and easy to miss.

Anytime you reflow a pad more than once, you have
an increased likelihood that this will happen.
The copper adhesive isn't intended to be melted
over and over...

the pa and ps share the same hs so I put the 2 caps on the track side of the
board, mainly so easier to temporary change the timing 100uF to 1uF if
necessary.
the 4.7uF smooths the rectified ac for power-off sensing but with it
intermittant contact , when that IC pin goes less than 0.7V in next mains
cycle, it triggered muting.

Anyway second time of desoldering the small cap, the pad came away, with the
usual few grams of force when blob molten. As a wired C , i desoldered with
normal soldering iron.
Amp back with owner now.