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TECHNET  July 2009

TECHNET July 2009

Subject:

Re: NTC (It's not Friday, but this is so true!)

From:

Inge <[log in to unmask]>

Reply-To:

TechNet E-Mail Forum <[log in to unmask]>, Inge <[log in to unmask]>

Date:

Tue, 14 Jul 2009 18:15:08 +0200

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (181 lines)

Wayne,
this was a classical High Current/High Temperature/Solder 
Decomposition/Fatigue issue. (Is there one word for this?). Famous failure, 
world wide during a couple of years with Philips Z12 TV chassis many years 
ago. Philips decided to solder mount the image frequency transformer by 
means of a number of pins in through holes on the PWB. The transformer was 
very warm, heavy and lots of current passed through the pins. The 
transformer had no screws or bolts, was just hanging in the solder joints. 
After some time, the solder joints looked EXACTLY like the one in my AC 
controller board. Those who were familiar with the technology at that time 
knew what to do: open the TV swing out the monster chassis, suck up the old 
solder around the transformer pins and redo the soldering. This had to be 
repeated once a year or so. Philips made a redesign and fixed a mechanical 
suppor which removed all mechanical force to the solder joints. All old dogs 
recognize this kind of issue, younger don't .

AND....this type of failures exist still. The below lines are written just a 
few years ago:

TV and Monitor Manufacturing Quality and Cold Solder Joints
Low cost no-name (or unknown name) computer monitors tend to be particularly 
prone to bad solder connections. However, so are many models of name brand 
TVs including those from RCA/GE/Proscan and Sony. We'll touch on these at 
the end of this article.
Any intermittent problems with monitors that cause random sudden changes in 
the picture brightness, color, size, or position are often a result of bad 
connections. Strategically placed bad connections can also cause parts to 
blow. For example, a bad connection to the SCR anode in a phase controlled 
power supply can result in all the current passing through the startup 
resistor, blowing it as well as other components. I had a TV like this - the 
real problem was a bad solder joint at a pin on the flyback. Thus, erratic 
problems, especially where they are power or deflection related, should not 
be ignored!

Bad solder joints are very common in TVs and monitors due both to poor 
quality manufacturing as well as to deterioration of the solder bond after 
numerous thermal cycles and components running at high temperature. Without 
knowing anything about the circuitry, it is usually possible to cure these 
problems by locating all bad solder connections and cleaning and reseating 
internal connectors. The term 'cold solder joint' strictly refers to a 
solder connection that was either not heated enough during manufacturing, 
was cooled too quickly, or where part pins were moved before the solder had 
a chance to solidify. A similar situation can develop over time with thermal 
cycling where parts are not properly fastened and are essentially being held 
in by the solder alone. Both situations are most common with the pins of 
large components like transformers, power transistors and power resistors, 
and large connectors. The pins of the components have a large thermal mass 
and may not get hot enough during manufacturing. Also, they are relatively 
massive and may flex the connection due to vibration or thermal expansion 
and contraction.

Why Can't TV Manufacturers Learn to Solder Properly?
I can think of several potential reasons - all solvable but at higher 
manufacturing cost.
  1.. Mass of large component leads (like shields) does not get adequately 
heated during manufacture leading to latent cold solder joints. While they 
may look OK, the solder never actually 'wetted' the heavy pins and therefore 
did not form a good mechanical or electrical bond.

  2.. Thermal cycles and differential thermal coefficients of circuit 
boards, traces, and solder. While it is not easy to do anything about the 
material properties, using plated through-holes or a similar mechanical via 
would greatly increase the surface area of the joint and prevent the 
formation of cracks.

  3.. Vibration. This is also directly related to the single sided circuit 
boards without plated through-holes to strengthen the joints.

  4.. Lack of adequate mechanical support (single sided circuit boards 
without plated through-holes (vias).
I believe that the single most significantimprovement would come about by 
using plated through-holes but this would add to the cost and apparently the 
consumer is not willing to pay more for better quality and reliability! Some 
designs have used rivlets - mechanical vias instead of plated ones. While 
this is good in principle, the execution has often been flawed where cold 
solder joints resulted between the rivlets and the circuit board traces due 
to lack of adequate process control.
The Sony and RCA/GE tuner shield problem is interesting because this could 
have been solved years ago at essentially no additional cost as other 
manufacturers - and their own repair procedures - have proven.

The End


Inge

----- Original Message ----- 
From: "Thayer, Wayne - IIW" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, July 14, 2009 4:02 PM
Subject: Re: [TN] NTC (It's not Friday, but this is so true!)


Hi Inge!

I am also someone who takes stuff to pieces.  You should see the scars on my 
knuckles from all of the sheet metal cuts you can get on a car.

Now that you've put your car back together....

Are you absolutely sure the solder joint was bad when it left the line?  I 
mean, the rest of the joints look just fine, so what is it about that one? 
In all liklihood this was wave soldered.

What if the real problem is that too much current is going through that 
joint, which is considerably smaller than the adjacent joint on the same 
trace?  You may have only peeled back the first layer of the onion!

I hate to put doubts in the mind of someone who did an excellent job getting 
the thing repaired, but I know you are tough!

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Tuesday, July 14, 2009 3:55 AM
To: [log in to unmask]
Subject: Re: [TN] NTC (It's not Friday, but this is so true!)

Inge - wow!  It's all in the hunt!  Good find!

Steve - You are right!  That is a cool trailer!

Steve C




http://stevezeva.homestead.com/files/Inge_s_Car_Fun_.pdf



http://stevezeva.homestead.com/files/Unusual_Trike.jpg

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