eHam Forums => Mods And Repairs => Topic started by: K8AXW on February 15, 2013, 01:05:22 PM

Title: Cooling media
Post by: K8AXW on February 15, 2013, 01:05:22 PM

I've encountered a problem that requires some assistance. 

I have a PCB on a transceiver that is densly populated and access is restricted.  I have a gradual LOS (Loss Of Signal) someplace on this board within 3 square inches.  The LOS takes place completely in about 3 minutes or so, much quicker than I can get to various points with a scope or meter probe.

I can kill the signal in 10 seconds with a hair dryer set on LOW heat.  I can bring the signal back by blowing cold air on it for about a minute or two.

I thought I found the bad component but it was like the farmer who dropped his chewing gum on the chicken house floor.  He thought he found it...... 3 times!

What I'm looking for is a method of cooling ONE component down quickly without cooling surrounding components.

Can any of you "bench men" suggest something??

Title: RE: Cooling media
Post by: KB4QAA on February 15, 2013, 02:17:03 PM
Buy a can of freeze spray with a tube nozzle/adapter.   Available from all the standard supply houses.

Title: RE: Cooling media
Post by: K8AXW on February 15, 2013, 05:06:36 PM
I know that'll work.  I've been there; done that.  I used it once and then the remainder of the can just set there and eventually went bad.

However, I'm looking for a more economical method.  Thank you.

Title: RE: Cooling media
Post by: KA4POL on February 15, 2013, 09:50:29 PM
However, I'm looking for a more economical method.  Thank you.
Hi Al,

take a piece of copper wire, a real thick one, put it in the freezer and then quickly apply it where needed. Not as effective as spray but real cheap, I meant economical  ;)

Don't forget to also check the vias and traces.

Title: RE: Cooling media
Post by: KE3WD on February 16, 2013, 05:17:22 AM

When using frozen bits of metal or wire instead of freeze spray, be careful about Relative Humidity and  condensation.  While Freeze Spray itself attracts the moisture, the extra surface area of the metal wire can attract a lot more -- don't let it get down into the circuit, such as the area between IC pins, etc.  Small bits of Aluminum do work rather well, I've found. 


Title: RE: Cooling media
Post by: KA4POL on February 16, 2013, 06:37:32 AM
Thermal conductivity of copper is 4.01 W/(cmK), Al only 2.37 So the cooling effect will be much better with copper.

Title: RE: Cooling media
Post by: K5LXP on February 16, 2013, 09:06:34 AM
I'm looking for a more economical method. 

What's your time worth?  A $10 can of freeze spray seems like a bargain to me if it can pinpoint the fault.

Usually though the problem is reversed - the circuit works normally but then intermittently fails.  As you describe it, you can get it into a fault condition and it stays there.  That's about as good as it gets for troubleshooting. 

Mark K5LXP
Albuquerque, NM

Title: RE: Cooling media
Post by: K8AXW on February 16, 2013, 09:30:00 AM
LXP:  I appreciate your comments and concern.  However, to answer your specific question, "How much is your time worth?" Right now, at age 77 it's worth quite a bit!

However, in reality I have to PAY for this solution.  That boils down to $10 for the spray, $6.00 for the shipping.... if I'm lucky and I use it once.  It MIGHT be the solution and then it might just as well not work..... which means $16.00 down the tubes and a week minimum shipping. 

I'm thinking, "there has to be a cheaper."  (I prefer the word "cheaper) Dieter.   ;D

I like your idea and will give it a try and get back to you.  In the meantime I've been thinking of alcohol and a small blower to evaporate it faster.  I intend to use a Q-tip to apply alcohol to a component and then blow it dry with the small blower I have.  I'll also report back on this idea. 

In the meantime guys, please continue to make suggestions. 

Title: RE: Cooling media
Post by: KA4POL on February 16, 2013, 10:25:35 AM
Alcohol (isopropyl alcohol, ethanol and isopropanol) is a polar solvent (very conductive) and is potentially corrosive (contains water). So be careful about using alcohol, I mean for cooling components  :P
It also may dissolve something on the PCB.
But this brought me to think about dry ice. If you could get your hands on some of that.

Title: RE: Cooling media
Post by: KE3WD on February 16, 2013, 11:50:04 AM
Very small pieces of dry ice held in tweezers and not held there long does work.  Don't ask my how and why I know that, its a long story about two technicians sent to the end of nowhere with nothing. 

Be careful of applying dry ice too long.  It can create such a delta temperature that composite component cases can crack. 

The good news is that it only takes less than a second for a leaky semiconductor junction to wake up and play right. 

Yes, the thermal co of Cu is better than that of AL.  But the thermal co is only one of several parameters to look at.  I was talking about what can happen in high humidity environment when using frozen bits of metal to find leaky junction.  Either choice can work anyway.  The Cu may just attract much more condensation over the same period of time, due to the thermal co. 


Title: RE: Cooling media
Post by: AA4PB on February 16, 2013, 12:40:15 PM

Title: RE: Cooling media
Post by: LA9XNA on February 16, 2013, 11:49:58 PM
If you by bay chance have got the possibly to have the board filmed by a thermal camera, The hot part will show up like a white spot.
The other options is as mentioned form other members.
Another option is to use a freeze spray with a straw to cool down smaller areas or the board.

Title: RE: Cooling media
Post by: K8AXW on February 17, 2013, 09:42:57 PM
OK guys..... looks like the spray coolant is the way to go, regardless of the cost/shipping time. 

Since the board is very dense I had second thoughts about sticking a piece of heavy copper wire down in there.... although I'm still thinking of wrapping it in tape first....

Dieter..... you made some very good points about alcohol being a conductor.  Never knew that and as for being corrosive.... I wasn't concerned about that because what I have is like 92% not the ordinary rubbing alcohol.  But, there's the possibility of removing the silkscreening from the PCB.  Not a good thing.

I don't have access to dry ice, which I thought of in the beginning.  I also don't have access to thermal imaging and since this is low voltage stuff I don't think thermal imaging would work anyhow.  I can't detect an temperature rise in anything with my finger.

4PB, thank you for the link.  The price Amazon shows with the very reasonable shipping costs makes the freeze spray an attractive buy. 

Thanks guys.  'ppreciate the input(s).

Al - K8AXW

Title: RE: Cooling media
Post by: KD0REQ on March 01, 2013, 10:12:39 AM
aw, heck guys, be creative.  get a common can of computer dusting "air", turn it upside down, and spray the suspect component.  you get a nice frosty cold shot from a can of sneeze you can buy anywhere nowadays.

danger, warning, fattening, etc. department:  much of this product is canned propane.  some of it is another fast-evaporating hydrocarbon.  no longer does The Borg (tm) permit you to buy Freon for the purpose.  so if you have a hot iron, sparking bad connection, HV arc going in the linear, or a smoke going, there is the possibility of a big surprise of the ugly variety.

but then, same thing applies to the freeze spray now.  about the only difference is the labelling, and maybe the ability of the can's valve to spray liquid right-side-up or upside-down.

Title: RE: Cooling media
Post by: K8AXW on March 01, 2013, 10:40:36 AM
REQ:   ;D  This is what is meant by "not seeing the forest for the trees" or is it the other way 'round?  I've done this very thing many times and never once considered that it can be used as a coolant!  Thanks for the great idea.

Suggestion:  Send this to the ARRL as a "Hints 'n Kinks item.  There's no doubt in my mind they'll publish it!

Al - K8AXW