IR thermometer problem

Thread Starter

birckcmi

Joined Jan 1, 2018
149
I'm working on my soldering technique and I'm having trouble getting a reliable indication of the joint temperature with my IR thermometer. It's a Kintrex IRT0421, and it has always delivered what seemed to be accurate measurements in the past. Now, if I use it as a "forehead" thermometer, it gives me a reading within reason, but if I try to measure the temp. of a soldered joint, or even the temp. of the functioning soldering iron, it gives me a figure lower than human body temperature, e.g., 77.5˚F. IF I point it at the space heater, I get a believable reading, but if I switch to the soldering iron, I don't. What am I doing wrong?
 

Sensacell

Joined Jun 19, 2012
2,673
Two factors come to mind:

Measurement spot size and IR emissivity of the thing you are measuring.
Shiny metal tends to reflect other IR sources, giving erroneous results.
Make sure the measurement spot is small enough to get the joint and not the large surrounding area.
 

jpanhalt

Joined Jan 18, 2008
10,933
Maybe it is not aimed right at the soldered spot and/or its field of view (FOV) is so much larger that the recorded average temp is correct.

Why do you need that aid anyway? Isn't the presence or molten solder enough when the iron is at the correct temperature?
 

Thread Starter

birckcmi

Joined Jan 1, 2018
149
So the size of the area to be measured and the reflective qualities of the surface are throwing it off? There's one other experience that may bear on the issue: I used the same thermometer to gauge the heat emitted by a 20ga nichrome wire being used for a plastic bender, and got believable results. At a particular measured temperature, the wire softened the plastic, but didn't melt it. The wire was about 2mm thick, against a matte foil background, and that seemed to work fine. Does that sound right?
 

Thread Starter

birckcmi

Joined Jan 1, 2018
149
Maybe it is not aimed right at the soldered spot and/or its field of view (FOV) is so much larger that the recorded average temp is correct.

Why do you need that aid anyway? Isn't the presence or molten solder enough when the iron is at the correct temperature?
That's a reasonable question. I'm trying to arrive at the lowest temperature I can to complete the joint and not endanger the through-hole components being mounted on the board. Thanks for the comment.
 

jpanhalt

Joined Jan 18, 2008
10,933
That's a reasonable question. I'm trying to arrive at the lowest temperature I can to complete the joint and not endanger the through-hole components being mounted on the board. Thanks for the comment.
There are so many variables in hand soldering, I doubt your no-touch temperature will be much use. All of the inexpensive ones I have seen have a FOV is it quite large based on the distance of the detector from the object.

On the other hand, beautifully soldered joints of temperature sensitive components have been made for many decades without such aids. Practice soldering. If temp remains a real concern with good soldering technique, you can switch to low-melting point solder, which is usually has bismuth in it.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
149
Well, OK. You are probably right. I know I need practice more than I need the precise temperature. I'll look into low-melting-point solder. I'll keep playing around with the IR thermometer, just to see if I can figure out the problem, but I'll get back to practicing too. Thanks for the rational answer.
 

neonstrobe

Joined May 15, 2009
127
You will have trouble with IR measurements if you do not know the emissivity. Surfaces can vary considerably, and to standardise you either have to present the same surface (e.g. with a layer of known paint or tape over a hot body) to the thermometer or calibrate the subject you are measuring. Molten solder will be quite different from a head. A way of calibration might be to compare your solder at a known temperature using a separate thermometer. Got a meter with a thermocouple?
 

Thread Starter

birckcmi

Joined Jan 1, 2018
149
Thank you for the suggestion. The importance of the surface at which the IR thermometer is pointed is starting to sink in. And I may have a meter-with-thermocouple. If I can find it, I'll try it. While I work on my soldering chops.
 

kaindub

Joined Oct 28, 2019
45
I have hand soldered SMD components with a 30w non temperature controlled iron. But I prefer my temp controlled Hako clone.
I think others have made the point that its all in the technique.
My recipe for good solder joints
Have the iron at a temperature that will melt the solder
Make sure both surfaces to be joined are reasonably clean (don't need to be anal about this though)
Make sure the iron tip is clean . Wipe it across a rag or some steel wool
Use resin cored solder. I have not found a brand that does not work
Apply the solder tip to both surfaces. Like in the corner of the board and component lead
Never tin the soldering iron and then take it to the joint. The flux burns off and you can't get a good joint.
Heat for about 2-3 seconds and then feed in solder. Solder should melt and flow across and around both surfaces if sufficient heat has been applied
Immediately remove solder and soldering iron
Joint should have a shiny surface if sufficient heat has been applied.
Unless I am soldering big fat wires I find I never have to apply heat for more than 5 seconds. Any more and you risk damaging components (But you will be surprised how much heat abuse components do take)
 
I'll add my two pennies worth.
Surfaces need to be freshly cleaned and a glass fibre pencil does the job.
For SMD stuff like 28 pin TQ packages, I use a Weller TP iron with a very small conical tip and temperature selected to suit the solder. That tip doesn't get used for anything else.
I like a low melting point solder (bismuth content noted), such as Loctite 362 SN62 that has no bismuth and find that it wets better.
I never rely on the flux in a cored solder and always add a drop of a high quality SMD flux like Chipquik SMD291NL.
This tacky flux keeps the component in place and the blanket of molten flux spreads the heat and produces a lovely shiny joint.
Sometimes, it's better not to try and solder the component directly, but heat the pad or track and let the flux and solder do the rest. After the first fixture, go round the pins again with a dry tip to tidy things up.
Never needed or used separate temperature monitoring - eyes and experience tell you all you need.
 
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