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In the shop -- measuring PCB temperatures in the cooker

My cheezy Chinese temperature meter ($9 on eBay, but it looks like a Fluke* from a distance so it must be good) seems to do a pretty good job of measuring the temperature of my skillet when I'm doing skillet reflow, but not so good at measuring the temperature of the actual board.

I'm doing an experiment with soldering two-sided boards of soldering all the little components on the back side, then spacing the board up 1/8" and doing the components on the front side, while the back-side components stay stuck with surface tension. It's worked well once (and maybe twice -- that board is cooling as I type).

But both times the temperature reading on the circuit board itself have obviously been wildly off -- seeing a reading of 120C on a board with melted solder is a clear indication that the meter and the solder have different opinions of the board temperature.

The thermocouples themselves are the welded-bead type, and seem to track fairly well when they're both in good thermal contact with the skillet -- although I'm wondering if those temperature readings are really accurate.

So -- any suggestions on accurately measuring board temperatures in a reflow oven? Is it just that a welded-bead thermocouple, even with some thermal grease, is not the right choice? Is there some other thermocouple that will do, or should I try measuring the thing with an IR thermometer or other means?

I'm open to suggestions.

Thanks.

  • "If it works, it's a Fluke!" I'm not sure why they never adopted that as a company slogan.
--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott
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How thick are the leads on the TC? It could be heat going out that way... perhaps stick a few inches of the wire to what you are measuring. Hey, are you sure it's the right TC for the meter? It wants type K and you are using a J or something. Is the meter sitting on top of the oven and getting hot too? (perhaps poor cold junc. comp.)

George H.

Reply to
George Herold

Argh -- it was NOT that hot. I'm not sure how I hallucinated a good solder joint (distance and geezer eyeball may have contributed), but I hit on the notion of clipping a bit of solder off of the roll and leaving it on the board -- when it melts, I'll know.

Oh boy -- thrice-baked circuit! I wonder how it'll be?

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Updates in real time! The thermocouple read somewhere between 180 and

190C at the point where I noticed that the solder lump had melted. Since I'm being a bad boy and using 63/37 solder I guess that it's reasonably accurate.

I need to cobble a fan onto my entirely cobbled-together apparatus -- with the board spaced off of the skillet it seems to heat faster if the air is moving around.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Updates in real time! The thermocouple read somewhere between 180 and

190C at the point where I noticed that the solder lump had melted. Since I'm being a bad boy and using 63/37 solder I guess that it's reasonably accurate.

I need to cobble a fan onto my entirely cobbled-together apparatus -- with the board spaced off of the skillet it seems to heat faster if the air is moving around.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 
=============================================================================== 

Sounds like you need to splurge on a convection toaster oven.  You could  
still use the skillet and only heat from below if you think having the heat  
sink mass will give you better results, or you could try a piece of .25" or  
thicker aluminum if you can scrounge a piece that will fit the oven.  Should  
heat quicker and be more uniform than the skillet. 

----- 
Regards, 
Carl Ijames
Reply to
Carl Ijames

The best way I know of to measure temperature on a board is to borrow a diode junction for the sensor:

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Transistor base-emitter junctions work fine and the rest of the circuit doesn't need to be powered.

You can use the 183C melting point of 63/37 eutectic lead solder to check the calibration.

Measuring correctly with thermocouples is difficult unless they are embedded within the material. I clamp a thermocouple between two copper washers under the lid knob of my Farberware pots, to know when they reach boiling on the woodstove downstairs. The readout is about

10C lower than the same probe immersed in the pot.

-jsw

Reply to
Jim Wilkins

So do you think a diode would do better than a thermocouple as far as actually being the temperature of the board?

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

The thermal response of diodes is *extremely* linear within a range of sensible temperatures. I made a tire thermometer out of one when I was racing, on the recommendation of an electronics engineer. Checked against a precision scientific mercury thermometer, it was +/- about a half-degree C over a two-decade range.

--
Ed Huntress
Reply to
Ed Huntress

It would likely do better at actually being AT the temperature of the board instead of at some average of the board and the thermocouple's wire leads.

I have better luck measuring surfaces with 30 gauge thermocouples. Try measuring the external temperature of a boiling pot of water with a thermocouple to see how tricky it can be.

-jsw

Reply to
Jim Wilkins

Throw it out. I have a nice 3-channel Extech one that' the bee's knees and cost $100 iirc.

Cramming the bare ends of the TC wires into the digitizer of a PIC knockoff is not going to work well. ;)

Cheers

Phil Hobbs

Reply to
Phil Hobbs

My $5 reflow skillet, or my $9 Fluke knockoff?

(I assume you mean the Fluke knockoff).

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

The single thermocouples I have seen on boards are RTV in a small hole in the board just for that purpose.

The ECD MOLE for use in a convection oven uses many thermocouples that are fastened to board locations to be monitored, with RTV, as I recall. We haven't used ours in 5 years or so, so the procedure is kind of vague!

Get a used toaster oven at a thrift store and use it. Not so much air movement and easier to watch both sides of the board.

Paul

Reply to
Paul Drahn

Some photos of my successful attempt to reflow 18(?) HP JetDirect printer cards in a cheap Black and Decker toaster oven: Instrumentation:

What I found was that the temperature varied wildly with the location of the thermocouple sensor in the oven. Correlation between the thermocouple and the IR thermometer was non-existent. I was initially trying to simulate the correct thermal profile, but soon discovered that it was impossible. The temperature varied too much across the boards and across the oven. If you attach thermocouples to the PCB's located in various parts of the oven, you will probably see wide variations in temperature. Sorry, but I didn't record the numbers.

I took my best shot (after a dry run), and found that it worked perfectly. In other words, it does not seem as critical as I initially expected. Since then, I've reflowed a few dozen board with good success using a few more thermocouple sensors just sitting on the BGA chips without silicon grease or any pressure that might cause the chip to move.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

This transport company has used a variation:

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Seems a bit dubious to me. Same with any variation on "just in time". I'd like the stuff there in plenty of time, thanks.

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8 
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
Reply to
Spehro Pefhany

It oughtn't be difficult to mount a thermocouple; maybe just weld thermocouple wires to a paper clip? or a cotter pin, mounted in a through-hole with the legs spread?

Reply to
whit3rd

I upgraded an underperforming Coleman camp stove oven with an insulating blanket of fiberglass mat within a fiberglass cloth envelope, held together with staples around the folded-in edges. First I stabilized the loose edge threads by melting them with a propane torch.

-jsw

Reply to
Jim Wilkins

Two decades is e.g. 4.2K to 420K. Pretty good quality mercury, that. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Right, that's what Net 30 is for. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

On Fri, 22 Jan 2016 14:54:12 -0600, Tim Wescott Gave us:

Get an IR imaging device. Hell, they even have units you can attach to your smartphone.

It will be the best few hundred bucks you ever spent.

Otherwise, just use some common sense.

Put some paste into the skillet (direct contact) and observe how long it takes to come up to the point at which the paste reflows, the research what that temp is by the maker of the paste.

Then, you can assume that when you see top side reflow happening on your PCB assembly, that it begins at that temp and should begin at some point after the pan has been on the heat for the pre-determined time period.

Since you need not ramp-up the board for that entire period of time, add the PCB at some point before the pan comes up to reflow temp. A point at which you want your ramp-up dwell time to start at.

Reply to
DecadentLinuxUserNumeroUno

How many years are there in a decade and in a century in Phil's world?

Reply to
jnLsS? ?????? ? ???? ??? ?dRIfd

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