Homebrew PCBs

Bought a GOOD lab hot plate to do solder reflow. Half inch thick aluminum top, temp setting dial.

When I first fired it up I very carefully found the settings on the dial that corresponded to 160C and 230C, the two temps I was interested in, for Toner Transfer, and for solder paste reflow.

The IR thermometer said the dial was WAY off. By a factor of like 1.5.
But this is OK, I just marked the spots on the dial and went about my
business.

Just did some toner transfer with the plate on the bottom and the normal iron on the top. I.E, the board and paper sandwiched between the hot plate and the iron. Set the hot plate for where the IR thermometer had found the setting for 160C

After being on the hot plate for like 15 seconds the paper started to
turn brown and smoke.

WTF?

I took the temp again. Yep, says right at 160C. But paper doesn't burn
at that low a temp... ????

I turned it down to the setting on the dial for 160, and tried again.

I after it had cooled off I put a piece of paper on top of the hot plate and took *it's* temperature. Just about exactly 160C.

After cleaning the board up and scraping off the burnt paper I ran it
through again and it worked fine a the dial setting of 160C.

Odd, no? IR non-emission coating on the plate or something?

I have an IR thermometer and experience the same thing.

I believe this is related to the concept of 'black body radiation'. The IR
thermometer is calibrated to measure the radiation of a pure black object
(absorbs light). If it's not pure black, then it will give a false reading.

That's my vague understanding, try googling 'black body radiation'.

If you use a thermocouple you won't have this problem! Or you can try
putting a black object on the hotplate, and measuring the temp of that.

On Fri, Jul 9, 2010 at 12:56 PM, rlspell2000 <rls@...>wrote:

>
>
> Bought a GOOD lab hot plate to do solder reflow. Half inch thick aluminum
> top, temp setting dial.
>
> When I first fired it up I very carefully found the settings on the dial
> that corresponded to 160C and 230C, the two temps I was interested in, for
> Toner Transfer, and for solder paste reflow.
>
> The IR thermometer said the dial was WAY off. By a factor of like 1.5.
> But this is OK, I just marked the spots on the dial and went about my
> business.
>
> Just did some toner transfer with the plate on the bottom and the normal
> iron on the top. I.E, the board and paper sandwiched between the hot plate
> and the iron. Set the hot plate for where the IR thermometer had found the
> setting for 160C
>
> After being on the hot plate for like 15 seconds the paper started to
> turn brown and smoke.
>
> WTF?
>
> I took the temp again. Yep, says right at 160C. But paper doesn't burn
> at that low a temp... ????
>
> I turned it down to the setting on the dial for 160, and tried again.
>
> I after it had cooled off I put a piece of paper on top of the hot plate
> and took *it's* temperature. Just about exactly 160C.
>
> After cleaning the board up and scraping off the burnt paper I ran it
> through again and it worked fine a the dial setting of 160C.
>
> Odd, no? IR non-emission coating on the plate or something?
>
>


[Non-text portions of this message have been removed]

Be aware that there is a difference between a visibly black object and a true blackbody (an object with an emissivity of 1.0 at all wavelengths.)  Normal IR thermometers assume an emissivity of 0.90 or 0.95 or such; high quality IR thermometers allow you to dial in an emissivity.  Of course you need to know what value to dial in for that to work, but it does help.  Normal IR thermometers are most useful at finding relative temperatures more than anything else.  And require calibration against the material under consideration for "real" applications.

Donald.
--
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----- "James Bishop" <bishopaj@...> wrote:

> From: "James Bishop" <bishopaj@...>
> To: "Homebrew PCBs" <Homebrew_PCBs@yahoogroups.com>
> Sent: Thursday, July 8, 2010 11:40:09 PM GMT -05:00 US/Canada Eastern
> Subject: Re: [Homebrew_PCBs] interesting
>
> I have an IR thermometer and experience the same thing.
> 
> I believe this is related to the concept of 'black body radiation'. The IR
> thermometer is calibrated to measure the radiation of a pure black object
> (absorbs light). If it's not pure black, then it will give a false reading.
> 
> That's my vague understanding, try googling 'black body radiation'.
> 
> If you use a thermocouple you won't have this problem! Or you can try
> putting a black object on the hotplate, and measuring the temp of
> that.
> 
> On Fri, Jul 9, 2010 at 12:56 PM, rlspell2000
> <rls@...>wrote:
> 
> >
> >
> > Bought a GOOD lab hot plate to do solder reflow. Half inch thick aluminum
> > top, temp setting dial.
> >
> > When I first fired it up I very carefully found the settings on the dial
> > that corresponded to 160C and 230C, the two temps I was interested in, for
> > Toner Transfer, and for solder paste reflow.
> >
> > The IR thermometer said the dial was WAY off. By a factor of like 1.5.
> > But this is OK, I just marked the spots on the dial and went about my
> > business.
> >
> > Just did some toner transfer with the plate on the bottom and the normal
> > iron on the top. I.E, the board and paper sandwiched between the hot plate
> > and the iron. Set the hot plate for where the IR thermometer had found the
> > setting for 160C
> >
> > After being on the hot plate for like 15 seconds the paper started to
> > turn brown and smoke.
> >
> > WTF?
> >
> > I took the temp again. Yep, says right at 160C. But paper doesn't burn
> > at that low a temp... ????
> >
> > I turned it down to the setting on the dial for 160, and tried again.
> >
> > I after it had cooled off I put a piece of paper on top of the hot plate
> > and took *it's* temperature. Just about exactly 160C.
> >
> > After cleaning the board up and scraping off the burnt paper I ran it
> > through again and it worked fine a the dial setting of 160C.
> >
> > Odd, no? IR non-emission coating on the plate or something?
> >
> >

Plain aluminium is off by 50% or more. Same with heatsinks. It's
almost impossible to measure with an optical thermometer or thermal
imager.

To get around this put a piece of plain white office paper on it.

I did tests with a thermocouple probe and a heatsink and the most
accurate results are achieved by simply sticking a plain white paper
sticker on. Of course the glue doesn't stand the temperature so it's
no good to use a sticker on the hotplate.


Generally optical thermometers are problematic. At least you noticed
the problem, I know plenty of people who just accept the reading at
face value and think they do precision work when indeed all they do is
waste time and space.

It is easier to get precise values with a thermocouple. I drilled a
hole in the edge of my hotplate to hold the thermocouple.

ST

On Fri, Jul 9, 2010 at 4:56 AM, rlspell2000 <rls@...> wrote:
> Bought a GOOD lab hot plate to do solder reflow. Half inch thick aluminum top, temp setting dial.
>
> When I first fired it up I very carefully found the settings on the dial that corresponded to 160C and 230C, the two temps I was interested in, for Toner Transfer, and for solder paste reflow.
>
> The IR thermometer said the dial was WAY off. By a factor of like 1.5.
> But this is OK, I just marked the spots on the dial and went about my
> business.
>
> Just did some toner transfer with the plate on the bottom and the normal iron on the top. I.E, the board and paper sandwiched between the hot plate and the iron. Set the hot plate for where the IR thermometer had found the setting for 160C
>
> After being on the hot plate for like 15 seconds the paper started to
> turn brown and smoke.
>
> WTF?
>
> I took the temp again. Yep, says right at 160C. But paper doesn't burn
> at that low a temp... ????
>
> I turned it down to the setting on the dial for 160, and tried again.
>
> I after it had cooled off I put a piece of paper on top of the hot plate and took *it's* temperature. Just about exactly 160C.
>
> After cleaning the board up and scraping off the burnt paper I ran it
> through again and it worked fine a the dial setting of 160C.
>
> Odd, no? IR non-emission coating on the plate or something?
>
>

Thanks for the feed back folks - I have one of those IR thermometers and it is almost impossible to get a decent temperature with hot stuff I'm interested in - copper, aluminum, etc.... even stainless steel.

I've tried to check temperature of PCB coming out of the laminator with my IR and it just does not work!

I though I had a cheap (it is) crappy IR - but maybe it's the application?

Ken H.

No IR thermometer can do that.
I don't think even with a better one (where you can adjust emission
coefficient) can you measure shiny metal properly.

ST

On Fri, Jul 9, 2010 at 3:51 PM, sailingto <sailingtoo@...> wrote:
> Thanks for the feed back folks - I have one of those IR thermometers and it is almost impossible to get a decent temperature with hot stuff I'm interested in - copper, aluminum, etc.... even stainless steel.
>
> I've tried to check temperature of PCB coming out of the laminator with my IR and it just does not work!
>
> I though I had a cheap (it is) crappy IR - but maybe it's the application?
>
> Ken H.
>
>
>
> ------------------------------------
>
> Be sure to visit the group home and check for new Links, Files, and Photos:
> http://groups.yahoo.com/group/Homebrew_PCBsYahoo! Groups Links
>
>
>
>

> 
> I after it had cooled off I put a piece of paper on top of the hot plate and took *it's* temperature. Just about exactly 160C.
> 
> After cleaning the board up and scraping off the burnt paper I ran it
> through again and it worked fine a the dial setting of 160C.
> 
> Odd, no? IR non-emission coating on the plate or something?
>


IR thermometers don't work on plain aluminum, I ran into this myself when I was building a hot plate. The temperature reported by the IR was WAY lower than that reported by a thermocouple attached to the plate.

What I ended up doing was taking a piece of 3/8" aluminum stock and boring four 0.250" holes down the side into which I inserted 60W ceramic heating elements made for soldering irons. I'm currently using an ebay PID controller to set the temperature which works very well, but I'm working on my own design that I can customize for this application.