Electronics > Manufacturing & Assembly

Measuring laminate temp while using toner transfer method

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ampdoctor:
I was trying to think of a way to come up with some sort of baseline measurement system for the time it takes for a household iron to bring the copper laminate up to a specific temperature while using the toner transfer method of making home rolled pcb's. 

The idea that I came up with was to utilize the resistivity of copper and calculate the required change in the resistance of the laminate for a given temperature.  At which point I could apply the iron and measure the amount of time it takes to reach the calculated resistance value.  Effectively, this would be a variation on how one would typically evaluate the core temperatures of a transformer. This would be used as a one shot for a few different board sizes and the measurements would not be taken while actually attempting to transfer the image.

While it wouldn't be exact by any means, and it's intent is to come up with a quick and dirty reference value, I think it would be good enough to take a lot of the guesswork out of the process.  The down sides that immediately come to mind are that you'd need to use the 4 wire method and a good bench meter to measure the VERY low resistances along with the need to have the probes fixed to the laminate during the heating process which may or may not damage them.  I suppose you could use one of dave's microCurrent fixtures too but again, I'm not sure this would fly either.

Has my cheese slipped off the proverbial cracker or does this seem reasonable? 
Comments, suggestions?

SeanB:
Thin insulated steel wire attached to the copper, and use the copper as the other wire in an iron copper thermocouple. simple, and accurate enough at this temperature. You will need a millivolt meter and a correction table along with the temperature of the meter to calculate the temperature. Otherwise find a display that will handle an iron copper thermocouple, or if you can get thin insulated nichrome wire use a copper nickel curve.

poorchava:
Don't use the iron. Use an office document laminator. Cheapest chinese stuff can be had for under $7. You just remove the bimetallic switch and thermal fuse and replace those with an AC switch. Then use a thermocouple to measure rollers' temperature and when it reaches 190*C pass the pcb multiple times through the laminator.

It has a huge advantage over the cloth iron that it applies very even pressure across whole pcb and doesn't smear the toner. You don't need any precise temperature control. Just pass the pcb through the laminator and examine it when it comes out. If you can see shape of the traces easily and paper is laminated to the surface, then you are good to go. Check the temperature every 3-4 passes and heat up as necessary.

ampdoctor:
The whole intent was to come up with something that was quick and dirty and could be done easily with available resources.  While the thermocouple idea would certainly provide better results it's a bit of a pain to have to source some of the components if you don't live in a major metropolitan area so that was pretty much rejected off hand.

As an interesting aside, and I have no idea why I never tried it before, I rolled a board this week using magazine paper instead of glossy photo paper.  It was a proof of concept design and I wanted to do it NOW so I got a bit impatient.  I have to say it worked like a charm. Dwell time with the clothes iron was significantly shorter, and the traces transferred perfectly without any need for Sharpie rework all the way down to 10 thou with and spacing.  There was also much less mess in soaking the paper off as there's really no backing on it like photo paper. So in short, if I run a few more and am able to get consistently similar results the original question may be moot.

notsob:
Why not try one of these 'el cheapo" IR guns, and see what happens.

http://www.ebay.com/itm/Non-Contact-IR-Infrared-LCD-Digital-Temperature-Gun-Thermometer-Laser-Point-Y362-/280875627907?pt=LH_DefaultDomain_0&hash=item41657df583

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