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Thermocouple calibration on reflow oven


Thermocouple calibration on reflow oven | 15 April, 2004

Hi everyone, Our QC manager is bugging me about calibrating the TC readings on our reflow ovens. I keep telling him that doing a regular reflow profiling with a mole is sufficient to see if the oven temperature are normal. The manuals state that TC calibration is necessary only if you replace a TC. So why should I waste time checking the calibration on a TC when I know it's working fine?

Is it really an established industry procedure to check the calibration of the TC in reflow ovens?

The only impact this calibration has on the process is the the temperature displayed on the screen will be right within a minimal % of error. But that is not the temperature on the PCB. In our plant, we have six lines with one oven each and they are all running only 1 profile. Nobody ever looks at the temperatures on the screen. I do a profile 4 times a year, and in the past 7 years never noticed anything unusual.

So is calibration of the TC really necessary?

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Thermocouple calibration on reflow oven | 15 April, 2004

We never really use to calibrate the thermocouples on our reflow oven or wave solder systems, but we did disconnect the T/C's and input a calibration standard to check that the computer cards were correctly reading the proper temperature for a given mV input. In 15 years of working in this feild I have never really hear of a Thermocouples going bad slowly or out of cal. although a loose connection can cause a junction and effect the mV output. Omega sells the calibration units for under $300...

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Thermocouple calibration on reflow oven | 15 April, 2004

There are ledgitimate reasons for checking furnace calibrations, but let me clarify what that is.

1. TC's are not necessarily calibrted. Not unless you need "absolute" accuracy. TC's operate against a (non-linear) standard +/- a tolerance. Most furnaces uses TC grade for temp chamber and EXTENSION grade TC wire for the connection to the a-d converter. If you wanted absolute accuracy each and every wire you have would have to be calibrated...and calibrated IN your machine AND every wire you use to profile boards with. 2. The analog-to-digital converters of the furnace are checked against NIST standards for accuracy. Example if 500 degrees Fahrenheit yields a 500mv output voltage for a type K TC, then a TC simulator can feed this to the A-D and the furnace displays what IT THINKS the temp is.

Why should I check this? Electronics fail. I have had Three furnaces fail. One failed 15 deg Celsius. Motorolla once asked me, "How do you know your furnace is working correctly?" It gets hot is not what they wanted!!!

I have run evaluations with "golden board" profiles to evaluate furnace stability. On one particular furnace, a upper zone had failed (heater open). The lower heater (directly below the failed heater) was compensating for the failed unit. Heat rises and the TC above was reporting all systems fine. They were not. The golden board never detected the failure within the UCL and LCL statistical limits derived for that furnace

Do you have your profiler calibrated? If yes, then how could you not have your ovens calibrated?

Profiling is not absolute. Just because your profiler says you reached a peak of 220C on channel 3, DOES NOT mean you actually reached that temp. This is only true if all junctions are cold-junction compensated, a-d converters are linearized and compensated AND your Thermocouple wires are individually calibrated (this means each wire has a calibration compensation table). You can even factor cross-zonal heating uniformity as another variable which WILL affect the resultant. Oh, and furnace TC feedback method also affects results of "golden board" profiling. The uncertainty factor for profiling is larger than you think.

Profiling is not absolute. I have only seen this in laboratories.

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Thermocouple calibration on reflow oven | 16 April, 2004

Ken, thanks for your answer, it is much better presented than what I was fed by our qc. By "calibrating TCs" I meant the readings the controller is displaying on the screen. So that would be the converter that we would check and calibrate. Thank you for clarifying. Now, on our ovens, if a heater was open on a top zone, the bottom would send more power. The display on the screen shows how much power is sent to each zone. I guess it depends also how large the process window is on the range of products that go through the ovens. All of our products (~50) use the same oven recipe and the temp difference on the PCB is consistently less than 10 deg C. So how much precision do I need?

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