Thermocouple Attachment Results are in!

The Rochester Institute of Technology under the guidance of Dr. S. Manian Ramkumar Ph.D. just conducted (October 2009) the most comprehensive study to date on thermocouple attachment methods.  Part I of II was to determine the most accurate and reliable method of thermocouple attachment.  Part II that has yet to be released is to determine the best attachment methods for BGAs, with the goal of seeing if there are reliable non-destructive methodologies, so stay tuned.

Results in a nutshell:

Aluminum Tape out performed all materials even Kapton! In an ideal word, the best attachment method of a thermocouple to a component is what I like to call a naked TC.  Aluminum double sided conductive tape was the closest thing to having nothing at all to attach the thermocouple.  Kapton tape is less responsive (deflecting and insulating heat), never mind if you have ever seen a saw-tooth TC plotted on a profile you know it has a very hard time staying in place on your PCB.   Additionally, High Temperature Solder which I have always considered the gold standard, is the least accurate or responsive.  When you get to the critical peak temperature of your profile, high temperature solder is sluggish to respond to the rapid change in temperatures, thus distorting your readings. As Phil Zarrow and Jim Hall discuss in Board Talk, “mass” on your thermocouple is not your friend.  Phil Zarrow:

any measurement method, the key element is to get the thermocouple in good contact with what you are trying to measure and to do it in a way that does not modify the area with a lot of extra mass or material that is going to give you an inaccurate reading….

Bingo!  This is actually what this study shows, now with the numbers to back it up.

Study Methodology:

The study looked at:

  1. Aluminum Tape
  2. Kapton Tape
  3. Chemtronics – CircuitWorks CW2400- Two Part Epoxy
  4. High Temperature Solder
  5. Loctite – 382 Instant Adhesive

The study used a KIC Explorer with standard type K thermocouples.  Multiple runs of a substrate coupon (62 mils thick plain copper coated with silver) was routed into 12 uniform 0.24″ isolated sections.

Test bank2Three identical test coupons were used and run multiple times.  KIC’s air-TC was utilized as the control to which each thermocouple was measured as the coupon traveled through all heated zones.

A total of three boards were used, running each board through twice, allowing the internal temperature of the KIC device to drop below 40 degrees C before rerunning the profile.

The tape attach methods were measured uniformly for each RTD connection, using a dial caliper, while the high temperature solder and epoxy quantities for attach were found to be visually uniform.

Mean

This graph indicates the mean temperature differential that was noticed within the oven for the various attach methods. The readings are based upon the complete profile starting at room temperature and ending at the peak temperature. The data from the cool down zone was eliminated from the analysis.

The graph shows the mean differential and the 95% confidence interval for each attach method. The Aluminum tape had the least differential (-0.48) followed by Kapton Tape, Loctite Adhesive, CW-2400 and then HT-Solder. The Confidence intervals among most of the attach methods do not overlap except Kapton and Loctite, indicating that the means of the attach methods are significant. Significant differences exist between the methods except between Kapton and Loctite as there is overlap. Clearly Aluminum tape outperforms all of the other methods.

Zone Differences

The thermocouples seem to behave similarly within each of the zones of the oven. Zone 6, where the soldering takes place or the peak temperature is reached, the thermocouple attach methods show a much higher temperature than the air temperature, indicating that the PCBs have attained much higher temperatures than the air. A closer examination of ZONE 6 reinforces the selection of Loctite or Aluminum Tape for Phase III of this project.

Conclusion:

When considering accuracy, repeatability and responsiveness, Aluminum Tape is a winner.   There are of course advantages and disadvantages to each material.  For example one can argue you can re profile a PCB set up with high temperature solder, but considering that the mass of the solder distorts your readings, this study even brings into question this bedrock of thermocouple attachment.  Never mind high temp solder destroys your PCB as well as there is little control over the size of the blog from TC to TC and board to board.   Also don’t forget every time you profile the same board again it loses some mass, which will be the focus of more blogs to come.

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Comments

  1. Miriam,

    A formal study will be coming, it should be completed by year end. The size of the aluminum tape was about 1/4″ square. Kapton was not used with the aluminum tape.

    Brian

  2. Very interesting article Brian, thanks! Will there be a more compreshensive report written like the one in 1999? I want to know for example how much Al tape was used and whether Kapton tape was used over it as well?

    Thanks, Miriam

  3. Dick,

    Great question. Several years ago we (KIC) took a look at Saunder probes which I now believe are called Temprobes from ECD. Now this was many many years ago and at the time we found the attachment method to be too unstable to warrant further investigation. I came across these probes again at a customer about a year ago and do not know how old they were, but I still observed real difficulty keeping them in place. You could see them bounce around when running a live RF profile since the TC reading looked like a saw-tooth. Also I had a few contact points that were not perpendicular to the probe which made attachment nearly impossible. In the end, the operator gave up in frustration and went for aluminum tape. The idea of external probes seems like a good one, but getting everything to line up just right I am not sure how practical it is. What is your experience? How complex are your boards? I am also told if you have low clearance in your oven you can’t use them, but not sure about this. Frankly they were not considered for the RIT study since they are not widely used despite being on the market I believe for more than 10 years. Kapton tape is still king as the mostly widely used attachment method, perhaps more than ¾ of the time and what this study showed is aluminum tape performed better thermally while also having the convenience of tape that it can be “sized/cut” to the attachment area and used in just about any situation. With ever more denser boards there is not a whole lot or real estate to work with so having the ability to conform your tape to a tight spot is pretty important. Of course the downside to Kapton is it doesn’t want to stay in place also giving you that similar saw-tooth reading. If you do not have issues keeping the probes in place, I suggest you conduct your own mini-study. Compare the same contact points with more than one attachment method. For example run a TC using aluminum tape to the same contact point as the probe, of course if you connect them at the same time don’t let them touch. This is not exactly scientific but it will at least give you an idea if both attachment methods give you the same reading.

    Also, I have no idea if all the mass of the temp probes comes into play with the overall thermodynamics of the board? The arm of the probe looks beefer than the wire of a TC and the hinging mechanism I wonder if it has a thermal impact? ECD, I am sure has considered this and I would assume it doesn’t have an impact, but I don’t know. Keep in mind though, the whole point is to measure your PCB with the least amount of foreign material attached, since whatever you are introducing into your production environment from TCs to even the profiler doesn’t belong there. For example, most users will trail a profiler a foot or longer behind the board as to not introduce the mass of the profiler to the production board being measured. Dick, I will be exploring further how mass plays into profiling. My gut tells me that my industry is getting a little carried away with all these tools we are introducing to make your life easier. Their mass could perhaps be causing their own problems in interfering with accurate data collection,……but I ramble…. Thanks for the question.
    Brian

  4. Is there a comparison of the “Thermocouple Attachment Results are in!” method to the mechanically clamped probe method?
    Thanks.

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