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.
The study looked at:
- Aluminum Tape
- Kapton Tape
- Chemtronics – CircuitWorks CW2400- Two Part Epoxy
- High Temperature Solder
- 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.
Three 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.
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.
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.
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.