There are several methods to attach thermocouples to PCBs, some better than others. When attaching TCs, they should be strategically attached to areas that are dissimilar in terms of mass, location and known trouble spots. They should also be isolated from air currents. Lastly, the placement of several TCs should range from populated to less populated areas of the PCB for the best sampling conditions.
Several methods of attachment have been used over the years. These include: epoxy, high temperature solder, Kapton® and aluminum tape. I have observed all four methods in the field with various levels of success for each method.
I find that epoxy is very good at securing TC conductors to the profile board to keep them from becoming entangled in the oven during profiling. Epoxies come in both insulator and conductor formulations, so you need to check the specs, otherwise an insulator can play a negative role in the collection of profile data. The ability to apply this adhesive in similar quantities and thickness is very difficult and even harder to measure in quantitative terms. This, of course, decreases reproducibility.
If you insist on epoxy, it is always wise to check the website of the epoxy that you are using to review the properties and the specifications of the epoxy. Epoxy will function within a wide range of temperature tolerances.
High Temperature Solder
The properties of solder used for TC attachment is quite different from that of electrically connective solder. Of obvious consideration, is the melting point of the attachment solder, which must be higher than the melting point of the reflow solder paste.
Also, keep in mind that the flux used for attaching high temperature solder must not interfere with your reflow process. This is especially important if the profile board is used as a finished product!
High temperature solder is not the best choice to use for TC attachment for a couple of reasons. First, it has the same pitfalls as epoxy, in that the quantity of solder needed to adhere the TC to a substrate varies from location to location. Secondly, solder is conductive; and therefore, it has been known to short- circuit thermocouples. Keep in mind that a thermocouple works when two dissimilar metals are apart from one another and only joined at the bead. Take a minute to look at the welded end of a thermocouple.
Generally, there is a short length of conductor that is exposed to the temperature gradient. Together, this exposed area, along with the physical weld produce an EMF(Electromotive Force). It is essential that the conductors and the weld are in a homogenous environment within the temperature gradient. It would be very difficult to simulate this environment with solder for each thermocouple on a PCB since several thermocouples would be required. The same is true for epoxy.
Kapton® tape is one of the most widely used tapes and methods for TC and TC conductor attachment. There are several sources for Kapton® and most properties are the same. Some advantages are: ease of use, cost and non-permanence.
Although most tapes are the basically the same, similar tapes and specifications can be different enough to affect attachment. Also, when several layers are applied, each layer will have an additive effect on the insulation and can negatively impact your profile. It is best to apply a thin layer. Another important disadvantage of this tape is that the PCB has to be very clean and smooth to achieve an airtight cover over the thermocouple weld and conductors. For this reason, it is not my first choice for TC attachment.
There are several sources of aluminum tape with various thicknesses and density. I prefer to have a less dense and thinner aluminum tape to minimize the effects of the tape directly on the TC weld. Heavier aluminum tape can defuse the heat transfer through the tape and act as an insulator. Keep in mind that you are looking for the method of TC attachment that has the least effect thermally but with maximum ability to adhere the TC to the contact point of data collection.
Low density aluminum tape allows for heat transfer to the EMF-producing area of the TC. The thermal conductivity of the aluminum tape allows for even conduction when the thickness of the tape is fairly consistent in the EMF-producing area of the thermocouple. Additionally, aluminum tape is not permanent, which is good because the board you are profiling can actually be sold, but you might need to reapply tape to the same profiling board after a couple of profiles.
Aluminum Tape Used Along with Kapton®
Now for a dynamite combination: Kapton® and Low Density Aluminum tape. Used together, they produce the least invasive methods of attachment. Using Kapton® tape to hold the aluminum tape in place and secure the thermocouple conductors is an effective use for Kapton® tape. A common method of TC attachment is called Window Paning (see figure 7-6). Use Kapton® around the aluminum tape as an anchor, while keeping only the area of attachment in contact with the aluminum tape. This gives your thermocoupled board many more uses before it needs to be re-taped.