Thermocoupling BGA

Perhaps the most challenging components to thermocouple are BGAs, since the area of reflow is hidden underneath the component. The most accurate methodologies are destructive.  The simplest way to thermocouple a BGA may be to drill a hole on the underside of the BGA and thread the TC bead into the drill-out hole that allows access to the target area without having to remove and re-attach the BGA.

Another method is to use a very thin gauge TC wire (40 AWG) and separate the two dissimilar wires, as shown below.  Then attach the TC and place the BGA on top of the TCs.  Again, the point of this exercise is to achieve an accurate “direct” reading.

tcbga1

Special thanks to Scott Nelson at Harris Corporation for providing this example.

A non-destructive method for thermocoupling BGAs is to simply mount the TC on top of the BGA and, perhaps, to the underside of the PCB directly below the BGA, and develop an offset. There is no right or wrong answer; much depends on your production tolerances and a whole host of other variables that have been discussed in this guide. The point is that only you know your process and limitations with respect to product and tools.

Note: This is an area of particular interest to me since it is a concern that just about everyone has an opinion on and no true right or wrong way has been developed.  Stay tuned more to come soon.

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Characterize your Thermocouples

characterizetcsTCs are sensitive instruments of measurement. TCs are abused mainly because they are misunderstood. First, go out and purchase a set of TCs with the proper rating for your process with the least amount of variability.  Look at the thermocouples that you are using.  Organize the TCs according to observed temperature values by plugging them into your profiler. For now, we are not going to enter into a discussion about electromotive force, thermometric sensitivity, linear vs. non-linear, TC conductor length, weld type, or even conductor material. What we are looking for here is that we are dealing with Type “K” thermocouples.

To characterize the thermocouples, you will need a few items.  First, get a new set of thermocouples, where the welded bead has been pre-formed by the same procedures. The length of the conductors (the length of the TC) should be more or less the same for each TC, with the method of TC attachment remaining constant.  My preferred method is Aluminum Tape (see TC attachment in Chapter 7 – Thermocouple Attachment Methodology and Materials).

Here are some suggested materials to use for characterization:

1. Three (or more) sets of Thermocouples  (Type “K”, all from the same manufacturer).

2. One stainless steel plate (if the same plate is used for each run, it can be made of any material as long as it is capable of withstanding the designated highest temperature for extended periods of time).  I recommend 1 or 2 mm. stainless steel, 12 inches long and 8 inches wide.

3. Squares of Aluminum tape to attach the TCs to the plate. Squares should be of equal size, with method of attachment consistent from TC to TC.  Remember, we are looking for reproducibility.

Run the plate and profiler through the reflow oven. Be certain to document as much data as possible for the sake of the experiment and for reproducibility, especially with respect to oven set points, conveyor speed, TC attachment, distance of the profiler from the plate, length of the TCs, internal profiler temperature, and any other data point that varies, having an effect on the reflow process in the future.

Repeat the run several times. Like most data, the bigger the sample, the better the results. Take your time to set up this effort.  If you are not getting similar results each time, then there is something wrong with the set-up of your experiment.  Go back and check the repeatability of each step.

Here is an example of a run where I dumped my TC readings into a spreadsheet.

characterizationtcexcel

I had over 300 samples for seven TCs.  The average TC reading for each TC is shown in the last row.  The spread from the lowest value of 149.5°C to the highest value of 150.6 °C, shows a variation of 1.1°C, which is within the +/- 1.2°C  rating for type K thermocouples.  Since I went to all this effort, I decided to pick from my study: TC#2, #4 and #7 for profiling my next PCB, knowing that my variability is no longer 1.1°C, but now .3°C, which is the difference of 150°C and 149.7°C.

The point is this, it may seem like you are splitting hairs, but 1°C here and 2°C there has a cumulative impact.  If you had a process that only allowed for, perhaps, a 5°C process window, which is not unusual today and you could cut out half of your variability due to your facility, equipment and TCs, wouldn’t it be worth it?

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Thermocouple Attachment

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.

Epoxy

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®

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.

Aluminum Tape

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.

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