This is dependent on many factors including thermal exposure, substrate material, and even thermocouple attachment method.

In general for the substrate, FR4 test boards are only good for 10-20 runs, where polyimide or hybrid boards may last 50-100. This is mainly due to the substrate de-laminating from repeated thermal cycles. As the PCB acts as an insulator (and a thermal mass), this de-lamination can cause thermal variation in your measurements.

The thermocouple attachment method is also important, as only high temp solder or high temp epoxies will give repeatable results at lead free temperatures over time (tapes will release after only 2-5 runs).

The lifetime of a test board is dependent on the type of substrate and the PEC construction. As long as the board does not delaminate then you can continue to use it many times - I've seen them lasting of the order of 6 months with more than 100 reflow cycles.

Its a good idea to store the test board in a dry cabinet between reflows to prevent any moisture build up. Discoloration of the laminate etc will occur but will not affect the test result.

Note that if you are using it for lead free profiling then it may not last as long due to the higher temperatures involved.

The short answer is "It depends." The material make up and physical attributes of the profile board, the thermocouples selected, the attachment method of the TCs and handling of the board will all contribute to the life expectancy of the test vehicle.

Most of the time thermocouples detach or become suspect by displaying erratic behavior. Attachment of TCs must bust both accurate and robust. Don't count on tape to last. Thermally conductive adhesive is a must.

One way to gauge profile board degradation is to create an identical golden board to validate your profile board. There are several commercially available profiling products that may prove useful.

You are fortunate to have profile test boards and I applaud your efforts to make sure your ovens are producing consistent profiles. They are however fragile, and each time you use them to profile you damage it a little more.

For the most part the PCB itself is at greatest risk. The layers will begin to separate or delaminate. This will affect the thermal conductivity of the board as air pockets develop. Traces will begin lift as the adhesives break down. This will destroy the test board's thermal nature.

When will this happen? Depending on the thermal profile, how hot and how long for the most part, it can start to happen in as little as 10 profiles, or it may last for a many as 50 profiles.

Either way, there is a limited life. Test profile boards should be reserved for major changes to your thermal processes, such as a new oven or significant changes to the oven's performance.

Many tools exist to Verify your oven is ready to produce consistent thermal profiles on a daily or some frequent period. These tools are made of materials that can withstand thousands of profiles without any degradation. These tools can and should be use often as part of a good thermal process quality program.

P.V. you are way ahead of the game for using an actual product, which has been argued on this forum many times from profiling companies to oven manufacturers as the only true representation of a profile.

To answer your question, a lot will depend on the board material, the temperature, and the amount of time that the board spends above the "glass transition temperature."

Above a certain temperature the FR4 material outgases and actually loses weight. In our tests at KIC, once the board loses about 3-5% of it's original weight, it is no longer a close enough representation of the original board to create a valid profile.

In other words, your golden board is not so golden anymore. Our tests showed somewhere between 5 and 10 runs before your PCB is compromised. You also need to watch the thermocouple attachment closely.

We found when attaching the TCs with aluminum tape, we would have to re-attach the TCs at least every five runs. Again a lot depends on the profile temperature.

Lead Free profiles are typically hotter and will wear out the TC attachment quicker. You might be tempted to use high temperature solder or epoxy, but a recent study concluded by RIT to be released soon shows this to be a very poor choice.

Another alternative often proposed as a solution is to use a solid state fixture, but a large bulky thermal mass is no closer to a true representation of your product than your PCB that has been profiled repeatedly. I wrote a somewhat provocative blog on this topic, where I compare some fixtures to sending a brick through your oven.

There are better alternatives to all of this. My customers will use KIC's Auto-Focus to create their first profile. What is nice about this tool, it does not require you to run a physical profile on your first go around, since a solution is given to you based off of a database of prior work.

In practice, many customers will use this solution as their set-up and run production or they decide to thermocouple up their board anyway and run the profile but optimize using another tool of ours called Navigator that drives the profile deep within their process specs.

Either way, you never profile a "sellable" production board more than twice. You have dead on accurate profiles because you are basing your work on true production boards, no bricks no scrap no guesses.

Hope this helps.