Mounting a thermocouple in a hole under a BGA pad can be a complicated task. Roger Saunders, president of Saunders Technology (Hollis, NH), explains some of the techniques currently in use:
One elaborate but effective method starts with removing the BGA. Once accomplished, drill a hole in the board at a center pad, insert a fine-gauge thermocouple into the hole, solder the thermocouple to the pad, (keeping the junction 0.002 to 0.004 inch above the pad), epoxy the wire into the hole, and then replace the BGA.
An easier but less reliable method is to drill a hole through the bottom of the board into one of the center pads and use a high-temperature adhesive to secure the thermocouple to the pad. The risk here is that inaccurate readings can result if the thermocouple lifts off the pad slightly during adhesive cure.
A disadvantage of both of these techniques is that it is necessary to retain the instrumented test board and re-profile it whenever the profile must be checked.