How To Prevent Thermocouple From Acting As Heat Sink?

What type of object are you trying to measure the process of? "Top of junctions" is too vague of a description to tell for sure.

Are you concerned with temperature measurement errors caused by the thermocouple wires wicking away thermal energy and thereby cooling the tip? Are you concerned with this same effect cooling off the thing being measured (is it REALLY small??)?

It is a junction temp of a capacitor, inside capcitor body. It is a Vishay part number CS20108D1.

Thanks

Hang on a second, isn't the capacitor you are measuring only rated to 125 deg? Why is the temperature to be measured up to 260 deg C as this is way outside its specification!

Can you provide more details on your application please as I am confused.

Yes, that is functional temp. However, wave soldering temp is 260 deg C for 3 to 5 seconds.

Given the application I think your best option is to use thermocouples with good thermally-insulating sheaths (possibly adding a second sheath of high temperature braided glass sleeving) and keeping the junction as short as possible (ie- not sticking out of the capacitor unprotected.

The weak point is going to be the capacitor entry point where the sheath(s) meet the capacitor, so you could also look at encapsulating the entry point in epoxy (or some other thermally-insulating compound).

How to prevent TCs acting as heat sinks?

You could try a non-contact method such as a FLIR thermal imaging camera. Yes they are expensive but you can hire them if your budget doesn't stretch that far.

use very thin wires, much thinner than capacitor leads. Try to lay down - attach initial inches of wire in the place having temperature similar to measured at junction.

May be going down with TC wires (in teflon insulation) to allow them to be flashed by soldering wave at the same time as capacitor leads? (If You are measuring temperature during soldering process)