Attaching thermocouples in Vapor Phase Soldering (VPS) presents a potential problem that’s often overlooked. To illustrate this potential problem, let’s look at how VPS works. All the work needed to heat the components and solder joints to the melting temperature of the solder take place in the very thin “condensate” layer of liquefied vapor phase fluid. This 0.2 mm layer is very much like the layer of water that forms on a cold drink on a humid summer day. (See figure 1.) The vaporized fluid (cloud) is at or very near the boiling temperature of the fluid, which is typically 200ºC to 240ºC, depending on the fluid you choose to use.
Figure 1: Vapor Phase Soldering and the condensation layer
When your room-temperature assemblies are immersed in the vapor cloud, the vapor quickly condenses onto the cold surfaces of all the parts, releasing the heat of vaporization, or the heat energy that was put into the liquid to evaporate it into a vapor. This energy release begins heating your assembly rapidly toward the 200+ºC vapor temperature.
To measure the thermal profile of the process to prove you are staying within the components’ thermal limits, (always a prudent step) you must attach a thermocouple to the component or solder joint you wish to profile. Here is where you must take care. (See figure 2)
Figure 2: Typical thermocouple attachment to component lead
The thermocouple may be attached with solder or epoxy. However the thermocouple wires extend away from the point of measure and may be exposed to the vapor. The vapor will condense onto the thermocouple wires in the same way it condenses onto all the parts. Exposed bare wires will heat quickly because they are very small. These heated wires will conduct that heat toward the thermocouple bead, where it is sensing the temperature, and artificially heat the sensing point. This will actually show as a “false” temperature rise in the early part of the ramp up in temperature because the heat is conducted in to the part by the thermocouple wires. The insulated part of the wire will not heat as fast because the insulation slows the heat flow of the condensing vapor.
So when connecting thermocouples to components to measure the thermal profile, it is very important to insulate any exposed thermocouple wire beyond the thermocouple sensing bead. (See figure 3)
Figure 3: Insulated thermocouple wire
This is best done with a thin layer of epoxy covering the exposed bare wires of the thermocouple, up to and a bit past the insulation on the thermocouple wire, being careful not to cover the sensing bead of the thermocouple. The epoxy does not need to be very thick, just a thin covering. Using too much would have the “chilling” effect of preventing the heat from getting to the component or solder joint, giving an artificially cooler profile result.
You may also try Kapton® tapes or high-temp heat shrink around the end of the thermocouple. These may shrink back or come loose, however; and if you plan to profile more than once, these methods may not hold well over time.
SMT Magazine, "Vapor Phase Soldering: The Comeback Kid" by Ray Prasad
Thanks to Kevin Syverson, Silicon Forest Electronics, Inc., for the use of their Vapor Phase soldering system.