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.

References:
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.

Test Thermocouples attached with UV cured Epoxy

UV Cure Epoxy® provides solid contact with very short cure times

What are some advantages of UV Cure Epoxy?

Stable – good long term
Cures in 10 seconds
Can be attached to metals and plastics
Does not require heating to cure

What are some of the disadvantages of UV Cure Epoxy.

Requires about $1500 to $2000 UV light source
Difficult to remove without damaging product
Difficult to recover thermocouple

Aluminum foil promotes solid contact with surfaces

What are some advantages of Aluminum Foil?

Easy to apply
Easy to remove without damage to product
Stable – good long term
Accurate – Can be attached to very small components

What are some of the disadvantages of Aluminum Foil.

Does not work well with very small contact areas such as component leads
Cannot see through it to know where thermocouple is placed