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

Thermocouple Attachment – Cyanoacrylate (Super Glue)

Cyanoacrylates (Super Glue) used to attach T/Cs for testing

Cyanoacrylate glue is sometimes used but is not the best

What are some advantages of cyanoacrylates?

It is easy to apply
Hardens quickly
Easy to remove
Available on most manufacturing floors
Makes an OK wire strain relief

What are some of the problems with cyanoacrylates?

Not made to take solder temperatures
Tends to “evaporate” at solder temp and will burn away with repeated runs resulting in increasing temperature readings with each use
Is a good insulator, so if too thick, will suppress the temperature value.

The main factors affecting thermocouple response time are thermocouple bead size and the conducting medium including attachment method.

Thermocouples response time is measured as a “time constant.” The time constant is defined as the time required for a thermocouple’s voltage to reach 63.2% of its final value in response to a sudden change in temperature. It takes five time constants for the voltage to approach 100% of the new temperature value.

Thermocouples attached to a heavy mass will respond much slower than one that is left free standing because its value is governed by the temperature of the large mass.  A free standing (exposed or bare wire) thermocouple’s response time is a function of the wire size (or mass of the thermocouple bead) and the conducting medium.  A thermocouple of a given size will react much faster if the conducting medium is water compared to still air.

Here are some typical time constants of various free standing thermocouple bead sizes (bead size is typically 2 times the diameter of the wire) in these conducting mediums:

    Wire (AWG)    Bead Size (inches)      Still Air (sec)        Water (sec)
           42                    0.003                              0.07                      0.003
           40                    0.005                              0.25                       0.02
           36                    0.010                                 1                            0.05
           30                    0.020                                4                            0.17

NOTE: Remember it takes five time constants for a thermocouple to reach 100% of the final temperature value so the above time constants must be multiplied by 5 to get the total time.

So the most common sizes (30 AWG or smaller) of thermocouples used to attach to surfaces or components will have fast enough response time to accuracelly measure the temperatures of reflow solder process which tend to change no faster then 5 degrees/second. If one wishes to measure the air temperature, 36 AWG is common since the air is always moving, and the chart reflects “still air” response times.

The main reason for selecting thermocouples of a specific size is to match the size of the surface or point where the thermocouple is to sense temperature. 36 AWG is a good compromise between cost, size, and strength. Much smaller and it is too easy to break. Much bigger and it may be bigger then the component or attachment point.

One other factor in selecting thermocouples is the heat source. In wave soldering, the heat is typically from the bottom of the assembly and the thermocouples are attached on top. Here the thermocouples will be cooler then bottom of the assembly causing them the sink the heat as it flows from the bottom to the top. Smaller thremocouples will reduce  the heat sinking effect.

In reflow soldering, both the top and the bottom of the assembly are heated at about the same rate causing the thermocouple wire and its bead to heat as fast as the assembly.  Some times the thermocouple can heat faster then the assembly because it is closer to the heat source and can act as a heat source to the component. This is often true where there is exposed thermocouple wire where the insulation has pulled back from the bead more then 0.5 inches. Keeping the insulation closer to the bead prevents this in most cases.

Box-whisker plots are a great way to show how stable the different thermocouple attachment methods are relative to each other.

The top dot is the Maximum value and the bottom dot is the Minimum value from the data set. The top of the box is the 75th Percentile (AKA: 3rd Quartile) and the bottom of the box is the 25th Percentile (AKA: 1st Quartile). This makes the Median, the red dot, the 50th Percentile (AKA: 2nd Quartile). Percentile is a number describing the data set such that the K-th Percentile is a number such that K % of all data values are less and (100 – K) % are larger than it, or to be more precise, at least K% of the sorted values are less than or equal to it and at least (100 – K) % of the values are greater than or equal to it.

Aluminum foil promotes solid contact with surfaces – Kapton tape adds adhesion insurance

What are some advantages of Aluminum Foil Framed in Kapton Tape?

Easy to remove without damage to product
Stable – good long term
Allows use of smaller pieces of foil while maintaining adhesion

What are some of the disadvantages of Aluminum Foil Framed in Kapton Tape.

Two step operation requires more time

The Sticky T/C is easy to attach and remove multiple times

What are some advantages of Sticky T/C?

Quick and easy to attach and remove

Does not damage product
Can be attached to metals and plastics
Provides identification of instrument input channel number
Able to see through and accurately place thermocouple

What are some of the disadvantages of Sticky T/C.

Not usable with small or irregular surfaces such as component leads

Product Information Here

The Temprobe is the easiest to use and remove attachment method

What are some advantages of Temprobe?

Quick and easy to attach and remove

Works well on “wet” solder paste

Does not damage product
Can be attached to metals and plastics
Sheathed construction protects thermocouple

What are some of the disadvantages of Temprobe.

Requires 1/2- 3/4  inch virtical clearence above board in oven

Air Dry Epoxy provides solid contact but requires time to cure

What are some advantages of Epoxy?

Stable – good long term
Can be attached to metals and plastics

Easy to see

What are some of the disadvantages of Epoxy.

Proper cure may require hours
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