18 May 2011
When using thermocouple leads to measure temperature, the bead of the lead needs to be attached to the surface being monitored to assess any temperature change
When using thermocouple leads to measure temperature, the bead of the lead needs to be attached to the surface being monitored to assess any temperature change. The bead is the physical junction of the two dissimilar wires.
In the case of a printed board assembly this may be on the surface mount pad/lead junction. During the reflow process the board is normally the surface which takes longer to rise in temperature than the lead.
It is, however, the combination of lead, paste and pad which needs to be monitored for correct set-up of the reflow process. In the case of wave or selective soldering it is the top and bottom side temperature of the board that needs to be monitored; the through hole and the actual leads may also be areas to monitor.
To ensure that the temperature reading is correctly monitored it is recommended that high temperature solder be used to attach the leads. A 90% lead/10% tin alloy is normally used as it will not become dislodged during normal soft soldering operations between 179-250°C. It is recommended that, when making up a profiling board, it is worth tack soldering a couple of additional leads with high melting point solder around the device.
If any strain is placed on the component or leads during reflow, the joint can be dislodged if only one joint is soldered with the thermocouple lead. The SMART Group offers interactive CD ROMs and training posters covering these procedures (contact: firstname.lastname@example.org).
First the thermocouple lead is placed on the surface of the board and secured with high temperature 'Kapton' tape making sure that the thermocouple bead will rest on the joint area. The tape assists the mounting operation by holding the lead on the board while the soldering operation is conducted. When the soldering operation is complete additional tape may be used to secure the leads onto the board.
Using a temperature controlled soldering iron set to reflow the high temperature cored wire, solder the bead to the pad surface. Make sure that the bead is held in place with the minimum solder possible. Excessive solder tends to affect the temperature reading as the joint can be larger than the actual area being monitored. Experiment with cored wire from different suppliers as some tend to have less flux or lower activity cores which make the soldering of beads difficult. Make sure the thermocouple leads are not touching!
Mounting additional thermocouple leads continues so that the maximum information may be obtained during profiling the board assembly. A minimum of three monitoring points is necessary to monitor and set-up a reliable process. However, with more complex boards and area array components six should be used.
Alternatives to solder attachment are high temperature metal tape and temporary adhesives. In both cases these are not the preferred technique. Kapton tape tends to lift during profiling and it is very difficult to hold the thermocouple bead on the surface of the pad/lead combination.
Using adhesive will only last a limited number of temperature cycles before separating from the board. Metal tape is also good for holding thermocouple beads on the surface of components like QFP and BGA for profiling rework systems where the temperature on the top and under the part must be checked.
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