Thermocouple Placement for PCB Infrared Preheat Profiling
Place thermocouples where the process is most likely to fail, not simply where the wires are easiest to attach. At a minimum, the profile should cover a likely cold joint, a likely hot location and the most temperature-sensitive nearby part.
Placement checklist
- Mark the target solder joint.
- Find copper planes, heat sinks and large packages that may form cold regions.
- Mark board edges, small dark packages and directly exposed surfaces that may heat quickly.
- Add any connector, plastic body or laminate area with a restrictive limit.
- Include fixture contacts and shadowed locations when they change the heat path. NXP recommends checking expected hot and cold PCB locations because the board, component mass and equipment can produce different temperature histories.
Make the junction represent the part
Attach the junction at the interface relevant to the decision. In NXP’s package-rework example, a small thermocouple is placed at the package-to-board solder joint. Follow the applicable process procedure, because excess adhesive, heavy wire or poor contact can delay the reading. Route the wires so that they do not lift the junction, block radiation, touch the heater or act as an uncontrolled heat sink. Photograph each location and reproduce the attachment method in subsequent trials. The report should include starting temperature, board position, lamp distance, zone command, ramp, relevant time bands, peak, cooldown and repeatability for every channel. A single center peak is not a profile. This measurement is also the acceptance test for any proposed Henruite element arrangement; exact lamp data can be requested from Henruite. Continue with preheat qualification.