Why Preheat a PCB Before Soldering or Component Rework?
Preheating is useful when it reduces the temperature difference between a solder joint and the rest of the assembly without pushing another part beyond its limit. It is not necessary for every PCB.
Is this board a preheat candidate?
Answer these questions before choosing a heater:
- Does a copper plane, heat sink or heavy package pull heat away from the joint?
- Does the top-side tool need excessive time or temperature to complete the joint?
- Are nearby connectors, batteries, plastics or laminate regions temperature-sensitive?
- Can the expected hot, cold and vulnerable locations be instrumented? A “yes” to either of the first two questions supports running a preheat trial. A “no” to the final question is a warning: a heating process that cannot be measured cannot be qualified.
What the trial must prove
Start with limits from the solder alloy, flux or paste, component documentation and board material. Attach sensors at the joint, a high-mass location and the most vulnerable nearby part. NXP’s LGA guidance notes that temperatures can vary substantially across a PCB and recommends checking expected hot and cold locations. Apply bottom heat with the board held in a fixed position, then introduce the local rework source. Record the complete temperature history, not just the preheater setting. Across repeated runs, compare the joint response, temperature gradient across the board, local-heater time and finished solder joint. If IR is selected, a Henruite quartz or halogen element is only one candidate component. Its voltage, power, wavelength, active length, reflector and mounting must be matched to the measured field through Henruite. The acceptance criterion remains the board result. Next: place thermocouples at risk locations or compare IR with a hot plate.