PCB Thermal Mass: Predicting and Measuring IR Preheat Response
Board weight may indicate that one assembly requires more energy than another, but it cannot predict the IR preheat time on its own. Use prediction to rank the risks; use measurements to establish the actual recipe.
What can be predicted before testing?
| Available information | Reasonable prediction | What it cannot prove |
|---|---|---|
| Board thickness and area | Relative bulk heat capacity | Local hot and cold points |
| Copper distribution | Likely heat-spreading paths | Exact joint temperature |
| Packages and heat sinks | High-mass regions | Radiant absorption |
| Fixture and airflow | Additional loss paths | Complete ramp time |
| Surface finish and shields | Possible absorption differences | Safe peak temperature |
| Use this information to select sensor locations and a conservative starting condition. Do not convert it into a precise heating-time formula unless the material properties, boundary conditions and heat-transfer model are known. |
Measure the missing physics
Profile a representative board from a defined starting state. Record the lamp command and electrical power together with temperatures at high-mass, low-mass and vulnerable locations. Compare the ramp rate, time within the permitted band, peak temperature, center-to-edge spread and repeatability during both cold starts and consecutive cycles. If a copper-heavy area remains cold while a small package approaches its limit, increasing the total wattage is not the appropriate first response. Test the field overlap, board position, reflector direction or separate control zones. Only after this map exists should Henruite lamp voltage, power, active length and reflector be screened through Henruite. Related calculations: installed watt density and edge/center zoning.