The core contradiction exposed in thick copper board welding on actual production lines is thermal response lag. Since the heat capacity of copper substrates above 2.0mm far exceeds that of conventional FR-4 boards, its actual heating rate under the same heating power is only about 60% of that of ordinary PCBs, while the high thermal conductivity of copper continuously dissipates heat deep into the board body and toward the edges. This requires the reflow oven to provide longer energy replenishment during the soaking zone; otherwise, the Solder Paste cannot complete the continuous reaction process of Flux activation, solvent evaporation, and alloy melting.

When we were debugging a power module thick copper board on the JJY SMT line, we found that when the soaking zone was set to 120 seconds and the peak temperature was maintained at 235℃, AOI continuously detected 12% of QFN device pad wetting defects. Microsectioning confirmed that the solder had not fully spread. After further extending the soaking zone to 165 seconds, the IMC layer thickness of solder joints met the standard under the same temperature conditions, and the wetting angle was generally less than 30°, indicating that extending the soaking time is not simply raising the peak temperature, but giving the solder paste sufficient time to complete interfacial oxide film removal and intermetallic diffusion.

It is worth noting that extending the soaking zone must be coordinated with lowering the chain speed and fine-tuning the zone power. If only the time is extended without reducing the conveyor speed, it will lead to insufficient preheating in the front section and increased risk of solder paste collapse. We once ignored chain speed matching, and after extending the soaking zone, the offset rate of 0402 resistors reached 8.3%. Later, we reduced the chain speed from 90cm/min to 75cm/min, and simultaneously lowered the power of the second zone by 5%, which completely eliminated solder beads and tombstoning.

Thick copper board welding is far more sensitive to the temperature profile than conventional boards. On the same board, in areas where the copper foil area difference exceeds 40%, the measured temperature difference can reach over 18℃. Therefore, multi-point thermocouple measurement must be used instead of single-point simulation. JJY uses K-type thermocouples to synchronously sample at the pad center, copper foil edge, and dense via area of a 3.0mm thick copper board, confirming that the temperature fluctuation at each key point within the soaking zone is controlled within ±2.5℃ to be considered qualified. Exceeding this range, even if the average value meets the standard, will cause local cold soldering.

In addition, the selection of solder paste must match the characteristics of thick copper boards. The rosin carrier in no-clean solder paste is prone to carbonization and residue under long soaking zones. We switched to low solid content (<9>