The essential contradiction in SMT Soldering of thick copper PCBs lies in the fact that the heat capacity of the copper substrate is far higher than that of conventional FR-4 boards. The heat capacity of 2oz copper thickness (70μm) is approximately 1.9 times that of 1oz copper thickness (35μm), resulting in a board surface temperature response lag of 8 to 12 seconds under the same heating power. If the standard FR-4 reflow temperature profile is applied, it is highly likely that the Solder Paste Flux enters the reflow zone without sufficient activation, or excessive local temperature difference during solder melting causes the solder joint void rate to exceed the standard. Our on-site measurement on a power module production line found that when the actual pad temperature of the thick copper board was 14°C lower than the oven cavity temperature measured by the thermocouple, the void area ratio of the bottom solder joints of QFN devices reached as high as 32%, far exceeding the 25% upper limit allowed by IPC-A-610H Class 2.

IPC-J-STD-020D explicitly requires strict peak temperature control for devices of MSL3 grade and above, where the peak temperature upper limit for MSL3 devices is 260°C, and the duration must not exceed 30 seconds. However, due to the large heat absorption inertia of thick copper boards, the phenomenon of "overheating on the surface and underheating on the bottom layer" often occurs in traditional reflow ovens. We once used an infrared thermal imager to continuously monitor the temperature gradient of a 2mm thick double-sided thick copper board, and found that at the end of the peak zone, the top layer pad temperature reached 263°C, while the corresponding bottom layer pad was only 241°C. This temperature difference directly leads to poor wetting of BGA bottom solder joints, and this deviation cannot be compensated by adjusting the oven speed. It must be improved by extending the soaking zone time and increasing the preheat ramp rate to enhance thermal uniformity.

The selection of solder paste must be co-designed with the thermal response characteristics of thick copper boards. Although no-clean solder pastes such as the NC-550 series have low residue, their rosin system has a relatively high activation temperature (initial activation point around 150°C), which is prone to premature carbonization during the slow heating process of thick copper boards, and instead hinders solder spreading. We compared and tested the wetting angle changes of three solder pastes on 2oz copper thickness OSP boards, and found that Kester NXG-45 began to significantly reduce the wetting angle at 145°C, while NC-550 did not show a significant decrease until 158°C. Therefore, for thick copper boards, it is more recommended to use solder pastes with low activation temperature, high metal content (90.5%), and thixotropic index >1.8. At the same time, printing parameters need to be adjusted synchronously: the stencil aperture area ratio should be no less than 0.66, and the squeegee pressure should be maintained at 4.2kgf±0.3kgf. Otherwise, the Solder Paste transfer rate will decrease by more than 12% due to the low-temperature adsorption effect of the copper board.

The optimization of the reflow temperature profile cannot be carried out in isolation, and must be comprehensively judged in combination with the furnace zone capacity and hot air circulation uniformity. Although a domestic eight-zone reflow oven has a nominal temperature control accuracy of ±1°C, the measured temperature fluctuation in zones 3 to 5 reaches ±3.7°C when fully loaded with thick copper boards, resulting in an actual soaking zone time deviation of more than 23 seconds. We finally adopted a segmented ramp setting method: set the preheat zones 1 to 3 to 0.8°C/s, and reduce zones 4 to 5 to 0.4°C/s to suppress solder paste splashing. Zone 6 (soaking zone) is strictly maintained at 150°C±2°C for 95 seconds to ensure solvent evaporation. The reflow section adopts a dual-peak strategy: the first peak is maintained at 217°C for 12 seconds to activate the solder, and the second peak impacts 260°C from 238°C with a liquid phase time of ≥4 seconds. This setting increased the yield of a certain automotive MCU thick copper board from 89.7% to 99.2%, and the solder joint void rate in X-ray sampling inspection was stably controlled at ≤18.3%.

On-site verification must use real PCB-mounted thermocouples, and it is strictly prohibited to rely on the readings of the oven's built-in sensors, because the latter reflect the air duct temperature rather than the board surface temperature. We once found that after the same equipment was replaced with copper boards of different thicknesses, the maximum deviation between the oven display temperature and the measured pad temperature reached 22°C, and the deviation direction was unpredictable. It is recommended to arrange 5 K-type thermocouples at the four corners and the center of each batch of first-article boards, and fix them with high-temperature tape + heat-resistant silicone grease. The sampling frequency should be no less than 2Hz. After importing the data into CurveTracer software, focus on checking whether the liquidus time above 183°C falls within the 45 to 90 second range. Less than 45 seconds is prone to cold soldering, while more than 90 seconds will exacerbate the risk of copper migration. This range requirement is derived from the accelerated aging constraint on the growth rate of solder joint intermetallic compounds in JEDEC JESD22-A108F.