Thick copper board soldering is not simply a matter of Printing Solder Paste and sending it through the reflow oven; the entire process chain must be recalibrated around the high thermal conductivity, low solder wettability, and large heat capacity characteristics of the copper substrate. Otherwise, solder joints are highly prone to interfacial delamination, abnormal IMC thickening, or local burn-through under full-load operating conditions in new energy vehicle OBCs or main drive inverters.

Copper thickness and pad structure must be redesigned synchronously

The heat conduction speed of a 2mm thick copper layer is nearly 7 times that of conventional 1oz copper. If the pad dimensions and solder mask openings of traditional FR4 boards are retained, heat is rapidly drawn away by the copper substrate during reflow, preventing the solder from fully melting and spreading. Our measurements show that for every 0.5mm increase in pad outer diameter relative to copper thickness, the pad must be synchronously expanded by 0.3mm, and the solder mask opening must extend at least 0.15mm beyond the pad on each side. Otherwise, the Solder Paste is drawn sideways during the preheating stage, resulting in insufficient solder joint volume.

Solder paste must match the wetting lag characteristics of thick copper

JiajinYuan has verified three alloy systems. The wetting onset temperature of 63/37 on thick copper is 22℃ higher than on conventional boards, while SAC305, due to its high silver content, exhibits obvious solder climbing delay at 185℃. The final selected formulation is SAC405, which has a wider melting range and a more gradual surface tension gradient, maintaining stable wetting progression within the 217℃～228℃ range. Combined with a 0.035mm stencil thickness and a 90° squeegee angle, it ensures that the fullness of one-time formed solder joints meets the standard.

The reflow profile must counteract the thermal inertia of the copper substrate

A conventional peak temperature of 245℃ is only the starting point for thick copper boards. We forced the soak zone to be extended to 90 seconds and maintained at 205±3℃ to allow Flux residues to fully decompose, while reducing the ramp-up rate from 2.5℃/s to 1.3℃/s to avoid forming an excessive thermal gradient between the copper substrate and the pad solder. Otherwise, microcracks will form at BGA bottom solder joints during the initial cooling stage due to uneven shrinkage, and the void rate detected by X-ray generally exceeds 25%.

Post-soldering inspection cannot rely solely on appearance finish

First article confirmation after thick copper board soldering must include three points: using a thermal imager to capture the surface temperature field distribution of solder joints under a full load of 120A, with a temperature difference exceeding 8℃ judged as an abnormal thermal path; using ultrasonic scanning to check for ring-shaped delamination inside the solder joints, which is a typical feature of poor bonding in the copper substrate-solder IMC transition zone; finally, performing 50 cycles of -40℃～125℃ thermal shock to observe whether there is micron-level lifting at the solder joint edges, a phenomenon with a 73% recurrence rate on boards without copper surface roughening.