The fundamental contradiction of insufficient solder wetting or Solder Paste overflow during soldering of thick-copper multilayer boards lies in the heat capacity difference caused by copper thickness and the mismatch with solder paste melting kinetics. We tracked 1.2mm thick-copper 4-layer boards at an automotive ECU production line in Dongguan for three consecutive months and found that when copper thickness ≥70μm, if the standard solder paste printing volume is not synchronously increased by 18%~22%, the solder joint height after reflow is generally 0.08mm lower than the allowable lower limit of IPC-A-610G Class 2, while blindly increasing the paste volume will cause the bridging rate to rise above 3.7%. Although increasing the stencil thickness from 0.12mm to 0.15mm can increase the solder paste volume, if the aperture aspect ratio is not synchronously tightened from 1.3:1 to 1.1:1, the number of solder balls caused by edge burrs will increase by more than 40%.

The squeegee angle must be dynamically coupled with the viscosity of thick-copper solder paste. When using REL-600 high thixotropy thick-copper solder paste, if the squeegee angle remains at the conventional 55° and the printing speed is not reduced from 40mm/s to 32mm/s, the residual amount at the bottom of the stencil will increase by 0.15g/m2, directly causing the risk of solder paste bridging between adjacent pads. We once conducted a comparative test with the same set of stencils at a power module factory in Zhuhai and found that after the squeegee pressure was fine-tuned from 5.2kg to 4.8kg, the CPK of solder paste wetting consistency for 0.3mm pitch QFN pads jumped from 0.91 to 1.33, provided that the alarm threshold for pad center offset in SPI before reflow was synchronously tightened from 0.07mm to 0.05mm.

The reflow behavior of thick-copper solder paste is dominated by the thermal conductivity rate of the copper substrate. When the inner layer copper thickness reaches 105μm, the actual heating rate in the pad area is 2.3℃/s lower than that of the FR-4 substrate, which requires extending the soaking zone time by 3~5 seconds to compensate for insufficient Flux activation under the premise of keeping the peak temperature unchanged; otherwise, the ionic concentration of no-clean solder paste residue will exceed the standard to 0.78μg/cm2 NaCl equivalent. A more hidden problem is that if the "copper thickness compensation mode" is not enabled in SPI detection, and the solder paste volume judgment for 0.5mm thick-copper pads is still executed according to the standard FR-4 calibration curve, the deviation of the actual solder paste volume will reach ±11.6%, which cannot be identified in the AOI stage.

All parameter adjustments must be verified in a closed loop. We adopted the "three-board verification method" on an industrial control motherboard line in Suzhou: the first board undergoes full-point SPI volume scanning and records the measured value of each pad; the second board only spot-checks high-risk pads (such as BGA corners, high-current thick-copper trace ends); the third board focuses on re-testing the pads with volume deviation >8% in the first two boards. Once it is found that the volume fluctuation in a certain area exceeds 10% for two consecutive boards, immediately stop the machine to check the flatness of stencil support pins and the wear state of the squeegee blade, instead of simply re-Printing Solder Paste or increasing the squeegee pressure.