The combination of large pads and thick copper boards is highly prone to insufficient solder in actual production. The root cause is that the significantly increased heat capacity of the copper layer leads to local heating lag, which prevents the Solder Paste from fully melting and completing intermetallic compound formation during the reflow stage. Meanwhile, as the pad area expands, the relative proportion of solder paste volume decreases. If the stencil aperture is not compensated in a stepped manner according to IPC-7527, the solder volume after printing will already be below the critical value.

Stencil design must match copper thickness and pad dimensions

When a 1.6mm board thickness is paired with 2oz copper, if a conventional 0.12mm thick laser stencil still uses a 1:1 aperture, the measured solder paste volume is 23% lower than the standard value. After we switched to a 0.15mm thick stencil with an elliptical aperture of 1.15 times the pad area, the insufficient solder rate was reduced from 12.7% to 0.9%. The 0.10mm nickel-steel composite stencil provided by JJY performs more stably in the 0.8mm fine-pitch BGA area, but it is prone to collapse at large pads. It is necessary to simultaneously increase the squeegee pressure to 5.8kg and reduce the printing speed to 25mm/s to maintain demolding integrity.

Squeegee parameters are strongly coupled with the rheological properties of solder paste

The viscosity of JJY T6 Solder Paste at 25℃ is 780Pa·s. When the squeegee angle is set to 55° and the pressure is below 4.2kg, its shear thinning effect is insufficient, leading to discontinuous solder paste filling at the edges of large pads. At this time, even extending the squeegee dwell time cannot improve the situation. The angle must be tightened to 48° and paired with 6.3kg pressure to ensure the paste completes effective displacement in the thick copper area.

Reflow oven temperature zone settings must counteract the heat absorption inertia of copper substrates

When thick copper PCB passes through the oven, if the heating rate in the front section is below 1.2℃/s, the pad center temperature remains below 183℃ before the peak zone, causing partial areas of the solder paste to be blown away before melting. We increased the temperature of the second preheating zone from 150℃ to 162℃, extended the holding time from 65 seconds to 92 seconds, reducing the board surface temperature difference from 22℃ to within 8℃, and then stabilized the peak temperature at 238±2℃, after which insufficient solder points basically disappeared. If the nitrogen concentration is below 99.95%, the IMC growth thickness will decrease by 0.3μm, which also induces poor wetting-type insufficient solder.

Placement accuracy deviation will amplify the risk of insufficient solder

When the XY offset of the placement machine for large pad components exceeds 0.08mm or the θ angle deviation is greater than 0.15°, the solder paste undergoes lateral contraction under asymmetric surface tension traction in the early stage of reflow. Under these conditions, the probability of insufficient solder for QFN48 packages increases by 3.7 times. It is necessary to enable the pad coverage algorithm in the JJY AOI system for real-time interception, rather than relying on manual sampling inspection.