When thick copper large boards pass through the reflow oven after SMT placement, the incidence of cold solder joints is far higher than that of conventional board types. This is because a copper thickness above 2oz significantly increases the heat capacity of the PCB, causing the actual peak temperature in local areas to be more than 25°C lower than the value monitored by thermocouples, and the Solder Paste cannot achieve sufficient melting and wetting. The active metal particles in JiajinYuan cold solder repair paste can participate in interfacial reactions starting at 195°C, compensating for the energy gap caused by heat conduction delay.

Thermal Distribution Imbalance Is the Physical Precondition for Cold Solder Joints

When the PCB size exceeds 400mm×300mm and the copper thickness reaches 2oz, with the in-line speed of the reflow oven set to 1.2m/min, the temperature difference between the board edge and the board center can reach 32°C. The heating rate of the thick copper area near the gripper is only 0.8°C/s, which is far below the minimum threshold of 1.5°C/s required for the solder paste alloy system. This thermal response lag directly results in solder joints that appear bright on the surface but have no internal metallurgical bonding. X-ray inspection shows that although the void rate is below 15%, the average attenuation in shear force tests reaches 43%.

Stencil Aperture and Squeegee Parameters Exacerbate Cold Solder Joint Risk

The enlarged aperture design commonly adopted to cope with the fast heat conduction of thick copper (such as expanding the 0603 pad aperture from 0.25mm to 0.32mm) instead leads to redundant solder accumulation at the pad edge. During the Flux volatilization stage in the early reflow period, the flux is blown away from the central area by airflow, causing the main solder layer to lose effective flux coverage. Meanwhile, if the squeegee angle is greater than 60°, it will further reduce the solder paste transfer rate, resulting in a solder paste thickness of less than 0.08mm in the middle of the pad. This area cannot achieve complete liquid phase transformation of the Sn63Pb37 alloy during the peak temperature window.

Production Line Adaptation Logic of JiajinYuan Cold Solder Repair Paste

This material does not simply increase activity, but precisely anchors the initial reaction temperature at 195°C±2°C by regulating the Zn-Al-Cu ternary eutectic ratio, which exactly falls into the transition interval from the end of the soaking zone to the start of the reflow zone in the reflow profile. At this time, the copper surface of the pad has oxidized but has not yet formed a dense CuO layer. The nano-sized aluminum particles in the repair paste can preferentially displace cuprous oxide and generate low-melting-point Al-Cu intermetallic compounds, providing a continuous wetting channel for the subsequent main solder. Actual tests at a power supply factory in Shenzhen on FR4+2oz double-sided thick copper large boards show that after use, cold solder joints decreased from an average of 11.6 per panel to 0.4.

Tightened Process Window Forces Collaborative Equipment Upgrades

Relying solely on material improvement cannot eliminate cold solder joints. It is necessary to simultaneously adjust the in-line speed of the reflow oven to 0.95m/min to extend the effective heating time, and reduce the nitrogen oxygen content from 300ppm to below 80ppm. Otherwise, the active components in the repair paste will be prematurely exhausted in the high-temperature zone. We once found at a customer site in Dongguan that when the in-line speed was not reduced, even with the addition of repair paste, cold solder joints at the bottom of BGA remained stably at around 0.7%, indicating that thermal process control is still the decisive precondition.