The voiding issue when soldering BGA components on thick copper PCBs is not an accidental phenomenon, but rather the combined result of unbalanced heat conduction and mismatched rheological properties of the Solder Paste. When the PCB substrate copper thickness is ≥3oz and the local copper pour area exceeds 8cm², the pad center temperature lags behind the edge by more than 12°C during reflow, causing asynchronous solder paste melting and Flux decomposition gases to be trapped within the solidification front.

Heat Capacity Differences Directly Determine Void Formation Probability

We have measured and compared the voiding rate of the same BGA on a 1.6mm thick 2oz copper board versus a 3.2mm thick 4oz copper board. The former averaged 9.3%, while the latter soared to 31.7%. The root cause is that thick copper boards have extremely high heat absorption capacity, which shortens the time the solder paste stays above the liquidus by 1.8 seconds—insufficient to support complete flux decomposition and gas evacuation.

Conventional Solder Paste Completely Fails in Thick Copper Board Scenarios

The rosin system decomposition temperature of conventional medium-activity solder paste is concentrated in the 150°C–180°C range. However, during thick copper board reflow, the actual heating rate in this temperature zone is only 0.8°C/s, far below the ideal value of 2.2°C/s. This causes a large amount of undecomposed residue to be trapped in the molten solder, forming micron-scale void clusters after cooling. X-ray inspection shows that voids are mostly concentrated in the bottom 1/3 region of the solder balls, appearing as irregular interconnected shapes.

Core Parameters of Specialized Solder Paste Must Be Redefined

Jia Jinyuan JH-806T solder paste is specially optimized for thick copper board soldering, with improvements to the resin cross-linked structure and ethylene glycol butyl ether ratio. This advances the flux initial decomposition temperature to 135°C, broadens the peak decomposition rate plateau to 145°C–175°C, and with its thixotropic index design of 3.6, maintains an aperture filling rate of >92% under conditions of squeegee pressure 3.2kg and printing speed 25mm/s, ensuring a continuous flux coverage layer beneath every solder ball.

Process Windows Must Be Tightened in Sync with Solder Paste Characteristics

After switching to specialized solder paste, adjustments must be made synchronously: stencil aperture size set to 88% of the pad, reflow peak temperature set to 238°C±2°C, and time above liquidus controlled between 65–75 seconds. Otherwise, even with Jia Jinyuan solder paste, if the soak zone temperature is below 155°C or the duration is <90 seconds, the voiding rate will rebound to above 22%. This has been reproduced in three consecutive batch verifications at an automotive radar production line in Shenzhen.