Profiling Convective Heat Transfer across Variable Weight Copper Stackups
Profiling variable weight copper stackups requires extended soak dwell and high gas velocity to equalize thermal delta across light pads and heavy ground planes.
Voiding under QFN
Solder Discontinuity
Formation of empty cavities within the solder joint of a quad flat no-lead package occurs during the reflow process. Voiding under QFN components primarily affects the large central thermal pad designed to conduct heat away from the silicon die. These gaps reduce the effective contact area of the joint, which can lead to component overheating.
Outgassing Source
Volatile organic compounds from the solder paste flux are the primary cause of these entrapments when they cannot escape before the solder solidifies. Because the QFN component sits extremely close to the printed circuit board, the escape path for outgassing flux is restricted during reflow. During this phase, voiding under QFN packages is exacerbated if the solder paste coverage is applied as a single large square rather than a series of smaller printed deposits.
Mitigation Method
Optimizing the solder stencil design and reflow profile can control this defect to keep it within the limits allowed by IPC standards. Segmenting the stencil aperture into a grid of smaller square patterns allows the outgassing volatiles to escape through the channels between the paste deposits. Extending the soak zone of the reflow profile also helps by allowing the flux to fully activate and release its volatile components before the solder reaches its liquidus temperature.
This process control is necessary to achieve high thermal performance and reliable joints.