How Solder Voiding Causes Overheating in BGA and QFN Pads
Excessive outgassing voiding inside a Ball Grid Array (BGA) or Quad Flat No-lead (QFN) thermal pad causes localized die overheating because the trapped gas pockets act as severe thermal insulators, blocking the primary heat dissipation path from the semiconductor die to the printed circuit board (PCB). This overview explains the mechanics of solder outgassing, why these voids drastically increase thermal resistance, and how the resulting trapped heat creates destructive hot spots that degrade and destroy power electronics.
What is Outgassing Voiding?
During the surface mount technology (SMT) reflow soldering process, solder paste is heated to melt and bond the component to the PCB. Solder paste contains flux, which chemically cleans the metal surfaces to ensure a strong metallurgical bond. As the paste heats up, volatile compounds within the flux vaporize.
Outgassing occurs as this gas attempts to escape the molten solder. In components with large surface area connections, such as QFN thermal pads or large BGA ground balls, the gas often gets trapped before the solder solidifies. These trapped gas bubbles become permanent empty pockets within the hardened solder joint, known as voids.
The Physics of Thermal Resistance
Power electronics generate significant amounts of heat during operation. BGA and QFN packages are designed with exposed metal thermal pads on their undersides specifically to pull this heat away from the internal silicon die and transfer it into the PCB's copper planes, which act as a heatsink.
This process relies on high thermal conductivity. Standard solder alloys have a thermal conductivity of roughly 50 to 60 W/m·K (watts per meter-kelvin), allowing heat to flow through the joint easily. In contrast, the trapped air and flux vapors inside a void have a thermal conductivity of approximately 0.026 W/m·K. Because the gas inside the void is nearly 2,000 times less conductive than the surrounding metal, the void acts as an insulator rather than a conductor.
How Voids Cause Localized Die Overheating
When a large void or a cluster of smaller voids forms directly beneath the silicon die, it interrupts the direct path of heat transfer. This creates a phenomenon known as thermal crowding.
Because heat cannot pass through the void, it must detour laterally through the thin silicon die and the component's package base to find a solid section of solder. This longer, bottlenecked path increases the overall junction-to-board thermal resistance. Consequently, the area of the silicon die situated directly above the void cannot shed its thermal energy fast enough. Heat accumulates rapidly in that specific microscopic region, resulting in a localized hot spot on the die, even if the overall average temperature of the component appears normal.
Consequences for Power Electronics
Power electronics—such as MOSFETs, voltage regulators, and high-power amplifiers—push massive currents through tiny areas of silicon. Localized die overheating in these components leads to immediate performance and reliability failures:
- Thermal Runaway: In certain semiconductors, as a localized area gets hotter, its electrical resistance drops, causing it to draw even more current than the cooler areas of the die. This generates more heat in the exact same spot, creating a fatal loop that melts the silicon.
- Parameter Drift: Excessive heat alters the electrical characteristics of the die, causing shifts in switching speeds, voltage thresholds, and efficiency.
- Mechanical Stress: The severe temperature gradient between the hot spot above the void and the cooler areas of the die causes uneven thermal expansion. Over time, this mechanical stress cracks the die, delaminates internal bond wires, and fractures the solder joint itself.
To prevent localized overheating, industry standards such as IPC-A-610 generally mandate that thermal pad voiding must be kept below 25% to 50% of the total joint area, though high-reliability power electronics often require voiding to be under 10% to ensure uninterrupted thermal transfer.