Development of Accelerated Method for Thermal Cycling in Electronic Packaging Application

The method is based on a microheater integrated next to a wire bonding pad (test pad) on a test chip. It is fabricated in CMOS technology without additional micromachining. The microheater consists of two polysilicon resistor elements, placed at opposite sides of the pad, operated in parallel using a constant voltage, each element extending over 30 × 70 μm with a resistance of ≈140 Ω at room temperature, and is operated based on Joule heating. The polysilicon is located at least 20 μm but not more than 50 μm from the pad aluminum. To characterize the microheater, Al serpentine resistors are placed on and between the heaters next to the pad, serving as resistive temperature detectors, having resistances of about 9.4 Ω at room temperature. With a constant operation voltage of 15 V, ≈140 mA of current and ≈2.1 W of heating power are generated, resulting in a heat flux of ≈500 MW/m². The thermal resistance of the heater is 200 K/W (i.e., loss coefficient of 5 mW/K). The maximum temperature measured on one of the microheater resistors was above 396 °C and was reached using 18 V within less than 5 s of voltage application starting at room temperature. When heating from 101 °C to 138 °C, even faster heating is possible, allowing the performance of highly accelerated thermocycles. These cycles are applied to a ball bond on the test pad. Compared to the 20 min cycles used by a standard test, the new microheater device performed cycles lasting 10 ms (5 ms on, 5 ms off) which is 5 orders of magnitude faster. The released energy is typically 10 mJ per cycle. A 50 μm diameter ball was made using 25 μm diameter Au wire and bonded to the test pad. The effect of the microheater-cycling on the contact resistance values of ball bonds is described. Starting with typical contact resistance values around 2.5 mΩ, the increase observed is between 4% and 7% after 5 × 10⁶ 10 ms cycles (≈14 h).