Determination of measurement limit for open solder bumps on a flip-chip package using a laser ultrasonic inspection system

The adoption of surface-mount technology has spurred a rapid decrease in the size of electronic packages. Advanced active component packages, such as flip-chips, chip scale packages (CSPs), and ball grid arrays (BGAs), are attached to a printed wiring board (PWB) with small solder bumps between the bottom of the device and the board. The attachment method reduces package size, but the solder connections are no longer visible for inspection. Although lead frame packages can be inspected using automated camera equipment, other techniques, such as scanning acoustic microscopy (SAM) and automated X-ray inspection (AXI), have been used for screening these advanced packages for missing and shorted solder joints online in a manufacturing facility. However, they have trouble identifying open solder joints, especially when applied on-line. An inspection system based on laser-generated ultrasound and interferometric techniques has been developed. The system uses short pulses of high-intensity infrared energy from an Nd:YAG laser to excite the bulk of a chip into vibration through the principle of laser-generated ultrasound. The vibration response of a reference good chip attached to the substrate is measured and compared with vibration responses from other chips. Changes in the vibration response indicate a change in the quality of the chip or its attachment to the PWB. Previously, this system has been used to identify misalignment in CSPs and flip-chips, to identify missing solder bumps, and for detecting structural cracks in chips. In this paper, the ability of the current inspection system to identify open solder joints is investigated. Peripheral array, 48-bump daisy-chain flip-chips (PB18) without underfill were used as a test vehicle. This flip-chip has a higher I/O count and smaller, more closely spaced bumps than previous flip-chip specimens. Individual solder pads were removed from the PWB design, and test specimens with zero to seven adjacent open solder bumps were created. After exciting the chips with laser pulses, the impulse responses at 48 points on the surface of each chip were recorded. Data analysis is performed to compare the responses from reference chips and chips with open solder joints. Results show that the system has the ability to detect open joints for the flip-chip package, and statistical analysis of the results from repeated comparisons between the good and defective chips is employed to establish a detection threshold.