The packaging of integrated circuit (IC) chips is one of the most important steps in the manufacturing process, contributing significantly to their overall cost, performance and reliability. Packaging of IC chips account for a considerable portion of the cost of producing the device, and failure of the package can lead to costly yield reduction. One of the approaches taken to solve such packaging problems is the development of “flip-chip” semiconductor packages.
A flip-chip packaged device includes a direct electrical connection of face down (that is, “flipped”) electronic components onto substrates, such as ceramic substrates, circuit boards, or carriers using conductive solder bumps formed in a ball grid array (BGA) on bond pads of the chip. Flip-chip technology is quickly replacing older wire bonding technology that uses face up chips with a wire connected to each pad on the chip. Flip-chip technology fabricates bumps (typically Pb/Sn solders) on aluminum bond pads on the chips, and interconnects the bumps directly to the package media, which are usually ceramic- or plastic-based.
The bumps of the flip-chip assembly serve several functions. The bumps provide an electrical conductive path from the IC chip (or die) to the substrate on which the chip is mounted. A thermally conductive path is also provided by the bumps to carry heat from the chip to the substrate. The bumps also provide part of the mechanical mounting of the chip to the substrate. A spacer is also provided by the bumps, which prevents electrical contact between the chip and the substrate connectors. Furthermore, the bumps also act as a short lead to relieve mechanical strain between the chip and the substrate.
In addition to bumps and spacers, metal heat spreaders or heat sinks can be utilized to dissipate the considerable amount of heat generated during operation of flip-chip packaged devices. The chips are attached to the metal heat spreaders with a thermal interface material (TIM) to decrease the thermal resistance between the chip and the metal heat spreaders.
Despite providing numerous advantages, such flip-chip packaged devices or assemblies are very delicate structures, the design and manufacturing of which creates difficult and unique technical problems. For example, the substrate onto which the flip-chip may be mounted can be a single layer structure, or the substrate may comprise two or many more layers of materials. Often these materials tend to be quite diverse in their composition and structure. The coefficient of thermal expansion (CTE) for these different layers may be considerably different and may result in uncontrolled bending or thermally induced substrate surface distortions.
Furthermore, there may also be potential CTE mismatch between the chip and the substrate resulting in additional warpage or distortion. Such distortions can cause failure of the flip-chip or other components of the substrate. In particular, the TIM can suffer from vertical compression and be pumped out from between the chip and the heat spreader after long power-cycle periods. This is especially true when the TIM is a non-solid material such as a silicon-oil-based AlN (aluminum nitride) filled thermal grease. The thermal grease “pump-out” issue can therefore lead to early failure of the flip-chip packaged devices resulting in poor reliability and thermal performance. Thus there exists a need to minimize the “pump-out” issue by improving the TIM reliability within a flip-chip package.