Flip chip is a die attach method where the electrical connections between the chip and package/substrate are made directly by inverting the die face-down onto the substrate/package. The bond pads of the die are physically, mechanically and electrically connected to the bond pads of the package/substrates by conductive bumps.
Advantages of Flip Chip:
The first step is to deposit electrically conductive bumps on the surface of the bond pads of a die. Electrically conductive bumps can be deposited on the die using many different process and material combinations. These processes include, solder bumping, stud bumping and adhesive bumping.
A metal layer is deposited over the bond pad using processes such as plating and sputtering. This under bump metallization acts like an interface between the bond pads and the solder bumps. This process helps passivate the oxide layer on the bond pads surface and acts like an adhesion layer for the solder which wets it during the bumping process. The under bump layer is often deposited at wafer level.
Stud bumping is a very similar process to wire bonding where a wire is melted into a sphere and attached to a bond pad. After the attachment, the wire is removed at the end to leave a stud on the bond pad. Stud bumped flip chips are bonded to a substrate using either conductive adhesives or thermosonic process. Stud bumping can be carried out using standard wire bonding equipment.
Adhesive bumping consists of forming conductive bumps on the bonds pad of the die using conductive adhesives. The adhesive bumps are normally formed over an under bump metal layer. Adhesive bumped filp chips are mounted on the substrate using conductive adhesives. Adhesive flip chip technologies can make use of conductive adhesive polymers, anisotropic conductive adhesives and in some cases non-conductive adhesives. Adhesive technologies do not require any cleaning however the disadvantage is that they are more difficult to rework. They also often have higher thermal and electrical resistance which may not be suitable for some applications.
Flip chip process is used for packaging a broad range of electronics which include Integrated circuits, infrared sensors, large area pixelated detector arrays, optical devices, MEMS and surface acoustic wave (SAW) devices. It is compatible with a wide range of substrates such as laminate PCB, polyimide, glass, ceramic, silicon and plastic package lead-frames.
Flip chip process can be used for directly attaching dies to the PCB board which may or may not have surrounding components packaged using flip chip technology. It is often referred to as flip chip on board. It can also be used for packaging multiple dies in a single package known as flip chip in packages. The flip chip technology can be used for producing 3-D packages where multiple dies can be packaged together in a cascaded arrangement.
The Advanced Packaging Facility has a range of equipment required to make use of the flip chip technology. It contains a high precision Finetech FINEPLACER® lambda flip-chip bonding machine with placement accuracy to 0.5 µm. It has manual, semi-automatic and automatic wirebonders for the bumping process. The facility contains robotic dispensing and jetting system for underfilling the flip chip assemblies. It also has a range of curing equipment which can be used for thermal and UV curing encapsulants.