Flip Chip Bonding

Advantages of Flip Chip:

Larger number of electrical connections can be made in a specific area on the die and the substrate which increases the I/O and packaging layout flexibility.
Flip chip assemblies generally lead to smaller package sizes as wire bonds are not needed.
Flip chip produces connections with lower parasitic inductance and capacitance which leads to superior electrical performance.
More suitable for high frequency components.
Lower electromagnetic emissions as unlike wire bonds there are no loops in the connections.

Flip Chip Bonding Work Flow

Conductive Bumps

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.

Alignment

The die with conductive bumps is flipped and aligned so the conductive bumps on the die align with the contact pads on the package/ substrate. High accuracy placement is one of the most important requirement of the flip chip process. The accuracy is expected to be of the order of a few microns for reliable function. The smaller the die and the die bond pads the higher the placement accuracy requirement.

Reflow

Once a die is aligned and flipped on the substrate, the conductive bumps are reflowed to spread the conductive material evenly across the bond pads. This improves the wettability of solder and reduces the gap or standoff between the die and substrate.

Encapsulation

A flip chip assembly is encapsulated after reflow by filling the gaps between the die and the substrate. The encapsulant is deposited on the edges of the die, it flows across the gap between the die and substrate by capillary action filling the space between the bumps. An additional deposition is then carried out on the edges of the die to complete the encapsulation. The underfill in addition to mechanical strength and reliability helps with the with the thermal expansion mismatch between the die and substrate. It reduces the thermal fatigue on the conductive bumps prolonging their life.

Curing

The final step in the flip chip process is to cure the underfill.

Types of Conductive Bumps for Flip Chip Bonding

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.