3. Under Bump Metallurgy (UBM)

3.1 Nature of layers

The main role of the UBM is to ensure compatibility between the chip's metallization and that of the card. Its nature varies according to the eutectic alloy providing the electrical connection; the UBM determines the region of the finish metallurgy wetted by the solder on the chip surface. Very often, the finishing layer of chip connection pads is made of aluminum, but gold (mainly for AsGa chips) and copper (as it improves electrical performance) are also used.

The UBM also helps protect the chip's metallization from corrosion caused by the diffusion of ionic contaminants from the environment (or even from the encapsulant).

It includes :

• an adhesion layer covering the...