Fig. 1. Structure of a Flip Chip BGA

The flip chip is structurally different from traditional semiconductor packages, and therefore requires an assembly process that also differs from conventional semiconductor assembly. Flip chip assembly consists of three major steps: 1) bumping of the chips; 2) ‘face-down’ attachment of the bumped chips to the substrate or board; and 3) under-filling, which is the process of filling the open spaces between the chip and the substrate or board with a non-conductive but mechanically protective material. Given the many different materials and technologies used in the bumping, attachment, and underfilling steps, the flip chip now comes in a vast array of variants.

Flip-chip Bumping

Physically, the bump on a flip-chip is exactly just that – a bump formed on a bond pad of the die.  Bumps serve various functions: 1) to provide an electrical connection between the die and the board or substrate; 2) to provide thermal conduction from the chip to the board or substrate, thereby helping dissipate heat from the flip chip; 3) to act as spacer for preventing electrical shorts between the die or chip circuit and the board or substrate circuit; and 4) to provide mechanical support to the flip-chip.

There are many known processes for flip-chip bumping. Solder bumping consists of placing underbump metallization (UBM) over the bond pad by sputtering, plating, or a similar means. This process of putting UBM removes the passivating oxide layer on the bond pad and defines the solder-wetted area.  Solder may then be deposited over the UBM by a suitable method, e.g., evaporation, electroplating, screen-printing, needle-depositing, etc.

This entire process of solder bumping is done at wafer level. Solder-bumped wafers are sawn into individual flip-chips that get mounted on a board or substrate by subjecting the assembly to a temperature that’s high enough to melt the solder, forming the interconnection.