Solder Bump Flip Chip

This is the second in a series of flip chip tutorials intended for flip chip users and potential users. Tutorial #2 presents an overview of solder bump flip chip bumping and assembly processes. Concurrently, FlipChips Dot Com’s Technology News Updates present industry experts describing the newest developments in their fields; our Literature and Photo pages give supplemental material.

GENERAL
Flip chip assembly by means of solder connection to the bond pads was the first commercial use of flip chip, dating to IBM's introduction of flip chip in the 1960's. Solder bump has the longest production history, the highest current and cumulative production volumes, and the most extensive reliability data of any flip chip technology. Delco developed their solder bump processes in the 1970's; Delco Delphi now assembles over 300,000 solder bumped die per day for automotive electronics.
More recent solder bump flip chip process variations have lowered the manufacturing cost, widened flip chip applicability, and made solder bumped die and wafers available from several suppliers to the commercial market. This introductory survey discusses the operations performed in solder bumping and assembly, and describes several of the solder bump processes now commercially available. The references listed at the end of the tutorial provide details.

PROCESS OVERVIEW
The solder bump flip chip process may be considered as four sequential steps: preparing the wafer for solder bumping, forming or placing the solder bumps, attaching the bumped die to the board, substrate, or carrier, and completing the assembly with an adhesive underfill.

Under-Bump Metallization
The first step in solder bumping is to prepare the semiconductor wafer bumping sites on the bond pads of the IC's. This preparation may include cleaning, removing insulating oxides, and providing a pad metallurgy that will protect the IC while making a good mechanical and electrical connection to the solder bump and the board.
This under-bump metallization (UBM) generally consists of successive layers of metal with functions described by their names. The "adhesion layer" must adhere well to both the bond pad metal and the surrounding passivation, providing a strong, low-stress mechanical and electrical connection. The "diffusion barrier" layer limits the diffusion of solder into the underlying material