Flip chip die attach is a method for making electrical connections to a chip, where the chip is inverted and its bond pads are connected to a corresponding pattern of bond pads on a substrate - see illustration.
Flip chip assembly is an alternative to the chip and wire assembly technique and is most commonly used where space is an issue, where there are a high number of chip connections, where good high frequency performance is required, or a combination of these factors.
Types of chips that have been flip chip bonded include integrated circuits (ICs), infrared sensors, large area pixelated detector arrays, optical devices, micro electrical mechanical systems (MEMS) and surface acoustic wave (SAW) devices. Types of substrate on to which flip chips have been bonded include laminate printed circuit board (PCB), polyimide, glass, ceramic, silicon and plastic-package lead-frames.
The electrical/mechanical connection between the bond pads on the chip and those on the substrate are typically made by bumps that have been deposited onto the bond pads of the chips while in their wafer form. These bumps can be made of many different materials and connected using a number of different processes. The following is a list of common process/material combinations:
- temperature reflowed solder bumps
- thermosonic or thermocompression bonded gold bumps
- adhesively bonded gold bumps
For most applications, the use of underfill (a material that fills the gap between the chip and substrate, forming a strong bond to each) is recommended for reliability, especially where the chip and substrate have a large thermal expansion mismatch - such as flip chip on board (FCOB), where a chip is flip chip bonded to a laminate PCB. The exception is where anisotropic conductive adhesive (ACA) is employed, where the anisotropic conductive paste (ACP) or film (ACF) forms the electrical connection and underfill.
The decision to use flip chip assembly, and the method, depend heavily upon the application.