flip chip bonding

Flip-chip bonding is the type of mounting technique that does not require
any wire bonds. It is the direct electrical connection between the
face-down die and the substrate or circuit board, through the conductive
bumps on the chip’s bond pads.

The space between the die and the substrate is filled with a nonconductive underfill
adhesive. The underfill protects the bumps from moisture or other
environmental hazards and provides additional mechanical strength to the
assembly. However, its most important purpose is to compensate for any
thermal expansion difference between the chip and the substrate. The underfill
mechanically locks together the chip and the substrate so that differences
in thermal expansion do not break or damage the electrical connection
of the bumps.
Re: flip chip bonding

The advantages of the flip-chip technique include small size, high performance,
great flexibility, and high reliability. Flip-chip packages are smaller
because the package and bond wires are eliminated, thus reducing the required
board area and resulting in far less height. Weight is reduced as well.
By eliminating the wire bonds, the signal paths are significantly shortened
and, thus, greatly trim the delaying inductance and capacitance of the interconnection.
This results in high-speed off-chip interconnections and, thus,
high performance. In the case of wire bonds, the I/O connections are limited
to the perimeter of the die, driving the die sizes up as the number of connections
increases. Flip-chip connections can use the whole area of the die, accommodating
many more connections on a smaller die. Mechanically, the
flip-chip connection is the most rugged interconnection method.