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Flip-Chip Assembly (Page 2 of 2)
Another type of
flip-chip bumping is what’s known as
plated bumping. Plated bumping removes
the oxide layer on the Al bond pad through wet chemical cleaning
processes. Electroless nickel plating
is then employed to cover the Al bond pad with a nickel layer to the
desired plating thickness, forming the foundation of the bump. An
immersion gold
layer is then added over the nickel bump for protection.
Stud bumping is
another flip-chip bumping process. This technique is very similar to
gold ball bonding in the sense that it starts by
melting the end of
the wire to form a free-air ball or sphere, which is then attached to the
bond pad. Unlike wirebonding though, the wire is broken off the ball
bond after the latter has been attached to the bond pad. Gold
stud-bumped flip chips may be mounted on a board or substrate using
conductive adhesives or by thermosonic gold-to-gold interconnection.
Adhesive bumping
is a flip-chip bumping process that stencils electrically conductive
adhesive over an underbump metallization placed over the bond pad. The
stenciled adhesive serves as the bump after it has been cured. Mounting
of adhesive-bumped flip-chips also uses conductive adhesives.
Fig.
2. Example of a Solder Bump Structure
Flip-Chip Underfilling
The open spaces
between the flip chip surface and the board or substrate is filled with
a non-conductive adhesive ‘underfill’
material to protect the bumps and the flip chip surface from moisture,
contaminants, and other environmental hazards. More importantly, this
underfill material mechanically locks
the flip chip surface to the board or substrate, thereby reducing the
differences between the expansion of the flip chip and the substrate.
This prevents the bumps from being damaged by shear stresses caused by
differences between the thermal expansions
of the chip and the substrate.
Flip-chip
underfilling is achieved by needle dispensation (Fig. 5) along the edges of the
flip-chip.
Capillary action
then draws the dispensed underfill inwards, until the open spaces are
filled. Thermal
curing
is then performed to form the permanent bond.
Fig.
3. Photos of an underfill dispensing machine (left)
and an
underfill dispensing tool (right)
Front-End
Assembly Links:
Wafer Backgrind;
Die Preparation;
Die Attach;
Wirebonding;
Die Overcoat
See Also:
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