TWO WAYS IN

How a die's pads reach the outside

wires, or bumps

A die's pads are tiny metal squares on its surface, and something has to connect each one to the package around it. There are two dominant approaches: wire bonding, which loops a fine wire from each pad outward, and flip-chip, which flips the die over onto a grid of solder bumps.

Every packaged chip uses one of these two connection methods, or occasionally both.

TWO GEOMETRIES

Face up with wires, or face down on bumps

Wire bonding keeps the die face up and loops a fine gold or copper wire from each pad to the substrate below. Flip-chip flips the die face down, so solder bumps on its pads land directly on matching pads on the substrate, with no wires at all.

Flip-chip trades the wire loop for a direct, much shorter connection.

THE TRADE-OFFS

Cheap and proven, or dense and fast

Wire bonding Mature and inexpensive, but pads must sit around the die's edge

Flip-chip Pads can cover the whole die face, for far higher connection counts

Electrical performance Flip-chip's short bumps beat a wire loop's added length and inductance

Cost Flip-chip generally costs more per die to set up

Neither method is strictly better: the choice depends on I/O count and budget.

WHY IT MATTERS

I/O count outgrew the die's edge

perimeter pads ran out of room

Wire bonding needs pads spaced around the die's perimeter, and perimeter length grows much more slowly than the I/O counts modern chips need. Flip-chip spreads connections across the whole die face instead, which is why high-performance chips increasingly rely on it.

Once a chip needs thousands of connections, the edge alone can't hold them all.

UNIT 38 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered the two main ways to connect a die's pads to the outside world.