Bonding

After the die has been mechanically attached to the substrate through die attach, it still needs to be electrically connected to the substrate connection structures. This process is referred to as bonding. Electrical contacting is achieved in about 90% of cases via wire bonding, while the mechanical attachment of the chip – as described in the previous chapter – is mainly carried out by bonding with epoxy resins.

In wire bonding, thin wires made of gold or aluminum are used, which are cold-welded to the contact pads of both chip and substrate. Three common variants are distinguished:

• Thermosonic ball-wedge bonding
• Ultrasonic wedge-wedge bonding
• Thermocompression ball-wedge bonding

In all three methods, a small ball first forms at the end of the wire, created by locally melting the wire tip. Due to the strong surface forces of the molten gold, the melt solidifies into a ball whose larger diameter compared to the bond wire ensures a more reliable mechanical and electrical connection.

In addition to classic wire bonding, flip-chip technology offers an alternative method that requires no long bond wires at all and represents the most compact form of connection between chip and substrate.

To create a flip-chip connection, small solder bumps must be present on the chip and/or the substrate. To form the connection, the chip is placed face-down, but with very precise alignment, onto the substrate. During a subsequent reflow process, the bumps melt and simultaneously form a mechanical and electrical connection between chip and substrate.

The key advantage of flip-chip technology lies in the very short connection paths from chip to substrate via the solder bumps – in contrast to the comparatively long wire bonds used in classic wire bonding. This reduces parasitic inductance and capacitance in the connection, which is electrically advantageous at very high signal frequencies.

Since a flip-chip connection typically leaves an air gap between chip and substrate, it is usually necessary to additionally introduce a so-called underfiller. This fills the remaining gap between chip and substrate and compensates for mechanical stress arising from the differing thermal expansion coefficients of the two materials.