As electronic devices trend further towards smaller, thinner, lighter, and higher performance characteristics, bump size likewise decreases meaning fine pitch becomes essential. ASE's Cu pillar bumps in flip chip packaging technology is the most effective method for fine-pitch interconnection for these package types.
Standoff between chip and substrate is more stable. Good electrical performance and high reliability - Cu pillar bumps do not easily cause fatigue by electro migration and thermal. Fine pitch is possible below 20um. Cu pillar bumps do not cause bump bridging between adjacent bumps and they distribute current uniformly. Higher elastic modulus. Lower cost.
Underfill does not easily flow between adjacent bumps, when the bump pitch passes below 20 um, therefore thermo-compression reflow (TCR) technology is best applied. During the TCR process, underfill is dispensed onto a substrate. The chip is then picked by attachment, and bonded to the substrate. The temperature of attachment is increased by heating under force. After a period of time, the solder joint becomes formatted, and following, the package is post-cured.