Gold Stud Bump Flip Chip Bonding on Interconnect Devices

Dick Pang, Weifeng Liu, Anwar Mohammed, Elissa Mckay, Teresita Villavert and Murad Kurwa
FLEXTRONICS Intl., Milpitas, CA

A molded interconnect device (MID) is an injection molded thermoplastic substrate which incorporates a conductive circuit pattern and integrates both mechanical and electrical functions. The thermoplastic material is doped with a metal-plastic additive which can be activated by laser. A laser beam is pointed on the surface of the injection molded plastic to form a metallization track by aggregating the metal additives. The metallized path is then plated with copper, nickel and gold finishes subsequently. MID Technology offers great advantages in design flexibility, device miniaturization, and true 3D integration of complex shapes.

Flip chip bonding of bare die on MID can be employed to fully utilize MID's advantage in device miniaturization. Compared to the traditional soldering process, thermo-compression bonding with gold stud bumps provides a clear advantage in its fine pitch capability. However, challenges also exist. Few studies have been made on thermocompression bonding on MID substrate, accordingly little information is available on process optimization, material compatibility and bonding reliability. Unlike solder reflow, there is no solder involved and no "self-alignment," therefore the thermo-compression bonding process is significantly more dependent on the capability of the machine for chip assembly alignment.

This paper presents the studies on flip chip thermo-compression bonding (TCB) of gold stud bumps on MID substrate. Non-conductive paste (NCP) is applied on the MID substrate before attachment of bare dies, and subsequently the dies are compressed at elevated temperatures to bond the gold stud bumps to the substrate pads and to cure NCP simultaneously. Daisy chained test vehicles were designed and built to demonstrate this process with multiple assembly challenges resolved.

The test vehicles successfully passed long term reliability testing based on IPC standard IPC-SM-784, although the substrate bond pads experienced excessive deformation during the thermo-compression bonding process at higher bonding forces. Regardless of the bonding forces evaluated, a certain degree of atomic bonding is observed between gold stud and gold plating on the substrate, However, such small scale bonding is not adequate to secure the chip in place, the assembly relies on the contraction of non-conductive paste during the cure process to maintain a reliable bonding interface. Based on reliability test results, the bonding force can be further reduced to minimize the substrate pad deformation while maintaining bonding reliability.

The flip chip thermocompression bonding has been successfully demonstrated on the molded interconnect device substrate.

Significant pad deformation and cracks during the thermocompression flip chip bonding process were observed, primarily due to high bonding temperature and bonding force, and softening of MID substrate material at the elevated bonding temperature.

A certain degree of gold to gold bonding is formed at the bonding tips and surrounding areas for all the evaluated forces. However, the tiny bonding area does not provide adequate strength to secure chips in place; contraction of NCP during curing is essential for a stable interconnect.

Bonding force needs to be further optimized to reduce the substrate pad deformation while maintaining a stable and reliable bonding interface.