Minimized Intermetallic Compound Formation at Cu-Pillar Bond Interfaces Through a Dual-Step Laser-Assisted Reflow and Laser-Assisted Bonding Process

Abstract:

Intermetallic compound (IMC) growth rates in solder contacts associated with conventional reflow technologies pose an obstacle to the further miniaturization of Cu-pillar geometries, as described in [1]. PacTech’s unique laser-assisted process technology, LAPLACE®, enables interconnect formation with minimal thermal load by combining solder cap reflow immediately after plating with laser-assisted bonding during assembly.

This work introduces a process flow for forming domeshaped solder caps after plating, followed by a flip-chip assembly step using a near-infrared (NIR) laser to create Cu-pillar interconnects with a minimized IMC layer. The study also aims to establish the correlation between IMC thickness and applied laser energy and determines the minimum laser energy required to form stable and reliable solder interconnects. Furthermore, the results are compared with those obtained using conventionally ovenreflowed round solder caps.

For this study, square Cu pillars with a total height of 20 μm and an edge length of 25 μm are fabricated on 8” Si wafers. The plated solder cap has a thickness of 10μm and consists of SnAg. In the first step, the wafer undergoes a laser reflow process to round off the solder caps. The wafer is then diced and the chips are singulated. In the second step, the chips are bonded onto a corresponding substrate with copper pads using a laser-assisted flip-chip bonding process. A 1.5 kW NIR laser system is used for both the laser reflow and bonding process to irradiate a Cu-pillar array of up to 50 mm x 50 mm in one step. Between the processing steps, the Cu pillars are analyzed to evaluate the bond interface quality, IMC growth as reflected by IMC thickness distribution, and the mechanical strength. The
morphological and metallurgical properties of the resulting solder deposits are analyzed using optical microscopy, Xray and SEM (scanning electron microscope). The IMC layer is characterized by cross-sectional grinding and polishing, followed by examination under a SEM, including focused ion beam cutting (FIB-Cut). The mechanical strength of the assembly is assessed by shear testing.

Finally, the potential of the presented soldering technologies is discussed in terms of economics, scalability and manufacturing capacity.

Keywords — laser reflow, laser bonding, LAB, LAR, LCB, solder, Cu pillar, IMC, laser