Fluxless flip-chip bonding using a lead-free solder bumping technique

Abstract

With the LHC exceeding the nominal instantaneous luminosity, the current barrel pixel detector (BPIX) of the CMS experiment at CERN will reach its performance limits and undergo significant radiation damage. In order to improve detector performance in high luminosity conditions, the entire BPIX is replaced with an upgraded version containing an additional detection layer. Half of the modules comprising this additional layer are produced at DESY using fluxless and lead-free bumping and bonding techniques. Sequential solder-jetting technique is utilized to wet 40-µm SAC305 solder spheres on the silicon-sensor pads with electroless Ni, Pd and immersion Au (ENEPIG) under-bump metallization (UBM). The bumped sensors are flip-chip assembled with readout chips (ROCs) and then reflowed using a flux-less bonding facility. The challenges for jetting low solder volume have been analyzed and will be presented in this paper. An average speed of 3.4 balls per second is obtained to jet about 67 thousand solder balls on a single chip. On average, 7 modules have been produced per week. The bump-bond quality is evaluated in terms of electrical and mechanical properties. The peak-bump resistance is about 17.5 mΩ. The cross-section study revealed different types of intermetallic compounds (IMC) as a result of interfacial reactions between UBM and solder material. The effect of crystalline phases on the mechanical properties of the joint is discussed. The mean shear strength per bump after the final module reflow is about 16 cN. The results and sources of yield loss of module production are reported. The achieved yield is 95%.

Laser-Assisted Micro-Solder Bumping for Copper and Nickel–Gold Pad Finish

“…It should be noted that the semi-closed chamber of the flip-chip bonder is not sufficient to process copper surfaces at elevated temperatures due to the oxidation issues. However, in case of Ni-based UBM, the pre- and post-bonding reflow steps can also be performed in the flip-chip bonder [ 27 ]. Moreover, Ni-based UBM with clean and noble surface finishing do not need to go through the cleaning processes but can directly be moved into the solder-ball bumper, as illustrated in Figure 3 .…”

“…At 200 °C, the reduction reaction is completed by desorbing CO 2 and H 2 , leaving the pure Cu metal. Using an integrated formic-acid module in the flip-chip bonder, tin oxides are effectively removed from the bump surface prior to the tacking process [ 27 ].…”

“…The molten ball is finally directed to wet the targeted pad through the nitrogen pressure. A detailed process description limited to 40 µm solder balls can be found in our previous paper [ 27 ]. For this work, the solder-ball bumper was equipped with a new bond head capable of using 30 µm balls, and optimization of the laser energy for the individual samples was performed.…”

“…This study is based on proven in-house processes utilizing a flux-less and lead-free laser-assisted bumping and bonding technique [ 27 ]. Here, we address upgrades with copper as a direct under-bump and pillar material, 30 µm bumps at 50 µm pitch, low-force bonding and full IMC solder joints.…”

Process Optimization of Via Plug Multilevel Interconnections in CMOS Logic Devices

“…In addition, the hole sizes of vias used to connect the wiring layers have decreased, whereas the thickness of the inter-metal dielectric (IMD) [3], which determines the height of the vias, has hardly changed, resulting in a sharp increase in the aspect ratio (A/R; via height/via hole size) [4,5]. In this situation, the via has reached its filling limit with the conventional Al reflow or 2-step Al deposition process [6,7]. Alternatively, the plug process using chemical vapor deposition tungsten (CVD W) has been applied after deposition of a barrier film [8].…”