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

Abstract

Flip-chip bonding is a key packaging technology to achieve the smallest form factor possible. Using copper as a direct under-bump metal and performing bonding under little force and at a low temperature eliminates the processing step for the deposition of a suitable wetting metal and offers an economical solution for electronic chip packaging. In this paper, various samples with copper and nickel–gold surface finishes are used to apply an in-house solder bumping, flip-chip bonding and reflow process to exhibit the bump-bond feasibility. Native oxides are reduced using process gases, and copper surface protection and solder wetting are achieved using copper formate. Lead-free 40 µm solder balls were bumped on 80 µm copper pads and 120 µm copper pillars to demonstrate a full intermetallic Cu–Cu bond as a base study for stacking applications. Using a low-force bonding technique, various chips with different dimensions were bonded at 0.5–16 MPa, followed by a reflow step at a maximum temperature of 270 ∘C. Then, 30 µm solder balls are utilized to bump the samples with NiAu and Cu bond pads at 50 µm pitch. A mean shear strength of 44 MPa was obtained for the 30 µm Cu samples. To the best of our knowledge, 30 µm solder bumping directly on the copper pads by producing copper formate is a novel research contribution.

Monolithic MHz-frame rate digital SiPM-IC with sub-100 ps precision and 70 μm pixel pitch

“…The major difference between the old and new concept is the interconnection approach of sensor and readout electronics. In the old concept, the readout IC is flip-chip connected to the sensor chip utilizing 30 μm solder spheres at 50 μm pitch [14]. This hybrid approach enables the direct readout of each sensor pixel by its corresponding pixel electronics occupying the same pixel area at about 50 μm pitch, without affecting the fill factor.…”

Effects of Cu/SnAgCu Powder Fraction and Sintering Time on Microstructure and Mechanical Properties of Transient Liquid Phase Sintered Joints

“…TLPS thus is accompanied with a long bonding time [18][19][20]. Many approaches have been employed to accelerate the sintering time such as high-temperature/pressure bonding [19,21,22], incorporation of reactive additives [23][24][25][26], surface modification [27,28], and electric/laser-assisted bonding [29][30][31][32][33][34]. For instance, Ramli et al employed TiO 2 particles to reduce the thickness of the interfacial IMCs for the enhancement of the shear strength and hardness of the solder joints [24].…”