Selective Post-Soldering Volume Adjustment for Improved Co-Planarity of C4 Bump Interfaces

The objective of this study is to introduce and qualify a local and selective solder-jet fusing process with previously deposited solder bumps in order to optimize the solder bump height distribution quality for improved semiconductor substrate Controlled Collapse Chip Connection (C4) interfaces. With the ongoing miniaturization of structural dimensions in semiconductor devices and the change of conventional FR4 substrate materials for the advanced packaging the requirements for solder bump surface co-planarity significantly increases. With the aim of improving the coplanarity of C4 bump interfaces this study investigates whether the additional introduction of minimal solder volumes down to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$33.5 \boldsymbol{\mu} \mathbf{m}^{\mathbf{3}}$</tex> using “PacTech's” unique SB <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${ }^{\mathbf{2}}$</tex>-Jetting technology as described by Kasulke et al. [1] can be used to modify the local height of individual solder bumps within a bump array. The approach involves merging the existing solder joint into a larger homogeneous solder bump by adding liquefied solder balls and remelting it using a laser. With the SB <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${ }^{\mathbf{2}}$</tex>-Jet process different solder volumes can be dispensed individually. The sample material used for the experimental series consisted of flexible FR4 substrates with a copper-gold pad finish and silicon chips showing a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5 \mu ~\mathrm{m}$</tex> NiAu under-bump metallization (UBM). In the first step, SAC305 solder balls were applied to the samples to form a conventional C4 bump interface followed by height measurement, shear-test and inspection. In the second step, smaller solder balls were selectively and locally dispensed in their molten state onto the initial solder bumps and the whole joint reflowed with a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2^{\text{nd }}$</tex> laser pulse. The identified process parameter will be presented together with the corresponding results obtained from the metrology analysis in order to provide a comprehensive understanding of the process capabilities. The measurement results include bump height measurements, shear tests, optical microscopy, solder bulk analysis by X-ray, cross-sectional polishing and scanning electron microscopy (SEM). Finally, further prospects for extended qualification activities and the use of this technology for mass production are outlined.