WLCSP Wafer Thinning Sequence to Minimize Solder Ball Cracking
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Solution Overview
Problem
The challenge in manufacturing wafer level chip scale packages is to reduce the overall thickness of the semiconductor substrate while minimizing the risk of cracking, especially when attaching solder balls, which is exacerbated by conventional processes that perform backgrind after bumping.
Innovation Solution
A method involving partial dicing of the semiconductor substrate before thinning and attaching a handle for support, followed by a bumping process after thinning, to form trenches and create wafer level chip scale packages, allowing for controlled thinning and minimizing the risk of cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the substrate thickness is reduced to achieve thinner packaged integrated circuits, then the overall thickness of the package is reduced, but cracks will form in the die at the locations where solder balls are attached
Solution Approach 1:
The patent applies preliminary action by performing the backgrind process to thin the substrate before the bumping process that attaches solder balls. By reducing the substrate thickness beforehand, the mechanical stress and strain during bump attachment are minimized, preventing cracks from forming in the thinned substrate at the bonding locations.
Solution Approach 2:
The patent segments the manufacturing process into distinct sequential steps: first performing backgrind to thin the substrate, then performing bumping to attach solder balls. This segmentation allows each process to be optimized independently, with the thinning process preparing the substrate without the added stress of attached bumps.
2Length of stationary object
If conventional processes perform backgrind after bumping, then the substrate can be thinned, but the risk of cracking increases during the thinning process
Solution Approach 1:
The patent inverts the conventional sequence of operations by performing backgrind before bumping instead of after. This reversal eliminates the harmful effect of cracking because the substrate is thinned while it is still mechanically robust and free of attached solder balls that would create stress concentration points during the thinning process.
Data Source
AI summary
Trenches are opened from a top surface of a production wafer that extend down through scribe areas to a depth that is only partially through a semiconductor substrate. Prior to performing a bumping process, a first handle is attached to the top surface of the production wafer. A back surface of the semiconductor substrate is then thinned to reach the trenches and form a wafer level chip scale package at each integrated circuit location delimited by the trenches. A second handle is then attached to a bottom surface of the thinned semiconductor substrate, and the first handle is removed to expose underbump metallization pads at the top surface. The bumping process is then performed to form a solder ball at each of the exposed underbump metallization pads.