Polymer-Island Bump Structure for WLCSP Solder Joint Reliability
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Solution Overview
Problem
Wafer-level chip-scale packages face reliability issues due to stress concentration at solder joints during drop impacts, which can lead to failures, particularly in mobile applications where flex between the PCB and chip applies stress to solder joints.
Innovation Solution
A four-layer bump structure is developed, comprising a conductive pad, a passivation layer, a re-distribution layer (RDL) with a via opening, a second polymer layer with an island acting as a stress buffer, and an under-bump metallization (UBM) layer, where the island under the bump relieves stress at the interface between the conductive pad and the RDL via, using materials like aluminum, silicon nitride, copper, polyimide, and multi-layer stacks for enhanced durability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional solder ball bump structure is used in WLCSP, then the packaging achieves wafer-level chip-scale integration, but stress concentration occurs at the solder joint interface during drop impact, leading to reliability failures
Solution Approach 1:
The bump structure is segmented into multiple functional layers: conductive pad, passivation layer, polymer layer, RDL, UBM layer, and bump. This segmentation distributes the stress across multiple interfaces rather than concentrating it at a single solder joint, with each layer serving specific mechanical and electrical functions to enhance overall reliability
Solution Approach 2:
The bump structure employs composite materials including polymer layers (for stress buffering), metallization layers (for electrical conductivity), and passivation layers (for protection). This composite approach combines materials with different mechanical properties to reduce stress concentration while maintaining electrical performance
2Stress or pressure
If the bump pad is made larger to reduce stress concentration, then stress distribution improves, but the area available for other circuit elements decreases
Solution Approach 1:
The conductive pad is positioned in the central portion of the semiconductor die, utilizing the vertical dimension and central area efficiently. This dimensional arrangement allows adequate stress distribution while preserving peripheral areas for other circuit elements and interconnections
3Reliability
If a multi-layer bump structure is implemented to reduce stress, then reliability under drop and thermal shock improves, but manufacturing complexity increases
Solution Approach 1:
The polymer layer is formed with a via opening that partially exposes the conductive pad before the RDL is deposited. This preliminary structuring allows the RDL to be completely filled and form an integral via, establishing the stress-buffering configuration before subsequent UBM and bump formation, thereby reducing stress concentration from the outset
4Reliability
If the via opening is completely filled with RDL to form an integral via, then electrical connection reliability improves, but material usage and processing time increase
Solution Approach 1:
The RDL completely fills the via opening and an RDL via is integrally formed with the bump pad, merging the redistribution layer and via formation into a single continuous structure. This integration eliminates separate via drilling and plating steps, reducing processing time while ensuring reliable electrical connection
Data Source
AI summary
A bump structure includes a conductive pad on a semiconductor die; a passivation layer covering a perimeter of the conductive pad; and a first polymer layer on the passivation layer. The first polymer layer includes a via opening partially exposing the central portion of the conductive pad. A RDL is disposed on the first polymer layer and patterned into a bump pad situated directly above the conductive pad. The via opening is completely filled with the RDL and a RDL via is integrally formed with the bump pad. A second polymer layer is disposed on the first polymer layer. An island of the second polymer layer is disposed at a central portion of the bump pad. UBM layer is disposed on the bump pad. The UBM layer covers the island and forms a bulge thereon. A bump is disposed on the UBM layer.