Under-the-ball Metal Electrode for WLCSP Adhesion
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Solution Overview
Problem
Current wafer level chip scale packaging (WLCSP) technologies face issues with poor adhesion and reliability due to a small contact area between solder balls and metal electrodes, leading to oxidation and the formation of intermetallic compounds and voids, which affect the mechanical strength and longevity of solder joints.
Innovation Solution
The formation of an under-the-ball metal electrode with an electrode body and tail, combined with a diffusion preventing and wetting layer, increases the contact area and adhesion between the solder ball and the metal electrode, preventing intermetallic compound formation and enhancing reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional metal electrode structure is used in WLCSP packaging, then the manufacturing process is simple, but the contact area between solder ball and metal electrode is small leading to poor adhesion
Solution Approach 1:
The metal electrode is segmented into multiple functional layers including a copper electrode body, nickel diffusion barrier layer, and tin wetting layer. This segmentation increases the contact area between the solder ball and electrode while preventing harmful intermetallic compound formation, thereby improving adhesion without excessive complexity
Solution Approach 2:
The patent uses composite material structure for the metal electrode, combining copper (for electrical conductivity), nickel (for diffusion barrier), and tin (for wetting and adhesion). This composite approach resolves the contradiction by achieving both simple manufacturing through established plating processes and improved reliability through enhanced contact area and adhesion properties
2Reliability
If the metal electrode side is left bare to simplify manufacturing, then the manufacturing process is easier, but the exposed metal surface oxidizes easily losing reliable connection
Solution Approach 1:
The metal electrode employs a composite layered structure where copper provides conductivity, nickel prevents oxidation and acts as diffusion barrier, and tin enhances wetting. This composite approach maintains connection reliability by protecting against oxidation while using standard plating processes that do not significantly complicate manufacturing
Solution Approach 2:
The nickel layer creates an inert environment for the copper electrode by preventing oxygen access, thereby preventing oxidation without requiring complex manufacturing changes. The tin outer layer further protects the underlying metals while providing good solderability
3Strength
If solder ball is placed directly on copper electrode, then the manufacturing process is simple, but intermetallic compounds and voids form between solder ball and electrode affecting mechanical strength
Solution Approach 1:
The nickel layer acts as an intermediary between the copper electrode and tin solder ball, preventing direct contact that would form brittle intermetallic compounds. The tin wetting layer serves as another intermediary that promotes good adhesion while preventing harmful diffusion. This intermediary approach strengthens the solder joint while using established plating technologies
Solution Approach 2:
The multi-layer composite electrode structure (copper-nickel-tin) prevents direct copper-tin contact that forms brittle intermetallics, while maintaining good mechanical strength through the graded material composition. The composite structure uses conventional plating processes, balancing strength improvement with manufacturing complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution significantly improves the adhesion and reliability of solder joints by increasing the contact area and preventing intermetallic compound formation, leading to enhanced mechanical strength and longevity of the chip packaging.
Implementation Method 1
diffusion preventing layer formed on the under-the-ball metal electrode... tin atoms can diffuse into the copper electrode and copper atoms can also diffuse into the solder balls... intermetallic compound (IMC) and voids may be formed
Implementation Method 2
wetting layer formed on the diffusion preventing layer... increases the contact area and adhesion between the solder ball and the metal electrode
Data Source
AI summary
An IC packaging method is provided. The method includes providing a semiconductor substrate. The semiconductor substrate has a metal pad and an insulating layer and the insulating layer has an opening to expose the meal pad. The method also includes forming an under-the-ball meal electrode on the exposed metal pad. The under-the-ball metal electrode has an electrode body and an electrode tail, the electrode body is located at a bottom portion of the under-the-ball metal electrode and is in contact with the metal pad, and the electrode tail is located at a top portion of the under-the-ball meal electrode. Further, the method includes forming a solder ball on the under-the-ball metal electrode.