Solder Ball Oxide Coating for Flip-Chip Interconnect Reliability
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Solution Overview
Problem
Conventional solder joints in flip-chip mounting are susceptible to oxidation and necking, leading to degradation of electrical and mechanical properties, and are prone to cracking due to thermal stress cycling, especially at small pitches.
Innovation Solution
A method involving the formation of a substantially uniform oxide coating on solder balls, which is then converted to a non-oxide protection layer, allowing for flux-free reflow and enhanced mechanical and electrical connectivity by preventing oxidation and reducing solder wicking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional solder joints are used in flip-chip mounting, then electrical and mechanical connections are established, but the solder joints are susceptible to oxidation and necking, degrading their electrical and mechanical properties
Solution Approach 1:
A nickel underbump metallization layer is introduced as an intermediary between the solder ball and the contact pad. This nickel layer acts as a barrier that prevents oxidation of the solder joint and eliminates solder wicking, thereby improving reliability without compromising electrical and mechanical connection properties
Solution Approach 2:
The interconnect structure uses a composite metallization system consisting of copper pillar, nickel underbump metallization, and solder ball. This composite structure combines the advantages of each material: copper provides electrical conductivity, nickel provides oxidation resistance and prevents wicking, and solder provides mechanical bonding
2Reliability
If conventional solder joints are used, then connections are formed, but the solder joints are prone to cracking due to thermal stress cycling, especially at small pitches
Solution Approach 1:
The nickel underbump metallization serves as a stress-absorbing intermediary layer between the rigid copper pillar and the solder ball. This intermediate layer accommodates thermal expansion differences and prevents stress concentration at the solder joint, thereby improving crack resistance under thermal stress cycling
Solution Approach 2:
The invention changes the metallurgical parameters of the interconnect by introducing nickel with specific physical properties (oxidation resistance, controlled wettability) that differ from conventional solder-only structures. This parameter change fundamentally alters the stress distribution and crack propagation behavior under thermal cycling
3Quantity of substance
If solder balls are used for interconnection, then electrical and mechanical connections are established, but solder wicking occurs which depletes solder material from the joint
Solution Approach 1:
The nickel underbump metallization layer acts as an intermediary barrier that prevents solder wicking. The nickel layer has controlled wettability that stops solder from migrating along the contact pad, thereby retaining solder material at the joint location and preventing depletion
Solution Approach 2:
The invention extracts the wicking problem from the system by introducing nickel as a separate functional layer that specifically addresses solder migration. This extraction isolates the wicking issue to the nickel-solder interface rather than allowing it to propagate through the entire interconnect structure
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
The non-oxide protection layer provides robust protection against native oxide formation, enabling reliable and crack-resistant solder connections with improved thermal stability and reduced probability of solder joint cracking.
Implementation Method 1
exposing the solder ball to an oxygen containing environment to form an oxidation layer on the solder ball
Implementation Method 2
converting the oxidation layer on the solder ball to form a non-oxide solder protection layer on the solder ball
Implementation Method 3
reflowing the solder ball to remove the non-oxide solder protection layer. Reflowing the solder ball also mechanically and electrically connect the first IC die to the second IC die
Data Source
AI summary
An integrated circuit interconnects are described herein that are suitable for forming integrated circuit chip packages. In one example, an integrated circuit interconnect is embodied in a wafer that includes a substrate having a plurality of integrated circuit (IC) dice formed thereon. The plurality of IC dice include a first IC die having first solid state circuitry and a second IC die having second solid state circuitry. A first contact pad is disposed on the substrate and is coupled to the first solid state circuitry. A first solder ball is disposed on the first contact pad. The first solder ball has a substantially uniform oxide coating formed thereon.