Copper Passivation Coating for Fluxless Flip-Chip Bonding
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Solution Overview
Problem
Copper oxidation during the bonding process of flip-chip packaging leads to non-wettability challenges and corrosion risks, which are not effectively addressed by existing methods like using 'no-clean' flux or formic acid environments, posing reliability issues for high-density I/O devices.
Innovation Solution
Applying a Cu-selective passivation coating to copper features, followed by a cleaning and rinsing process, and then using chemical vapor deposition to ensure the coating adheres only to copper surfaces, thereby preventing oxidation and enhancing bond reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If no-clean flux is used to chemically eliminate copper oxides during solder bonding, then copper oxidation is addressed, but corrosion and electrical leakage risks increase due to flux residues
Solution Approach 1:
The invention extracts and removes the harmful flux residues from the bonding process by implementing a fluxless bonding approach. Instead of using chemical flux to remove oxides, the process mechanically removes oxides through surface preparation techniques (such as plasma treatment or chemical-mechanical polishing) before bonding, thereby eliminating the source of corrosion and electrical leakage while maintaining bond reliability
Solution Approach 2:
The invention introduces an intermediary surface treatment process between copper oxide removal and bonding. This intermediary step uses controlled oxidation followed by mechanical removal or plasma cleaning to prepare the copper surface without requiring chemical flux, thus mediating between the need for oxide removal and the avoidance of harmful residues
2Reliability
If formic acid and nitrogen environment is used to dissolve copper oxides, then oxidation is minimized and contaminants are reduced, but equipment cost and process complexity increase significantly
Solution Approach 1:
The invention replaces expensive, specialized formic acid bonding equipment with conventional, readily available bonding tools. The process uses disposable or easily regenerable surface treatments (such as single-use plasma treatment or simple mechanical polishing) instead of requiring costly formic acid vapor generation and control systems, thereby achieving similar interface cleanliness with much simpler equipment
Solution Approach 2:
The invention changes the bonding process parameters from requiring formic acid vapor environment (high temperature, controlled atmosphere) to conventional bonding conditions. By modifying the surface preparation method rather than the bonding environment, the process achieves oxide removal without needing specialized equipment for chemical vapor treatment
3Quantity of substance
If copper is used instead of gold for bonding material, then cost is reduced and electrical conductivity is improved, but oxidation resistance deteriorates significantly
Solution Approach 1:
The invention applies preliminary surface treatment to copper bonding surfaces before bonding to remove oxidation-prone characteristics. Through techniques such as plasma cleaning, chemical-mechanical polishing, or controlled etching, the copper surface is prepared in an oxidation-resistant state prior to bonding, enabling reliable copper-to-copper or copper-to-solder bonds without the protection gold would naturally provide
Solution Approach 2:
The invention converts copper's natural tendency to oxidize from a harmful characteristic into a beneficial one by using controlled oxidation followed by mechanical removal or plasma cleaning. This creates a fresh, highly reactive copper surface that bonds more reliably, transforming the oxidation issue into a surface preparation opportunity that enhances bond strength
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 solution minimizes copper oxide formation, ensuring robust and reliable Cu-to-solder bonding, meeting Automotive Electronics Council (AEC-Q100) grade 1 safety standards and facilitating next-generation packaging technology.
Implementation Method 1
a Cu-selective passivation coating to copper features... preventing oxidation
Implementation Method 2
using chemical vapor deposition to ensure the coating adheres only to copper surfaces
Data Source
AI summary
The invention provides improved techniques for bonding copper to solder or other types of flip chip devices using a passivation coating on copper. The surface of a substrate is cleaned prior to mounting a flip chip device onto the substrate. The substrate is rinsed to remove residual artifacts remaining on the surface subsequent to the cleaning. Subsequent to the rinsing, a protective coating is applied to the surface of the substrate to produce a coated substrate. Copper pillars with solder caps extending from the flip chip device are bonded to metallic features on the surface of the coated substrate.