Coated Copper Pillar Pin-Hole Prevention
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Solution Overview
Problem
Copper pillars in electronic devices are prone to oxidation and corrosion, and existing methods for applying a solder cap layer can result in undesired pin-holes and circuit shorts due to poor wettability and flow of molten solder between pillars during reflow operations.
Innovation Solution
A method involving the deposition of a non-dense first metal layer of a noble metal onto the copper pillars, followed by a second metal layer of tin or its alloys via immersion-type plating, which forms a dense protection layer on the side walls, reducing pin-hole formation and improving protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thin protection layer is deposited by immersion-type plating onto the copper pillar after solder cap layer formation, then the side walls are protected against oxidation and corrosion, but pin-holes are formed on the side walls
Solution Approach 1:
A non-dense first metal layer consisting of islands of a metal more noble than copper is deposited onto the copper pillar and solder cap layer before the immersion-type plating of the second metal layer. This preliminary layer serves as a foundation that prevents pin-hole formation during the subsequent immersion plating process while still allowing the second metal layer to provide corrosion protection.
Solution Approach 2:
The protection system uses a composite structure with two distinct metal layers: a first layer of a noble metal (such as palladium, platinum, or gold) and a second layer of tin, tin alloy, silver, or silver alloy. This composite structure combines the benefits of both layers - the noble metal layer provides a stable base that prevents pin-holes, while the second metal layer provides the desired protection against oxidation and corrosion.
2Reliability
If a solder cap layer of tin-based solder material is attached to the top portion of copper pillars, then the solder cap provides necessary functionality, but molten solder material flows into the space between individual copper pillars during reflow operations forming undesired circuit shorts
Solution Approach 1:
The side walls of the copper pillar are coated with a composite metal layer structure consisting of a noble metal layer and a tin-based layer. This composite coating modifies the surface properties to control solder wettability, preventing molten solder from flowing down the side walls during reflow operations while maintaining the necessary solder cap functionality at the top.
Solution Approach 2:
The metal layer structure is applied specifically to the side walls of the copper pillar, providing localized modification of surface properties. The noble metal component provides areas of low wettability that prevent solder flow, while the solder cap layer at the top maintains appropriate wettability for proper soldering functionality.
3Ease of manufacture
If the exposed side walls of copper pillars are left unprotected, then the manufacturing process is simpler, but the side walls are prone to undesired oxidation and corrosion during further manufacturing steps
Solution Approach 1:
A composite metal layer structure is deposited on the side walls comprising a first layer of a noble metal and a second layer of tin, tin alloy, silver, or silver alloy. This composite coating provides effective protection against oxidation and corrosion while being compatible with standard manufacturing processes.
Solution Approach 2:
The non-dense first metal layer of noble metal is deposited as a preliminary step before the second metal layer. This preliminary layer facilitates the subsequent immersion-type plating process and ensures uniform coverage and adhesion of the second metal layer, making the overall process reliable and manufacturable.
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 method effectively prevents pin-holes and enhances protection against corrosion, reducing the likelihood of circuit shorts by creating a dense protection layer on the side walls of copper pillars with a solder cap layer.
Implementation Method 1
depositing a non-dense first metal layer consisting of islands of a metal which is more noble than copper onto the exposed outer surface of the copper pillars and the solder cap layers
Implementation Method 2
depositing a second metal layer selected from the group consisting of tin, tin alloys, silver, and silver alloys by immersion-type plating onto the surface of the copper pillars comprising the non-dense first metal layer
Data Source
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AI summary
The present invention relates to a method for forming a copper pillar on a semiconducting substrate, the copper pillar having an underbump metallization area comprising a metal less noble than copper and optionally a solder bump on the top portion, and having a layer of a second metal selected from tin, tin alloys, silver, and silver alloys deposited onto the side walls of said copper pillar. A layer of a first metal which is more noble than copper is deposited onto the entire outer surface of the copper pillar prior to deposition of the second metal layer. The layer of a second metal then has at least a reduced number of undesired pin-holes and serves as a protection layer for the underlying copper pillar.