Die Backside Stiffener via Cold Spray for Warpage Control
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Solution Overview
Problem
Current semiconductor packaging solutions, such as picture frame stiffeners and die-backside-stiffeners, face challenges like increased assembly steps, z-height, and reduced effectiveness in controlling package warpage due to coefficient of thermal expansion (CTE) mismatch between the die and substrate, as well as limitations in material choices and manufacturing efficiency.
Innovation Solution
A semiconductor package with a stiffener directly formed on the back side of the die using additive manufacturing processes like cold spray, which deposits metal or metal-ceramic composites without an adhesive layer, enhancing mechanical coupling and reducing warpage by optimizing material modulus and CTE, while allowing simultaneous deposition and patterning at the wafer level.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a picture frame stiffener is used to control package warpage, then warpage control is improved, but assembly complexity increases due to additional assembly steps at the unit level
Solution Approach 1:
The stiffener structure is merged with the die itself by forming it as an integral part of the die backside through additive manufacturing. This eliminates the need for separate assembly steps to attach a distinct stiffener component, thereby reducing assembly complexity while maintaining warpage control functionality.
Solution Approach 2:
The stiffener is formed on the die backside before the die is mounted to the substrate. This preliminary formation of the stiffener structure allows subsequent assembly steps to be simplified, as the stiffener is already in place and does not require separate attachment operations.
2Reliability
If a picture frame stiffener is used to control package warpage, then warpage control is improved, but z-height increases
Solution Approach 1:
Instead of using a wide picture frame stiffener that increases overall package height, the stiffener is applied locally only on the die backside area where it is most needed for warpage control. This localized application maintains effective warpage mitigation while minimizing the increase in z-height.
Solution Approach 2:
The stiffener covers only the necessary portion of the die backside rather than extending to the full periphery. This partial coverage provides sufficient warpage control for the die area while avoiding the excessive z-height increase that would result from a complete picture frame structure.
3Reliability
If a DBS is attached with an adhesive layer to control package warpage, then warpage control is improved, but mechanical coupling strength decreases at high temperatures
Solution Approach 1:
The adhesive layer is completely removed from the structure. Instead of attaching a separate DBS with adhesive, the stiffener is formed directly as an integral part of the die backside through additive manufacturing, eliminating the weak mechanical coupling issue associated with adhesive layers at high temperatures.
Solution Approach 2:
The stiffener and die are merged into a single integrated structure through direct formation on the die backside. This integration eliminates the adhesive interface entirely, providing strong mechanical coupling that maintains integrity at high temperatures while still achieving effective warpage control.
4Reliability
If a DBS is attached with an adhesive layer to control package warpage, then warpage control is improved, but thermal conductivity decreases due to thermal bottleneck
Solution Approach 1:
The adhesive layer is completely removed from the structure. Instead of attaching a separate DBS with adhesive, the stiffener is formed directly as an integral part of the die backside through additive manufacturing, eliminating the thermal bottleneck associated with adhesive layers and improving thermal conductivity.
Solution Approach 2:
The stiffener and die are merged into a single integrated structure through direct formation on the die backside. This integration eliminates the adhesive interface that acts as a thermal bottleneck, enabling efficient heat transfer from the die through the stiffener structure.
5Ease of manufacture
If lithography is used to pattern DBS layer to create dicing streets, then dicing streets are formed, but manufacturing complexity and costs increase
Solution Approach 1:
The lithography process is replaced with additive manufacturing technology. Instead of using photochemical lithography to pattern the DBS layer, the stiffener is directly formed on the die backside through material deposition and consolidation processes, eliminating the need for lithography chemicals, masks, and associated cleaning steps.
Solution Approach 2:
The manufacturing approach changes from a planar lithography process to a three-dimensional additive manufacturing process. This parameter change allows direct formation of the stiffener structure with integrated dicing street features, reducing the number of manufacturing steps and associated costs.
6Area of stationary object
If picture frame stiffener width is reduced to increase die-to-package area ratio, then die-to-package area ratio is improved, but warpage control effectiveness decreases
Solution Approach 1:
The stiffener is applied locally on the die backside with optimized distribution of stiffening material. This local quality approach concentrates the stiffening effect where it is most needed to control die warpage, maintaining effectiveness even with reduced overall stiffener width and improved die-to-package area ratio.
Solution Approach 2:
The additive manufacturing process enables use of composite materials with optimized mechanical properties for the stiffener. These composite materials provide enhanced stiffening efficiency per unit width, allowing reduced stiffener dimensions while maintaining warpage control effectiveness and improving die-to-package area ratio.
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 approach reduces package warpage, increases manufacturing throughput, and decreases costs by eliminating the need for lithography and adhesive layers, while enabling the use of materials not typically available in standard manufacturing, thus improving the die-to-package area ratio and miniaturization capabilities.
Implementation Method 1
a stiffening layer formed on the back side surface of at least one of the one or more dies... formed using one or more additive manufacturing (AM) processes (e.g., a cold spray (CS) process)
Data Source
AI summary
A foundation layer having a stiffener and methods of forming a stiffener are described. One or more dies are formed over the foundation layer. Each die has a front side surface that is electrically coupled to the foundation layer and a back side surface that is opposite from the front side surface. A stiffening layer (or a stiffener) is formed on the back side surface of at least one of the dies. The stiffening layer may be directly coupled to the back side surface of the one or more dies without an adhesive layer. The stiffening layer may include one or more materials, including at least one of a metal, a metal alloy, and a ceramic. The stiffening layer may be formed to reduce warpage based on the foundation layer and the dies. The one or more materials of the stiffening layer can be formed using a cold spray.