Die Edge Fillet Structure for Thermal Stress Crack Buffering
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Solution Overview
Problem
Semiconductor packages face cracking issues due to differences in coefficients of thermal expansion (CTE) and modulus of elasticity among various materials used, leading to deformation and failure during thermal cycling and reliability tests.
Innovation Solution
Incorporating a fillet, either formed from capillary underfill epoxy or carbon nanotube material, within the semiconductor package to absorb stresses and prevent cracking by filling the cavity between the dies and the solder mask, thereby enhancing the modulus of elasticity and resistance to cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional adhesive bonding is used to bond the memory package substrate with the functional circuit die, then the basic structural integrity is achieved, but cracks occur at the copper traces near the adhesive edge due to thermal expansion differences and stress concentration
Solution Approach 1:
A fillet is formed at the edge of the adhesive bonding interface before final assembly, creating a stress-absorbing cushion that prevents crack propagation into the copper traces. The fillet acts as a pre-positioned protective element that cushions thermal and mechanical stresses before they can cause damage to the trace lines.
Solution Approach 2:
The fillet material is engineered with specific mechanical properties including a modulus of elasticity between 0.5-5.0 GPa and yield strength between 5-50 MPa, creating a gradual transition in mechanical parameters from the rigid copper traces through the adhesive to the substrate. This parameter gradient reduces stress concentration and prevents cracking.
2Strength
If the adhesive layer is made thicker to improve bonding strength, then the bond between substrate and die is enhanced, but stress concentration at the adhesive edge increases, leading to more frequent cracks in the copper traces
Solution Approach 1:
The fillet is applied locally at the critical edge region where the adhesive meets the substrate, creating a zone with different mechanical properties specifically where stress concentration occurs. This localized modification addresses the harmful stress concentration without requiring changes to the entire adhesive layer or bonding interface.
Solution Approach 2:
The fillet is formed from a composite material containing carbon nanotubes dispersed in an epoxy matrix, combining the adhesive properties of epoxy with the exceptional mechanical strength and flexibility of carbon nanotubes. This composite structure provides both bonding capability and stress distribution, eliminating the trade-off between bond strength and stress concentration.
3Ease of manufacture
If standard epoxy fillet material is used, then the cavity is filled and some stress absorption is achieved, but the modulus of elasticity and crack resistance are insufficient to fully prevent trace line fractures
Solution Approach 1:
The fillet material is formulated as a composite containing carbon nanotubes at a concentration of 0.1-5.0 wt% dispersed in an epoxy matrix. The carbon nanotubes provide exceptional tensile strength and flexibility, enabling the fillet to absorb thermal cycling stresses and prevent crack propagation while maintaining ease of dispensing and curing characteristics similar to standard epoxies.
Solution Approach 2:
The addition of carbon nanotubes fundamentally changes the mechanical parameters of the fillet material, achieving a modulus of elasticity between 0.5-5.0 GPa and yield strength between 5-50 MPa. These parameter changes enable the fillet to function as an effective stress buffer that protects copper traces from fracture during thermal cycling and reliability testing.
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 fillet effectively reduces the occurrence of cracks in the conductive trace lines by absorbing stresses, increasing the semiconductor package's robustness and reliability through improved stress resistance and tensile strength.
Implementation Method 1
a fillet formed from a capillary underfill
Data Source
AI summary
A semiconductor package having a fillet is provided. The semiconductor package includes a trace disposed within a solder mask that has a top surface. A first die is over the solder mask and mechanically couples with the trace. A first adhesive is between the trace and the first die where sides of the first die and the first adhesive define a die edge. The semiconductor package includes a fillet adjacent the die edge and a second die above the first die. The semiconductor package also includes a second adhesive having a bottom surface where the second adhesive is between the first die and the second die. The solder mask top surface, the first die surface, and the second adhesive bottom surface define a cavity where the fillet is within the cavity at the die edge.