Step Cavity in BGA Encapsulant for Drop Test Stress Relief
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Solution Overview
Problem
Ball grid array (BGA) semiconductor packages face reduced drop test performance as package size increases, leading to uneven loading and stress on solder balls due to mechanical shock, resulting in premature failure during drop tests.
Innovation Solution
A step cavity of selected depth and width is formed in the encapsulating compound layer around the edges or corners of the BGA package, reducing the thickness at the periphery and providing stress relief to the outermost solder balls, making the package more compliant to bending.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the size of the BGA package is increased, then the functionality and performance of the package is improved, but the drop test characteristic decreases exponentially due to increased stress on peripheral solder balls
Solution Approach 1:
The patent applies local quality by creating step cavities at specific locations (edges or corners) of the encapsulating compound layer. This modifies the local structural properties at the periphery where stress concentration occurs during drop tests, making those specific regions more compliant while maintaining the overall package size and functionality.
Solution Approach 2:
The patent changes the physical parameter of the encapsulating compound layer by introducing step cavities with specific depth and width parameters. This alters the stiffness distribution of the package, creating a gradient where the periphery has reduced stiffness compared to the center, thereby improving stress distribution during impact events.
2Strength
If the thickness of the encapsulating compound layer is increased to provide more protection, then the strength of the package is improved, but the compliance to bending is reduced, increasing stress at edges and corners
Solution Approach 1:
The patent segments the encapsulating compound layer by creating step cavities that divide the uniform thickness into varying thickness regions. This segmentation allows different parts of the package to have different compliance characteristics - the periphery becomes more compliant while the center maintains sufficient strength and protection.
Solution Approach 2:
The step cavities create local quality variations in the encapsulating compound layer, where the periphery has reduced thickness and increased compliance, while the center maintains full thickness and strength. This local differentiation resolves the contradiction between overall strength and peripheral compliance.
Data Source
AI summary
A ball grid array (BGA) package includes a substrate layer having first and second sides. A semiconductor chip is attached to the first side of the substrate layer by a dielectric adhesive layer. A plurality of solder balls are attached to the second side of the substrate layer. The solder balls may be set out by rows and columns. A plurality of wires electrically connect the semiconductor chip to the solder balls. A layer of encapsulating compound is deposited over the semiconductor chip. A step cavity of a selected depth and shape is formed in the layer of encapsulating compound at or near the edge or periphery of the layer of encapsulating compound. The step cavity is separated from the solder balls by the substrate layer but spans over a plurality of selected solder balls.