BGA Package with Non-Planar Ball Placement for Joint Strength
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Solution Overview
Problem
Conventional Ball Grid Array (BGA) packages using leadframes as chip carriers face issues with high costs, poor moisture reliability, and weak solder ball joints due to planar ball-placement surfaces, leading to dropped solder balls during handling and stress-induced cracks.
Innovation Solution
A BGA package with a lead-on-chip (LOC) configuration is developed, featuring non-planar ball-placement surfaces on leadless leadframes with embedded solder balls in cavities, enhancing joint strengths and stress resistance by adjusting etching conditions to create roughness variations and using a plating layer to prevent oxidation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If planar ball-placement surfaces are used on leadframes, then the manufacturing process is simple, but the joint strength of solder balls is weak leading to dropped balls
Solution Approach 1:
The patent applies curvature by forming the ball-placement surface as a spherical or convex surface rather than a flat plane. This curved geometry increases the contact area between the solder ball and the leadframe surface, thereby enhancing joint strength and preventing solder ball drop during handling and assembly processes.
Solution Approach 2:
The patent implements local quality by creating a convex or spherical surface specifically at the ball-placement area, while other portions of the leadframe may remain flat or have different geometries. This localized geometric modification targets the critical joint area without unnecessarily complicating the entire leadframe structure.
2Ease of operation
If solder balls are placed on flat surfaces, then the placement process is easy, but cracks occur during SMT processes and operations
Solution Approach 1:
The convex or spherical ball-placement surface creates a geometric configuration that distributes mechanical stress more effectively during SMT processes and package operations. The curved geometry reduces stress concentration that would otherwise occur on flat surfaces, preventing crack formation and improving overall package reliability.
3Ease of manufacture
If leadframes are used as chip carriers, then the cost is reduced, but moisture reliability is poor
Solution Approach 1:
The patent applies composite materials by forming a coating layer on the leadframe surface, particularly on the ball-placement area. This coating layer (which may be an oxide layer or other protective material) provides moisture barrier properties to the otherwise moisture-sensitive leadframe structure, maintaining cost-effectiveness while improving moisture reliability.
4Strength
If non-planar ball-placement surfaces are created, then joint strength increases, but manufacturing complexity increases
Solution Approach 1:
The patent employs spherical or convex surfaces to achieve enhanced joint strength. While this introduces geometric complexity, the spherical form factor is relatively simple to implement through standard manufacturing techniques such as spherical indenters during pressing operations or specialized molding processes, balancing performance improvement with manufacturing feasibility.
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 LOC configuration increases the ball-joint areas and stress resistance of solder balls, reducing voids and enhancing joint strengths, stability, and reliability, while minimizing package dimensions and avoiding direct stress on solder balls.
Implementation Method 1
using a plating layer to prevent oxidation
Data Source
AI summary
A BGA package primarily includes a leadless leadframe with a plurality of leads, a chip disposed on the leads, a die-attaching layer adhering to an active surface of the chip and the top surfaces of the leads, a plurality of bonding wires electrically connecting the chip to the leads, an encapsulant, and a plurality of solder balls. Each lead has a bottom surface including a wire-bonding area and a ball-placement area, moreover, a plurality of lips project from the bottom surfaces of the leads around the ball-placement areas. The encapsulant encapsulates the chip, the bonding wires, the die-attaching layer, and the top surfaces, the bottom surfaces except the ball-placement areas. The solder balls are disposed on the ball-placement areas.