Conductive Ball With Elastic Core and Resin Buffer
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Solution Overview
Problem
Conventional solder balls used in BGA IC packages face issues with reliability due to cracking and decreased conductivity under thermal stress, and high elastic modulus leads to inadequate contact area and increased electrical resistance when pressed against electrodes.
Innovation Solution
A conductive ball with a sphere made of an elastic body coated with a thermal expansion-resistant resin shell and a conductive metal shell, where the elastic body has a heat resistance of 200°C or more and the resin shell has a tensile strength of 1 to 700 MPa, preventing the metal shell from cracking and maintaining conductivity under thermal history.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the elastic modulus of the solder ball is high, then the solder ball maintains its shape and structural integrity, but the contact area with electrodes is reduced to a point contact and electrical resistance increases
Solution Approach 1:
The patent applies parameter changes by controlling the elastic modulus of the sphere to be 0.1 to 10 MPa, transforming it from a rigid structure to a softly elastic one. This enables the sphere to deform elastically under compression, increasing the contact area with electrodes from point contact to surface contact, thereby reducing electrical resistance while maintaining structural integrity through elastic recovery
Solution Approach 2:
The patent employs composite materials by combining a sphere made of elastic body (such as silicone rubber or polyurethane) with a metal shell coating. The elastic body core provides the necessary softness and deformability for increased contact area, while the metal shell maintains conductivity and structural integrity, creating a composite structure that resolves the contradiction between shape maintenance and contact area
2Strength
If the elastic modulus of the solder ball is high, then the solder ball resists deformation, but smaller solder balls are not contacted to the electrodes and cannot act as connectors
Solution Approach 1:
By changing the elastic modulus parameter to a low range (0.1 to 10 MPa), the sphere becomes sufficiently soft to deform elastically even when small in size. This allows solder balls of various diameters (0.04 to 2 mm) to all make adequate contact with electrodes, ensuring that size variations do not prevent functional connectivity while the elastic body maintains overall structural integrity
3Reliability
If the sphere is made of elastic body with low elastic modulus to increase contact area, then the contact area between sphere and electrodes is increased, but the coefficient of thermal expansion of the elastic body is higher than that of the metal shell causing the metal shell to rupture and crack under thermal history
Solution Approach 1:
The patent introduces an intermediary layer of resin shell between the elastic body sphere and the metal shell. This resin shell acts as a buffer that absorbs differential thermal expansion stresses, preventing the metal shell from rupturing or cracking while allowing the elastic sphere to maintain its low elastic modulus for adequate contact area. The resin shell mediates the conflicting thermal expansion properties of the elastic body and metal shell
Solution Approach 2:
The patent creates a three-layer composite material structure consisting of the elastic body core, resin shell intermediate layer, and metal shell outer layer. Each layer serves a specific function: the elastic body provides deformability for contact area, the resin shell provides thermal expansion buffering, and the metal shell provides conductivity and structural protection. This composite structure resolves the contradiction between electrical conductivity requirements and structural integrity under thermal stress
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 conductive ball ensures high conductivity and prevents conductivity decrease due to thermal history, providing a stable electrical connection with increased contact area and usable temperature range.
Implementation Method 1
the elastic body having a heat resistance of 200°C or more... allowing the deformation of the solder ball when the solder ball is pressed on the flat surface of the electrode
Implementation Method 2
a thermal expansion-resistant resin shell applied so as to coat the surface of the sphere... the coefficient of thermal expansion of the elastic body as the component of the sphere is generally higher than that of the metal as the component of the metal shell
Implementation Method 3
a conductive metal shell applied so as to coat the outer surface of the thermal expansion-resistant resin shell... which can be used as a connector by intervening between electrodes to apply a current between the electrodes
Data Source
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AI summary
The present invention provides a conductive ball, which can be used as a connector by intervening between electrodes to apply a current between the electrodes with relatively high conductivity, and which is prevented from decreasing the conductivity due to the following thermal history. The conductive ball of the present invention comprises a sphere formed of an elastic body; a thermal expansion-resistant resin shell applied so as to coat the surface of the sphere; and a conductive metal shell applied so as to coat the outer surface of the thermal expansion-resistant resin shell. For example, the sphere is formed of a silicone rubber, the thermal expansion-resistant resin shell is formed of a polyimide, and the conductive metal shell is formed of copper, gold, silver, or palladium, or an alloy containing it.