Solder Transfer Sheet Solid Phase Bonding
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Solution Overview
Problem
Existing methods for forming solder bumps on circuit boards with fine electrode pitches suffer from bridging issues due to the melting of solder particles on non-electrode portions, leading to inconsistent solder distribution and increased manufacturing costs.
Innovation Solution
A method using a solder transfer sheet with a solder layer that undergoes solid phase diffusion bonding at a temperature below the solder's solidus temperature, allowing selective bonding to electrodes without melting, thereby preventing bridging and ensuring accurate solder distribution on circuit boards with electrode pitches as small as 50 µm or less.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If solder particles are melted using a transfer sheet to form solder bumps, then solder distribution is achieved, but bridging occurs between electrodes when electrode pitch is fine
Solution Approach 1:
The patent changes the temperature parameter from above-solidus (melting) to below-solidus (solid phase), preventing solder particle melting and subsequent bridging while maintaining effective solder transfer through solid phase diffusion bonding to electrodes
Solution Approach 2:
The patent utilizes solid phase diffusion bonding instead of melting, maintaining solder in solid phase throughout the process to avoid liquid solder bridging between fine-pitch electrodes while still achieving effective bonding through atomic diffusion at the solid-liquid interface
2Ease of operation
If a transfer sheet with depressions matching electrode pattern is used, then alignment-free solder transfer is achieved, but manufacturing cost increases
Solution Approach 1:
The patent extracts the pattern-matching requirement from the transfer sheet structure, using a uniform solder layer without depressions or complex features, thereby simplifying manufacturing while maintaining alignment-free operation through selective wetting of electrode areas
Solution Approach 2:
The patent employs a uniform, homogeneous solder layer across the entire transfer sheet surface without localized depressions or variations, simplifying manufacturing while achieving selective solder transfer through the inherent wettability differences between electrode and non-electrode areas
3Productivity
If electrode pitch is reduced to increase circuit density, then circuit board functionality improves, but solder bridging increases
Solution Approach 1:
By changing the temperature parameter to remain below the solidus point, the patent enables processing of fine-pitch electrodes without melting-induced bridging, thus allowing higher circuit density while maintaining reliability
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 enables the formation of solder bumps with minimal variation and high reliability on fine-pitch circuit boards without bridging, eliminating the need for precise alignment and reducing manufacturing costs by using a uniform solder layer that selectively adheres to electrodes.
Implementation Method 1
heating the circuit board to a temperature lower than the solidus temperature of the solder alloy constituting the solder layer, thereby selectively producing solid phase diffusion bonding between the solder layer and the portions to be soldered
Data Source
Figure 1(a)~2
Figure 3(a)~3(c)
Figure 4~5
AI summary
A method of forming solder bumps on electrodes of a circuit board without producing bridging using a solder transfer sheet which does not require alignment includes superposing a circuit board and a solder transfer sheet having a solder layer adhered to at least one side of a supporting substrate, performing heating under pressure to a temperature lower than the solidus temperature of the solder to selectively perform solid phase diffusion bonding of the solder layer to electrodes, and peeling the transfer sheet from the circuit board. The solder layer is in the form of a continuous solder coating or in the form of a monoparticle layer of solder particles which are adhered to the supporting substrate by an adhesive layer.