Pd-Plated Wiring Substrate for Low-Temperature Solder Reliability
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Solution Overview
Problem
The challenge is to achieve high reliability in solder mounting using solders with a melting point lower than 140°C, particularly for Flip Chip-Ball Grid Array (FC-BGA) techniques, while avoiding the limitations of existing low-melting-point solders such as Sn-Bi alloys which are brittle and have poor impact resistance.
Innovation Solution
A wiring substrate is developed with a Cu or Cu alloy electrode and a Pd-plating film, where solder with a melting point below 140°C, including Sn, Bi, or Sb, is bonded by controlling the thermal conditions to prevent the formation of a Pd concentrated layer between the electrode and the solder, ensuring stable bonding and improved reliability through uniform dissolution of Pd in the solder.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If solder with a melting point lower than 140°C (such as Sn-Bi alloy) is used to lower mounting temperature, then thermal degradation of components is prevented and substrate bending is avoided, but the solder becomes hard and brittle resulting in degraded impact resistance
Solution Approach 1:
The invention changes the chemical composition parameters of the solder by adding specific elements (Sb, In, or Ga) to the Sn-Bi base alloy. This modifies the crystal structure and mechanical properties of the solder, transforming it from a hard and brittle state to a more ductile state while maintaining the low melting point characteristic, thereby resolving the contradiction between low mounting temperature and impact resistance
Solution Approach 2:
The invention creates a composite solder material by combining Sn-Bi base alloy with additional elements (Sb, In, or Ga). This composite structure leverages the low melting point of Sn-Bi while the added elements contribute to improved mechanical strength and ductility, achieving both low mounting temperature and high impact resistance simultaneously
2Strength
If Sn-Ag-Cu based solder is used for reliable bonding, then bonding strength is improved, but reflow temperature increases causing thermal degradation of low heat resistance components
Solution Approach 1:
The invention fundamentally changes the compositional parameters of the solder from Sn-Ag-Cu to Sn-Bi-based alloy with specific additions. This parameter change lowers the melting point and reflow temperature while maintaining adequate bonding strength through the synergistic effects of the alloy composition, enabling reliable bonding at lower temperatures suitable for heat-sensitive components
3Reliability
If reflow temperature is increased for Sn-Ag-Cu solder bonding, then bonding reliability is improved, but the intermetallic compound layer grows thick degrading impact resistance
Solution Approach 1:
The invention changes the chemical composition parameters to Sn-Bi-based alloy with Sb, In, or Ga additions, which alters the intermetallic compound formation kinetics. This allows achieving bonding reliability at lower reflow temperatures, thereby preventing excessive growth of intermetallic compound layers and maintaining impact resistance
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 high reliability solder mounting even after multiple reflows, enhancing the impact resistance and bonding stability of the solder, suitable for applications requiring repeated reflow processes like FC-BGA, by dissolving Pd uniformly in the solder bulk, thus preventing the formation of a Pd concentrated layer.
Implementation Method 1
bonding solder having a melting point lower than 140°C by heating to dissolve Pd in the solder
Implementation Method 2
bonding solder having a melting point lower than 140°C by heating
Data Source
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AI summary
There is provided a wiring substrate including an electrode including Cu or a Cu alloy, a plating film having a film including at least Pd, formed on the electrode, and a solder which is bonded onto the plating film by heating, has a melting point of lower than 140°C, and includes Pd dissolved therein, a Pd concentrated layer being absent between the solder and the electrode.