What Is a Gold Solder?
Gold-tin (AuSn) alloy paste is a form of gold (Au) and tin (Sn) alloy primarily used as solder material, provided in paste form.
Essentially, it comprises a mixture of gold-tin alloy powder and a binding material called flux. Gold-tin alloys have a higher melting point (217°C and above) compared to common lead-free solders, and they excel in heat conductivity and post-bonding reliability. They are used for joining and sealing precision components such as high-frequency devices, optical devices, and for applications requiring heat resistance during subsequent low-temperature reflow, as well as for soldering applications needing tensile strength and corrosion resistance.
Applications of Gold-Tin (AuSn) Alloy Pastes
Gold-tin (AuSn) alloy paste is primarily used for bonding and sealing precision components such as:
Die bonding for high-brightness LEDs, Peltier elements, power semiconductors
Sealing materials for crystal devices, SAW devices
The former is used in automotive, lighting, and thermoelectric module applications, while the latter is used in mobile communications, base stations, MEMS sensors, among others. It is a solder material used for bonding high-frequency devices and optical communication devices.
Moreover, gold-tin (AuSn) alloy paste’s paste form allows for high adaptability to various bonding methods such as screen printing, dispenser coating, and pin transfer, making it suitable for various joint shapes.
Principles of Gold-Tin (AuSn) Alloy Pastes
Gold-Tin (AuSn) Alloy Composition
Typically, gold-tin (AuSn) alloys with around 20% tin content are commonly used. The main compositions of gold-tin (AuSn) alloys along with their melting points and densities are as follows:
AuSn18: 278-360°C, Density 14.89
AuSn20: 278-300°C, Density 14.52
AuSn21: 278°C, Density 14.35
AuSn21.5: 278°C, Density 14.26
AuSn22: 278°C, Density 14.17
AuSn90: 217°C, Density 7.78
Gold-tin alloys have a higher melting point than common lead-free solders, excellent heat conductivity, post-bonding reliability, and good wetting properties. They offer superior thermal conductivity compared to thermosetting Ag epoxy resins, leading to high joint strength. They are particularly suitable for applications requiring heat resistance during low-temperature reflow, as well as for applications requiring tensile strength and corrosion resistance.
Comparison Between Foil Material and Paste Material
Besides paste materials, gold-tin (AuSn) solders are also available in foil form (preform solders). Using foil materials requires remaking molds when there are changes in bonding surface shape or size, incurring additional costs. Paste materials do not require molds, offering flexibility in supply for various sizes and shapes. Paste materials are also compatible with various supply methods like printing, dispensing, and pin transfer, making them versatile solder materials.
Types of Gold-Tin (AuSn) Alloy Paste
Gold-tin (AuSn) alloy paste comes in various compositions, mainly with around 20% tin content like AuSn18 or AuSn22. The powder size of the alloy in the paste varies depending on the intended application, with typical types including:
Printing: 16-53um, 16-32um, <32um, 5-16um
Dispensing: 5-16um, 16-32um
Pin transfer: <11um
Thin-film formation: <11um
Non-Cleaning Gold-Tin (AuSn) Alloy Pastes
Traditional gold-tin (AuSn) alloy pastes using nitrogen gas during reflow required a cleaning process due to residual flux after reflow. Recent developments using glycol reflow processes revealed that high-temperature-resistant rosin and activators, previously needed for oxide removal in traditional pastes, are unnecessary. This dramatic reduction in post-reflow flux residue has led to non-cleaning gold-tin (AuSn) alloy pastes, eliminating the need for cleaning processes.