Gold-tin solder and its application in the field of electronic device packaging

Gold-tin solder and its application in the field of electronic device packaging

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Soldering is an important technology for assembling electronic products. In order to obtain an ideal solder connection, the selection of solder is crucial. The solderability, melting point, strength, Young's modulus, thermal expansion coefficient, thermal fatigue, creep and creep resistance of the solder can all affect the quality of the solder connection.

Eutectic gold 80% tin 20% solder alloy (melting point 280℃) has been used in semiconductor and other industries for many years. Due to its excellent physical properties, gold-tin alloy has gradually become the best solder material for optoelectronic device packaging.

2 Physical properties of Au80%Sn20% solder

Some basic physical properties of Au80%Sn20% gold-tin solder.

(1) Moderate soldering temperature

The soldering temperature is only 20-30℃ higher than its melting point (i.e. about 300-310℃). During the brazing process, based on the eutectic composition of the alloy, a small superheat can make the alloy melt and wet; in addition, the solidification process of the alloy is also very fast. Therefore, the use of gold-tin alloy can greatly shorten the entire brazing process cycle. The brazing temperature range of gold-tin alloy is suitable for the assembly of components with high stability requirements. At the same time, these components can also withstand the subsequent assembly using lead-free solder at a relatively low temperature. The assembly temperature of these solders is about 260℃.

(2) High strength

At room temperature, the yield strength of gold-tin alloy is very high. Even at a temperature of 250-260℃, its strength can meet the requirements of airtightness. The strength of the material is comparable to that of some high-temperature brazing materials, but the brazing process can be completed at a much lower temperature.

(3) No need for flux

Since gold accounts for a large proportion of the alloy composition (80%), the degree of oxidation on the surface of the material is low. If a vacuum or a reducing gas such as a mixture of nitrogen and hydrogen is used during the brazing process, there is no need to use chemical flux.

(4) Good wettability and no corrosion of lead-free tin solder on gold-plated layer

The composition of gold-tin alloy is similar to that of gold-plated layer, so the degree of dissolution of very thin coating by diffusion is very low, and there is no migration phenomenon like silver.

(5) Low viscosity

The liquid gold-tin alloy has very low viscosity, so it can fill some large gaps.

In addition, Au80%Sn20% solder also has high corrosion resistance, high creep resistance and good thermal and electrical conductivity. The disadvantages of Au80%Sn20% solder are that it is expensive, brittle, has low elongation and is not easy to process.

3 Thermodynamic properties

Since the thermodynamic properties of gold-tin alloy determine many of its performance, it is necessary to understand some basic thermodynamic properties of the alloy. Gold-tin can form a eutectic alloy at a composition ratio of 80 wt% gold and 20 wt% tin.

The eutectic reaction of gold-tin alloy at 280℃ is liquid phase L<-->ζ+δ. Near this reaction of the alloy, it mainly includes ζ'' (Au5Sn) phase, ζ phase and δ (AuSn) phase. In the ζ'' phase, the weight percentage of tin is 10.7%. It has a hexagonal structure and is a stable phase below 190℃. The ζ phase is formed by the peritectic reaction β+L<-->ζ. At 521℃, the weight percentage of tin in the ζ phase is 5.7%; at 280℃, this percentage is 11.3%; and at 190℃, this percentage is 8.8%. The ζ phase has a magnesium-type hexagonal close-packed structure. Phase ζ is an intermetallic compound with a melting point of 419.3℃ and a NiAs-type hexagonal structure. The composition of the δ phase can fluctuate within a certain range, with the atomic percentage of tin ranging from 50.0% to 50.5% (weight percentage of 37.5% to 37.9%).

4 Gold-Tin Solder Preforms

There are many forms of solder that can be used for microelectronic packaging, the most important of which are wire, sheet, solder paste and preforms. Due to the brittle nature of gold-tin alloy, these forms of wire or sheet are difficult to process and shape according to specifications. In the process of processing, there is often a waste of materials, a lot of labor is required, and the quality is also inconsistent. Among all these forms, solder paste is the most ideal form for electronic packaging. However, one of the ingredients of solder paste is flux, which is prohibited in many applications. Even if flux can be used, the assembled components must be cleaned of their residues after the soldering process is completed. Therefore, in order to obtain stability in applications such as device production and packaging, the right choice should be stamped preforms. Preforms can ensure the precise amount and accurate position of solder to achieve the best quality at the lowest cost. In the 1960s, preforms were first used to produce some components such as metal-encapsulated tantalum capacitors. Now it is mainly used in the production and packaging of some passive components and optoelectronic devices. Preforms have the following advantages:

① By adopting the preforming method, the amount, composition and surface state of the solder can be accurately controlled, thereby providing a larger brazing process window and the best assembly quality to improve the reliability of the brazing connection, which is the high Cpk value and low cost under the condition of quality assurance usually required by the industry.

② Using preforms in a controlled atmosphere can eliminate the use of flux that is easy to pollute and difficult to control. By controlling the brazing process, the costly cleaning process after welding can also be eliminated.

③ Preforms are usually the best solution for high-performance welding that requires high reliability and good thermal conductivity.

④ There are almost no restrictions on Jinxi solder preforms for changes in the substrate materials that need to be connected and special performance or environmental protection requirements.

⑤ After proper design and application, preforms can achieve a higher performance-price ratio, making the solder joints have a high yield and electrical reliability.

5 Application of AuSn solder sheet

Since the melting point of AuSn eutectic solder (280℃) is much higher than that of Sn96.5％Ag3.5％SnAg eutectic solder (221℃), it cannot be used with organic materials widely used in electronic packaging at the same temperature. However, AuSn solder is the best choice for some special applications that require good mechanical and thermal conductivity to achieve high reliability. These applications include airtight sealing cover, RF and DC isolation bonding in optoelectronics|laser device packaging process, laser diode die bonding, etc.

Part of the airtight electronic packaging product needs to be welded to some ceramic parts. In this case, the main consideration is that ceramics have some physical properties that metal parts cannot achieve, such as low thermal expansion coefficient, electrical insulation, high strength, etc. One of the applications is to bond the active device chip to a housing when both the active device chip and the substrate need to have a low thermal expansion coefficient (see Figure 2). In this case, a stamped AuSn preform with a thickness of about 25μm can be used as solder. Another application in ceramic packaging is capping, which is sealing a metal or glass cover to a ceramic housing (see Figure 2). In this case, a 25μm thick gold-tin preform in the shape of an open frame is also selected. For the capping, a parallel seam welder is generally used to locally heat the connection. This will not affect the active device chip brazed with gold-tin alloy in the package. The third application related to ceramics is the welding of lead insulators (see Figure 3). In this case, high-strength ceramics are used instead of glass as insulators. For these applications, the brazing material is required to have good wettability, corrosion resistance and high Young's modulus. High Young's modulus ensures that the material can be processed to a very thin layer while still maintaining flatness over a large area. As mentioned earlier, the surface of the gold-tin preform is clean and free of oxides, allowing the use of a production process without flux. Even if the substrate surface is slightly oxidized, a nitrogen-hydrogen mixture can be used to remove the oxides. After the oxides are removed, the gold-tin preform can be heated and melted to start the welding process. Usually, it is very effective to use nitrogen-hydrogen mixed gas to remove oxides above 235℃. However, if a low melting point solder is used (for example, below 235℃, at which the nitrogen-hydrogen mixed gas has not yet taken effect, the oxides on the surface of the substrate will exist at the welding point. This is one of the main reasons for the poor quality of some welding. Devices soldered with gold-tin alloys can withstand long-term thermal stress cycles.

In the through-bonding of optoelectronic devices such as transmitters, receivers and amplifiers, gasket-type gold-tin preforms are the best choice. During the connection process, the molten gold-tin gasket will fill the gap between the outer conductor of the insulator and the package substrate (both made of Kovar alloy and plated with nickel and gold) under capillary action, as shown in Figure 3. Since the gap between the insulator and the substrate is very small, too much solder will cause a short circuit. As one of the advantages of the preform, a precise amount of gold-tin solder can be made into a gasket-type preform to prevent short circuits. Road.

The use of gold-tin preforms when assembling high-power laser diodes (LDs) using die bonding technology has been accepted by more and more manufacturers. As shown in Figure 4, the laser diode chip and the copper heat sink are welded by gold-tin preforms. Since the luminous efficiency of the laser diode drops sharply with the increase of temperature, it is very important to dissipate the heat generated by the diode in time when it is emitting light. The excellent thermal conductivity of gold-tin solder can play a very effective role here, ensuring the best performance of the laser diode. In addition, because the Young's modulus of gold-tin alloy is high, it can maintain flatness and certain bending resistance even in very thin (5-25μm) conditions. Therefore, the possibility of fire and pores in the solder layer during the welding process is greatly reduced, which reduces the thermal resistance of the solder joint, thereby greatly improving the reliability of the laser diode. Gold-tin alloy is also used for flip-chip welding. In flip-chip welding, the excellent thermal conductivity and electrical conductivity of gold-tin alloy are particularly important because the active area of the device is connected to the substrate. In addition, gold-tin alloy preforms are also used in microwave system assembly and other fields. As the excellent properties of gold-tin alloy and the advantages of its preforms are increasingly recognized, its application in the packaging field will become more extensive and important.

6 Precautions for using gold 80% tin 20% solder

Gold-tin solderand must be used correctly to achieve good results. The main factors affecting welding quality are: gold-tin solder composition, surface quality of weldment and solder (such as oxides, contamination, flatness, etc.), process factors (furnace temperature wire, maximum temperature, gas composition, fixture, etc.) [2]. As shown in Figure 1, the melting point of gold-tin alloy is very sensitive to composition near the eutectic temperature. When the weight ratio of gold is greater than 80%, the melting point increases sharply with the increase of gold. The welded parts often have a gold-plated layer, and the gold in the gold-plated layer will immerse into the solder during the welding process. In the case of too thick gold-plated layer, too thin preformed solder sheet, and too long welding time, the amount of gold leached into the solder will increase, causing the melting point to rise. Therefore, all the above welding parameters need to be optimized [2]. Usually, the furnace peak temperature should be selected at about 350℃, the welding time is 2~4 minutes, and the welding yield can be above 98%.

Because the melting point (280℃) of gold 80% tin 20% eutectic alloy is moderate, it has high strength, no need for flux, good thermal and electrical conductivity, excellent wettability, low viscosity, easy welding, corrosion resistance, creep resistance, etc., it is widely used in ceramic packaging covers of microelectronic and optoelectronic devices, chip bonding, metal-packaged ceramic insulator welding, and chip welding of high-power semiconductor lasers. It can significantly improve the packaging reliability and electrical/thermal conductivity of these devices.

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