What alloys are generally required for military solder paste

The extreme operating environments of military electronics (e.g., high temperatures, high humidity, strong vibrations, huge temperature differences) place almost exacting demands on the materials used in their assembly - especially solder paste alloys. The core of their selection is always centred around the three principles of reliability, durability and resistance to extreme conditions.

I. In-depth analysis of mainstream alloy system

According to different application scenarios, a clear spectrum of alloys has been formed in the military field.

1. Gold-tin alloy (Au80Sn20): the ultimate solution in the field of high reliability.

As the "king of performance" in the military field, gold-tin eutectic alloys take on the most critical tasks with their unique characteristics.

Ultimate performance: its high melting point of 280°C makes it the only choice for high-temperature environments such as aircraft engine compartments and missile guidance heads. At the same time, it offers excellent thermal conductivity (for heat dissipation in high-power devices), high mechanical strength (resistance to vibration and shock) and innate corrosion resistance (against salt spray).

Process Specificity: Au-Tin solders are usually supplied as preforms and reflowed under vacuum or inert gas protection to achieve a high purity connection without flux, avoiding any risk of corrosion.

Typical applications: infrared focal plane array packages, satellite/airborne high power microwave assemblies, high reliability optoelectronic device coupling.

2. Tin-silver-copper alloys (SAC series): the mainstream pillar of the lead-free era

SAC series has become the mainstay of replacing traditional tin-lead solder, balancing performance, environmental protection and cost.

SAC305 (Sn96.5Ag3.0Cu0.5): As an industry benchmark, its melting point of around 217°C provides a good process window, and its comprehensive mechanical properties and thermal fatigue reliability make it widely used in a variety of high-density packaging (BGA, CSP) scenarios ranging from military computers and communication radios to ground radar systems.

Low-silver formulations (e.g. SAC0307): Optimising cost by reducing silver content, although some properties (e.g. creep resistance, fatigue life) are slightly sacrificed, they still meet the needs of many non-extreme environment military devices, reflecting the art of balancing cost and reliability.

3. Tin-Bismuth Alloy (Sn-Bi): Specialist in Low Temperature Processes

Core value: The ultra-low eutectic melting point of 138°C is its greatest strength, designed for heat-sensitive components (e.g., MEMS sensors, lasers) and step-welding processes. In the latter process, a high-temperature alloy can be used to solder most of the components first, and then the remainder can be soldered with a tin-bismuth alloy, preventing secondary melting of the soldered joints.

Important note: Tin-bismuth solder joints are brittle and have poor resistance to mechanical shocks, so they should not be used in scenarios where there is a high risk of vibration and shock.

4. Tin-antimony alloys (Sn-Sb): Mechanical property strengtheners

Function orientated: The addition of antimony (Sb) is designed to increase the hardness, strength and creep resistance of the alloy. Another key role is to effectively inhibit the growth of Tin Whisker, eliminating the significant risk of failure due to short-circuiting of Tin Whisker in long-term service.

Application Scenario: Ideal for shipboard electronic equipment, military vehicle control systems, and other applications that are subject to constant vibration.

5. Sn-Pb alloys: a traditional solution for limited use

Despite its 183°C melting point, excellent process maturity and fatigue resistance, Sn63Pb37 has been severely restricted by environmental regulations due to its lead toxicity. Currently, it is only used on a limited basis in certain ultra-high reliability aerospace and aviation applications where it is irreplaceable and exempt, and its scope of application is shrinking.

II. selection considerations: a systematic decision-making process

Selection in the military field is never a simple comparison of performance parameters, but a multi-dimensional systematic decision-making:

Environmental adaptability is the first threshold: the working environment of the equipment must be defined first. Select gold-tin or high silver SAC for high temperature; tin-bismuth for heat sensitivity; tin-antimony or gold-tin for high vibration; gold-tin for highly corrosive environments.

Mechanical requirements determine material strength: continuous mechanical stress or shock and vibration environments require prioritisation of alloys for fatigue and creep resistance (Sn-Sb, Au-Sn).

Process compatibility affects manufacturing yields: the package form of the component (fine pitch requires finer powder size), the number of layers and materials of the PCB (heat capacity), and the soldering process (whether or not step soldering is required) all have a direct impact on the choice of alloy and solder paste type.

Strategic balance of cost and reliability: This is the key to the final decision. On critical systems such as missile guidance and spacecraft, no cost is spared and only the highest reliability is sought (gold tin); on common ground communication equipment or logistic equipment, cost can be optimised under the premise of meeting the basic military specification requirements (low silver SAC).

III. Future trends: innovation and customisation

Complete implementation of lead-free: As the industry matures and performance improves, lead-free alloys (SAC, AuSn, etc.) will fully replace leaded solder, even in the traditional "exempted" areas.

Research and development of high-performance multi-alloys: it is difficult to meet all the needs of a single alloy system, and the future focus will be on the development of complex alloys such as quintuplets and hexaplets (e.g. Sn-Ag-Cu-Bi-Ni), with micro-alloying technology to finely control the melting point, microstructure and reliability.

Customised solutions for extreme environments: For specific scenarios such as deep sea, space and hypersonic vehicles, there will be more special alloy formulations and a full set of process solutions (including special fluxes, welding processes and testing standards) to match them.

In summary: Selection is a sophisticated process of starting from application scenarios, deducing material requirements in reverse, and ultimately finding the best balance between performance, process and cost. The four systems of gold-tin, SAC, tin-bismuth and tin-antimony constitute the core material matrix of current military manufacturing, supporting the electronic spine of modern defence equipment.