Metal alloys used to create electrical connections between electronic components and printed circuit boards without the use of lead meet international environmental regulations and safety standards. These lead free solders emerged as the primary material for surface mount technology after the adoption of the restriction of hazardous substances directive in the early twenty first century. Most formulations rely on a combination of tin, silver and copper to achieve the necessary melting point and wetting properties required for high volume manufacturing.
The transition away from traditional tin lead compositions changed the thermal requirements of the assembly process because the new materials require higher temperatures to reach a liquid state. Manufacturers must carefully select the specific alloy based on the heat sensitivity of the components and the long term reliability requirements of the final product. The mechanical properties of these joints differ from their leaded predecessors, often exhibiting higher stiffness and a different sensitivity to thermal cycling.
Combination of different metallic elements in precise proportions determines the melting behavior and the physical characteristics of the bonding material. The most common lead free solders are known as sac alloys, which stand for tin, silver and copper. The percentage of silver in the mix is a major factor in the cost of the solder and the strength of the resulting joints.
Higher silver content generally improves the fatigue resistance of the material but can also lead to the formation of large, brittle intermetallic plates. Other additives like bismuth or nickel are sometimes used to lower the melting point or to refine the grain structure of the alloy. The purity of the metals is also a concern, as even trace amounts of contaminants can affect the flow of the molten solder and the quality of the final bond.
Each alloy has a specific liquidus and solidus temperature, which must be carefully matched to the capabilities of the reflow oven.
Application of heat to melt the alloy and form a metallurgical bond requires a precise control of the temperature over time. During the assembly of a printed circuit board, lead free solders are typically applied in the form of a paste that contains metal powder and chemical flux. The board is then passed through a reflow oven where the temperature is gradually increased to the peak melting point of the alloy, usually around two hundred and forty degrees celsius.
This is significantly higher than the temperatures used for lead based solders, which necessitates the use of components and substrates that can withstand the increased heat. The duration of the liquid phase must be long enough to allow the solder to wet the pads but short enough to prevent excessive growth of the intermetallic layer. Cooling the board at a controlled rate is also important for the formation of a fine grain structure that is resistant to cracking.
Any deviation from the optimal temperature profile can lead to defects such as cold joints or solder balls.
Performance of the electrical connections under mechanical stress and environmental exposure determines the overall life of the electronic device. Lead free solders are known to be more prone to the formation of tin whiskers, which are thin metallic filaments that can grow from the surface of the solder and cause short circuits. They also tend to be less ductile than lead based solders, which makes them more susceptible to failure under mechanical shock or rapid vibration.
Thermal cycling is another major concern, as the mismatch in expansion between the solder and the board can lead to the initiation of fatigue cracks at the corners of the components. To mitigate these risks, engineers conduct extensive testing to qualify the solder for specific applications, such as automotive or aerospace environments. The development of more robust alloys and improved flux formulations continues to be a major area of research in the electronics industry.
The final reliability of the product depends on the successful integration of the alloy, the process and the design of the assembly.
Coupled hydrostatic stress models predict thermal shock vacancy voiding across ternary intermetallic joints before thermal cycling failure occurs.
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