The performance of lead-free solder balls in low-temperature soldering processes depends largely on the compatibility of their alloy components with low-temperature environments. Traditional lead-free solder balls rely on components such as tin, silver, and copper. The melting points of these alloys are often high. Under low-temperature conditions, the melting process becomes slow and incomplete, and inadequate soldering is prone to occur. To meet low-temperature requirements, specially formulated lead-free solder balls introduce elements such as bismuth and indium. These elements can effectively reduce the melting point of the alloy, allowing it to melt smoothly at lower temperatures, providing basic conditions for low-temperature soldering.
In a low-temperature environment, the fluidity of lead-free solder balls is the key to affecting soldering quality. Lowering the temperature will increase the viscosity of the molten solder ball, slow down the flow rate, and make it difficult to fully infiltrate the soldering area. Especially on plastic substrates or thin metal foils commonly used in flexible electronic devices, if the fluidity is insufficient, the solder ball cannot be spread evenly, and it is easy to form local accumulation or voids, resulting in a decrease in the bonding strength of the solder joints. However, by adjusting the proportion of each element in the alloy, the fluidity can be improved to a certain extent. For example, increasing the indium content can reduce the viscosity of the molten state and help the solder ball better fill the solder gap at low temperatures.
Wettability is another important indicator for measuring the low-temperature soldering performance of lead-free solder balls. Under low-temperature conditions, the oxide film on the metal surface is more difficult to break, and the wetting effect between the solder ball and the substrate will be affected. If the wettability is poor, there will be a small gap between the solder ball and the substrate after solidification, which will not only reduce the conductivity, but also make the solder joints susceptible to external environmental erosion in subsequent use. To address this problem, the surface treatment process of the solder ball can be optimized, such as pre-coating a thin layer of flux to help remove the oxide film, improve the wettability at low temperatures, and ensure the tight bonding of the solder joints.
Flexible electronic devices have special requirements for the flexibility and fatigue resistance of solder joints, which are closely related to the low-temperature soldering performance of lead-free solder balls. Flexible devices will experience repeated bending and stretching during use, and the solder joints need to have a certain degree of elasticity to buffer deformation stress. The solder joints formed by low-temperature welding have relatively fine internal grain structures and good toughness, and are more adaptable to deformation than solder joints formed by high-temperature welding. However, it should be noted that if the soldering temperature is too low and the solder ball is not completely melted, microcracks may exist inside the solder joint, which will reduce its fatigue resistance and affect the service life of the device.
The anti-oxidation ability of lead-free solder balls during low-temperature soldering should not be ignored. In a low-temperature environment, the protective atmosphere (such as nitrogen) during the soldering process will be weakened, and the solder ball will be more easily oxidized when it comes into contact with air after melting, forming an oxide layer on the surface of the solder joint. This oxide layer will hinder the conduction of current and may gradually expand during subsequent use, causing the solder joint to fail. By forming a local sealed space in the soldering area and reducing air contact, the oxidation reaction can be effectively inhibited to ensure the stability of the electrical properties of the solder joint after low-temperature soldering.
The diverse substrate materials of flexible electronic devices pose a challenge to the compatibility of lead-free solder balls. For example, flexible substrates such as polyimide have poor heat resistance, and high-temperature soldering will cause deformation or aging of the substrate, while low-temperature soldering can avoid this problem. The soldering process of lead-free solder balls at low temperatures causes less thermal damage to the substrate and can maintain the original mechanical and electrical properties of the substrate. However, the metal coatings (such as copper and nickel) on the surfaces of different substrates have different reactivity with lead-free solder balls. Incomplete reactions may occur at low temperatures, and it is necessary to optimize the interface bonding by selecting matching coating materials.
Specially formulated lead-free solder balls can show performance that meets the requirements in low-temperature soldering processes. Through reasonable component design, it reduces the melting point, improves fluidity and wettability, and takes into account the flexibility and oxidation resistance of solder joints, which can meet the requirements of flexible electronic devices for welding temperature, bonding strength and reliability. Of course, the effect of low-temperature soldering is still affected by process details, such as the control of parameters such as welding time and protective atmosphere. It needs to be optimized in combination with the characteristics of specific devices to give full play to the role of lead-free solder balls in flexible electronic welding.
