In modern industry, there is an ever-increasing need for efficient processing and/or electrical micro-connection of thin metallic materials. In many fields, the compatibility of materials or processes is insufficient for conventional heat treatments such as welding, brazing and soldering, or the use of adhesives and mechanical fasteners is not desired. This situation is likely to be very common in the energy storage industry, as next-generation batteries, a key component of the emerging power battery industry, require the use of thin foils to make cathodes and anodes. In the consumer electronics industry, high-density packaging and miniaturization continue to drive innovation and challenge traditional connectivity technologies.
From a laser perspective, there are a number of challenges that make microwelding of thin metallic materials extremely difficult. Successful welding requires avoiding weld perforations, distortion, and bending, all goals that require careful control of the heat input to the process. Overcoming material thresholds often requires high average power in conventional laser penetration welding processes. The average power required for welding of highly reflective materials and dissimilar metals may be higher, and one of the fundamental challenges is whether to use a thermal conduction welding process or a deep penetration welding process. Wider, weaker heat sources tend to produce higher heat input and heat affected zones when conducting heat conduction welding, so they are generally not recommended as a solution to sheet metal welding problems. In deep penetration welding, a highly concentrated, high-intensity heat source minimizes the weld pool, helping to control heat input. Therefore, tuning of deep penetration welding parameters is crucial to obtain high-quality results.
One method that is widely used in welding is the use of nanosecond (ns) pulsed fiber lasers. These short-pulse, high-peak-intensity lasers may be better suited for marking, engraving, and other material removal processes, so intuitively, they might work the opposite way when used in material welding processes. But the pulse control provided by the Master Oscillator Power Amplifier (MOPA) has excellent parameter flexibility, enabling the possible processing of metal bonding. Nanosecond pulsed fiber lasers operate at pulse energies of a few microjoules to >1mJ, pulse durations ranging from 10-1000ns, and can reach peak powers of >10kW, operating at frequencies as high as 4MHz, which are significantly different from continuous wave (CW). ) and even quasi-CW (QCW) long-pulse lasers, but many still operate in these ranges.
Using nanosecond micro-welding as a welding tool is suitable for a variety of applications and for overcoming welding challenges from foils to dissimilar metals. Bonding of thin metal foils (<50 μm) is particularly challenging because it requires a very delicate energy balance that is sufficient to melt the metal, but not produce significant vaporization and plasma. Foils are easy to weld using lap joints, a process in which close contact between the foils is necessary for good results, but this presents a significant challenge for fixtures. Today's battery production process has many stringent requirements for multi-layer foil stack welding. The existing technology is ultrasonic welding, but manufacturers are increasingly looking to use laser welding to increase production efficiency, quality and improve foil stack limitations. Lasers offer many potential solutions, but infrared (IR) nanosecond lasers have demonstrated the ability to weld up to 20+ layers of copper or aluminum foil using a 200W EP-Z laser, but eliminating porosity in this application is highly challenging.
The higher peak power of nanosecond pulsed fiber lasers means that highly anti-metallic metals such as copper can be accessed more easily with small average powers. Research into directly attaching components to copper printed circuit board (PCB) tracks using the nanosecond microsoldering process as an alternative to soldering has shown great promise. So far, copper leads up to 150 μm thick have been successfully attached to >60 μm deposition tracks without any significant delamination from the FR4 substrate. This provides an alternative to the bonding of heat-sensitive components or components whose operating temperatures may exceed the limits of traditional soldering.
Butt welding of foils is difficult due to the challenges of lamination. But this can be achieved by edge welding technology, where two foils are clamped together, cut with a laser, and the edges of the upper and lower foils are welded together by the parameters used. The subsequent remelting process can significantly increase the strength and quality of the joint, resulting in consistent tensile strength. Welding a 10 μm copper foil to a 25 μm aluminum foil yielded a tensile strength >2.5N, while welding a 50 μm aluminum foil to a 50 μm aluminum foil yielded a tensile strength >25N.
Another major application area is welding standard cells together to form larger battery packs for devices such as power tools, vacuum cleaners, e-bikes and electric vehicles. The requirement is straightforward to produce welds with high conductivity, high strength, and high reliability without burning through or leaving traces on the battery contacts. Materials range from pure metals such as aluminum and copper to coated materials such as nickel-plated steel and nickel-plated copper, which can be joined in all conceivable combinations, each presenting unique challenges. The thickness of these contact joints is typically in the 100-300 μm range, well within the capabilities of nanosecond microsoldering processes.
Control of heat input is critical for these welds because of the high risk of weld perforation in batteries. The nanosecond micro-welding process offers a variety of options for weld design, as the weld can be obtained in a spiral welding pattern using a galvo beam delivery system. This allows each weld to be tailored to the application, making the diameter and spacing of each weld key to welding specific material combinations and thicknesses, resulting in better control of the heat input to each weld.
The low average power of these lasers makes it difficult to achieve high productivity, but a 200W laser can weld up to 20 0.8mm diameter solder joints per second (depending on material and thickness), which is fast enough requirements for most applications.
