Challenges in the laser microwelding of thin metallic materials

AUTHOR: Dr Jack Gabzdyl, VP of marketing and business development, SPI Lasers

In today’s modern industry, there is a growing demand for the efficient mechanical and/or electrical microjoining of thin metallic materials. There are many areas where material or process compatibility is insufficient for conventional thermal processes, such as welding, brazing and soldering, or the use of adhesives and mechanical fasteners is not desired. This is perhaps most prevalent in the energy storage sector, where the use of thin foils for cathode and anode manufacture is needed for next-generation batteries; a critical component of the burgeoning e-mobility sector. While in the consumer electronics industry, there is a constant push for innovation through high-density packaging and miniaturisation, challenging conventional joining techniques.

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From a laser perspective, there are multiple challenges that make the microwelding of thin metallic materials exceptionally challenging. For successful joins, there is a need to avoid over-penetration, distortion and warping, all of which relate to the need for careful control of the heat input of the process. In conventional laser keyhole welding processes, overcoming material thresholds often requires relatively high average powers, which can be greater for joining bright materials and dissimilar metal combinations. One of the basic dilemmas is whether to use a conduction limited or keyhole process. In conduction limited welding, a broader, less intense heat source tends to produce a higher heat input, so it is often discounted as a solution for thin sections. In keyhole welding, a highly focused, intense heat source minimises the melt and thus helps to control the heat input. The management of parameters for keyhole welding is therefore crucial to achieving a high-quality end result.

One approach that is gaining widespread adoption in joining is the use of nanosecond (ns) pulsed fibre lasers. These short pulse, high-peak intensity lasers are perhaps better known for marking, engraving and other material removal processes, so their use for material joining processes are perhaps counter intuitive. However, the pulse control offered by the master oscillator power amplifier (MOPA) provides exceptional parametric flexibility, enabling processing regimes where the joining of metals is possible. Ns pulsed fibre lasers operate at pulse energies of just a few microjoules to >1 mJ, affording pulse durations in the 10–1,000 ns range, and are capable of peak powers of >10 kW and operation up to 4 MHz, thus significantly differentiating them from conventional lasers such as continuous wave (CW) and even quasi-CW (QCW) long-pulsed lasers, however many also operate in these regimes.

The use of ns microwelding as a joining tool is suitable for a diverse range of applications and joining challenges from thin foils to dissimilar metal combinations. The joining of thin metallic foils (<50 μm) is particularly challenging, since it requires a very delicate balance of energy, sufficient to melt, but insufficient to create significant vaporisation and plasma formation. The foils tend to be joined in a lap weld configuration where intimate contact between them is a requirement for achieving a good result, however this poses a significant fixturing challenge. In the manufacture of today’s batteries, there are requirements for multi-foil stack bonding. The incumbent technology is ultrasonic bonding, but manufacturers have increasingly been looking to lasers to increase productivity, quality and foil stack limits. Lasers offer a multitude of potential solutions, but infrared (IR) ns lasers have proved capable of joining stacks of up to 20 foils in both copper and aluminium using a modest 200 W EP-Z source, however, eliminating porosity in this application is extremely challenging.

The high peak powers of ns pulsed fibre lasers mean they can couple into highly reflective metals, such as copper, with relative ease and with little average power. Studies on the direct bonding of components onto copper printed circuit board (PCB) tracks using the ns microwelding process as an alternative to soldering are showing considerable promise. Copper leads as thick as 150 μm have been successfully bonded onto >60 μm deposited tracks without any visible delamination from the FR4 substrate. This offers alternative options for bonding thermally sensitive components or ones with in-service operating temperatures that may exceed the limits of conventional soldering.

Butt microwelding of thin foils is extremely difficult based on the challenges of fit up. However, this can be achieved using an edge welding technique, whereby two foils are clamped together and the laser is used to cut through them, where the parameters used result in the edges of the upper and lower foils being welded together. A subsequent re-melting pass significantly improves the joint strength and quality to the point where a consistent tensile strength can be achieved.

Joining 10 μm copper to 25 μm aluminium foils has achieved a tensile strength of >2.5 N, and joining 50 μm to 50 μm aluminium foils has achieved a tensile strength of >25 N. (Image 1)

Image 1: Examples of laser microwelding; 25 μm aluminium to 25 μm aluminium (left) and 25 μm aluminium to 10 μm copper (right).

Another major application area is the joining of standard battery cells to form larger packs for anything from power hand tools to vacuum cleaners, through to e-bikes and e-vehicles. The requirements are relatively straightforward, in that a strong reliable weld with high electrical conductivity needs to be generated with no burn through or witness mark on the battery contact.

The range of materials are broad, from pure metals, such as aluminium and copper, to coated materials, such as nickel-plated steel and copper, and these are joined in all conceivable combinations, each bringing its own specific challenges. These contact tabs are typically in the 100–300 μm thickness range, which is well within the capability of the ns microwelding process. (Image 2)

Image 2: Cross section of a nickel platted copper to nickel plated copper spot microweld.

The control of heat input is critical to these joints, where the risk of over-penetration into the cell is significant. The ns microwelding process offers multiple options for joint design in that spots are created by using a spiralling pattern enabled by the
scanner-based beam delivery system. This enables each spot to be tailored to the application, with the diameter and pitch of each joint being key to the specific material combinations and thicknesses being joined, giving greater control of the heat input to each spot. The low average power of these lasers makes achieving high productivity levels challenging, but a 200 W laser can make up to 20 0.8 mm diameter spots/sec, depending on materials and thicknesses, which is sufficient for many applications. (Image 3)

Image 3: Examples of battery spot microwelding of dissimilar metals.

The flexibility of the process means that alternative joint geometries can often be considered, and this is nicely demonstrated by the possibility of using grid patterns to cover large areas at very high speeds. This technique has been shown to be extremely effective at joining a wide range of dissimilar metals with very low heat input. (Image 4)

As technology continues to evolve at an ever-increasing rate, working to ever smaller dimensions presents an ongoing challenge that manufacturing processes need to keep up with. The ns microwelding technique is just one of many fibre laser manufacturing processes that is increasingly used to solve today’s industrial manufacturing challenges, but one that is key to helping enable the current technological revolution.

Image 4: Various material combinations of a ~4 mm diameter grid-patterned geometry, which were microwelded in ~1s.

About the author

Over the past 11 years Dr Jack Gabzdyl has held numerous roles at SPI Lasers with the most recent being VP of Marketing & Business Development. Jack has 33 years of industrial laser materials processing experience and has been widely published with over 60 technical conference papers and articles/publications.

He holds a degree in Metallurgy and Materials Science as well as a  PhD in Laser Material Processing from Imperial College London, focusing on laser cutting.

Through his career Jack has been actively involved in the wider promotion of laser technology having been chair of the UK ‘Make it With Laser’ programme, UK delegate to the IIW and a committee member of AILU. His contribution has recently been recognised by his election as a Fellow of the Laser Institute of America (LIA).

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