Laser micromanufacturing is just like traditional manufacturing, except that it uses photons (light) instead of drill bits and saw blades. Lasers are used for machining, marking, welding and surface treatment. It is, however, important to define micromanufacturing as, perhaps simply, the process involving lasers for material removal, addition or alteration. Furthermore, the feature sizes on target are less than 1mm (usually much less) and the material thickness is also less than 1mm (again, usually much less).
LASER (light amplified stimulated emission of radiation) light has some unique properties that make it attractive for machining purposes. So why use lasers for materials processing? Firstly, they are non-contact, meaning that there is less chance of damage to the part and no tool wear. Secondly, they can be selective in the material-removal or joining process. By choosing the right laser wavelength and energy density on target, it can, in many cases, remove one material selectively over another or weld dissimilar materials together. A third reason is that lasers can be very flexible.
There are frequently situations where the high-volume manufacturing of a part may be more affordable using custom hard tooling, but this is expensive and cannot usually be done for prototypes. Lasers make exceptional prototyping devices because of this flexibility. They can also frequently replace other technologies that have their own inherent disadvantages.
Seeing the light
There are a lot of choices concerning which laser to use. In general, IR lasers are red lasers and material removal is by a first-order thermal mechanism. UV lasers, on the other hand, can have enough photon energy to break chemical bonds without heating the material, and the first-order removal mechanism is via a non-thermal or ‘cold’ process.
For UV lasers, longer pulse lengths can increase the thermal component of the laser processing. So, for material removal, it is recommended to work with the shortest pulses possible, given cost and reliability requirements. Pulse lengths over 1ns (10-9 seconds) tend to show thermal side effects, whereas pulse lengths below 1ns may not, depending on the material, for example. In any case, the key to clean and low-taper processing seems to be peak power intensity (PPI).
High PPI is achieved by any combination of short pulse length, high energy per pulse and focusing to a small spot size. Fortunately, new lasers have recently been commercialised that go down into the picosecond (10-12 seconds) and even femtosecond (10-15 seconds) pulse-length regime, which turns out to be extremely valuable for micromachining applications.
For joining and deposition, longer wavelengths and longer pulse lengths or continuous wave (CW) are used because the purpose is to precisely deliver heat to a certain point. Longer wavelength lasers inherently introduce heat into the system by exciting the vibration-rotation bonds.
The most important thing in material-removal applications is that there is strong absorption of the incident photons. At least 50% absorption is needed and the closer to 100% the better. Absorption depth is a function of the material, the incident energy density and the laser wavelength. As a general rule, UV photons are absorbed within fractions of microns of the material surface whereas IR photons have a penetration depth to the order of tens of microns or more per pulse. All things being equal, this means that UV photons are capable of higher precision and controlled ablation, but are slower than IR lasers.
Market applications
Because of the wide range of available lasers with differing pulse lengths, wavelengths and power outputs, almost any material can be a candidate for laser materials processing as long as the thickness and absorption are within the boundary conditions. In addition, these laser tools are used in a variety of markets including medical devices (usually disposable), microelectronics, defence, semiconductors and alternative energy.
Currently, about 50% of the disposable medical products used in the world are made in the US, and about half this capacity is contracted out to smaller vendors. This makes laser contract manufacturing of medical devices, especially disposable devices, a lucrative marketplace.
There are some barriers to getting fully immersed in this marketplace, such as long lead times between concepts and regulatory approval, the small percentage of ideas that actually make it to manufacturing, and the tedious paperwork and certification process. However, on the positive side, the volumes of an established product can be high, the profit can be good and the customers can be locked into one or a few vendors – as long as those chosen vendors deliver quality products on time and at a competitive price. Requirements and trends in the medical device industry include miniaturisation, accuracy, repeatability, traceability, minimal impact on material without post-processing, cost and compliance.
Laser cutting can be done on a variety of materials with a variety of lasers, but perhaps one of the highest revenue-generators is the cutting of stents for cardiovascular applications. Metal stents are cut using a fixed-beam laser or a USP laser for bio-absorbable stents. Drilling of holes for drug delivery, liquid flow, gas (air) flow and plating operations are also quite common. Hole diameters in the micron range can be drilled using high-precision UV lasers. Insulin inhalers and microfilters are also two large applications areas.
Perhaps the biggest application area is laser marking. These marks usually add no functionality to the part (except in some cases, for instance where graduations are needed on a catheter), but they do add cost. Marking is mandated and is part of the traceability needed for this generation of devices. It is also useful in anticounterfeiting.
Laser deposition and joining are two other growing areas. Lasers can be used to weld similar and dissimilar materials, and even plastics. Since welds are harder to inspect and are made after there is already a lot of value in the manufacturing process, this tends to be done in-house rather than shopped out to a contract manufacturer.
Cost-effective
There are many laser contract manufacturers possessing the equipment and expertise to do precision laser manufacturing, just as there are a number of systems integrators that will build a laser system for in-house production environments; some companies provide services and systems. Contract manufacturers are an ideal resource for small runs and prototypes and can, in some cases, be more cost-effective (even in high-volume manufacturing) than large companies, because of lower overheads. There is no need for a large capital expense, no equipment to maintain and no need to hire specialised laser operators and engineers.
Typically, there is some recurring set-up cost incurred whenever a particular job is run and there may be NRE (non-recurring engineering) if special tooling is needed. Piece pricing is dictated by the laser, complexity, number of parts per run, and handling and shipping requirements. Hourly rates vary from about $100 an hour for marking jobs to $500 an hour for high-power femtosecond lasers or other more ‘exotic’ laser choices. For medical devices, the laser machining probably needs to take place in a clean room or at least a controlled environment, and this may affect cost too.
In-house systems minimise or eliminate shipping and handling issues, can be more cost-effective long-term and can allow on-site quality control and retention of process control. This is accompanied by a large capital expense and the need to make sure that the appropriate facilities and personnel are available so that the laser system works in the best manner possible.
In summary, lasers provide valuable and unique opportunities for high-precision materials processing, especially in the medical device industry. Lasers, however, are just ‘fancy light bulbs’ that need the addition of a variety of other hardware and underlying expertise in order to make them suitable for manufacturing environments. Systems integrators can provide in-house laser solutions, while contract manufacturers provide a valuable resource for prototyping, R&D and high-volume production. As parts get smaller and smaller, lasers will continue to play an increasing role in the manufacturing of next-generation products and devices.
