John Connolly, engineering manager, Blueacre Technology
Commercial advantage relies on producing high quality micro at an affordable cost. As tolerances on components are moving towards the micro-precise +/-5 μm and even ultra-precise +/-2.5 μm, laser technology is becoming the manufacturing process of choice for design engineers. This article examines some of the latest, high-precision lasering techniques afforded by companies such as Blueacre Technology, a provider of laser systems and contract manufacturing services for medical devices and electronic parts.
What is precision?
Accuracy, precision and repeatability are terms that are often used by engineers, and they are fundamental in determining process capability. As precision engineering becomes more common, designers of micro-devices, or devices containing micron-scale features, have become used to obtaining parts with +/-25 μm features as a matter of routine. To gain commercial advantage, functional requirements are continually expanding, and design engineers are seeking parts with ever tighter tolerances. It is not uncommon to have parts designed with some features in the micro and ultra-precise range. Due to limitations with conventional micromachining techniques, laser technology is fast becoming the process of choice for such devices, enabling these exacting requirements to be achieved.
What are the challenges in achieving precision?
Irrespective of the machining process, whether the tool is a conventional drill bit or a laser beam, positioning of it relative to the workpiece is generally highly accurate and highly repeatable. As an example, the robotics used at Blueacre Technology have a positional accuracy of +/-1 μm and bi-directional repeatability of +/-500 nm (figure 1). These methods of motion control can comfortably be classed as nano-precision, however that is where their benefit ends. Once the tool comes into contact with the material, a number of different factors control the size of the feature machined and whether the repeatability of the feature can move below micro-precision.
Figure 1: A coronary stent undergoing laser cutting.
Every part manufactured at Blueacre Technology starts with using a laser to produce a hole. For the majority of processes, the hole is approximately 20 μm, and larger features are created by overlapping a series of holes in the desired pattern. Irrespective of the pattern, the repeatability of the part is determined by the spread in size of the individual holes, both within the part and between parts in a batch.
Why will the laser hole vary in size?
The main difference between conventional machining and laser machining is that the laser is a non-contact process. It depends on the laser light interacting at an atomic level with the workpiece, breaking chemical bonds and turning solid material into a plasma. This plasma, which consists of an ionised gas, is ejected from the surface of the workpiece and captured by an extraction system.
Therefore, the laser hole is determined firstly by the parameters of the laser beam such as the laser wavelength and power, secondly by how small the laser beam is focussed, and thirdly by how the laser light interacts with the workpiece.
The laser parameters will vary pulse to pulse but given that it takes hundreds of individual laser pulses to drill a hole, the variations average out and should not cause short-term variation. Similarly, no two lenses are exactly the same but within one machine they will not change and can be taken as static. The interaction of the laser beam with the material is where the majority of variation occurs, and it is the controlling of this interaction that makes a process more repeatable.
Why does the laser material interaction vary?
Heat is the main driver of variation in the laser process and minimising the heat input by the laser into the bulk material is of the upmost importance. When light is incident on a material, it is either reflected, absorbed or transmitted (figure 2). The more light that is absorbed, the more efficient the machining process and the lower the cost.
Figure 2: Light incident on a material.
However, absorption of the laser light on its own is not enough; the laser light that is absorbed has to be coupled into the creation of a plasma to eject the material. For most materials, the shorter the wavelength of the laser light, the more absorption occurs and the better the plasma creation. Therefore, UV lasers are generally preferred for micromachining, with the emittance of light at 355 nm being the most common choice.
In addition to wavelength, the shorter the laser pulse, the better the quality of the laser process. This is because the less time a laser pulse interacts with a material, the less time there is for heat to build up and transfer into the bulk of that material. For this reason, laser pulses with nanosecond, picosecond and even femtosecond pulses are employed. The main driver for which pulse width to use is as much down to cost as functionality. Femtosecond lasers offer better quality but do cost more to purchase and run. Therefore, they tend to be used for the production of high-end micro devices seen in the medical and electronics industries.
What can be achieved?
As mentioned previously, a hole forms the building block of any design, therefore the variation on hole diameter must be kept to a minimum in order to increase the repeatability of any part. In order to determine repeatability, Blueacre Technology compared a hole drilled by a nanosecond laser and a femtosecond laser in a nylon tube (figure 3). Both lasers and optics were setup so that the spot size at the focal plane was 25 μm, and a set of 30 holes was drilled with each laser. The femtosecond laser was able to drill a circular hole with minimal thermal load into the bulk nylon material. The nanosecond laser showed signs of melting, leading to a non-uniform hole shape with a raised lip.
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Figure 3: Comparison of a hole drilled by a nanosecond laser (top) and a femtosecond laser (bottom).
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Figure 3: Comparison of a hole drilled by a nanosecond laser (top) and a femtosecond laser (bottom).
To check repeatability, the ranges of the values were measured and are shown in the table. The femtosecond laser has a repeatability of +/-3.5 μm, which is moving into the area of ultra-precision. It should be noted that laser machining a hole in a clear polymer such as nylon is a challenging process. It would be expected that results for metals, which have better thermal dissipation properties, would have a still lower range.
Metal micro components
Micro devices are becoming more common in areas ranging from household products to medical devices. Micro components rarely stand alone and in general require further assembly to form functioning parts. Joining technologies such as welding require close fitting of surfaces, which in turn increases the need for slots and holes machined with high tolerances.
Micro moulding
The production of micro moulds has long been a target application area for laser processing. The ability to produce micron-scale features with +/-5 μm repeatability enables Blueacre Technology to produce moulds for critical applications. One such application is the manufacture of moulds for the production of micro needle drug delivery systems. These systems normally take the form of an array of conical or pyramidical shaped needles that range in height from 50 to 1,000 μm, with a tip that is 1 μm in diameter. Each needle is made from a bio-absorbable material that carries the drug. As they are small, they cause no pain and can deliver large molecule drugs directly into the blood stream, without getting rejected by the liver.
In general, the micro needle arrays, as shown in figure 5, are produced by moulding. The mould can be made from various materials, but soft, low-melt temperature polymer moulds are considered optimum as they can be easily peeled from the micro needle array without causing damage. As with all drug delivery systems, the volume of pharmaceutically active material delivered into the blood stream must be well controlled, which means the moulds must be made to very high tolerances.
Figure 5: An array of needles manufactured using a laser micro mould.
Drug infusion
Drug infusion catheters are medical devices that can be entered into a large vein or artery to deliver drugs directly into the blood stream. To enable the drug to leave the catheter, holes as small as 10 μm are laser drilled through the sidewall. As the amount of drug released depends on the size of the hole, the tolerance in hole diameter is generally <+/-5 μm. As detailed above, by ensuring the right laser conditions are used, these tolerances can be routinely achieved. The process can be made more complicated when multilayer catheters are used.
Multilayer catheters can consist of up to seven layers of different polymer materials, generally with a metal braid sandwiched in between two polymer layers. Due to the different melting temperatures of the various metals and polymers, these structures can prove difficult to machine. Figure 6 shows holes laser machined through such a catheter by Blueacre Technology. As can be seen, the ideal process produces holes with little or no heat affected zone (HAZ) and clean sidewalls.
Figure 6: Laser machined holes in a braided polymer tube.
Summary
The article has described some of the issues around achieving micro and ultra-precision processing with laser technology and how companies such as Blueacre Technology are striving to eliminate these for a range of micro devices. Micro precision of +/-5 μm is readily achievable and ultra-precision of +/-2.5 μm is certainly within reach.
Blueacre Technology