Excimer lasers drill small holes
in plastic packages and metal containersIntroduction:
Improved leak test equipment sensitivity, increased market competition and potential legal fallouts are three reasons why calibrated leak holes in packages are getting smaller. Pharmaceutical, medical device and beverage packaging companies guarantee package sterility and integrity by asking their respective quality groups to perform destructive and non-destructive tests on their packages.
Excimer lasers are used to drill small holes in various types of plastic and metal packages such as metal containers from the beverage industry, foil packages from the medical device industry or blister packs and plastic ampoules from the pharmaceutical industry. All three industries have different requirements; yet all share a converging leak test requirement. All three industries are demanding that their packages are drilled with smaller holes, with capabilities approaching hole diameters as small as 5 micron with diameter tolerances of +/- 1 micron.Excimer Laser Hole Drilling Technology:
Traditionally, calibrated leak holes can be introduced into packages by mechanical means such as needles, punches or drills. However; as the hole diameter decreases there is a proportional reduction in diameter tolerance. Excimer laser technology provides a cost effective solution.
Why excimer lasers?
An excimer laser is an ultra-violet light source. It produces a multi-mode beam of light. The laser wavelength is typically 193nm or 248nm (0.00001") with a maximum power average of 80 Watts and a repetition rate up to 200 pulses per second. The short wavelength enables the laser to drill such small holes. The optical resolution of the drilling system is proportional to the wavelength. As a general rule, the theoretical optical resolution is twice the laser wavelength. From a practical point of view, the excimer laser can drill holes approaching 1 micron in diameter since the laser wavelength is four times smaller. Compared with solid-state laser sources, the excimer laser is especially suited for calibrated hole drilling in packages because its wavelength is four times shorter than the Nd:YAG laser.
The method for controlling an excimer laser beam makes it suitable to drill precise calibrated leak holes in packages. Due to the laser's mode structure and its resulting spatial distribution, a technique known as mask projection is used to drill calibrated leak orifices. The laser beam illuminates a non-contact mask that contains a hole whose diameter is ten to thirty times larger than the required hole diameter of the part. Downstream from the mask is a special lens that "images" the hole. The rays of the laser beam are directed by this lens into the package. This technique produces consistent, repeatable leak hole diameters with tolerances approaching +/- 1 micron. This is because there is a true "optical image" of the calibrated leak orifice at the package surface.
If a Nd:YAG laser is deployed, with its four times larger wavelength, then the laser beam will pierce the package wall. This piercing effect will not produce as repeatable or as accurate results as the excimer laser because the beam is not imaged on to the package work piece; instead it is focused to a spot with a highly divergent light rays after the focal point.What does this mean?
Think of the excimer laser as a circular shaft of light whose diameter matches the calibrated leak orifice diameter precisely. The laser beam will drill the hole uniformly as the beam etches its way through the package wall. The Nd:YAG laser has a Gaussian beam (hotter in the center that the surrounding edges), whose drilling process is characterized by piercing the center of the leak hole first and then opening up the hole. If the hole diameter is larger than the beam spot size (typically 25 micron spot size), then the hole is formed by trepanning or moving the laser spot along the circumference of the calibrated leak hole. On the other hand, the large size of the excimer laser beam allows the entire hole to be drilled at one time, providing a smoother replication of the desired hole shape.
Superior hole edge quality is another reason why excimer lasers are used to drill calibrated leak holes. The short wavelength allows materials such as plastics to be "ablated" or vaporized when exposed to the excimer laser. This ablation phenomenon transforms the package material from the solid state to the gas state, creating smooth, uninterrupted edges around the calibrated leak hole. Other methods, such as mechanical or longer wavelength laser sources will leave burrs or will "heat" the material surrounding the hole, leading to inconsistent results.Leak Test Equipment
Leak test equipment manufacturers have kept up with the market's needs by improving such methods as force and pressure decay, helium leak testing, mass flow test and acoustic micro imaging, among others.
Pressure decay technique1. is a differential method involving the measurement of the pressure of an inflated test part. After a certain time, the pressure is re-measured to calculate the leak rate.
Mass flow leak detector1. is a single point measurement technique that measures the leak rate by measuring the inlet air flow required to keep the test part at equal pressure.
Helium leak testing or helium mass spectrometry2. is the most sensitive leak testing method, capable of detecting a leak rate of 5 x 10-12 atm cm3/sec, corresponding to a hole diameter of 100 angstroms (.01 micron)!
Acoustic micro imaging equipment (AMI) is a non-destructive method that uses ultrasound to detects leak hole diameters as small as 5 microns.Medical Device Packaging:
In the medical packaging industry, microbial ingress challenge testing3. is used to test the package sterility by spraying microbes on to the package or by submerging into a microbe bath. By using excimer lasers to drill different hole diameters, the correlation between calibrated leak orifice diameter and sterility can be studied. Alternatively, medical device packaging can be tested by physical means such as mass flow, pressure decay or helium testing, among others.
Examples of medical devices that require calibrated leak orifices include catheters, IV sets, bags, check valves and canisters.Pharmaceutical Packaging:
Microbial ingress protection and consumer security are two reasons why pharmaceutical packages require excimer laser-drilled leak holes.
Excimer laser-drilled leak orifices are used to validate and calibrate filled pouches, blister packages, filled and sealed bottles, vials and plastic ampoules as well as TYVEK trays and pouches. In the case of plastic ampoules, excimer lasers at a wavelength of 193nm and 248nm are used to produce hole diameters ranging from 5 to 10.Beverage Containers:
Metal containers contain lids and bodies that together, form the package such as metal cans.
Excimer lasers are used to drill hole diameters in metal container lids ranging from flow rates of .08 cc/per minute to 3.0 cc/per minute, corresponding to leak rate orifice diameters of 10 microns to 45 microns.
Summary:
Computer chips are getting faster and smaller in today's ever-complicated world. The packaging industry is moving in the same direction, requiring smaller calibrated leak orifices that are detected by more sensitive leak test equipment. There may some debate in the packaging industry on how sensitive the leak test equipment must. (ie; correlation between microbial ingress challenge testing and the calibrated leak hole diameter). However it is clear that laser drilling, especially excimer laser drilling, is an ideal method to produce calibrated leak holes diameters as small as 1 micron. Equally important, the technology produces repeatable and consistent leak orifices, relying upon its shorter laser wavelength, mask projection imaging technique and unique "ablation" or vaporization process.
References:
1. Testing Medical Device and Packaging Integrity, Micheal and Stephen Franks, T.M. Electronics, 2006
2. Packaging Testing: A Touchy Subject, William Leventon, Medical Devicelink, Jan 9, 20023. Validation of Sterile Package Integrity-Overview for Medical Devices, Marvyn J. Frederick, NV Organon, PMP 2000 speaker
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