where d is the diameter of the beam entering the focus lens, and f is the focal length of the focusing lens.
A Gaussian beam is preferred for drilling uniform holes. However, the laser beam is in a transient mode for a period of time after the laser beam is switched on. The transient mode is characterised by variations of all the radiation parameters. The range of the variations and the duration of the transient mode depend on the pump energy, repetition rate, thermophysical properties of the active medium, cooling rate, and the cavity design. At the end of the transient mode, the pulse energy can increase by a factor of 5 -10 compared with the energy of the first pulse [6]. Concurrently, the pulse duration can also increase by a factor of 5 -10, while the divergence by a factor of 2-3 [6]. Obviously, identical holes could only be obtained after the transient period. The most important factor affecting the hole-size scatter is the instability of the laser-pulse parameters. For Nd:YAG laser, the energy instability is around 5% [6].
The best drilling results are obtained only when there is a proper combination of pulse energy, pulse duration, and pulse frequency. From equation (1), the overall energy , Eov, can be calculated by
The pulse duration, D, is defined as the subtraction of the times at which the laser power intensities are 50% of the maximum power intensity. The peak power is determined by the following equation [11] :
(9)
For high pulse frequency lasers, the peak power is usually expressed in terms of average power:
(10)
where P is the average power, and Q is the pulse frequency .
A high peak power is always desired in a drilling process for fast vaporisation. As shown in the equation (9), the peak power is determined by the pulse energy and the pulse duration. Shorter pulse duration leads to smaller heat-affected-zones, and thus better hole quality. However, it should be noted that the high pulse energy is usually obtained at high order beam modes, which produce large divergence angles. In the case of very fine drilling, this situation is undesired except for mask projection drilling.
3.5 Materials properties
Material properties are critical in evaluating the possibility of laser drilling. The most significant properties are as follows:
1) Properties that affect the beam absorption: the surface reflectivity at a given laser beam wavelength, and the absorption coefficient.
2) Properties that affect the heat flow: thermal conductivity and diffusitivity.
3) Properties that relate the amount of energy required to cause a desired phase change in the form of melting or vaporisation. These include density, specific heat and latent heat.
Two major types of materials are commonly used in PCB manufacturing: laminated dielectrics and deposited dielectrics. In laminated dielectric materials, non-woven glass reinforced materials such as aramid and polyimide provide good laser machineability [12], while the conventional woven glass reinforced materials such as FR-4 are not drilled well with CO2 lasers [2-3,13]. In deposited dielectric materials, photo-sensitive polymers could be the most desirable dielectric material for thin film deposition. Inorganic dielectrics such as silicon dioxide is a common interlayer dielectric for IC applications [12].
4. Hole quality characteristics and improvement methods
The requirements for PCB via holes are:
· precise in the individual hole geometry and the relative hole positions to meet the high precision alignment and registration requirements.
· free of debris/adhesives inside the hole and on the copper plate for better plating processes
· minimum HAZ with no delamination
· no thermal damage to the copper plates
· smaller or equal to 1:1 aspect ratio (hole size/depth), for easy metallization in the plating process. Higher aspect ratios tend to cause voids and thus result in poor yield.
There are a number of ways to improve the hole quality:
1) Projection technique
A mask cuts off the peripheral part of the light beam whose power density is insufficient for vaporisation. As a result, the zone irradiated becomes sharply defined and the hole diameter is greatly reduced.
2) Coaxial air assistant
The drilling process is influenced, to a certain degree, by delivering compressed air or other gases through the nozzle. The assistant gas has two functions: to blow away melts, and to protect lens from flying debris. However in via-hole drilling of polymers, the use of assistant gases has not been reported.
3) Post-laser treatment
Chemical etching process quickly eliminates the molten metal and burr inside the hole and thus considerably decreases the surface roughness [6].
4) Auxiliary plate
Cover plates can be applied to both sides of the real sample surfaces to produce circular holes [6].
5) Multiple-pulse hole drilling
This method is normally used to achieve high quality precision holes.
5 Comparison of hole-drilling techniques for PCB vias
Mechanical drilling has been the standard for via formation in PCB manufacturing. The cost of mechanically drilling a 0.340 mm through-hole is approximately US$ 0.0025 per hole. The calculation was based on drilling of one panel at a time [2]. However, the cost of mechanical drilling escalates dramatically when the hole size decreases to 0.150 mm. Drill bits alone cost US$ 0.0040 per hole. This makes it impractical for electronic manufacturers in high-volume, price-sensitive markets to exploit the density advantages of ultra-small hole drilling in their products.
A typical CO2 laser via drilling system is illustrated in Fig. 4. The focused beam drills one hole at a time. When drilling polymers and plastics, unwanted flow of melted material can substantially degrade the edge quality or limit the minimum thickness which can be processed. When drilling woven glass fibre-reinforced polymers such as FR-4, the laser beam is refracted off the glass fibres, producing irregular hole walls [2]. This causes difficulties in the plating process. For this laser drilling technologies to be more cost-effective, new and improved materials without the conventional glass weave need to be developed. Epoxy-coated Thermount material developed by DuPont has been demonstrated to be suitable for laser vias drilling [13, 14]. The cost of the LaserVias is estimated at US$ 0.0025 to US$ 0.0035 per hole [15]
A typical Nd:YAG laser via drilling system is shown in Fig. 5. The laser beam is scanned across a finite length and a pulse train is triggered when the beam is scanned to a via location. The XY stage steps in the perpendicular direction with respect to the beam scan. One row of vias at a time is drilled. The drilling process is also a thermal process.
Excimer lasers ablate organic materials very cleanly, leaving well-defined edges and resulting in minimum damage to the hole wall and to the surrounding material. Higher degree of depth control can be achieved as the material is removed layer by layer.
Three major techniques are identified for the excimer laser processing: projection imaging, contact-mask scanning and conformal mask scanning. In the projection imaging, the laser beam is projected onto the workpiece through a mask with the desired hole sizes and shapes. This technique is mainly used for generating small patterns (e.g. 1 cm x 1 cm). In the contact mask scanning, a metal mask is held close contact with the substrate and the beam is scanned over the entire surface. In the conformal mask processing, a thin metal layer is laminated onto the substrate. The process technique is the same as the contact mask scanning. The size of the workpiece is limited only by the range of the table movement. A conceptual excimer laser via drilling system is given in Fig. 6.
A mask is commonly used in excimer laser processing,, which allows simultaneously process an array of holes. The throughput for the excimer laser could thus be higher. An example is given in Fig. 7, where an array of approximately 1000 70 mm square holes have been machined. Traditional laser techniques would require each of the holes to be individually trepanned; while the excimer laser drilled all the holes simultaneously in a matter of seconds. Testing these holes in a thermal stress and thermal shock cycle in accordance with MIL specifications shows no abnormalities.
Photographs of the holes drilled by the three types of lasers are shown in Fig. 8, which indicates clearly the superior hole quality drilled by the excimer laser.
A comparison of the mechanical drilling with laser drilling techniques is given in table 1 to outline the major characteristics of each technique.
Table 1 Comparison of vias formation techniques
1. The processing speed depends on type of material, thickness, distance between holes, laser properties and X, Y table movement speed.
2. Based on drilling of one board at a time.
The advantages for LaserVias are: suitable for a wide range of materials, ultra-small holes thus extended design features, ease of introducing to the current board manufacturing process and elimination of a sequential of processes thus saving a great number of process cycles.
The challenges for excimer laser drilling to become the method of choice are: the equipment and operating cost, safety concerns in handling of hazardous gases and the etch rates.
6. Summary
In this article, the physical mechanism and parameters of laser drilling, the types of laser and comparisons of various processing techniques are reviewed. It is found that the excimer laser is a very promising tool for vias formation in the PCB manufacturing. The areas for improvements have also been discussed.
Acknowledgement
The authors gratefully acknowledge Dr. Chen Wei-Long for his encouragement and all the group members for their helpful discussions at Advanced Machining Group of Gintic Institute of Manufacturing Technology.
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, x and y are the co-ordinate, and Imax is the intensity at r=0
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