LASER DRILLING
Laser Drilling Technology has 3 main applications in printed circuit boards manufacturing industry, including processing Blind and Buried vias, Microvias and FPC Outline. In this PCB article, we introduce the first two applications -Blind and Buried vias processing and microvias processing.
PCB manufacturers now commonly fabricate high density interconnect or HDI boards, as these are the fastest growing technologies. Compared to the standard circuit boards, HDI PCBs have a higher circuit density because fabricators use blind or buried vias and microvias when making the boards.
With technology changing according to consumer demand, designers using the HDI technology can now place more components on both sides of the PCB. This is primarily possible because they can use the via-in-pad process that allows more technology on fewer layers. Use of HDI technology allows products to weigh less and be physically smaller. Along with thinner materials and shrinking components, HDI technology is allowing electronics to shrink in size while expanding functionality, quality, and speed.
The PCB material is a composite of various materials including copper foil for forming electric circuits, resin for ensuring electrical insulation, and glass fiber for enhancing mechanical strength. High-quality vias of 0.1mm(4mil) diameter or less are essential to the high-density interconnection of the composites. In fact, CO2 laser drilling has become central to the present HDI PCB manufacturing process by the development of fine quality via-hole fabrication on the composite, achieving excellent productivity and establishing economic efficiency
Mechanically Drilling Vias and Microvias
Laser drilling has become more widely-used recently as it can overcome many of limitations placed on mechanical drilling. Laser drilling can be used to form microvias in PCB manufacturing and is not limited by the same material constrains as mechanical punching.
As newer devices require higher component densities, microvias are becoming more common in PCB designs. Multilayer PCB boards with smaller traces at high-density also require microvias to route the signals. Laser drilling of microvias is the preferred fabrication method due to their small size, and standard drilling methods cannot accurately form vias with the high aspect ratio and density required in mobile, IoT, and wearable technologies.
The greatest benefit of microvia is the small footprint. Microvias are also important for routing signals through small traces in multilayer boards. Laser drilling allows the formation of microvias in a wide range of materials with profiles that meet IPC standards. This method will become more prominent as mobile and IoT devices become more complex and dominate the market.
Today’s multilayer PCBs feature both through holes and via holes. Through holes typically have diameters of 0.15mm(6mil) or larger and they are deepest holes, thus they have high aspect ratios and are most conveniently mechanically drilled. Via holes are either blind vias or buried vias. Blind vias are exposed to one side of the PCB board and stop at the next underlying copper layer. Buried vias start out as blind vias, and are thereafter embedded in the multilayer PCB connecting the two copper layers intermittent in the multilayer PCB structure. This is accomplished by stacking multilayers together.
HDI PCB manufacturers use CO2 lasers in conjunction with high precision, high-speed scanner systems. A laser beam requires focusing and works by ablating the material on which it is focused. Therefore, it is only possible to make holes of limited depth using lasers. However, this suits the HDI boards, as manufacturers use lasers only to make blind or buried vias.
The HDI board manufacturing process uses copper direct drilling with a laser beam for creating bling or buried vias. In the laser drilling process, a laser drills a blind via by first penetrating the surface foil, where the energy of the laser transfers to the copper foil and evaporates it. Then the laser burns through the substrate layer and stops on the surface of the inner copper layer. This is a high-speed process, and a 2-head CO2 laser-drilling machine can drill 1,500 – 3,000 holes per second for the copper direct processing of an HDI board.
HDI PCB Laser Drilling Microvia Process
For blind microvias with diameters of 40µm, although manufacturers use UV lasers, newer type of CO2 lasers are available. The latter are more economically efficient than UV lasers, as the machine splits the CO2 laser into four parts, uses special high-performance lens for each beam, and processes four holes at a time. Such machines are capable of drilling more than 4,500 holes per second.
Creating double-sided PTH vias in boards with copper layers on both the top and bottom sides requires a different approach. The fabricator starts by using a laser beam to drill a hole half-way to the board thickness, then turns the board over and drills a hole from the other side at the same position—thus creating one through-hole. As the laser drilling position accuracy is less than ±10 µm, double-sided drilling of through-holes of less than 100 µm diameter does not cause displacements. Manufacturers are adopting the laser PTH drilling technique rapidly as it offers several advantages over conventional mechanical drilling, such as better hole positioning accuracy, high productivity, and offsetting drill-bit costs.
Laser Drilling Advantages
- Laser drilling for vias and microvias with diameter as small as 0.075mm (3mil). As the vias take up less real estate on the board, the designer has more space for achieving higher wiring density. This has the effect of making an HDI board smaller and/or reduces the number of layers, thereby reducing the overall cost of the board.
- The high-peak-power CO2 laser pulse can process the surface copper foil cleanly. It produces fine-quality blind vias holes that have smooth wall surfaces. This helps the plating process fabricators perform after the laser drilling, thereby strengthening the integrity of the plating process, and the quality and durability of the HDI PCB overall.
- Conventionally, PTH drilling takes up a lot of real estate on the board. Rather than use PTH drilling, PCB fabricators prefer a combination of blind and buried vias and microvias, as they can be stacked on above the other to achieve the same functionality as a double sided PTH. The designer has the flexibility either to stack up the vias and microvias or to stagger them to achieve higher wiring density. Designers often use landless microvias and via-in-trace technology for achieving even higher wiring densities.
- Apart from using vias and microvias for conducting signal and power from one circuit layer to another, designers also use them for conducting heat away from a component generating heat while in operation. However, this requires filling the vias with copper or other heat-conducting material.
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