The growing demand for cell phones and other miniaturized electronic devices has created a strong need for small printed circuit boards (PCBs) and high-density interconnect (HDI) structures. A critical step in manufacturing these components is the production of small holes, called microvias, which enable electrical connection to be made between various different layers in circuit boards. Laser-based microvia formation has become one of the leading technologies to accomplish this task due to its processing versatility and moderate cost. Most of the laser systems for producing vias of over 50 µm in diameter are CO₂-based. This applications note examines how laser process parameters affect hole quality. G- and K-Series 9-Micron A DIAMOND K-225i CO₂ laser with output at 9.4 µm was set up on an optical table. This laser was chosen because many plastics, such as FR4, demonstrate higher absorption at 9.4 µm than at 10.6 µm. The output beam from the laser was directed into a 0.6X telescope to reduce its diameter from 6.5 mm to 4 mm. This beam was then sent through an acousto-optic modulator and re-expanded by a 1.33X telescope.

The beam was then passed through an isolator/circular polarizer assembly and then to a 63 mm focal length lens. The lens was 4.45 meters from the laser's output bezel and the beam size on the lens was 16 mm. For some tests, a 4.3 mm diameter aperture was placed in the beam path 2 meters from the laser.

The laser was pulsed externally with a function generator.

A 2-axis motor-driven table was attached to the optical table under the focusing lens. The table was driven by a PC-based numerical control that was set up to trigger the function generator on command.

Pieces of FR4 (1080) sample material were set on the motorized stage. The lens-to-workpiece distance was adjusted so that the sample was either at the focal point of the lens or a known distance from focus. Holes were drilled using various combinations of laser pulse duration and total pulse count, and at different focal positions with and without the aperture. Arrays of 10 X 10 holes were drilled using some of these processing combinations. These arrays were examined for hole diameter, hole uniformity, taper and debris generation. The best-looking holes were made with a 20 µsec laser pulse (pulse energy on the workpiece was 16 mJ).

Using Near Field Imaging:

The aperture was then added. Power transmission through the aperture was 38%. A series of holes were made at varying focal positions. Optimum hole shape was obtained using a defocus of 0.025" (0.64 mm). Arrays of holes were then drilled using pulse durations varying from 25 to 45 µsec. The 40 µsec array was thought to have the best appearance. Pulse energy for this condition was 18 mJ.

Fig. 1 - 10 X 10 Array of 40 µsec Pulses + 4.3 mm Aperture.

Fig. 2 - SEM Photograph of FR4 Holes Drilled.

The CO₂ laser has proven capable of producing high-quality microvias in FR4, and has also demonstrated the required reliability and cost of ownership characteristics for the production environment.

Material	Composite
Process	Drilling