Microvia Drilling

However, a greater emphasis will be placed on the laser-drilling process laser via hereafter) since it is the most popular process today and it seems its popularity will grow in the future. It must be understood that via hole formation is just one element of fabricating HDI wiring boards. Fabrication of HDI wiring boards with microvia holes involves many new processes not common to conventional board fabrication. Therefore, additional emphasis will be placed upon these new fabrication processes that are common to other microvia technologies. 

Nonphotoimageable, Nonreinforced Dielectric Materials. This group can be laser-drilled, plasma-etched, and/or mechanically treated to form microvias. As stated earlier, many of the photoimageable dielectrics are laser-drillable. 

FIGURE 22.14 Photos of conventional and skip-microvias laser-drilled in Thermount materials. (Courtesy of DuPont)... 

However, there are many variations. For the purpose of drilling microvia holes, there are four laser systems UV/Yag laser, CO2 laser, Yag/C02, and CO2/CO2 combinations.Then there are three dielectric materials RCC, resin only (dry-film or liquid resin), and reinforced prepreg. Therefore, the number of ways to make microvia holes by laser systems is driven by the permutation of four laser systems and these three dielectric materials. 

The most desirable shape of drilled microvia holes is tapered, which is accomplished by pulse drilling. Normally, two to three pulses are used. When three pulses are applied on the same spot continuously, the capture pad may be overheated and excessive epoxy smear may result. Therefore, when three pulses are to be applied, the entire panel is drilled first by two... 

The speed of drilling by a Yag/C02 machine is much slower than that of a CO2 laser. However, the number of microvia holes to be drilled for infrastructure boards is normally one magnitude less than that seen in cell phone applications, in which the number of microvia holes can reach as many as 650,000 to 700,000 per m. Furthermore, the price of infrastructure boards is much more than that of mass-produced cellular phone motherboards. Therefore, slower speed of driUing, which means higher drilling cost, does not affect the total cost of manufacturing. 

CLLAVIS. The CLLAVIS build-up technology is marketed by CMK of Japan. This laser-drilled microvia technology is the most common of HDI processes. The cross-sectional view in Fig. 23.13 shows the filled buried vias in the multilayer core, as well as the optional filled microvias that can be stacked. This structure is also available with the simpler, unfilled staggered microvias. 

Microvias that are created by laser drilhng can be located at a reduced pitch for CAF studies because one does not expect drill damage from a laser-ablated hole and it will not be subjected to the drill wander of a mechanically drilled hole. Thus, laser drill tolerances are below 0.025 mm (0.001 in.). 

FIGURE 6Z8 Microvia capabilities shown by base thickness and hole diameter. Also shown is the range of technologies from HDI to mechanical drilling required to create the vias. 

Microvia hole drilling Laser (excimer, UV YAG, carbon dioxide) Microhole punching High-accuracy x-y table Plasma etching, chemical etching... 

FIGURE 43.2 Standard manufacturing process for microvia holes in double-sided flexible circuits starting with copper clad laminate (CCL). The hole is created by drilling and then copper plated. 

The UV YAG laser is another choice for generating microvia holes smaller than 50 /mi in diameter on flexible materials. It has a higher productivity rate than the excimer laser. It can drill through both copper foil and flexible substrates. An issue with the UV YAG laser is that it takes a long time to generate large holes. 

Conformal masks for Microvia Generation. Mostly, the laser systems are capable for the drilling for both of base films and copper foils of the flexible laminates. It is possible to make microvia holes through copper foils by the same machine in one process. However, the... 

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