Removal of UV YAG or excimer laser debris after microvia drilling
BY TODD LIZOTTE
The formation of carbon residue while UV lasers drill microvias in polyimide or other dielectric materials is normal and expected. The UV lasers that drill microvias use a process called photoablative decomposition (or simply photoablation). The process employs the unique properties of high-energy UV photons to break chemical bonds of the material drilled. Sometimes this process is termed a non-thermal process because there is a minimal heat affected zone. The resulting carbon debris forms within the laser plasma plume and drops back to the surface of the material being processed. Because the carbon debris needs to be removed, the first cleaning processes used are typically solvent- or detergent-based, or in the worst case a reactive oxygen plasma cleaning technique is used. Either way, the use of chemicals or a reactive plasma process increases cost and does not always remove the carbon down to the sub-micron level, leading to potential shorts or defects at downstream plating or laminating. This article describes the use of water-soluble coatings that can be placed onto the surface, then removed using standard warm water pressure washing methods. This process was applied to an existing TAB process.
Carbon Debris
The debris that hits the surface surrounding the microvias being drilled takes the form of carbon and other materials that did not make it to a gas state when processed by the UV laser (Figure 1). This carbon is broken down into particles ranging from 10 microns to 0.1 micron. At this scale, the carbon is attached to the surface by a number of bonding mechanisms, including electrostatic forces, fluidic bonds and mechanical bonds.
 Figure 1. Carbon debris around via. |
The electrostatic forces are particularly problematic, because polyimides are high surface energy materials that tend to generate electrostatic charges simply by being handled, especially through the winding and unwinding operations in a TAB processing line. Although grounded rollers, ion bars and ionized air blowers can minimize the peak charges, a charge still is present and can exert a large holding force on micron-sized carbon particles.
Fluidic bonds are sometimes present because of humidity or the hydration level of the material at the time of processing. In most cases, fluidic bonds are very easy to overcome. Mechanical bonds are primarily due to energetic particles that may embed themselves into the material after falling to the surface; these particles are mostly adhesives or carbon particles that may still have enough energy to actually attach themselves to the material's surface roughness through a true mechanical bond, similar in nature to Velcro. There are other bonding forces that can act on the carbon particles, including many types of atomic bonds.
 Figure 2. Multi-layer TAB film with a water-soluble coating applied. |
In all cases, the obvious critical issue is debris falling back to the surface and under certain circum-stances embedding itself into the sur-face. The solution to the problem is to provide the particles with a sacrificial coating to interact with, which ulti-mately is washed away, taking with it the contaminants.
Sacrificial Coatings
Water-soluble coatings or reactive coatings can be deposited onto the TAB film before laser processing. These films are placed onto the surface of the TAB tape to a thickness that is adequate to trap energetic particles and trap and bond carbon and other debris (Figure 2).
 Figure 3. A meniscus process for applying a water-soluble coating. |
The sacrificial coatings are selected from standard off-the-shelf materials with some specific characteristics. The main characteristic is being able to be washed away using standard high-pressure warm/hot water washing techniques. The second characteristic is how environmentally friendly it is. In R&D situations where a quick test is necessary, many laser process develop-ers use hairspray or mix standard hand soap into a spray bottle or airbrush to provide a quick aerosol-delivered water-soluble coating.
One of the most common industrial water-soluble coatings is polyvinyl alcohol (PVA). With specific additives or used as-is, PVA is a very durable, yet easily applied material. Other pro-prietary coatings and techniques are also used for applications involving CO2 laser processes.
Applying Water Soluble Coatings
When drilling with a UV laser, it is desirable to minimize the amount of material being drilled to increase drill speeds. The addition of sacrificial coatings can add more time to a process even if it saves time in cleaning. The key parameter is to figure out the minimum sacrificial coating thickness to achieve the cleaning objective.
 Figure 4. Typical TAB circuits that can be cleaned with the process described here. |
Water-soluble coatings can be applied in several ways, although meniscus techniques are the most popular. To con-trol coating thickness, a meniscus technique provides the best degree of precision. The issue becomes the set-up of the process to achieve thin coatings and uniformities on the order of ±2 percent or less.
Meniscus coating methods involve the use of an applicator roller, similar to an ink roller used in printing. The TAB mate-rial is brought into a gap between the applicator roll and a gap roller that establishes a zone where a meniscus can form and transfer the liquid material to the TAB material (Figure 3).
Meniscus techniques allow several ways of adjusting coating thickness to high-precision tolerances: (1) adjusting roll speed (faster speeds result in thicker coatings); (2) adjust-ing the distance between the applica- tor and the TAB material; and (3) applying heat to the coating material to increase its viscosity. The process is very stable once it is set up properly and operated in a temperature- and humidity-controlled environment.
High-speed Pressure Washing After Drilling
After a roll of TAB material is coated and then laser processed, it is time for cleaning. The reel is brought to an aqueous washer specifically designed to wash and dry the TAB film. Depending on the TAB configuration (single layer or multi-layer), the washing machine can be run with additives, such as water-based corrosion-inhibiting agents, or a variety of cleaning chemistries, including aqueous (alkaline, acid or neutral pH) or semi-aqueous solutions.
 Figure 5. The pressure wash zone for a post-drilling cleaning process. |
Once the reel of TAB material is loaded, the washer has guide roller assemblies to accommodate a wide range of substrate widths, including 24, 35, 48 and 70 mm, in both super and standard sprocket formats (Figure 4).
The TAB material is placed on edge and threaded through the system to a take-up reel at the end of the machine. The TAB material first enters a pre-wash stage where it is soaked prior to entering the washer zone to allow the water-soluble coating to begin to hydrate and become soft. After pre-wash, the TAB material enters the pressure wash zone, which uses high-pressure spray technology as a means of removing the water-soluble coating that holds the carbon debris.
 Figure 6. The blow dryer zone for a post-drilling cleaning process. |
Upon exiting the pressure wash zone, the TAB material is rinsed with virgin deionized water and then into a first stage blow-off to remove a majority of the fluids. Once through the pressure spray cleaners and first stage blow-off, the material enters into the high-pressure air knives, which provide effective drying at high processing speeds. The final stage is the static dissipation stage where a set of anti-static bars is located to dissipate static electricity. The entire length of the TAB tape is supported by a series of backup rollers through-out the machine. Figure 5 shows the wash zone, and Figure 6 shows the dry zone.
Key parameters, such as sacrificial coating thickness, washer TAB transport speeds, water temperature, spray pressures, and the drying air pressure and temperature, can be varied to ensure that the TAB circuit is efficiently cleaned.
Conclusion
The techniques described here give an example of how to clean or nearly eliminate the debris formed during UV laser processing. Although applied to a reel-to-reel TAB application, the techniques can be applied to sheet- or board-based manufacturing operations using existing cleaning technology. AP
Todd Lizotte, chief development officer/vice president R&D, can be contacted at NanoVia LP, 4 Delta Drive, Unit #6, Londonderry, NH 03053; 603-421-0713; Fax: 603-421-0214; [email protected].