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Almost all double-sided and multilayer flexible circuits require through-holes or vias to enable an electrical connection between conductive layers. Creating a reliable plated via requires several critical steps. The first one is creating the through-hole.
Typical methods for creating through-holes in flexible laminates are:
- Numerically controlled drilling
- Laser drilling
- Hard tool punching
Numerically Controlled Drilling
Numerically controlled (NC) drilling is the most common method used in flexible circuit and printed circuit board manufacturing. Panels can be stacked several layers high and are sandwiched between entry and exit materials to ensure cleanly cut holes. Many NC drilling machines have multiple spindles, which allow multiple holes to be drilled simultaneously. Multiple spindle drilling combined with stacking make NC drilling a very efficient operation with a locational accuracy of about +0.001” from hole to hole. This technology has been around for decades but remains the workhorse process for making holes in panels of flexible and rigid circuit board material.
Drill bits can come in all shapes and sizes; most are carbide steel. The bit has angular sharp grooves that evacuate the material by a shredding action as the rotation of the drill cuts then pulls the material out of the way. The smaller the via diameter, the faster the drill should spin to achieve maximum cutting efficiency. Drill sizes may be as large as 0.250” for creating tooling locations and can go down to about 0.005” for the creation of electrical vias. While customers frequently ask “what is the smallest via you can drill?” practically speaking, an 0.008” diameter is close to the lower limit for efficient volume production.
While the side view of a drilled hole is smooth to the naked eye, it actually has spiraling microgrooves along the sidewall. Most substrates are composites with layers of copper, adhesive and polyimide. This complicates drilling as it must cut cleanly through several different types of materials. Drill bits can get hot and soften the adhesive, smearing it around the diameter of the hole. The bit temperature is a function of drill speed and the design of the bit. Optimal bit designs maximize material evacuation while minimizing temperature rise. Adhesive smear can create an insulation layer around the circumference of the drilled hole. This can prevent the copper plating from creating the layer-to-layer electrical connection. As a result, many double-sided and multilayer circuits are next processed through a plasma desmear operation to remove this dielectric layer.
In laser drilling, a highly focused beam of energy ablates or vaporizes the material. The laser beam consists of pulses of various intensities and wavelengths. Laminates comprised of multiple materials such as copper, polyimide and adhesive require specific process algorithms to address the material differences, since different materials ablate at different laser settings. Polyimide and adhesive are quickly removed while copper is much slower and requires increased energy. While NC drilling can handle a stack of multiple layers of material, laser drilling is only performed on a single substrate layer.
Virtually any size or shape opening can be created with laser drilling. The larger the hole, the more energy required and the higher the cost. Lasers can drill very small holes, but for flexible circuits, via sizes smaller than 0.002” may be difficult to plate due to the inherent limitations of the plating chemistries and processes. For high-density circuits with vias in the 0.002” to 0.006” range, laser drilling is often preferred over NC drilling. NC drilling would most likely be preferred for larger vias because of the higher throughput rate.
Laser drilling can be highly controlled in the X, Y and Z directions as well as be tuned to selectively ablate specific materials. This capability makes the technology useful for multilayer fabrication where blind and buried vias are required. It is also used to create dual access openings on top and bottom sides of a single-sided circuit by selectively removing polyimide and adhesive. Even with a finely tuned laser output, there is often some carbon debris remaining in the hole requiring removal. This is usually accomplished with a plasma etch process. Alternative chemical cleaning processes are also employed.
Punching vias is usually performed using a male-female die set. The upper section of the tool is a punch and the lower part is the die. Laminate is placed between the upper and lower plate. The top punch is pushed into the material and a sheering action creates the hole. The slug created by the punch is pushed through the lower die plate.
Punching is the least frequently used process for flexible circuit fabrication, especially for low volume or quick turn parts. It does find favor with ultra-high volume, lower density circuits and lends itself well to continuous reel-to-reel circuit fabrication. Die sets can be quite expensive depending on the number and size of holes in the pattern. A die set can be built to accommodate a variety of shapes and sizes but this process has limitations for creating vias in high density circuit applications. For most manufacturers, 0.015” to 0.020” diameter punches are the practical limitation for via size with a punch tool.
Of the three methods, punching puts the highest level of mechanical stress on the substrate. At a microscopic level, a punched hole will look deformed when compared to NC or laser drilling. A dull punch can result in the substrate layers pulling apart, or the adhesive layer smearing over the copper part of the sidewall. This can cause problems at plating and result in a poorly metalized through-hole. An undetected broken punch is also an obvious problem.
Hard tooling, NC drilling, and laser drilling all have advantages and disadvantages in flexible circuit processing. The preferred method depends on circuit density, production volume and laminate configuration. From a plating standpoint, there are no inherent advantages among the three alternatives.
Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.