One of the critical steps in the fabrication of a flexible circuit is the imaging process. The imaging process defines the circuit pattern. Most fine line circuits use photolithography for the imaging process. The process uses light to transfer an artwork image through a photo tool. There are a multitude of possible steps for imaging a circuit but a typical sequence is:
- Coat resist on copper laminate
- Photo expose circuit pattern onto resist
- Develop unexposed resist
- Etch exposed copper
To see more on the basics of imaging please see https://www.allflexinc.com/imaging-process-for-flexible-circuits/.
In the process sequence above, light sensitive photo resist is exposed and developed to create a pattern that selectively masks the copper from etching chemistry. After copper etching, the resist mask is stripped away and the remaining copper forms the desired circuitry pattern.
A common method for photo exposing involves using a photo tool. The photo tool is aligned to the resist coated substrate and high intensity UV light is flooded over the tool. The photo tool has openings that allow the UV light to selectively pass through. The UV light will then create a photo chemical reaction in the resist, causing it to harden. The resist that is not exposed will be selectively removed when immersed in developer solution during the next step. It is important to note that in most cases, the photo tool must be precisely aligned to the substrate.
The UV light source is high intensity and often collimated. Collimated light is perpendicular to the photo tool; non-collimated light is not perpendicular as it tends to originate from a single point.
Non-collimated light tends to transmit at angles to the photo tool; if the photo tool is not in intimate contact with the resist then the angular direction of the light will expose more area of resist than intended. This will result in poor circuit trace definition. For very fine lines, width and spacing violations could be the result. One can see from the image that the further the photo tool is from the resist, the wider the light pattern will be that exposes the resist.
Collimated light tends to transmit perpendicular to the photo tool. Ideally if there was some distance between the photo tool and the resist, the light pattern exposing the resist would match the opening in the photo tool and not spread out. For this reason, collimated light source tends to be used for fine line circuitry. Even with highly collimated light, there are limitations on how close the exposed pattern will exactly match the photo tool. For photo imaging lines and spaces of .005” or greater, the collimated light source and photo tool will suffice. There are however some limitations that come into play as the circuit density increases:
- Highly collimated light is not perfectly collimated. There will be some light spread.
- The photo tool glass itself may minutely refract light or bend light causing some light bleed.
- Any dirt or dust in the air or on the tool will refract light
- The photo tool is never perfect
- Aligning the photo tool to a flexible substrate that may have minute distortions can reduce registration tolerances
When dealing with tight tolerances and lines and spaces less than .005”, these minute issues may pose problems. There is another method of photo imaging that yields more precise results.
Laser Direct Imaging (LDI)
LDI exposes the trace directly with a highly focused laser beam that is NC controlled. Instead of flooded light passing through a photo tool, a laser beam will digitally create the image. The LDI process has several advantages over the traditional photo tool process:
- There is no light leak, the beam is highly controlled and focused.
- No photo tool is required
- Since there is no photo tool, there is no light
refraction to contend with
- Image alignment is more precise, the computer enhanced optical alignment can automatically compensate for material distortion
LDI removes or compensates for a lot of variables that come into play when using a photo tool. The result is that the image lines and spaces as well as the alignment to the substrate are more accurate.
Even with LDI there are variables that limit the density of the circuit pattern. The subsequent chemical etching of copper has its limitations on precision. Thickness of copper also will have a large affect on the tolerances.
The main disadvantage of LDI is processing time; a flooded light over a photo tool will take less than a couple of seconds. Since the laser beam must “raster” the entire circuit pattern, the process will take more time. However, for applications requiring very fine lines or very tight registration, LDI may be the only viable option.