This article is the third and final part of methods for imaging etches resist. The first part discussed screen printing; the second part discussed traditional photo exposing. The third part discusses laser direct imaging (LDI).

The basic process sequence for LDI is similar to photo exposing: 
  • The flexible substrate is coated with photosensitive resist.
  • The resist coated substrate is positioned in the LDI exposing unit
  • LDI digitally exposes the desired pattern
  • The resist is developed and the unwanted resist is washed away.
  • The copper pattern exposed by removed resist is chemically removed i.e. etched
  • The resist is stripped off. Only the copper pattern remains.

Unlike photo exposing, LDI does not use a photo tool, but directly exposes a digitally saved artwork pattern onto the resist. Photo resist is selectively exposed as the laser beam increments across the substrate in a rastering fashion. The image formation can be likened to the image formation on a CRT screen, which is formed from thousands of horizontal lines across the screen. Like photo exposing, LDI requires a photo resist, but the resist is normally specially formulated for laser printing since LDI resist is much faster acting than traditional photo resist. Like photo exposing, resist for LDI comes in liquid or dry film options and the resist application methods are identical to those employed when using an artwork photo tool.

The post exposing processing of a LDI processed flexible circuit is exactly the same as photo exposing.

While technically an LDI digital process does not have the same resolution capability as contact printing, LDI is actually superior for high density flexible circuit fabrication. The LDI imaging process is capable of reproducing features sizes down to 1 mil. This may be an issue for Integrated Circuit fabrication where features are much smaller, but for most PCB fabricators, this resolution is acceptable. LDI has become the standard used in the printed circuit industry for High Density Interconnect (HDI) circuit boards.

The main reason LDI has gained favor in the industry, especially for high density circuit fabrication, is it eliminates the photo tool. Consider the following issues associated with a photo tool.

Consider the following issues associated with a photo tool: 
  • There is an expense associated with the storage, preservation, tracking and constant inspection of a photo tool that LDI does not have.
  • As the photo tool is used, dirt, fibers, smears and scratches can degrade the photo tool and reduces its ability to recreate the desired pattern.
  • Even under ideal conditions, a photo tool will allow some diffraction of light.
  • Photo tools are susceptible to temperature and humidity variations that can distort the original image.
  • There are limitations in photo tool alignment to the substrate. As discussed in other posts, flexible circuit substrates may change dimensionally through normal processing. While a static photo tool can be adjusted to a “best fit”, LDI computer algorithms use optics technology to stretch or shrink the image pattern to precisely accommodate dimensional changes. This is particularly important for double sided and multi-layer circuit fabrication where registration to vias and other features are critical.

There are two main disadvantages of LDI: capital cost and maintenance expense. The purchase price for
an LDI is significantly higher than that of the traditional photo imaging lines. Soft demand in a highly
capitalized factory can have a pretty dramatic effect on profitability. Maintenance service contracts add an
additional annual cost.

While All Flex extensively uses LDI for its high density production, traditional photo imaging equipment
can be a better fit for some “non-traditional” applications such as Maxi Flex®.

Like photo imaging and screen printing, to avoid compromising yields LDI should be in a near particle free
environment. Each resist coated panel goes through a cleaner immediately before inserting into the LDI
chamber to remove any particles. Particles and fibers can block the UV light and create a short or open
in the circuit pattern. Class 10,000 cleanroom conditions are recommended with tight temperature and
humidity controls.

When LDI technology was first introduced around 20 years ago, throughput was an issue. LDI was often
restricted to low volume or prototype runs. Subsequent advances in equipment as well as faster acting
photo resist have made it practical for high volume circuit fabrication. When one considers the improved
yields, enhanced capability, reduced set up time, and lower costs associated with eliminating the photo
tool, investments in LDI technology can result in acceptable ROI’s for many circuit fabricators.

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