The leap of advancement from the conventional phototool to laser direct imaging (LDI) was fascinating but tough. Let’s find how we sailed this journey.
Introduction: Why the change?
We have often heard “old is gold”, but this does not apply to everything, especially when it comes to technology. For technology, it’s usually old is old and requires an upgrade. If you stay loyal to a specific technology for more than a specific time, you will eventually become obsolete. So, no matter what industry you are in, you need to update yourself to the latest version. Sounds quite simple, right? But that’s not the scenario, at least not always. Sometimes, it is quite a hustle to upgrade your existing technology. Technological advancements may sound very fascinating when we say it like that, but take in a lot of comprehensive efforts.
Our PCB world is no exception.
In our world also we have to keep up with customer demands, which always tend to rise upwards. As the fine line between need and desire is getting blurred, the technology industry is coming up with products fancier than our imagination. Our electronics is getting sleeker and miniaturized day by day. Today’s PCB manufacturers are driving for producing not only smaller, faster, lighter, less expensive, more complex and reliable PCBs but are also constantly adapting to new product designs.
PCB manufacturers are thriving to produce high-density interconnect (HDI) boards at significantly minimized cost and lower implementation time. Board complexity is growing every day. During the past five years, an average track width in conventional circuit boards went from 200 µm to 100 µm. Today the call for 50/50 µm or even 25/25 µm is real. Existing technologies are unable to offer acceptable options to our growing requirements. The conventional film exposure process generally used for the photolithography structuring of the conductive pattern was efficient until now. However, now it limits our capabilities, to track-widths and track-spaces of < 150 µm.
The inevitable change
Therefore, incorporating a few necessary modern strategies in production process became inevitable. The most important of these strategies was the exposure of the circuit patterns directly on boards by the laser system. As I said earlier, it’s not so easy as we make it sound. Before we could bet our money upon this challenge, we had developed and tested several new technologies. Laser direct imaging has managed to prove itself, as the best and most comprehensive imaging solution for HDI PCBs.
By this time, you must have understood laser direct imaging, as its name suggests, is an imaging process. It is the process of imaging circuits on boards directly, i.e., it doesn’t use phototool or mask. LDI processing requires a board with a photosensitive surface, positioned under a computer-controlled laser. Therefore, we expose the photosensitive resist by the means of a laser beam which is switched on and off by computer-controlled system and is scanned across the board panel. The laser used in this process is often assumed to be in the UV spectrum as this tends to suit most of the commonly available photoresists. There are also systems that operate in both the visible and infrared spectrum, working with specially formulated photoresists. The most common photoresist we use these days is UV sensitive.
History of laser direct imaging: How they barged into the conventional PCB process?
Since time inception in PCB the majority of manufacturers have been producing circuit boards using films to create circuit patterns in a photoresist. This technology utilized transparent film with a circuitry pattern on them. The exposure process is carried out inside UV exposure machines, which utilized high-power lamps as the source of UV light. This technology is called photolithography.
But since the late 2000s, since 2004 to be precise, laser direct imaging has become the most comprehensive imaging solution for HDI boards. It was introduced to the market by the Orbotech company with their Paragon 9,000 machine. LDI uses a focused laser beam to directly expose a circuit board panel coated with photoresist. Thus, it eliminates the use of masks, whereby inherent problems (which we will discuss in later parts). But when this type of imaging was first introduced, somewhat 20 years ago, throughput was an issue. Therefore, it was used only in low-volume or prototype runs. But with time, subsequent advancements in LDI equipment and faster acting photoresists have made it feasible for high-volume runs.
Old but not so gold traditional method problems
PCBs always consist of a base material on which a copper clad is applied. For the structuring of the conductive pattern, the material surface is coated by a photosensitive laminate (photoresist) called laminating process.
Imaging or creating a circuit pattern is one of the most fundamental steps of board fabrication. There is a multitude of possible steps to image a circuit but typically the sequence is:
- Coat resist on copper laminate
- Exposing the circuit design on to the resist
- Develop the unexposed resist
- Etch the exposed copper
This above sequence can be put as:
A light sensitive photo-resist is exposed and developed to create a pattern that selectively protects the copper from etching. After copper etching, the resist mask is stripped away and the remaining copper forms the desired circuitry.
Photo-exposing usually uses phototool. The phototool is aligned to the resist-coated substrate and a high-intensity UV light is flooded through it. The UV light selectively pass through it leaving desired circuitry unhardened. Therefore, the copper gets exposed as the circuit pattern. But, the problem in this method is the precise alignment of the phototool to the substrate.
Why? Let’s find out.
The UV light source is of high intensity and very often “collimated”.
Let’s start from the beginning. Collimated light tends to transmit perpendicular to the phototool. If we consider for an ideal situation, even if there is a distance between the photool and the resist (as discussed above), the light pattern exposing the resist would match the opening in the phototool and not spread out. Therefore, it is used for fine line circuitry. But there are still limitations like:
- When we say highly collimated light, it does not necessarily mean perfectly collimated light. So, it might just spread.
- Or maybe the glass of the phototool itself can refract or bend the light minutely but significantly enough to ruin the image.
- Dust or dirt in the air or phototool can also refract the light. Therefore, the phototool itself is not always perfect.
- Or flexible substrate can be really difficult sometimes, aligning the phototool to flex substrate can have minor distortion that in turn can reduce registration tolerance.
This all for collimated light, so, when it comes to non-collimated light; is not perpendicular as it tends to originate from a single point, there are poor circuit traces. That is because the light spreads even for a tiny gap between the phototool and the resist.
For 5 mils and above photo imaging lines and spaces collimated light will suffice but today’s boards are way denser. Therefore, it is no more “the best option” that can be offered.
Diving deep into laser direct imaging
LDI is our answer to most of our problems, if not to all, surfacing right now. HDI boards are the most popular in the PCB industry right now. Let’s first explore whether these are suitable for HDI boards.
It has been proved that these systems, which work in UV spectrum, are most suitable for obtaining fine lines and spaces below 2 mils. Capabilities of an LDI making it suitable for HDI.
- Fine lines, traces and spaces down to at least 2 mils and below are possible.
- Proper depth of focus ensuring imaging quality for high topography design. This is especially needed for uniform exposure of outer layers.
- The system design holds good for various product types, materials, thicknesses, manufacturing technologies and production steps.
- It is a flexible and precise registration system which is compatible with different manufacturing technologies and production.
Regardless of these and any other capabilities, LDI can be considered as an amazing break-through in the field of circuit board manufacturing.
Let’s now dig into LDI process, and see how can we describe it. We know how conventional PCB imaging works, trailing from there we have:
- The substrate is coated with photosensitive resist.
- The substrate is then positioned in the LDI unit.
- LDI digitally prints the desired circuitry.
- The resist is developed and the unwanted resist is etched off.
- The resist is stripped off. The desired copper pattern remains.
As we know, unlike photo exposing, LDI does not use a phototool, but directly exposes a saved artwork pattern digitally onto the resist. Photoresist is partially or rather selectively exposed as the laser beams increase across the substrate in a rastering way.
Raster, or here rastering, means composing of tiny rectangular pixels, or picture elements, that are arranged in a grid or raster of x and y coordinates (includes a z coordinate in case of 3D) in such a way that it forms an image.
The image formation in here is likely of that on a CRT screen, which is formed from hundreds of horizontal lines across the screen. Like phototool, LDI also requires photoresist but in general they are specially formulated ones. But the application of photoresists in both the processes are identical. Even the post exposing process of an LDI board is the exact copy of phototool process.
Therefore, laser direct imaging is way superior to phototool in terms of HDI board fabrication. Still wondering why? Let’s take note of the following differences and in short see how LDI overpowers phototool:
- Phototool demands regular expenses associated with storage, preservation, tracking and constant inspection. LDI avoids this entire process expense and hardships.
- As the phototool is handled manually, dirt, dust, fibers, smears and scratches can easily degrade the phototool. LDI is fully computer-controlled and hence free from such problems.
- Even under ideal conditions there can be light diffraction in a phototool.
- There are repeated defects occurring due to the handling of phototools and also off-contact exposure.
- The dimensional stability of phototools is not very up to the mark, i.e., its size changes with temperature and humidity.
- Phototools are anisotropic in nature, therefore when it comes to PCBs with not so flourishing tolerances, taking the same value for both x-axis and y-axis can lead to serious inaccuracies.
- There is no light leak, the beam is highly controlled and focused.
- Image alignment is precise, the computer-enhanced optical alignment can automatically compensate for material distortion.
We see LDI removes a lot of variable coming into play for phototools. Consequently, the image lines or traces and spaces along with the substrate alignment are quite precise. But even with LDI, there are variables that limit a PCB in this circuit pattern density. Like the proper thickness of copper. A precise chemical etching process has its limitations, etc.
Even LDI can have a few issues. The main point is the processing time, while flooding a substrate with UV light is a matter of a couple of seconds, rastering an entire circuit pattern for obvious reasons will require more time. However, for circuit boards with tight tolerances and requiring less than 5 mil traces and spaces LDI is the only viable option right now. Therefore, LDIs are effectively eliminating the phototools as of now, until we can find something more precise.
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