The reasons are laser direct imagers can provide much higher manufacturing yields across the board, can image traces that are finer than any previous methods, at much higher yields and consistency than traditional methods.
Laser Direct Imaging has been around since the early 1990’s, but it initially struggled for adaptation primarily due to its relatively slow exposure speeds and low productivity compared to traditional imaging methods.
To understand the impact of laser direct imaging, it’s helpful to know the previous methods for circuit trace creation.
There are three components that are needed to create circuits on PCB’s using traditional subtractive methods. One is the resist itself, two is the phototool that is used to mask the resist, and the third is the light source that will be used to expose the resist.
The Evolution of Photoresists
The first resists were not photoresists but were inks that were screen printed onto either the innerlayers or outerlayers of PCB’s. They were fast, with images being printed in seconds. But they were relatively coarse with circuit dimensions typically .015” (0.381mm) or larger. They were also vulnerable to contamination either from the environment, or material on the surface of the PCB, that would interfere with reproduction of the circuit traces.
The next progress in technology was the development of liquid photoresists. Liquid resists were dipped or spray coated into very thin coverings on the surface of the PCB and then exposed to a light source. The liquid resists were very thin and helped fabricators reduce their trace geometries. Their weakness was the coating process, which was also prone to inconsistency and was also vulnerable to any contamination either from the environment or the material surface itself. Liquid resists still exist, and often in very high technology applications, but they are rare.
0.005___(0.127 mm) surface mount pads and photoimageable liquid coverlayer imaged by laser direct imager
0.004___(0.1016 mm) free floating traces imaged by laser direct imager with perfect front to back registration
The Development of Dry Film Photoresists
The third photoresist is photoimagable dry films, which are the predominant materials used worldwide today. They offer pretty good resolution down to .003 – .004” (0.0762 – 0.1016mm) geometries, with excellent consistency and good yields. Conventional dry film photoresists require attention to cleanliness – typically coated and imaged in class 10,000 clean rooms with cleaning processes for the PCB surface and the phototool.
Conventional dry film photoresists also had very long exposures on the first Laser Direct Imagers, one side would typically be timed in minutes, rather than seconds. This led to the development of dry film photoresists specially designed for laser direct imaging, which are quite popular today.
How Phototools Shaped PCB Imaging
Phototools and tooling in general, are used to correctly place the circuit image on the PCB surface.
With screen printing, there were no phototools. The resist ink was printed directly onto the surface of the PCB. But there was tooling required to hold the PCB in place and register the screen to the PCB surface. Mechanical tooling is inaccurate and allows the panel to move. This inaccuracy does not provide good registration front to back sides of the panels, critical to overall PCB yields.
Photoresists require phototools. The PCB panels are coated with photosensitive resist and then blanketed with light across the surface. Phototools allow light through to expose the photoresists in some areas and blocking the light in others. The light hardens the photoresist wherever it is exposed. In all other areas, the resist stays soft and is washed away in the developer after imaging. Phototools were popular in other industries at the time – lithography printing, screen printing, etc. so they were readily adapted to PCB manufacturing.
The Limitations of Traditional Phototools
Phototools also required good discipline for cleanliness in the environment and in the materials. Any dust or scratches in the phototool emulsion will reproduce in the imaged circuits. They also depended on conventional tooling to hold front to back registration, often aligned manually by an operator. All of which are detrimental to yield as the PCB technology progressed. Front to back registration using conventional phototools was typically greater than ± 0.003” (0.0762mm). It was fast, with exposures measured in seconds, and sometimes equipment could image both sides at one time, further enhancing productivity.
The Introduction of Laser Direct Imaging
Laser Direct Imagers were introduced in the early 1990’s and saw limited production for the reasons stated above. They were relatively slow using conventional dry film resists.
But they offered advantages which became more compelling in time. The first is that there is no phototool. The designer’s artwork is printed directly onto the dry film photoresist. Wherever the laser hits, the emulsion hardens. Wherever it doesn’t touch, it stays soft to be removed in the developer after imaging. Without a phototool, all the associated defects of phototools are eliminated.
Laser direct imager
The Evolution of PCB Exposure and Light Sources
The screen-printed resists did not need a light source for the PCB’s but did need an exposure table to expose the screen used for screen printing. This process of making the screens was prone to defects and often had to be retouched by hand. The resulting circuits were coarse by today’s standards, usually resulting in features 0.015” (0.381mm) or greater.
With the development of photoresists, came the introduction of light sources to expose the PCB panels. The first models were screen printing exposure units, re-purposed to expose PCB panels. Equipment manufacturers developed PCB printers designed just for this task. Panels were held with conventional tooling, and drawers moved the panels into and out of the exposure chamber. They offered good productivity, at lower costs. As PCB technology progressed, they would reach the limit of what they could faithfully reproduce – typically circuit geometries larger than .0010 (0.254mm).
The Development of Fully Columnated Light Sources
The next development in exposure units was called “fully columnated light sources.” These units were similar in size and shape to conventional light exposure units. They differed in that they added parabolic mirrors in the exposure chamber, that focused the light onto the panel, and more importantly modeled the light so that it hit the panel nearly perfectly perpendicular to the panel. This feature, though expensive, allowed fabricators, who had good cleanliness disciplines in place, to image down below 0.003” (0.0762mm) consistently. The problem with columnated printers is that they imaged almost too perfectly. Any particle, any dust, any scratch was reproduced perfectly on the PCB panel. Attention to cleanliness was crucial to success with these printers.
How Laser Direct Imaging Improved PCB Production
Laser direct imagers addressed and solved all these problems. Though expensive, the first units were more than $ 1 million each. The laser is perfectly perpendicular to the surface of the PCB. It is highly accurate. Front to back registration is faultless, critical for class III registration. The artwork is digital and is not used to create an intermediary phototool for imaging. That intermediary step introduces additional yield loss. The laser direct imager scales the digital image to match the movement of the panel, critical for yield on thinner and flexible laminates and materials. And it is not susceptible to dust or contaminants compared to conventional light sources.
Additionally the laser only exposes the circuits; it does not blanket the whole panel with light. Because of this Laser Direct Imagers just aren’t as sensitive to dust or contaminants in the environment, further improving yields.
Improved Registration With Laser Direct Imaging
Laser Direct Imagers offer the fabricator two more advantages. The first is front to back registration. When the laser direct imager prints on the first side, it puts two small UV markers on that side. When the operator flips the panels, the laser direct imager locates the marks and then prints the second side. Registration side to side is typically ± 0.0005” (0.0127mm); very accurate, very reliable and far better than an operator’s capabilities. Laser Direct Imagers also can scale the designer’s artwork to match panel movement, dramatically improving layer to layer registration.
Why Laser Direct Imaging Became Widely Adopted
In time the market introduced photoresists that were compatible with laser direct imaging, optimizing the UV wavelength and energy delivered to the panel, to reduce exposure times and increase productivity.
These developments have led to the almost universal adaptation of laser direct imaging for PCB fabricators across the World. Further, the laser direct imager was one of the first pieces of “smart” equipment in PCB fabrication, relying on optics to find and register tooling fiducials to dramatically improve registration. Many other equipment processes followed suit, improving manufacturing technology and yields. The initial laser direct imagers acted as a measuring stick showing how the rest of the fabricator’s processes needed to improve to match their capability.
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