Photoplotter

A photoplotter is an electro-optomechanical device designed to expose latent images onto high-contrast monochromatic photographic film through a computer-controlled light source. Once exposed, the film undergoes photographic development to produce a finished image. Photoplotters are essential in the industrial production of printed circuit boards (PCBs) and integrated circuit (IC) packaging, where they generate photolithography masks with precisely defined patterns. Although originally associated with film-based processes, the technology has evolved alongside developments in microelectronics and computer-aided manufacturing.

Purpose and Industrial Applications

The primary use of photoplotters is the creation of photoplots, which serve as masks in the photolithographic fabrication of PCBs. These masks define the conductive copper traces, solder masks, and legend layers. As of the late 1990s, photoplots could reliably resolve features of approximately 25 micrometres, suitable for most contemporary PCB manufacturing requirements.
Integrated circuits are produced using a similar principle, though at much finer scales. IC photomasks often require sub-micrometre resolutions, traditionally achieved by photoreducing images produced by photoplotters. Beyond electronics, photoplotters have been applied in chemical milling, technical graphics, and specialised precision artwork, owing to their capacity for detailed optical pattern generation.

Historical Development

The first commercial photoplotter was introduced in the 1960s by Gerber Scientific, a company that also developed the now-standard Gerber format, a vector-based file standard for PCB image description. This format remains central to PCB design workflows, encoding layer information for copper features, solder masks, and component legends.
Early photoplotters employed a xenon flash lamp and projected images through a series of apertures mounted on a rotating wheel. An imaging head traversed the surface of the film without physical contact, producing two basic types of exposures:

• Draws: continuous lines or arcs formed while the light remained on as the head moved
• Flashes: fixed shapes exposed briefly through specific apertures to form pads or other discrete features

These machines translated vector instructions directly into optical patterns.

Modern Photoplotting Technologies

Contemporary photoplotters primarily operate as raster graphics devices, using laser beams that can be modulated at multi-megahertz rates. Lasers provide high resolution, uniform exposure, and improved speed. Multiple beam configurations may be used to increase throughput.
A significant advancement is laser direct imaging (LDI), which bypasses photographic film entirely. Instead, the laser directly exposes photoresist applied to the PCB substrate. LDI eliminates film handling, improves alignment accuracy, and accommodates finer, more complex designs demanded by modern electronics. It is also compatible with high-density interconnect (HDI) manufacturing, where microvias and narrow traces are common.

Input Formats and Control Systems

Photoplotters typically use vector graphic input files, with the Gerber format serving as the industry standard. The format allows precise specification of drawing commands, aperture definitions, and coordinate data. Raster systems internally convert these vectors into pixel-based instructions for laser modulation.
The machines are controlled by computer-aided engineering software and integrated into PCB production systems for automated workflow management.

Materials and Exposure Process

In traditional systems, the medium is a high-contrast black-and-white photographic film, chosen for its sharp edge definition and dimensional stability. After exposure, the film undergoes chemical development prior to use in the photolithographic process.
Modern processes involving LDI dispense with film, exposing photoresist-coated substrates directly. This approach reduces contamination risks, processing time, and dimensional distortions associated with thermal or humidity fluctuations.

Manufacturers and Industry Relevance

Major producers of photoplotter technology include:

• Gerber Scientific, pioneers of early photoplotters and developers of the Gerber format
• Orbotech, a significant supplier of PCB fabrication machinery
• Ucamco, current maintainers of the Gerber file specification and manufacturers of precision imaging systems

The continuing evolution of photoplotting technologies aligns closely with the demands of modern electronics manufacturing, where accuracy, speed, and reliability remain paramount.

Significance in Modern Fabrication

Despite the increasing use of direct-write technologies, photoplotters remain integral to many PCB production workflows, particularly for high-volume or cost-sensitive manufacturing. Their ability to produce consistent, high-resolution optical masks ensures their ongoing relevance. Furthermore, advances in laser imaging systems have expanded the capabilities of photoplotters, reinforcing their role in bridging traditional photolithography and emerging direct-exposure techniques.