Friday, March 13, 2009

Photoresist Imaging

Dry-film photopolymers used in the printed circuit industry (e.g. photoresist and soldermask) are exposed using ultraviolet (UV) radiation in the range of 300 nm (nanometers) to 440 nm. In a production environment, exposure equipment routinely provides as much as 6000 Watts of irradiation from banks of high-pressure mercury arc-lamps. The primary goal of these systems is to provide intense, collimated UV light at levels conducive to high board throughput and reliable, repeatable image transfer.

For the home shop, a much smaller system is usually adequate. Since throughput is a secondary concern, the radiant output can be much lower and still provide acceptable performance. Two things that cannot be traded off, however, are source collimation and uniformity of illumination.

Source collimation, or the degree of parallelism with which the UV light illuminates the artwork/sensitized substrate, ultimately determines the fidelity of image transfer and the minimum size feature that can be reliably resolved. With most practical light sources (lasers not withstanding), there is always going to be a tradeoff between collimation and the total radiant flux reaching the photoresist. At one extreme, you could place your source at infinity (or pretty far away if infinity is out of the question) and achieve a very high degree of collimation. The inverse square law , of course, would reduce the total amount to light actually striking your laminated copperclad to next to nothing, so exposure times on the order of millennia might be expected. At the other extreme, you could put the source right up against the board being exposed to achieve very short exposure times, but, the divergence of the incident light would be so extreme that resolving fine details would be impossible.

Uniformity of illumination determines the consistency of exposure from one point to another and affects the trace-width uniformity in the developed image. From a uniformity point of view, the ideal source would be an isotropic emitter whose emission area was as large as, or larger than the substrate you are imaging.

Any light source intended for use in PCB production must strike an acceptable balance between these two factors and the cost of implementation. Needless to say, you can significantly increase the efficiency of your illuminator by using reflective and/or refractive optics with intense point sources but the uniformity of illumination in the exposure plane usually suffers. Large area emitters, while technically feasible, sacrifice collimation in favor of uniformity (not to mention extreme cost).

There are a number of very good commercial exposure units available priced from US$500 to US$2,500 (not including shipping). If exposure times on the order of 4 to 7 minutes per side are not objectionable, you can make an exposure source capable of resolving 0.005" (0.13mm) traces on 0.010" (0.254mm) centers over a 12" by 18" (305mm x 457mm) exposure plane for under $200.

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