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Karl's Tech Talk
By Karl Dietz
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Quick-Turn Circuit Board Shops
Quick-turn circuit board shops specialize in the speedy delivery of small lots or prototype boards. Thus, they value any processing technique and technologies that reduce the time of the critical path between receiving the CAD data and shipping the finished boards. Some of the technologies that enable fast delivery are also suitable for mass production. The following are some examples of quick-turn processing.
The creation of a silver halide phototool involves a number of time-consuming steps such as plotting, development, fixing, drying, and equilibrating the phototool to the temperature and humidity of the working environment. Equilibration is particularly time-consuming.
Some quick-turn shops have tried to shortcut the equilibration step with limited success. The target dimensions of the phototool features are those of a damp phototool. Typically, the dryer temperature of the processor is set lower than normal so that the film exits the processor just a little damp and relatively cool. The film is measured within minutes after processing and sent directly to the imaging area. A film processed and used this way will slowly change size over the next several hours while it is in use, but when used quickly after processing and measuring, and if the tolerances are fairly forgiving, this approach can be viable in reducing overall processing time.
Laser direct imaging, or other digital imaging processes, has been first adopted by prototype and quick-turn shops for the obvious reason that they eliminate all phototool processing steps. The other big advantage of LDI is its ability to scale the image (i.e., change the dimensions slightly, for best fit to image features on another layer).
Direct Metallization Processes
Direct metallization processes allow copper plating in through-holes without having to go the lengthy electroless copper process. Direct metallization process uses either carbon, graphite, or palladium catalyst, or a conductive polymer to initiate copper electroplating on the dielectric hole wall. These processes avoid the use of problematic chemicals that are part of the electroless copper formulation; they use less water, typically have a smaller equipment footprint, and use fewer processing steps, which allow the quick-turn shop to reduce processing time. Table 1 compares a graphite-based direct metallization process with the sequence of steps that make up the electroless copper process.
Editor's Note: This article originally appeared in the May 2015 issue of The PCB Magazine.
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