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Issues: Fine Line Circuits and Spaces

Thin Line Widths (Often Diagnosed as Over-Etching)

Often, customers receive boards from their supplier that display fine line widths less than the allowable 20% per IPC (0.001” for most controlled impedance designs) and misdiagnose this as over-etching. While etching can be a primary root cause, many overlook the importance of the primary Imaging step to this technology. SEC Addresses Imaging Issues in the following ways:

Photo-Plotting

Photoplotters are used to “print” the artwork from which the circuitry is imaged. Typical photoplotters found in the industry may print at 1000-2000 dpi. We have installed two newer model photoplotters that plot at 8,000 dpi as standard, and can be increased to 24,500 dpi for extreme fine line applications. This insures that when we go to print a 3 mil line (for example) that the initial artwork displays a true 3 mil line.

Photo-Imageable Dry Film

We have worked with our suppliers to identify and stock a variety of dry films that have specific characteristics for specific features. When our operators encounter fine line circuits on inner and outer layers, they change out our laminators to replace the standard dry film with premium dry film that resolves images with greater clarity.

Exposure Units

The process for imaging works such that the artwork is placed over a production panel that is coated with a photo-imageable dry film. UV light is then turned on to expose the film per the artwork, much like taking a film picture. Typical exposure units have a single point light source that is spread across a production panel using reflectors. Unfortunately, the reflectors cause the light to hit the panel at an angle, which is not conducive to fine line imaging.

Saturn has recently purchased and installed 4 Olec AT-30 Automatic-Alignment, Hybrid Collimated Light Source exposure units. These units have a light source arrangement so that light hits the panel surface at a 90 degree angle across the entire 18”x 24” working area. This enables us to define fine line circuits across a wider work area, increasing our yields and lowering cost for our customers.

Etching

One part of the fluid dynamics of the etching process is called “puddling”. Since etching is a horizontal process, etching chemistry is sprayed on to the top and bottom of the panel. While the chemistry that is sprayed from the bottom falls back off the panel, the chemistry sprayed from the top remains on the panel, forming a puddle. This puddle then absorbs much of the force of the newly sprayed chemistry. Since etching is as much a function of the physical force of the chemistry hitting the copper as it is a chemical reaction, the etch rate on the top side of the panel is reduced. This requires the operator to slow down the line speed in order to properly etch the top surface. As a result, the bottom side is often over-etched.

Saturn has recently installed Hollmuller etching lines for both inner and outer layers. These lines have the latest anti-puddling spray technology incorporated within them. This technology uses a specific spray pattern and nozzle arrangement, along with an oscillating spray bar, to “sweep” away old etchant so that newly sprayed etchant is sprayed directly on to the copper surface. Furthermore, these lines automatically control temperature and chemistry concentrations to insure uniformity throughout the production process.


Registration

Fine line designs often incorporate tight annular rings and drill to trace spacing. This requires registration throughout the production process to be as tight as possible. Here are some of the ways in which we address Registration:

Scale Factors

We have studied a large extent of various stackup combinations and materials from Isola, Nelco, Rogers, Taconic, and Arlon to determine scale factors in standard processing in combination with copper retention percentages. A key instrument in these studies is the use of the PerfecTest system and resulting knowledge database. We consult this database each time a new part number is produced.

Automatic Alignment Imaging Systems

We have installed 4-camera automatic alignment imaging systems (4 units between 2007-2008). These units utilize a 4-camera system to align the top and bottom artwork to within 0,5 mils. They then make micro adjustments to the panel position for best-fit panel-to-artwork alignment.

MANIA Micronics 86 Drills

We have installed 4 new Mania drilling systems. Typically, drilled hole position is within +/- 2 mils of nominal with most drilling systems. Mania drills achieve drill registration to within +/- 0.5 mils. The key features on these drills that enable this characteristic are the use of linear motors in the X, Y, and Z axis. As opposed to the standard screw drive motion system, linear motors are able to start and stop faster since they induce less vibration and use linear scales to measure position.






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