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26 The PCB Magazine • June 2016 While technically an LDI digital process does not have the same resolution capability as contact printing, LDI is actually superior for high-density flexible circuit fabrication. The LDI imaging process is capable of reproducing feature sizes down to 1 mil. This may be an is- sue for integrated circuit (IC) fabrication where features are much smaller, but for most PCB fabricators this resolution is acceptable. LDI has become the standard used in the printed circuit industry for high density interconnect (HDI) circuit boards. The main reason LDI has gained favor in the industry, especially for high density circuit fabrication, is it eliminates the phototool. Con- sider the following issues associated with a pho- totool: • There is an expense associated with the storage, preservation, tracking and constant inspection of a phototool that LDI does not have • As the phototool is used, dirt, fibers, smears and scratches can degrade the phototool and reduce its ability to re- create the desired pattern • Even under ideal conditions, a phototool will allow some diffraction of light • Phototools are susceptible to temperature and humidity variations that can distort the original image • There are limitations in phototool align- ment to the substrate. Flexible circuit substrates may change dimensionally through normal processing. While a static phototool can be adjusted to a "best fit," LDI computer algorithms use optics technology to stretch or shrink the image pattern to precisely accommodate dimensional changes. This is particularly important for double-sided and multilayer circuit fabrication, where registration to vias and other features are critical. There are two main disadvantages of LDI: capital cost and maintenance expense. The purchase price for an LDI unit is significantly higher than that of the traditional photoimag- ing lines. Soft demand in a highly capitalized factory can have a pretty dramatic effect on profitability. Maintenance service contracts add an additional annual cost. As with photoimaging and screen printing, to avoid compromising yields LDI should be in a near particle-free environment. Each resist- coated panel goes through a cleaner immedi- ately before inserting into the LDI chamber to remove any particles. Particles and fibers can block the UV light and create a short or open in the circuit pattern. Class 10,000 cleanroom conditions are recommended with tight tem- perature and humidity controls. When LDI technology was first introduced around 20 years ago, throughput was an issue. LDI was often restricted to low volume or proto- type runs. Subsequent advances in equipment as well as faster acting photoresist have made it practical for high volume circuit fabrication. When one considers the improved yields, en- hanced capability, reduced set-up time, and lower costs associated with eliminating the phototool, investments in LDI technology can result in acceptable ROIs for many circuit fabri- cators. PCB Dave Becker is the V.P. of sales and marketing at All Flex Flexible Circuits and Heaters. To contact Becker, or read past columns, click here. IMAGING METHODS FOR ETCH RESIST, PART 3: LDI " Particles and fibers can block the UV light and create a short or open in the circuit pattern. Class 10,000 cleanroom conditions are recommended with tight temperature and humidity controls. "

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