Electroplating High aspect ratio

There has been a continual increase in size and complexity of PCBs with a concurrent reduction in conductor and hole dimensions. Conductors can be less than 250 p.m wide some boards have conductors less than 75 pm wide. Multilayer boards greater than 2.5 mm thick having hole sizes less than 250 pm are being produced. This trend may, however, eventually cause the demise of the subtractive process. It is difficult to etch such fine lines using 35-pm copper foils, though foils as thin as 5 pm are now available. It is also difficult to electroplate holes having high aspect ratio. These factors may shift production to the semiadditive or fully additive processes. 

Micromolding (LIGA ) Patterning of three-dimensional resist structures by high-aspect-ratio lithography, fabrication of a mold by electroplating... 

Use of X-ray exposure to produce a high aspect ratio resist structure that can serve as a mold for electroplating... 

Plated-through holes can be formed by additive (electroless) copper deposition, of which there are three basic methods fnlly additive, semiadditive, and partially additive. Of these, semiadditive involves pattern electroplating for PTHs with very thin snrface copper, bnt the other two form PTHs solely by electroless copper deposition. The additive process has various advantages over the subtractive process in forming fineline condnctors and PTHs of high aspect ratio. A detailed account of the additive process is given in Chap. 31. 

CuNiAu boards fabricated either with the NiAu as the Cu etch resist or by the SMOBC process followed by electrolessly plating Ni and Au can confer improved PTH reliability. There are two mechanisms for the observed improvement the enhanced rivet effect provided by the Ni and the elimination of Cu dissolution during solder shocks such as wave soldering or PGA rework. For high-aspect-ratio holes, electroless Ni confers an additional benefit because the plating thickness in the barrel is more consistent than for conventional electroplating. 

The SU-8 resist, optimized for photolithography at 365 nm wavelength (I-line), was found to be suitable for excimer ablation using a 248 nm KrF excimer laser. Stmctures with high aspect ratio, that is the ratio of the height to the width of a microstructure were demonstrated and used as a template for electroplating (alternative laser-LIGA) [26]. 

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