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14 The PCB Magazine • April 2015 material is dramatically smaller than with lon- ger pulse lasers. These new lasers work without any special treatment for the copper foil. B. Plating and imaging processes Design features e.g. line/space, dielectric thickness, final copper thickness are driving the selection of plating process (e.g., in 0.3 mm pitch BGA design with two lines between BGA pads, via hole size is normally 75 µm, and pad size is 150 µm). Fine line with 30 µm/30 µm line and space is required for the layout design. It is challenging to achieve this fine line struc- ture by existing subtractive process. The etching capability is one of the key factors where the to- tal copper thickness together with plating uni- formity needs to be optimized together with the imaging process. That is why the PCB industry is now adopting the mSAP process, which com- pared to subtractive process, can easily produce line with optimized conductor shape in which the top line width is almost equal to the bot- tom line width all over the whole production panel—easy control of the line shape. Another advantage of mSAP is that standard PCB pro- cesses such as drilling and plating are employed leveraging existing resources and know-how, and the use of conventional materials provides good adhesion between the copper and dielec- tric layer to guarantee the high reliability of the end product. The key benefit of mSAP process compared to subtractive process is that it can easily pro- duce line with optimized conductor shape in which the top line width is almost equal to the bottom line width all over the whole pro- duction panel—easy control of the line shape. Thinner conductors with controlled shape may provide some benefits in terms of signal integ- rity due to improved cross talk and noise ratio. In fact, moving into thinner conductor lines may be mandatory in order to reach required NExT-GENERATION ULTRA-THIN HDI PCB MANUFACTURING CHALLENGES continues FeAture Figure 4: Heat-effected zone to material. nanosecond level pulses compared to ultra-short pulse lasers. Picosecond pulses reduce the zone [5] .