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16 The PCB Magazine • November 2014 sary, the noble metal thickness can be increased to several micrometers by electroplating accord- ing to the desired properties (ohmic resistance or conductivity). The desired feature sizes and fine lines and spaces (< 25 µm) were achieved by implementing novel technologies like laser direct imaging and drilling with corresponding step and repeat registration as well as new aux- iliary materials for thin films. As a base several bio-inert substrate materi- als are available. For flexible applications poly- imide or LCP (liquid crystal polymer), for rigid applications glass, PEEK or a flexible material with a rigid stiffener can be used. Especially ca- pable are the properties of LCP, a very flexible thermoplastic base material with its biocompat- ible nature. LCP shows very low water absorp- tion (0.04%) in comparison to standard acrylic adhesives (8%), a temperature stability of up to 190°C (Tg > 280°C, Td > 320°C), excellent high- frequency properties (εR= 2.9, tan θ = 0.0025) and low weight (3.2 g/cm 3 ). For multilayer ap- plications no glue is needed due to its thermo- plastic properties (Figure 3). For applying the pure noble metals on the dielectric base material adhesion is a key fea- ture. To achieve good adhesion adequate surface preparation has to be chosen or an additional adhesion promoter has to be applied. The man- ufactured PCBs have passed the tape test in ac- cordance with IPC-TM-650 2.4.1. Furthermore the noble metal substrates have undergone a bending test by using a test coupon built up as a resistance loop (Figure 4). The traces are placed on a 100 µm single sid- ed polyimide. Cracks in the noble metal traces are detected on the first occurrence of resistance changes. figure 3: assembling of different Pi layer made by adhesives (left); several lCP layer thermally bonded (right). figure 4: (a) overview of the bending test machine; (b) test structure; (C) structure under bending including electrical testing. NOBLE METAL PCB MANuFACTuRING FOR DIRECT IMPLANTS continues