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June 2014 • The PCB Design Magazine 41 SURFACE FINISHES FoR HIgH-SPEED PCBS continues b) The risk of black pads (lack of balance within the ENIG plating chemistry) especially on smaller BGA pads. 2. Single-sided plugged via holes: As with most finishes, via holes plugged from one side partially plugged—are not recom- mended. Also, placing holes very close to SMD pads is not recommended, since the plating so- lution can become trapped inside and may con- taminate or reduce the solderability of the joint. 3. Solder mask bridges between SMD pads: As with immersion tin, this treatment is aggressive towards the solder mask. Therefore, larger solder mask bridges may be necessary, on fine pitch SMT components, at some fab shops. 4. The impact on the conductor's loss characteristics at high frequencies: Selection of low-loss plating is just as criti- cal as the selection of dielectric material when designing high frequency circuits. At approximately 2.7 GHz, the resonant behavior of the nickel component in ENIG increases insertion loss. This resonance is at- tributed to the ferromagnetic properties of the nickel layer. It is therefore wise to avoid using full body ENIG coating of microstrip traces at high frequencies. In fact, it may just be an odd 3 rd or 5 th harmonic that falls on this particular lossy region and causes radiation with much lower fundamental frequencies. Therefore, sol- der mask over bare copper (SMOBC) processing should be considered for all high-speed designs. Points to Remember • Electroless nickel/immersion gold (ENIG) finish has traditionally been the best fine pitch (flat) surface and lead-free option over recent years. • Nickel has a ferromagnetic property that can adversely affect electromagnetic fields in the high frequency domain. • The issue of the ferromagnetic properties has been addressed by introducing a nickel/ gold (NiAu) alloy. • Microstrip (outer) layers of a multilayer PCB suffer from wide variations in both trace width and thickness, hence impedance varia- tions. • It is very important not to pour copper fills on the signal layers of the board. beyond design Figure 2: immersion systems process (courtesy of Multitek).