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40 The PCB Design Magazine • June 2014 applied after solder mask. In the second case, nickel is not plated on the traces. However, the lands are protected by the plating, which are exposed to the environment prior to assembly. This is described as solder mask over bare cop- per (SMOBC) processing and is best for high- speed design. Plus, given the cost of gold, liquid photoimageable solder mask (LPISM) should be applied before the ENIG process in order to limit the plating area. In a microstrip configuration (Figure 1), electric fields (blue) exist between the traces and the reference plane. At high frequencies (> 1 GHz), the current density (red) tends to build up on the edges and surface of the plating due to the skin affect. Therefore, special consider- ation should be incorporated for any edge-cou- pled structures on the outer layers. This impact is more prevalent when ENIG is applied over all copper features, such as application of ENIG before solder mask. But, this effect can be mini- mized when only the pads (not the traces) are plated with the finish. ENIG plating thickness is specified by the IPC-4552 standard for ENIG plating for print- ed circuit boards. The gold plating is typically very thin, 0.075–0.125 µm. On the other hand, nickel plating, which is used to stop copper mi- gration to the gold, normally requires 3–6 µm thickness. These thicknesses can vary between fab shops and processes. Higher gold thickness requires extended solution dwell time or in- creased solution temperature. Plating finishes can impact the overall loss of the transmission line due to lower conduc- tivity metallization that covers the surface of the trace. Silver plating is an exception to this increased loss effect due to its similar conduc- tivity to copper. This finish, however, is not as preferred as other finishes like ENIG or immer- sion tin. As electronic devices have miniaturized, so too have their chip package sizes, along with the size and clearance of lands. As a result, short-circuit risk has increased. The plating of fine patterns (15 µm spacing) clearly favors pal- ladium/gold (Pd/Au) or EPIG. This is also a solu- tion to the lossy transmission lines, since this finish contains no nickel. EPIG is also ideal for wire bonding applications. Generally, most of the surface treatment dis- solves into the solder paste or wave solder dur- ing the soldering process and the solder joint is formed between the solder and the copper. One exception is ENIG, where the solder dissolves the thin layer of gold and forms a joint with the underlying nickel alloy. The immersion systems process (Figure 2) uses a chemical displacement reaction to de- posit a metal layer onto the exposed copper surface of the PCB. The base metal (copper) do- nates the electrons that reduce the positively charged metal ions present in solution. The im- mersion layer will continue to grow however, as the thickness of deposit increases, the rate of deposition falls. Therefore, the process is self- limiting. There are four design concerns associated with ENIG plating: 1. Solder mask defined BGA pads should be avoided, due to the risk of: a) Brittle joints (the inter-metallic com- pound that is formed when soldering against nickel). beyond design Figure 1: Microstrip electric fields and return current distribution. SURFACE FINISHES FoR HIgH-SPEED PCBS continues

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