Revolutionised physical properties of electronic devices Metallizing PCB Edges

Autor / Redakteur: by Arnold Wiemers / Claudia Mallok

One speciality of the PCB for the application platform “meltemi” is edge plating (or edge metallizing). Edge plating provides clear measurable advantages at comparably ignorable costs for devices with moderate or high demands regarding EMC, signal-integrity and de-heating. Practice has proved that the theory can be implemented and works.

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( Archiv: Vogel Business Media )

Regarding EMC aspects a device is operating stabile if it doesn’t radiate own interferences (emission) and doesn’t show any reaction to interferences being radiated by other devices (immission). In a technical view these interferences are caused by signal transmission between components of a device or between several parts or boards of a device system. Specific signal transmission in complex multilayer systems takes place via signal tracks on the PCB. Power supply is provided by plane areas or plane layers.

It has to be considered, that for each signal transmission a back current path has to be provided (the so-called GND plane) and that the transport of the information doesn’t take place in two-dimensional packets but three-dimensional electro-magnetic fields. Therefore any device (and any PCB) is a giant electronic microcosm with an hardly imaginable diversity of happenings in smallest spaces.

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Herein lies the key for an extended strategy for achieving EMC stability. The construction of a multilayer is done by laying signal layers and power plane layers above each other. During operation of a device created but unnecessary energy is emitted via the edge areas of the power planes. Schematic concepts are trying to compensate these effects, i.e. by termination of signals, by defined signal properties (impedance) or by high-capacitive MultiPowerSystems (MPS). Success of these linear signal transmission can’t be comprehensive. An effective control of the electromagnetic fields within the three-dimensional space of the PCB is missing.

However, control can be achieved if field areas are divided into compartments on and within a PCB. In principle the creation of these compartments can be provided via a structured shielding.

EMC behaviour of the PCB significantly improved

Analysing a multilayer construction leads to the simple cognition that each layer is ending rectangular at the PCB edge. In saying so the task is defined: control of the three-dimensional electromagnetic space in a PCB is provided by shielding the edges of the PCB. The prognosis is that in doing so the EMC behaviour of a PCB is significantly improved.

Plating PCB’s

In order to give an impression of the function of a shielding it is necessary to describe the electro-plating process. The production step of plating is an elementary one in PCB production. The galvano-technical connection of several layers in a PCB is done by a cathodic-anodic procedure for copper deposition at the inner wall of a drilled hole. In a topological sight the inner wall of a hole is part of the PCB surface.

The electro plating process therefore always causes a metallic deposition on the PCB surface. This surface can be modified in different ways by holes, slots or millings. If the subsequent PCB edge is to be metallised mainly minor logistic re-arrangement of production steps is necessary.

The edge to be plated isn’t processed (as usual) in one of the last production steps but has to be carried out already before the plating process.

To elaborate the contours milling tools are advisable. With these tools straight or rounded contours are produced without any problem. Cut-outs and/or partial milling of the contour is possible. Milling can be performed to open the contour over the complete thickness of the PCB. Anyhow, it is possible to have plated z-axis millings, opening the panel surface only down to a certain depth. If a plating of the complete outer contour of a PCB is desired it has to be considered that for the following production steps 1-2 mm wide pegs to hold the PCB within the panel are necessary which cannot be plated. The edges of all plated contour millings will have the same galvanic final surface plating as the actual PCB (see figure 1).

Manufacturing costs for plated edges are low. Any manufacturer of PCB’s has a galvanic production line and a milling facility. Thus no additional invest in machines or into inventing a new process will be necessary. Only in production process sequences or PPS a decision “edge plating yes/no” has to be invented.

De-heating

Primarily the motivation for edge plating was controlled EMC behaviour. Very soon it was obvious that also the active de-heating of a PCB is improved significantly. Developing of more and more capable and powerful electronics on decreasing space inevitably leads to thermal problems. Conducting the heat by additional measures (heat sinks, casing) is extensive and in-creases device costs.

One of the strategic aims of CAD design and device construction therefore has to be the integration of de-heating into the multilayer system. In addition to constructive properties of a multilayer (thick copper layers, MPS) the conductive pattern has to be included in the de-heating concept.

The fact that edge plating has more than a trivial share in this purpose arrises from the calculation of the edge surface of a standard euro PCB (100 mm × 160 mm).

The result is a edge surface of 2 (10 + 16) ⋅ 0.15 = 7.80 cm². In today designs this net area hardly can be kept free on outer layers for de-heating purposes. Additionally the quality of the de-heating via the PCB edge has to be considered. A compact area on an outer layer of a PCB never has the capability for heat spread and heat conduction as a closed area like band laid around the edge of the PCB (figure 2).

Signal-integrity

The component industry already accepted that edge plating in principle is an advantage for signal-integrity in the point of signal transfer from device to periphery. For any demanding electronic system (i.e. LVDS, high speed) the interface from device to cable and/or plug is a vulnerability. For a few millimetres on the way from PCB edge to the cable the shielding or the continuous reliable ground reference for signal transmission is missing. Clearly noticeable disturbances in signal quality can be the consequence. Signal quality can be secured if PCB, plug and cable form an entity.

This task is solved with plugs which are mounted on the plated edge of a PCB. The differential signals are transmitted from the upper and lower region of the plug to the top and the bottom side of the PCB. In the centre area the plug has a metallic joint face that simultaneously provides a reference plane. On the back side this joint face is connected to the partially metallised PCB edge (figure 3). By this means in ideal conditions each signal faces a homogeneous electro-magnetic field from cable via plug to the input component on the PCB.

The result

Edge plating provides clearly measurable advantages at comparable ignorable costs for devices with moderate or high demands regarding EMC, signal-integrity and de-heating. EMC stability of “meltemi” is impressive. And the quote from the EMC test facility “the probe must be damaged, I’m not detecting anything” is already legendary.

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