Dendrite Formation

07 May 2006

A copper dendrite is the most common found on a printed circuit board assembly, dendrites however can be silver, tin, copper or other metallic materials. They occur when contamination is present on the surface of printed board in the presence of moisture, equally it can be found on other surfaces like on a ceramic substrate in a hybrid circuit or even on the surface of a component between two terminations. Lowering of surface resistance between a track and pad or track and via hole and can lead to corrosion. This often results in the formation of metal filaments just like a fern or Christmas tree. Electrochemical reaction dissolves metal at one electrode and forms at the opposite electrode. Often but not always they form from the anode to cathode.
The example images shows the formation of a copper dendrite between two conductors. This fault may occur when flux residues remain on the board surface and are then subjected to high temperature and humidity. Flux residues or any ionic material which can come from a variety of sources and can cause these micro shorts to form. Residues left on the boards during the printed board manufacturing stages can also lead to dendrite formation. That is why many companies requested the board to be cleaned and a specific standard of cleanliness quoted. In use the more moisture present the faster the corrosion can occur and cause this intermittent fault. If the moisture level is very high and forms as a layer on the surface of the board and is unable to run off catastrophic failure can occur.

A printed circuit board with a voltage applied of 5-10 volts may allow the formation of a conductive path. It creates an intermittent fault which can be very difficult to pinpoint. In operation when the copper dendrite makes contact with the adjacent track a current will flow causing the copper dendrite to blow. This is why this type of fault is most commonly an intermittent problem.

The same thing can occur in the field if moisture is allowed to form on the surface of the printed board with the minimum of contaminates on the surface. If it is know that moisture can firm the boards should be protected with conformal coating in selected high risk areas or the board is sealed in a suitable case.

Using the expertise of your material suppliers, this fault should not occur. It normally happens when the incorrect materials are used for an application or inappropriate setting are used in the assembly process. Material suppliers spend significant time and energy to formulate materials that pass SIR specifications. Surface insulation resistance is effectively a measure of the rate or ability to form dendrites.

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