Porosity: real-world concerns

Porosity is not something that may immediately spring to mind when considering switches or connectors yet it is a real problem in the real world and can substantially affect the workings of these humble devices.

So what is porosity? Gold is often plated into plugs, sockets, connectors and switches to improve their electrical conductivity. However, sometimes the plating is faulty, leading to inferior performance. And although the faults are tiny and cannot be seen by the naked eye, they can cause major failures, sometimes in a matter of weeks from manufacture.

Faulty plating begins with the manufacturing process and can include substrate surface defects, unclean plating, micro cracks in deposits, the thickness of the plating and variables in the manufacturing process.

When considering gold deposition, the nickel underplate is important as a porosity control mechanism. Thicknesses of 2.5 to 3.8 microns are probably ideal but 1.27 micron can be acceptable by allowing a small degree of porosity.

Anything thinner than this is likely to cause big problems. It is also necessary that the underplate should be stress free and ductile otherwise cracking can occur leading to defects in the gold plating.

When it comes to performance there are two basic types of porosity that can influence the operation of the component. The types are passive and active. Passive is where the discontinuity is through the gold to the nickel underplate only. The nickel will form a nickel-oxide film which is self-limiting in thickness and is passive regarding corrosion. The nickel-oxide performs in the same manner as passivation in stainless steel. Assuming that the gold thickness is greater than 0.75 microns, a contact that comes to rest on a pore site will not contact the exposed nickel and will not cause a fault.

An active pore is where the discontinuity descends through the gold and the nickel, exposing the base metal, which is usually a copper alloy. The base metal will start to corrode and extend through the pore channel to the contact surface. This may cause the contamination to creep across the gold surface and the probability of failure increases. This is the main concern. Part of the concern is that pore corrosion is time-dependent. Porosity has caused field problems in as little as a few weeks and as long as years depending on the application and performance requirements, and a further problem is that the sites occur randomly.

The number of pores increases significantly as the gold and/or nickel thickness decreases. The effects of porosity may be controlled at gold thicknesses greater than 0.762 microns with the use of pore blockers. For gold thicknesses of less than 0.762 microns, porosity can increase dramatically.

With gold thicknesses of less than 0.25 microns, it is no longer an issue of pores but instead a lack of surface coverage, which results in large areas without gold plating. Thin gold can be used but with a great deal of care and it should not be used when durability requirements include more than 10 mating cycles, when fretting is a problem or when direct exposure to a harsh environment is involved. Sheltering the connector system is essential as anticipated field life is short.

Based on a paper by Max Peel, Senior Fellow at Contech Research in the US.

Robin Pearce, Bishop & Associates
rpearce@bishopinc.com