The rise of consumer electronics has brought challenges with the surface finish of connections. We have all experienced how, over time, the cables we use to charge cell phones or connect earphones stop working.
While there’s peopl that believe this is due to “programmed obsolescence”, that is far from true.
In reality, the root cause of the failing connections is due to 2 different processes:
- Wear and tear due to use
- Contact corrosion
Wear and tear can be limited by using harder metallic coatings to make connectors last longer.
However, contact corrosion is more difficult to control. In general, this corrosion is limited by using surface finishings with noble metals like gold. These surface finishings, in theory, should not oxidize easily. In practice, though, these surface finishings end up showing corrosion over time.
Why does this happen? We will explain the reason below.
What is contact corrosion?
To understand why connections end up showing corrosion, we must know what contact corrosion is.
Contact corrosion, also known as galvanic corrosion, is an electrochemical process that occurs when 2 different metals are in contat between themselves and with an electrolyte.
These conditions cause that one of the two metals, the most reactive or “less noble”, is consumed producing an oxide layer.
Why does contact corrosion happen?
All metals have what is known as the normal electrode potential or normale reduction potential. This potential determines the amount of voltage required to oxidize or reduce a metal and is measured against a known reference potential. Normally vs SHE.
Due to this potential, when 2 different metals are connected and in the presence of an electrolyte, what we have is, essentially, a shortcircuited battery. So, as this battery gets discharged, the most reactive metal corrodes until the potential difference is 0.
Is contact corrosion dangerous?
If contact corrosion is not adressed during the design stage, it can cause the failure of the structure or electronic device.
In the case of metallic structures, unmanaged corrosion can lead to the failure of the structure. Meaning that the metallic structure could collapse. This happens because the mechanical resistance of metals and metal oxides is considerably different. Therefore, corrosion must be managed to avoid catastrophic results.
In the case of electronic devices, contact corrosion can lead to shortcircuits and lower conductivity of electrical contacts. So, eventually, the device stops working. An example would be the contact corrosion observed in some Lightning Charger Cables, where the corrosion leads to an error in detecting the cable connection and slower battery charging.
Why is there contact corrosion in consumer electronics?
As we explained above, contact corrosion occurs when 2 different metals are connceted between them and in the presence of an electrolyte.
In the case of consumer electronics, contact corrosion occurs for 2 reasons:
- The gold surface finish is porous, which means that both the base metal and the gold finish is exposed
- We handle electronic devices with our bare hands. This exposes devices to our sweat, which contains salts and act as an electrolyte.
On top of that, as the device gets worn through use, more base metal gets exposed, accelerating the corrosion.
How to avoid contact corrosion?
| Metal | Anodic Index, V |
|---|---|
| Gold | 0 |
| Platinum | 0 |
| Silver | 0.15 |
| Nickel | 0.3 |
| Copper | 0.35 |
To avoid contact corrosion, the best is to use a single metal or electrically insulate them. For example, with a layer of paint.
However, depending on the application this may not be possible. In these cases the anodic index is used.
The Anodic Index is a measurement of the electrochemical potential difference between a metal and gold in contact corrosion conditions.
With this index, contact corrosion can be minimized. In fact, it is recommended that:
- For hostile environments, such as outdoor applications, the potential difference should not exceed the 0.15 V.
- For normal environments, such as consumer electronics, the potential difference should not exceed the 0.25 V.
- For controlled environments, such as indoor applications with controlled temperature and humidity, the potential differenc should not exceed 0.5 V.
One of the solutions to reduce corrosion in consumer electronics is to introduce a thick intermediate platinum layer. With these, the potential difference between the gold finish and the platinum base is reduced to 0 V. However, this solution is not viable from an economic point of view, since platinum as a scarce and expensive resource.
Hardened metal finish from Macias Sensors to avoid contact corrosion
At Macias Sensors we have developed a surface finish that allows us to use PCBs to make electrochemical measurements without experiencing corrosion.
This is possible because our process is able to completely cover the exposed electrode, avoiding the base metal to be exposed to the electrolyte and get oxidized. Therefore, our surface finish is an ideal solution to contact corrosion in consumer electronics.
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