In previous posts we have exposed the natural tendency of electronic devices to be integrated in a PCB. This tendency also applies to electrochemical sensors.
At the moment, the transition of electrochemical sensors towards printed circuits has been accomplished by screen-printing with the SPEs. This fabrication method, while it reduces costs and enables mass production, has its limitations. Especially when developing complex sensing systems.
The solution to these limitations is to integrate electrochemical sensors in PCBs. But until now, this has been a challenge. All traditional PCB surface finishings were not designed for electrochemistry, but to avoid copper oxidation in the PCB and facilitate solderability.
In this post we will explain the most common PCB surface finishes and why they do not work for electrochemistry.
Traditional surface finishings for PCBs
Why do PCBs need a surface finish?
The last step in the fabrication of PCBs is the surface finish. This surface finish is required, since the copper used for the connecting the different components ina PCB oxidizes with time when exposed to air. This oxide layer negatively affects the performance of a PCB. Therefore, the exposed connection pads need to be protected by a specialized surface finish.
Nowadays, the most common surface finishings for PCBs are HASL, ENIG, ENEPIG, Hard/Soft Gold and ImAg.
Hot Air Solder Level or HASL
HASL is the most economic surface finish. It consists in a thin layer of solder deposited on top of the exposed copper after the soldermask application.
This layer is applied by immersing the PCB in molten solder and removing the excess with “air knives”. These “air knives” are, essentially, pressurized air that works as a “knife” and removes the excess solder in a similar way as hand dryesr push excess water.
The HASL finish, while very economic is not suitable for electrochemical measurements. Since it can be corroded upon applying a potential in an aqueous electrolyte.
Electroless Nickel Immersion Gold or ENIG
ENIG is the most common surface finish in PCBs.
This means that, if gold is deposited onto copper directly, it will diffuse through the gold and end up oxidizing once it reaches the surface.
The immersion gold layer provides a good corrosion resistance. However, this can’t be deposited directly onto copper. Since copper diffuses into gold at room termperature.
To avoid this diffusion problem, a thick nickel layer is deposited between the copper base and the gold layer. This nickel layer acts as a diffusion barrier for copper and makes ENIG a stable surface finish for traditional PCBs.
However, if ENIG PCBs are used for electrochemical measurements, corrosion will appear.
This happens bevause the gold layer is very thin an porous. And while it work OK to prevent PCBs from corroding in air, it does not perform for electrochemistry measurements. Once a potential is applied inside an electrolyte, first the exposed nickel dissolves and then the copper base. Ultimately, the PCB device would be destroyed and useless.
Electroless Nickel, Electroless Palladium Immersion Gold or ENEPIG
This surface finish is very similar to ENIG. However, it displays an extra layer of palladium between the nickel and gold layers.
Thanks to this palladium layer, ENEPIG show an improved corrosion performance over ENIG. But this comes at a higher price.
While ENEPIG shows an improvement in corrosion over ENIG for traditional PCB applications, it is not enough to consider this surface finish for electrochemical measurements.
The porosities present on both the gold and palladium layers are enough to expose the nickel underplate during electrochemical measurements. So once a potential is applied inside an electrolyte, the results are the same as in ENIG coatings: nickel and copper dissolution leading to the destruction of the device.
Hard and Soft Gold
Both Hard and Soft Gold surface finishings are electrolytic coatings. Meaning, they are deposited by a process known as electrodeposition.
Similar to ENIG, this surface finish also has an intermediate nickel layer to block copper diffusion. However, in this case, the gold layer is significantly thicker, often reaching several microns.
Due to this increased thickness, and the need for specialized equipment, this surface finish is the most expensive.
As the title suggests, this surface finish has two variants:
- Hard Gold: in this case the deposited gold layer is not pure, but an alloy to provide gold with an increased hardness. The most common alloys are gold-cobalt and gold nickel, maintaining gold percentage in the plat > 99.5 %. This finish is used for edge connectors in PCBs to prevent excessive wear and tear due to repetitive connections
- Soft gold: in this case the deposited gold layer is very pure, with over 99.9% of gold content in the plated layer. As a result, this gold layer is softer and not suitable for repetitive connections. However, due to its high purity, it is often used in biomedical applications to avoid contact with toxic metals like nickel and copper.
The Hard/Soft Gold is the surface finish that provides the highest corrosion protection, since the gold layer is very thick.
Still, this surface finish is not good enough for electrochemical measurements,
Despite having a thick gold layer, this layer is still too porous and exposes the nickel layer. Therefore, once a potential is applied, the nickel and copper underplates dissolve and the PCB is destroyed.
Often, in an attempt to avoid this corrosion, thicker and thicker gold layers are deposited. Sometimes even reaching 100 micrometers! However, this does not guarantee the success of the plate. It must be noted that the problem is not thickness, but porosity.
Immersion Silger or ImAg
ImAg is a simpler surface finish than ENIG, ENEPIG or Hard/Soft Gold. Also, it is more economic and better for the environment due to the chemicals used.
This surface finish consists in a thin silver coating directly onto the copper base.
While silver also oxidizes in air, it does so at a slower rate than copper. Thus, ImAg finished PCBs offer some corrosion resistance, but not as good as ENIG, ENEPIG or Hard/Soft Gold.
This surface finish is interesting for electrochemistry, particularly to integrate reference electrodes. However, it is not suitable for working or counter electrodes. That’s because if a silver electrode experiences a current it will oxidise and dissolve. Therefore, ImAg is not suitable to develop PCB-based electrochemical sensors either.
Why none of the traditional PCB surface finshings do not work for electrochemistry?
The processes aforemention tend to leave pores between the layers. For this reason, even if gold is used as the top layer, the underplates corrode because the pores expose them.
Since this corrosion is only observed during electrochemical measurements, that is, when a potential is applied inside an electrolyte, this issue has not been addressed. After all, PCB surface finishings were developed to ensure shelf-life stability and solderability of PCBs.
The solution: a surface finish specially developed for electrochemistry.
At Macias Sensors we are aware that, for electrochemical biosensors to transition into PCBs, a suitable surface finish is required.
For this reason, we have developed our own process of hardened metal deposition. Specially designed for electrochemical measurements.
Thanks to our process, we can supply PCB-based electrochemical sensors that can perform electrochemical measurements without damaging the device.
Our surface finish enable the integration of electrochemical biosensors into complex PCB assemblies. As a result, we can rapidly develop complex biosensor systems that combine:
- teperature control
- internal calibration
- complementary sensors