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During the production of multilayer PCBs, the exposed surface of copper is prone to oxidation and corrosion when exposed to air and moisture. This can decrease the solderability, reduce the bond strength between the copper layers and prepreg during lamination, and impact the reliability of the PCB.
To solve this, PCB oxide coating treatments—specifically brown oxide and black oxide—are applied to inner-layer copper conductors before lamination. Once the inner-layer circuits are created, these are used to provide adhesion, enhance lamination strength and build greater reliability in multi-layered PCBs.
What Is Brown Oxide Treatment?
Brown oxide is a mixture of cuprous oxide (Cu₂O) and cupric oxide (CuO). With brown oxide treatment, a uniform layer of fluff is formed on the copper surface of the substrate, which enhances the bonding force of the prepreg.
Brown Oxide Formation Process
Brown oxide formation is the simplest and cost-effective solution for surface finishing. The forming process of brown oxide includes:
- Surface cleaning: Initially, the copper surface of the PCB is cleaned to remove any oils, dirt or impurities.
- Alkaline bath: PCBs are dipped in the alkaline solution of sodium chlorite.
- Oxide layer formation: A brown-colored oxide layer is formed when the solution reacts with the exposed copper of the PCB under controlled conditions.
- Rinse and prepare: Finally, PCBs are cleaned and then sent for the assembly process.
Typical brown oxide coating thickness: 8–25 micrometres (µm)
What Is Black Oxide Treatment?
Black oxide is an advanced copper oxide surface treatment consisting mainly of cupric oxide (CuO) with some cuprous oxide (Cu₂O).
Black Oxide Formation Process
There are three main methods of applying black oxide: black oxidation, brown oxidation and low-temperature black oxidation. Although the specific chemistry differs, all three methods follow a similar overall process.
The formation process of black oxide includes:
- Surface cleaning: The copper surface of the PCB is cleaned to remove any oils, dirt or impurities.
- Alkaline bath: PCBs are dipped in the alkaline solution of sodium chlorite.
- Aggressive oxidation: A black oxide layer is formed when the solution reacts with the copper more aggressively under high oxidation intensity.
- Hot water rinse: PCBs are rinsed with hot water.
Typical black oxide coating thickness: 1–13 micrometres (µm)
Difference Between Brown Oxide and Black Oxide
The basic function of the brown and black oxide treatment is to enhance the bonding force by increasing the contact area between the copper foil and resin. They also:
- Improve the wettability of the copper surface, enabling the resin to flow into dead corners effectively.
- Form a passivation layer on the copper surface, preventing hardener from reacting with copper under high-temperature and high-pressure conditions.
While oxide treatments are carried out to protect the copper surface from oxidation and corrosion, each type of treatment has its own criteria. Some differences between brown and black oxide are as follows:
Brown Oxide vs Black Oxide: Full Comparison Table
| Parameter | Brown Oxide | Black Oxide |
| Surface Structure | Granular / dense | Needle-like crystals |
| Fluff Thickness | Thinner | Thicker |
| Coating Thickness | 8–25 µm | 1–13 µm |
| Chemical Management Difficulty | Lower | Higher |
| Micro-etching Rate | Lower | Higher |
| Solderability | Poor | Moderate |
| Corrosion & Leach Resistance | Poor | Moderate |
| Thermal Transfer | Poor | Moderate |
| Surface Roughness & Coverage | Smaller roughness, limited coverage | Larger roughness, better coverage for minor repairs |
| Pink Ring Risk | None (good acid resistance) | Possible under poor process control |
| Process Cost | Low | Moderate |
Limitations of Each Oxide Treatment
There are some limitations of each oxide treatment process.
Brown Oxide Limitations
- Poor solderability: Brown oxide provides poor solderability as the oxide does not dissolve easily.
- Low corrosion resistance: It has poor resistance to corrosion and leaching.
- Thermal flow inhibition: A too thick coating of brown oxide can inhibit the thermal flow.
- Environmental sensitivity: Under harsh environmental conditions, it may cause delamination and contamination.
Black Oxide Limitations
- Chemical sensitivity: Black oxide has poor resistance to chemicals compared to the more modern oxide treatment.
- Process control demands: Inadequate control can negatively affect surface insulation resistance.
- Layer cracking: It gets easily affected by heat and chemicals.
- Pink ring risk: If the process control is not adequate, a risk of pink ring formation can be observed in multilayer PCBs.
PCB Application Guide: When to Use Brown Oxide vs Black Oxide
The oxide treatment application varies based on the choice of quality, performance, cost and different process control mechanisms. Some application areas of each oxide treatment are described below:
When to Use Brown Oxide
Brown oxide is the preferred choice for:
- HDI boards and high-layer-count PCBs – Brown oxidation is the preferred choice for high-density interconnect (HDI) boards and high-multilayer designs, as it provides a denser, more flexible oxide film
- High-speed and high-frequency electronics – Support signal integrity in applications such as smartphones and communication devices
- High thermal stability applications – Recommended where good inner-layer adhesion under thermal stress is critical
- Lead-free soldering processes – Black oxide is not suitable for lead-free soldering due to its poor leach resistance, which causes tin-copper intermetallics to leach out and degrade solder joints; brown oxide is the safer choice in lead-free environments
- PCBs under high mechanical stress – Provide reliable performance in mechanically demanding environments
When to Use Black Oxide
- Standard multilayer PCBs – Ideal for complex layer stack-ups requiring reliable adhesion where HDI-level precision is not required
- Cost-sensitive designs– Black oxide is often preferred when performance demands are lower and cost efficiency is the priority
- Consumer electronics and low-performance devices – Suitable for non-critical applications
- Legacy multilayer PCBs and impedance-controlled boards – Still widely used in older designs
Conclusions
During the manufacturing process of multilayer PCB, the adhesion between copper layers and prepreg plays an important role. Both PCB brown oxide and black oxide aim to achieve the same main goal: roughening the copper surface to enhance bonding with resin.
- Brown oxide is easier and cheaper to produce a copper surface with that layer of oxide film, which is denser and more flexible. As a result, it will give better results when subjected to thermal cycling and mechanical stress.
- With the rougher and thicker surface created, black oxide can give more initial adhesion strength to standard multilayer PCBs. However, the oxide film of black oxide can be very brittle, and the process has to be controlled very strictly, which means that black oxide cannot be used for very high-reliability or high-density applications.
Of course, one can hardly say that brown or black oxide treatments are always better. The choice of brown oxide or black oxide will depend on the design needs of the PCB, its manufacturing process, cost and reliability of the application. As PCB design continues to evolve towards higher densities and more demanding environments, understanding the advantages and disadvantages of these two processing methods helps to make informed engineering decisions.
FAQs
A1: Generally, black oxide can not be used for HDI boards due to the following reasons:
- Black oxide has low thermal stability, and the hard oxide film can peel off with repeated heating and cooling of HDI boards.
- HDI designs demand fine lines and tight spacing. But black oxide offers less flexible bonding compared to that of HDI designs.
- Under mechanical and thermal stress, a reliability problem may arise due to delamination.
A2: The surface roughness plays an important role in PCB lamination. It provides the following functions:
- Expands surface area for good resin bonding with copper.
- Creates micro anchor points that will bond resin and increase adhesion.
- Resists delamination when subjected to mechanical stresses and heat.
- Enhances the general lamination properties and the reliability of PCBs.
- The surface roughness must be carefully controlled. If it is too rough, poor resin flow occurs, and if it is too smooth, poor bonding occurs.
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