In PCB fabrication, copper conductors are coated with solderable surface finishes to protect the copper from oxidation and improve solder joint formation. The three most common surface finishes are:
- Brown oxide
- Black oxide
- Immersion tin
This article examines the processing, performance, and applications of these three surface treatments for PCBs. Their benefits and limitations are compared to help select the optimal finish.
Brown oxide is the simplest and lowest cost PCB surface finish. It involves:
- PCBs are immersed in an alkaline sodium chlorite solution
- The solution reacts with exposed copper to deposit a brown-colored oxide layer
- PCBs are rinsed and sent for assembly
The brown deposit comprises cuprous oxide (Cu2O) and cupric oxide (CuO).
The oxide coating thickness ranges from 300 to 1000 μin (8-25 μm).
- Simple room temperature process
- Very low cost
- Provides moderate protection against oxidation
- Brown color provides solder mask alignment aid
- Oxide dissolves into molten solder during assembly
- Low cost consumer electronics
- Non-critical connections not needing solderability
- PCBs with selective spot plating
- For solder mask alignment reference
- Not RoHS compliant due to high lead content
- Poor solderability as oxide does not dissolve well
- Thick coatings impair thermal transfer
- Can flake off and contaminate assemblies
- Limited component lead adherence
Black oxide produces a thinner and more solderable finish than brown oxide:
- PCBs immersed in alkaline sodium chlorite solution
- More aggressive chemistry converts copper to black oxide
- Followed by hot water rinse
Primarily cupric oxide (CuO).
50 to 500 μin (1-13 μm).
- Room temperature process
- Low cost
- More solderable than brown oxide
- Provides good oxidation resistance
- Black color gives high contrast for inspection
- Cost-sensitive consumer and industrial electronics
- Selective soldering requiring solderability
- Improved solderability versus brown oxide
- Inferior leach resistance to most other finishes
- Lower surface insulation resistance
- Can still flake off boards over time
- Not RoHS compliant due to high lead content
Immersion tin is an electroless plating process for depositing a uniform solderable tin layer:
- PCBs pass through concentrated tin salt solutions
- Tin ions simultaneously deposit onto exposed copper
- The deposited tin catalyzes further tin plating
- Fully coats copper traces with matte tin finish
Nearly pure tin (Sn).
20 to 500 μin (0.5-13 μm).
- Excellent solderability maintained over shelf life
- Reflows into bright shiny solderable surface
- RoHS compliant and lead-free
- Superior leach and corrosion resistance
- Consumer and industrial electronics
- Automotive electronics
- High reliability assemblies
- Lead-free soldering processes
- Higher cost than oxides
- Not suitable for press-fit connections
- Requires process control to minimize voids
- At high temperatures can oxidize to non-solderable surface
Comparison of Performance
|Parameter||Brown Oxide||Black Oxide||Immersion Tin|
|Environmental||Not RoHS Compliant||Not RoHS Compliant||RoHS Compliant|
- For moderate cost and solderability – Black oxide with selective immersion tin
- For lowest production cost – Non-soldered connections use brown oxide, soldered pads/terminals use immersion tin
- For high reliability assemblies – Immersion tin finish over copper pads, nickel/gold on component leads
Preparation – Thorough cleaning and microetching improves adhesion and uniformity.
Operating Parameters – Parameters like temperature, solution concentrations and dwell times need optimization for quality finishes.
Agitation – Solution agitation improves deposition uniformity.
Solution Maintenance – Regular analysis and replenishment of solution prolongs bath life.
Rinsing – Generous rinsing prevents staining or contamination of surfaces.
Drying – Ambient drying in clean air avoids water stains.
- Brown oxide provides the most cost-effective finish for PCBs not requiring soldering ormaximized performance.
- Black oxide offers moderately improved solderability and protection over brown oxide for low cost boards.
- Immersion tin provides the best solderability, oxidation resistance, shelf life and leach resistance for quality requirements.
The finish can be selected based on cost, performance and reliability needs and mixed on selective basis.
Frequently Asked Questions
What is the main composition of brown oxide and black oxide finishes?
Brown oxide contains a mixture of cuprous oxide (Cu2O) and cupric oxide (CuO) while black oxide is primarily cupric oxide (CuO).
Which provides better oxidation resistance – black oxide or immersion tin?
Immersion tin provides significantly better oxidation resistance compared to black oxide due to the formation of a uniform metallic tin layer.
What PCB metallization works best under immersion tin plating?
What is the typical thickness of immersion tin finish on PCBs?
A thickness between 50-500 microinches (1.25-13 microns) is commonly used for immersion tin coatings on PCBs. Thicker finishes improve leach resistance.
Is black oxide suitable for lead-free soldering processes?
Black oxide is not suitable for lead-free soldering because of its poor leach resistance causing tin-copper intermetallics to leach out, degrading solder joints. Immersion tin is preferred.
Research on the Causes of black-oxide copper process
The super-roughening liquid is a copper surface treatment process designed for the copper surface. The solder mask uses the ultra-roughening process, which have good adhesion between the soldering oil and the copper surface, prevent the soldering ink from falling off.
Therefore, the ultra-roughening process in the PCB industry has been widely used.
In the process of super-roughening, there are a large number of blackening of the copper surface. The picture is as follows:
3. Analysis of the reason of blackening of super-roughened copper surface
3.1 SEM comparison analysis of super-roughened blackening position and normal copper surface position; see the following figure:
3.2 SEM analysis: It can be seen from the above picture that the copper crystal lattice is irregularly irregular in the blackout position. The normal copper lattice is in the form of a honeycomb, and the copper lattice distribution is better. The preliminary analysis of the abnormal black copper surface shows that the copper lattice is poorly distributed or the copper lattice is too coarse, resulting in blackening of the over-roughened copper surface.
4 Test verification
4.1 Compare the simulated test with 20ASF, 27ASF and 38ASF respectively; the results of super-roughening are as follows:
4.2 Comparison test with different current densities, the results show that the current density of copper plating on the VCP pattern plating line is too large, resulting in blackening of the super roughened copper surface.
5 Improvement method and improvement results
5.1 Improvement method: Adjust the copper lattice. (In order to facilitate the over-roughening of the chemical water, the electroplated copper lattice distribution is better, the copper current density is lowered, and the copper plating time is extended).
5.2 Test PCB board verification:
5.2.1 Because there are only two program line speeds for the VCP diagram wire of our pcb factory, the first line speed is 0.8m/min copper plating time 30 minutes, the second line speed is 0.4m/min copper plating time 60 minute.
5.2.2 Quantity 400PNL. (Test board parameters: copper plating time of 60m for the second wire speed of 0.4m/min, copper current density of 18ASF)
5.2.3 Tracking test the effect after improvement.
Verification of VCP pattern plating by test plate The large current density production method has an effect on the super-roughening process of the solder mask. The current density of the VCP pattern plating copper plating is adjusted from 32ASF*30 minutes to 18ASF*60 minutes, which can solve the super roughening copper surface blackening problem.