What is Galvanizing?
Galvanizing is a process of applying a protective zinc coating to steel or iron to prevent rusting. The zinc provides a sacrificial anode that protects the underlying metal from corrosion. There are several methods of galvanizing including hot-dip galvanizing, electrogalvanizing, and sherardizing.
While galvanizing is commonly used on steel parts, cables, and structures to improve their corrosion resistance, the process can also be applied to copper traces on printed circuit boards (PCBs) to protect the copper from oxidation and corrosion. This is important for PCBs used in harsh environments.
Why Galvanize PCBs?
Protection from Corrosion
The main reason to galvanize the copper traces on a PCB is to protect them from corrosion. Copper is prone to oxidation when exposed to air, and the thin copper traces on a PCB are especially vulnerable. Oxidation increases the resistance of the copper traces and can eventually lead to breaks in conductivity.
In harsh environments with high humidity, salt spray, or corrosive chemicals, the copper traces on a PCB can quickly degrade if not protected. Galvanizing provides a durable zinc coating that seals the copper traces from the environment and provides sacrificial protection against corrosion. This greatly extends the lifespan and reliability of the PCB.
Improved Solderability
Galvanizing the copper pads on a PCB can also improve their solderability. The zinc coating acts as a solderable surface finish that makes it easier to wet the pads with solder and achieve good solder joints. Tin-lead solder alloys readily wet and bond to zinc.
Some alternative PCB Surface Finishes like ENIG (electroless nickel immersion gold) provide good oxidation resistance but have inferior solderability compared to galvanized surfaces. The gold layer must dissolve into the solder before bonding can occur.
Cost Effective
Compared to more exotic PCB surface treatments like ENIG, galvanizing is a very cost-effective option. The raw materials and process chemicals for galvanizing are inexpensive. Galvanizing also has a high throughput and yield since it is a mature, well-understood process.
For PCBs that require oxidation protection but not the wire-bonding capability of ENIG, galvanizing provides reliable performance at a much lower cost per board. It is a good choice for many industrial and automotive applications.
The PCB Galvanizing Process
Cleaning
The first step in galvanizing a PCB is to thoroughly clean the copper surfaces. Any oils, fingerprints, or contaminants will prevent the zinc from bonding to the copper. The PCB is washed with solvents and then etched with a mild acid to remove copper oxides.
Masking
Next, the areas of the PCB that should not be galvanized are masked off. This includes any connectors, through-hole pads, or test points. The mask is typically a temporary latex paint or tape that is removed after galvanizing.
Zinc Plating
The heart of the galvanizing process is electroplating the PCB with zinc. The board is immersed in an electrolyte bath containing dissolved zinc salts. The PCB is connected to a negative voltage source to serve as the cathode. Zinc metal anodes are also immersed in the bath.
When voltage is applied, the zinc anode oxidizes and dissolves into the electrolyte. The positively charged zinc ions are attracted to the negatively charged PCB and plate out onto the exposed copper surfaces. The zinc ions are reduced at the cathode surface and bond to the copper.
The thickness of the zinc plating is determined by the current density and plating time. A typical zinc thickness for PCBs is 10-15 μm.
Passivation
After zinc plating, the PCB is rinsed and then passes through a passivation step. The freshly plated zinc is reactive and can quickly oxidize or form white rust. Passivating the zinc with a chromate conversion coating improves its corrosion resistance and gives it a silver-blue color.
Masking Removal and Final Finish
Once the galvanizing process is complete, any masking material is removed from the PCB. The final step is to apply a topcoat over the zinc if desired for additional protection. Common topcoats include resin, acrylic, or epoxy coatings.
Galvanized PCB Design Considerations
When designing a PCB that will be galvanized, there are a few special considerations to keep in mind:
Trace Spacing
The minimum trace spacing on a galvanized PCB needs to account for some lateral growth of the traces due to the zinc buildup. The zinc will plate onto the sides of the copper traces as well as the top, increasing their width.
To prevent short circuits between adjacent traces, a minimum spacing of 8 mils (0.2 mm) is recommended for galvanized boards. High voltage circuits may require even more spacing.
Pad and Via Sizes
The zinc plating will also reduce the inner diameter of plated through-holes and vias. For this reason, the pad and via sizes should be slightly larger than the final desired hole size.
A typical via hole will shrink by 0.4-0.8 mils (0.01-0.02 mm) in diameter after galvanizing. Vias that will be galvanized should have a minimum drill hole size of 12 mils (0.3 mm).
Solder Mask Clearance
The solder mask opening around pads should be slightly oversized compared to a standard ENIG or OSP finish. This allows for tolerances in solder mask alignment and ensures that the zinc plating fully covers the copper pad.
A minimum solder mask expansion of 4 mils (0.1 mm) per side is recommended for galvanized PCBs. Critical RF circuits may require tighter control of the pad dimensions.
Controlled Impedance
The zinc plating will change the dimensions of the copper traces and therefore impact the impedance of controlled impedance transmission lines. If tight control of impedance is needed, this effect must be accounted for in the PCB stackup design.
The impedance of a microstrip trace is proportional to the trace width and inversely proportional to the square root of the dielectric constant of the substrate. The zinc plating effectively widens the trace and lowers its characteristic impedance.
To compensate, controlled impedance traces should be designed slightly narrower than their nominal width to account for the zinc buildup. The exact adjustment depends on the plating thickness but is typically in the range of 0.2-0.5 mils (0.005-0.0125 mm).
Galvanizing vs Other PCB Finishes
There are several common surface finishes used on PCBs to protect the copper from oxidation and provide a solderable surface. Each has its own advantages and tradeoffs compared to galvanizing.
| Finish | Advantages | Disadvantages |
|---|---|---|
| HASL | Low cost, excellent solderability, easy rework | Uneven surface, thermal stress on PCB |
| ENIG | Flat surface, gold wire-bondable, some oxidation resistance | Expensive, nickel corrosion in some environments |
| OSP | Low cost, flat surface, easy to apply | Short shelf life, no oxidation resistance |
| Immersion Silver | Excellent solderability, flat surface, some oxidation resistance | Prone to tarnishing over time |
| Immersion Tin | Good solderability, flat surface, better oxidation resistance than silver | Whiskers can grow from tin and cause shorts |
| Galvanizing | Good oxidation resistance, excellent solderability, low cost | Adds to trace/pad dimensions, must remove from some areas |
The choice of surface finish depends on the specific requirements and environment of the PCB. For many industrial and automotive applications, galvanizing provides the necessary corrosion resistance at a lower cost than ENIG.
Frequently Asked Questions
How thick is the zinc plating on a galvanized PCB?
The typical plating thickness is 10-15 μm (0.4-0.6 mils). Thicker coatings provide more corrosion protection but have a greater impact on trace dimensions.
Does galvanizing affect PCB solderability?
Galvanized pads have excellent solderability, even better than bare copper. The zinc readily wets with molten solder and helps the solder to flow across the pad. Galvanizing can improve solderability on boards that will be stored for a long time before assembly.
What is the shelf life of a galvanized PCB?
Galvanized PCBs have a very long shelf life since the zinc plating protects the copper from oxidation. Boards can be stored for several years before assembly without degradation. The exact shelf life depends on the storage conditions and any additional topcoats applied over the zinc.
Can press-fit connectors be used on galvanized PCBs?
Press-fit connectors should not be used on pads that are zinc plated. The dimensions of the plated holes will be smaller than designed due to the zinc buildup. This can cause damage to the connector pins or even prevent full insertion. Connector pads should be masked off during the galvanizing process.
Is it possible to selectively galvanize only certain areas of a PCB?
Yes, areas of the PCB that should not be galvanized can be masked off with tape or latex. This includes any connectors, test points, or other features that need to remain bare copper. The mask is removed after plating. For large volumes, a permanent plating mask can be made from sheet metal and fitted over the PCB.
