What Is PCB Corrosion?And What Causes PCB Corrosion?

What is PCB corrosion?

Corrosion is the process by which the PCB reacts chemically with the environment and loses its performance. Corrosion can cause a lot of problems in the PCB, such as short circuit, reduced current capacity, increased resistance, etc.

Corrosion is the oxidation process that occurs when oxygen combines with a metal, producing rust and causing the metal to peel off and lose its valuable chemical properties. Since PCBS are mainly made of metal and exposed to oxygen, they are vulnerable to corrosion.

However, not all metals are created equal when it comes to corrosion.

Highly corrosive metals include:

· graphite

· gold

· silver

· Copper-nickel alloy

Easy corrosive metals include:

· tinning

· guide

· copper

· Nickel plating

What causes PCB corrosion?

1. Electrolyte

When an electric current is applied to a conductor, an electric field is generated, which can cause electrons to flow from one terminal of the circuit to another, leaving behind ions/charged particles, which are attracted to the surface of the conductor and stick to it causing corrosion, a process called electrochemical migration.

This migration usually occurs in the presence of an electrolyte, the most common of which is water.

2. Water

Water penetrates into the metal of the PCB, which is particularly prone to corrosion, and if uncontrolled, enters other parts of the PCB, it may cause serious damage.

3. Humidity

If the humidity in the air is too high, it will cause the material used in the PCB to oxidize.

4. Flux residue

The flux residue is mainly left during the welding process, usually composed of chemicals such as borax, chlorinated press and lead acetate, which will corrode the copper trace on the PCB, and the longer the corrosion, the more serious.

Do you know what types of PCB corrosion are?

1. General attack corrosion (atmospheric corrosion or uniform corrosion)

The most common type of corrosion, also known as atmospheric corrosion or uniform corrosion, is usually caused by a chemical reaction between oxygen/moisture in the air and copper.

General corrosion attacks lead to the formation of copper oxide, low conductivity can cause problems with the PCB, but the mechanical properties remain the same, which is usually easy to diagnose and prevent.

2.Local corrosion

· Filiform corrosion

Moisture entering below the surface finish can cause defects in the copper, which then have the potential to spread to a wider PCB.

· Crevice corrosion

Areas below the components on the hardware or PCB may corrode in these gaps due to residual substances. Flux, cleaning solutions, or other contaminants in such gaps often cause this type of corrosion.

· Corrosive pitting

This type of corrosion is seen as voids or "holes" in copper. Local electrochemical reactions lead to deterioration, resulting in increased pit diameter and depth, and ultimately failure. Corrosion producing compounds often hide pits, making them difficult to detect.

3.Galvanic corrosion

Galvanic corrosion, also known as bimetallic corrosion, occurs when two different metals (such as copper and the component's metal, gold, or tin) are coupled in a corrosive electrolyte.

This is similar to pitting, but the main difference is that electrochemical corrosion occurs only between electrochemically different metals when they are in electrical contact and the metals are both exposed to the electrolyte.

4.Electrolytic dendrite formation

When there is ion contamination in the water, dendrites will form on the copper trace line. When the adjacent copper trace grows dendrites, it may short circuit and cause PCB failure.

5.Intergranular corrosion

Intergranular corrosion occurs when there are chemicals on the grain boundaries of copper traces. Grain boundaries typically contain higher impurities that are more sensitive to this type of corrosion.

6.Solder resistance residue corrosion

Flux residue from welding has always been a major cause of corrosion. Flux is very important in the welding process because it prevents oxidation during welding. The flux actually reduces the metal oxides that form at the electrical contacts of the weld due to heating. The flux also improves the wetting of the bead on the pad.

Older flux materials contain corrosive chemicals, such as chlorine, and the residue must be removed after welding. Newer fluxes use organic acids that decompose at high temperatures.

When reflow welding components, there is usually no solder resistance residue. But with wave soldering, the flux is unlikely to reach the temperature needed to break down the acid. Any remaining flux residue is acidic and highly corrosive.

7.Fretting corrosion

In fretting corrosion, the action of turning off the welding switch produces a wiping action that removes the surface oxide layer and oxidizes the underlying layer. Eventually, too much rust can build up and prevent the switch from activating.

How to prevent PCB corrosion？

1.Cover PCB with conformal coating

In cases where it is not possible to encapsulate the PCB in the housing, conformal coatings can help.Different forms of conformal coating, such as simple solder resistance, aerosol spray coating or epoxy resin coating, are effective corrosion protection measures. However, for PCBS with components that generate heat, such conformal coatings may have to be applied judiciously so as not to hinder heat management.

2.Keep the PCB dry

Do your best to avoid moisture around the PCB. Try to keep them in an environment that is not affected by humidity.

3.Prevent electrolyte from wetting PCB

By preventing moisture or other liquids from reaching the PCB, PCB corrosion can be effectively prevented. There are many ways to achieve this, such as placing the PCB inside a housing with the right IP grade.

4.Effectively remove the flux residue on the PCB

Older flux materials are notorious for producing chlorine or other harmful halogens, which can cause pitting of copper traces unless the flux material is removed after the welding process. However, the organic acids in the new fluxes do not contain halogens, and they break down at higher temperatures, such as during reflow welding. However, the circuit board that is wave soldering may not reach the decomposition temperature, and the residual flux residue may have to be thoroughly cleaned manually to prevent corrosion of the gap.

5.10 best PCB corrosion protection technology

In many cases, developers choose not to protect the board from corrosion. For example, prototype or disposable plates that will be used immediately are usually not protected against

corrosion. However, it may be better to develop some kind of PCB corrosion protection for all boards, and the following table lists 10 PCB corrosion protection technologies.

The most effective PCB corrosion protection method
Serial number	Corrosion protection method	Stage	Description
1	Follow a controlled lamination procedure	Manufacture	During this important manufacturing step, desiccant desiccant should be used and personnel should wear protective clothing
2	Use an appropriate solder stop layer	Manufacture	Solder stoppers are an underrated element of protection in circuit board construction.However, the type of solder stop layer is important and should be chosen appropriately based on design considerations
3	Use a mesh copper plane	Manufacture	Reticulated copper surfaces also offer a good option.They prevent moisture from moving between the laminated planes
4	Use surface treatment	Manufacture	The best way to protect the board between fabrication and assembly is to use a surface treatment, ENIG being the most popular but by no means the only option
5	Bake	Manufacture	To remove moisture, the board should be baked after assembly
6	Use tri-proof paint	Manufacture	Conformal coatings regulated by IPC (mainly in IPC-CC830B and IPC-A-610) should also be used.This is especially applicable if it will be used in hazardous environments
7	Use hot secondary molding	Manufacture	Another option is thermal secondary forming or packaging.This is ideal for high temperature environments
8	Use Microencapsulation	Manufacture	Microencapsulation is the encapsulation of some of the most corrosion-prone components or board elements
9	Use ESD bag for storage	Manufacture	The circuit board should always be stored in an ESD bag for electromagnetic isolation. These bags can also help prevent moisture buildup
10	Mount the board in the enclosure	Manufacture	Mounting plates in the housing can also provide protection; However, it should be well ventilated to prevent moisture buildup

How to remove the corrosion on the PCB - common chemical reagents?

1.Compressed air

Compressed air is a fairly common tool for cleaning electronic products. For simple repairs, compressed air provides an unobtrusive way to remove any dust from the inside of an electronic device or machine and blow it out. Use a short jet to inject air into the vent. If you are not satisfied with the dust removed, you can open the device with a screwdriver and operate around the assembly, carefully cleaning the circuit with air.

2.Baking soda

You can use baking soda to remove corrosion from PCB. Baking soda contains sodium bicarbonate, which works on PCB dirt and dust collection, breaking down dirt into small particles. The mild abrasive properties of soda water can eliminate corrosion without any side effects.

3.Deionized water

When you clean a circuit board with water, you have to make sure it is free of contaminants. The ions in ordinary water have conductive properties that reduce the properties of electrons. At the same time, deionized water has no damaging contaminants or ions.

4. Distilled water

Since there are no ions that conduct electricity to electronic devices, distilled water outperforms any other form of liquid when it comes to mixing cleaning solutions. Distilled water is specially treated to remove harmful contaminants so it won't damage the PCB.

It can also quickly become contaminated with dirt on hands or in the air, and must be stored in sealed distilled water when not in use and avoided with bare hands.

5.Isopropyl alcohol

When it comes to chemical cleaners, isopropyl alcohol or rubbing alcohol can remove all kinds of dirt, debris, flux and rust from the circuit board. Isopropyl alcohol is a good PCB cleaner because it is cheap and evaporates quickly. There are fewer chemicals in alcohol compared to other cleaners used for similar purposes.

It is important that the isopropyl alcohol content used to clean the circuit board is 90% or higher. High concentrations of isopropyl alcohol can cause adverse effects in contact with the body, so be sure to handle it carefully and use latex gloves and goggles.

Steps to remove PCB corrosion

After getting all the necessary cleaners and tools ready for the board to work, you can follow the given steps to clean the corrosion on the board:

1.First turn on the given electrical equipment or appliances, and remotely PCB by disconnecting all connections and power.

2.Record the layout and configuration of the PCB, must remember every detail, because after cleaning also need to reassemble the PCB.

3.In the use of the correct tool, you must check the user manual, the correct use of the tool.

4.After removing the circuit board, use a cleaning solution such as isopropyl alcohol or baking soda solution.

5.Now, use a soft brush to apply it to the circuit board, and then continue to apply until the rust and dust become soft.

6.you can then use a brush to remove dirt, and further use distilled water to remove the remaining dust and corrosion.

7.if there is still hard corrosion on the board, you can further apply more isopropyl alcohol, because it is a stronger agent.

8.Finally, clean with microfiber carpet and dry the circuit board using an air dryer, and then install it back into the electrical equipment to remove all corrosion.