PCB surface finishes are a core aspect of PCB fabrication. Printed circuit boards always feature cool finishes on their surface. The copper will oxidize if the surfaces are not protected. Therefore, this will make the circuit board useless. PCB surface finish is an important interface between the circuit board and the components. Furthermore, surface finishes shield the exposed copper circuitry. Also, they offer a solderable surface when placing components on the circuit boards.
There are different types of surface finishes. Choosing the right surface finish is as crucial as choosing the right material for your PCB. Also, the type of surface finish used for a PCB contributes to its functionality. Immersion tin and immersion silver are examples of PCB surface finish type. We will discuss immersion silver vs immersion tin in PCB.
What is Immersion Silver PCB?
Immersion silver is a type of surface finish used on PCB surfaces. PCB manufacturers protect the exposed copper pads with immersion silver. This material, immersion silver, is commonly used for EMI shielding and aluminum sir bonding. Also, immersion silver finish’s thickness ranges from 5 to 17 micro inches. You can store immersion silver PCB for at least 12 months under a good storage condition. It is important to solder the PCB immediately after you remove it from the storage.
Immersion silver is commonly used due to its environmentally friendly nature. For its surface treatment process, manufacturers place a thick layer on the copper surface. Therefore, this prevents the copper surface from tarnishing.
The fundamental of the immersion silver technology process is a displacement reaction. Here, silver ions remove metal copper from the surface of the PCB. Before this, the manufacturer uses a micro etching solution to roughen the copper surface. After this, a layer of immersion silver will build up with the help of a controlled speed of deposition. The deposition allows a sense of crystalline structure to build up. Therefore, this creates a layer of silver on the surface of the copper.
Also, the controlled speed of deposition prevents the growth of particles from precipitation and agglomeration. Immersion silver is a cheaper surface finish alternative. It is cheaper than immersion gold. Immersion silver is an ideal choice when functional connections are a concern. Also, it is a better choice when working with a tight budget.
The presence of silver in this surface finish enables solder to spread well. Therefore, the solder is stronger. Also, immersion silver provides a low contact resistance. This is one of the reasons it is more preferred to other surface finish types. Immersion silver PCB is RoHS compliant.
Advantages of Immersion Silver Surface Finish
The benefits of this surface finish outweighs its costs. Due to this, immersion silver has become popular in the PCB industry. It is now a preferred choice among PCB manufacturers. Immersion silver has several advantages which include:
Excellent flatness
Immersion silver has a very flat surface. This is very beneficial for SMD assembly. Also, it has exceptional surface planarity that makes it easy for manufacturers to work on. With this type of surface finish you can install surface components like Ball Grid Array (BGA).
RoHS Compliant
This is of the benefits of this surface finish. Immersion silver is environmentally friendly; hence, it complies with industry standards.
Good solderability
Immersion silver has great solderability after several thermal excursions. Also, the surface allows solder to spread evenly. Therefore, you can efficiently solder components.
Excellent conductivity
Immersion silver surface finish has great conductivity which makes it ideal for use in devices with high-speed signal design.
Low cost
Due to its low cost, immersion silver is a preferred surface finish among PCB manufacturers. Immersion silver is ideal for PCBs fabricated with low budgets.
Immersion Silver Surface Finish Process
The immersion silver surface finish involves some steps. You will have to go through this step to coat your PCB with immersion silver:
Cleansing
The first thing you need to do is to remove oxidation and dust residues on the PCB. By doing this, you are preparing the PCB for the process. Also, wet the surface to remove air trapped in the plated-through holes.
Micro-etching
This is the next step after cleaning the PCB. Micro-etching helps to modify the copper surface. Also, you create the liquid bath by using sulfuric acid.
Pre-dip
This stage involves immersing the PCB into acid. Pre-dip helps to prepare the copper surface for the coating process. Also, this can help to minimize the chances of oxidation.
Immersion Silver
Here, you will minimize copper metal by the silver. This will help to create a silver layer. You have to carry out this process slowly. This will help to create a consistent and uniform silver deposit layer.
Post-dip
In the stage, an antioxidant organic compound is generously applied on the board. Furthermore, this prevents oxidation.
Problems Associated with Immersion Silver Finish and Their Solutions
There are some common problems associated with the use of immersion silver finish. However, these problems have their own solutions.
Silver Whiskers
Silver whiskers occur when you expose the circuit board to high hydrogen sulfide and humid environments. Therefore, this may result in a short out of the PCB’s components. To prevent this, you can put a conformal coating on the silver layer.
Tarnish
Immersion silver PCBs react with chlorine and sulphur when exposed to air. Therefore, this discolors the silver layer and disrupts the solderability feature. The best way to prevent this is by adhering to proper storage rules.
Also, storing the PCB in a sulphur free package will prevent it from tarnishing. Make sure the package is airtight and reseal it when you remove a board from the package. In addition, an anti-tarnish bath can be very helpful in the final stages of the plating process.
Galvanic Effect
Galvanic effect can happen during the immersion silver process. It happens between the cracks of the copper surface and solder mask. Copper corrosion can result in copper ions during silver ion supply to immersion silver. You can prevent this by properly treating the copper surface during micro-etching. This can help to enhance uniform immersion silver.
What is Immersion Tin?
Immersion tin is a cost-effective surface finish type. PCB manufacturers utilize a thin metallic layer of tin on the copper layer of a PCB. The copper pads of immersion tin PCBs have a tin layer. Tin contains no lead. Therefore, immersion tin is an environmentally friendly option for PCB surface finish. Also, it can produce flat surfaces. The thin layer is whitish; hence, this finish is also referred to as “white tin.” Manufacturers apply immersion tin through a process known as electroless chemical bath.
Also, immersion tin prevents the copper from oxidizing. It offers oxidation protection that can last throughout the shelf life of the PCB. Immersion tin finish process requires careful handling all through the PCB assembly process. This is because this process is highly vulnerable to any damage that results from poor handling. Therefore, it is ideal to create a process that can reduce poor handling.
Immersion tin’s thickness is ≥1.0UM. Also, immersion tin is a budget-friendly option as it is less expensive than immersion gold.
Advantages of Immersion Tin Surface Finish
Using immersion tin as a PCB surface finish comes with a lot of advantages.
Lead-free
Immersion tin is lead free, which makes it an ideal option when manufacturing consumer electronics that demand RoHS compliance. Therefore, it is an environmentally friendly option. Also, the application of immersion tin on a PCB doesn’t require much chemicals.
Flat surface
Immersion tin gives a PCB a flat surface for manufacturers to work on. It provides a fine surface when soldering BGA surface mount components. Also, immersion tin offers you with great solderable surface. It can also withstand multiple thermal cycles.
Cost-effective
Immersion tin is a budget-friendly surface finish method. The process requires affordable materials. Also, it requires easy process steps. Immersion tin coating isn’t expensive. Also, it provides the best value, making it a cost-effective option for assemblers.
Allows re-works
It allows for re-works especially when there is an error during the soldering process. In addition, manufacturers can keep Immersion tin under storage for 6 months. Also, it is easy to rework immersion tin coating.
Limitations of Immersion Tin
Immersion tin has its own limitations regardless of its benefits.
Immersion Tin and copper have a strong affinity for each other. Therefore, this makes the two metals diffuse into one another. This may cause small strands referred to as ‘tin whiskers’ to form. Also, these tin whiskers’ can result in shorts in the circuits. Therefore, this can minimize the quality of the solder joints of a board. Also, this can adversely affect the functionality and shelf life of a board.
Another limitation of immersion tin is that it is highly sensitive to handling. The surface can tarnish when it has contact with bare hands. Therefore, manufacturers need to be careful when handling immersion tin PCBs. It is advisable to use protective gloves.
Immersion Tin doesn’t use the usual displacement process. The process depends on the exchange between the tin and the copper. Also, this process uses thiourea. This component is a carcinogen. It has health implications. Therefore, the process has to be highly controlled.
The Immersion Tin Process
The application of immersion tin on a PCB requires some steps. These steps include:
Pre-cleaning
This stage requires you to prepare your PCB. It is the first stage of the immersion process. Here, you will need to dip the PCB in a chemical solution to remove any contaminants on the surface. Also, you will dip the circuit board in an acidic solution to achieve this.
The pre-cleaning process helps to enhance the effectiveness of the plating process. Therefore, it is an important step. Furthermore, you need to use water to rinse the PCB surface. This will help to eliminate any debris and the remaining cleaning solution on the PCB surface.
Micro-etching
This requires you to dip the PCB in sulfuric acid. The purpose of micro-etching is to roughen the copper layer structure. Therefore, this helps to enhance the copper layer. Also, this will get the copper surface ready for the tin layer. The copper surface will bind with the tin layer effectively.
Furthermore, use water to clean the PCB surface. This will help to eliminate any residues.
Pre-immersion
Pre-immersion is a crucial step in the immersion tin process. This step helps to prevent the copper surface from oxidizing early. Here, the manufacturer immerses the PCB into an acid. Also, this offers a solid foundation for the deposition of the tin material.
Immersion Tin
This is the major step in the immersion tin process. The manufacturer prepares a solution that contains tin cations. You will need to dip the circuit board in this solution. Also, this helps to speed up the electroless reduction process. The tin ion in the solution reduces the copper metal.
Also, this forms a flat and thin layer of tin on the copper pads. You need to monitor this process closely. Also, ensure you adhere to the appropriate temperature as well as the solution composition. This will help you achieve good results.
Post-cleaning
This involves using water to clean the plated PCB. Ensure the water is very warm. Also, this step helps you to eliminate plating salts completely. It helps to prevent any stain when drying. If you don’t rinse properly, the PCB will appear dull.
Drying
Ensure you dry the boards after cleaning the boards. Also, this helps to get ready d of any moisture content in the PCB. You can use a warm oven baked or clean forced air drying. Also, it is very important to use the appropriate temperature when operating the oven.
In addition, you can preserve the PCB by considering the baking duration.
Features to Consider When Choosing Immersion Tin
Before you choose immersion tin, there are some features you have to consider. These features will help you determine if immersion tin is good for your PCB or not.
PCB quantity
This surface finish is an ideal choice when you need to fabricate a large volume of PCB. This is because the process is quite easy. Also, it isn’t expensive.
Cost
This is another criteria that determines your choice ofPCB surface finish. Immersion tin saves on cost. Therefore it is a budget-friendly option.
Fine pitch components
This is the right surface finish for you if you plan to assemble a high number of components in your board. Immersion tin provides you a flat surface to mount PCB components.
Shock drop
This is ideal for components that have concerns for brittle fractures. A tin-copper bond offers good strength.
Application environment
This indicates where the PCB would be used. Immersion tin might not be too good for applications subjected to extreme temperature.
Comparison of Immersion of Silver and Immersion Tin
Immersion silver and immersion tin are two different types of surface finished in PCB. They share some similarities. Here, we will look at the similarities and differences between these two PCB surface finishes.
Similarities
Immersion silver and immersion tin adopt the same process. These two surface finishes are environmentally friendly and budget-friendly. The cost of immersion tin PCB is very close to that of immersion silver.
Also, Immersion silver and immersion tin don’t have black pads. The process of immersion tin and immersion silver processes require micro-etching.
Both surface finishes require manufacturers to wear protective gloves during handling. This is because immersion silver and immersion tin can tarnish due to poor handling.
Immersion silver and immersion tin both experience whiskering. Therefore, this results in short circuits. Also, these two surface finishes need to be kept under good storage conditions. This is because they are prone to tarnishing.
Differences
Immersion silver has a longer shelf life than immersion tin. It can last for 12 months while immersion tin can only last for 6 months.
Immersion silver is more environmentally friendly than immersion thin. The presence of Thiourea during the immersion tin process makes it toxic. Also, Thiourea is carcinogenic.
Immersion silver consumes more power and water and power during its processing. However, immersion tin requires less power and water during its processing.
Also, immersion silver can withstand multiple reflows, unlike immersion tin. This makes it a preferred option for manufacturers. Immersion tin has a strong affinity to copper, which makes it react with copper. Therefore, the tin diffuses into copper. This reduces the solderability of the PCB surface. However, immersion silver doesn’t react with copper. This makes it have a longer shelf life.
How to Address Tin Whiskers in Immersion Tin PCB
Tin whiskers are thin crystalline structures that are very conductive. They appear from immersion tin PCB surfaces. Also, tin whiskers can exceed 10mm.
Tin whiskers result in short circuits which can further cause system failure. Also they develop connections between closely spaced circuit elements. Therefore, this negatively affects the performance of the PCB. Several theories have tried to explain what causes tin whiskers. Tin whiskers can take some time to grow before you notice them. They can take months or years.
According to some, their growth is majorly caused by the relief of compressive stress in tin plating. While others believe that abnormal tin growth and recrystallization cause tin whiskers.
Also, factors like temperature and moisture temperature can contribute to their growth. However, it is possible to prevent tin whiskers. One of the best ways is by using a tin alloy while plating instead of pure tin.
Furthermore, avoid using it on the copper traces of the PCB. You can treat the solder pads with immersion or ENIG. Try to utilize flat integrated circuit packaging. This will help to avoid any mechanical stress.
How to Handle and Store Immersion Silver PCBs
Immersion silver PCBs are very sensitive when it comes to storage and handling. It is important you adhere to the precautions below when handling immersion silver boards;
Ensure you put in gloves when handling these PCBs. Fingerprints can tarnish the PCB surface and this can cause some solderability issues.
Keep the PCBs in dry conditions for a period between 6 – 12 months.
Ensure you carry out a solderability test if the PCB has been stored for long. This helps to verify the solder before assembling the components.
Also, avoid using acid cleaners on immersion silver PCBs. You should rather use water washes that have a PH greater than 6.0. Use packaging materials that are sulphur-free. These are ideal for storing immersion silver PCBs. Also, they tarnish from occurring.
You need to reduce the air that passes through it once you open the package. Make sure you are baking the PCB at the right temperature and time. Baking helps to reduce stress or discharge moisture.
Frequently Asked Questions
What does Immersion PCB Baking do?
Immersion PCB baking helps to relieve stress and remove moisture from your PCB. PCB fabricators perform this under a controlled environment with the right temperature. Also, you can make use of an oven which is specifically designed to perform this process.
How thick is immersion silver PCB?
Immersion silver has an even thickness that ranges 0.12 to 0.40um. Also, immersion silver is thinner than the traditional electroplated silver. Therefore, it provides a flat surface for easy component assembly.
How thick is immersion tin PCB?
Immersion tin PCB has a thickness that ranges from 0.7 to 1.0 microns. Also, this coating is very flat. This gives the PCB a great soldering surface.
How is immersion tin applied?
PCB manufacturers apply immersion tin through the process of autocatalytic chemical reduction.
Conclusion
Immersion tin and immersion silver are examples of PCB surface finishes. Also, they are some of the most common PCB surface finishes. They have their benefits and limitations. Although they share some similarities, they have their differences. It is important to consider some factors when choosing PCB surface finish type. The type of board and the operating environment have a lot to determine. Not all surface finishes are ideal for a board.
In this article, we have compared immersion silver and immersion tin. The major difference is the type of immersion used. Immersion silver uses silver to protect exposed copper pads. However, immersion tin uses a thin metallic layer of tin.
