The Different Types of PCB Surface Finishes
As we detailed in our previous blog “An Introduction to the Types of Printed Circuit Boards,” PCBs can be found in a number of objects people use on a day-to-day basis. But for a printed circuit board to function properly, it needs a surface finish.
Without this specialized coating, the PCB can’t form the necessary connection that allows it to make electronic devices work. But what exactly is a surface finish and what is it made out of? As an electronics manufacturer, you need the answers to these questions if you expect your products to work appropriately and serve your customers.
Below, we explain exactly what a surface finish is and other related information so you better understand why this part is so vital to a PCB’s functionality.
What Is a Surface Finish?
Simply put, a surface finish is a coating around the outside layer of any printed circuit board. This coating dissolves into the solder paste to form the proper connection between the PCB and its main electrical component.
Surface finishes come in both metallic and organic materials. These materials protect the base copper of the PCB until it can be assembled into an electronic.
The surface finish not only influences how the PCB works but also the following:
- The amount of rework
- The field failure rate
- The process yield
- The scrap rate
- The test function
Additionally, the kind of surface finish used on the PCB will greatly affect the overall cost of the piece.
What Factors Should You Consider Before You Choose a Finish?
When you choose a PCB to use for your electronic devices, you want the component to be compatible with your products. You’ll want to consider the following factors before you select a specific surface finish:
- Chemical resistance
- Compatibility with other finishes
- Contact resistance
- Finish availability
- Press-fit requirements
- Shelf life
- Tarnish resistance
- Wear resistance
- Wire bonding
You should also think about how the user’s environment will impact a finish’s viability. For example, if product users live in an environment that could cause the finish and the PCB to corrode, you should select a material that can better withstand those external factors.
What Materials Are Used for PCB Surface Finishes?
Surface finishes come in various materials. Below, we’ve listed several of the most effective and commonly used finishes that you can choose from.
Electroless Nickel Immersion Gold (ENIG)
ENIG finishes consist of two layers made from metallic coating. The coating is a layer of soft gold that covers a layer of nickel. The gold protects the nickel while it’s in storage by providing a low contact resistance. Once the PCB is removed from storage and ready for use, the nickel layer gets soldered to other components.
This material is flat and has a long shelf life, usually about a year, and has an average fabrication cost.
Hard Electrolytic Gold
Like the ENIG, this finish material is made of a layer of nickel covered by a layer of gold. The gold layer is hard gold, however, to protect the nickel in high-wear environments (such as keypads). Hard gold finishes also have a year-long shelf life, but they’re much more expensive to manufacture.
Hot Air Solder Level (HASL) and Lead-Free HASL
An HASL surface finish is probably the most commonly used finish in the entire industry. It has an incredibly low cost and has a long shelf life. It can easily solder to other surfaces, making it the most flexible option available.
To make this surface finish, manufacturers immerse a circuit board into a pot of a molten alloy, usually comprised of tin and lead. In a lead-free HASL, other materials replace the lead in the liquid alloy. After the board has been dipped in this alloy, it is removed and put under air knives. These devices then blow hot air over the board’s surface to remove any excess material.
Organic Solderability Preservative (OSP)
This finish is usually made from organic preservatives that prevent the copper surface of a PCB from oxidizing. A thin layer of OSP is placed directly over the PCB to protect it before it gets soldered to another device.
The OSP is made of a water-based compound that bonds to copper selectively. And, because this finish is water-based and organic, it’s one of the greenest materials available. However, its organic structure gives it a short shelf life-usually about nine months with special storage techniques.
Immersion silver finishes are both metallic and organic: they’re made of silver and OSP layers. The OSP keeps the silver from tarnishing, which extends its shelf life to about a year. It’s a better alternative to ENIG finishes because it has a fine pitch, flat coating, and easy solderability.
However, the silver requires delicate care as the material is sensitive to electrical testing, packaging, and handling.
Now that you understand more about this vital component of a PCB, you can acquire better-quality printed circuit boards. If you have any questions about PCBs or you want to invest in newer technology and PCBs for your products, get in touch with the experts at Streamline Circuits.