Surface finish is an important aspect of PCB manufacturing. Printed circuit board needs surface treatment since oxidation can occur. Copper exist in the form of oxides, thereby affecting electrical properties and solderability of boards. PCBs would have soldering problems if the surface isn’t treated. Therefore, surface finish is crucial in the production of circuit boards.
Surface finish offers a lot of benefits for PCBs. Also, this finish creates a critical interface between the PCB and its components. There are different types of PCB surface finish. In this article, we will provide further details about PCB surface finish.
What is a PCB Surface Finish?
A PCB surface finish is a type of coating between a circuit board and its components. Also, surface finish protects exposed copper circuitry. It is an intermetallic joint between the solderable area of a PCB and the components.
PCBs surface is prone to oxidation if left without a coating. Therefore, there is need for surface finish in PCBs. Also, a PCB surface finish is crucial for creating a connection between the circuit board and its components.
Types of PCB Surface Finish
Surface finishes are available in different types. The intended application of the PCB determines the type to opt for when manufacturing a PCB.
Hot air solder leveling (HASL)
This finish involves coating molten tin-lead solder on the PCB surface and using heated compressed air to flatten it. Therefore, this process creates a coating layer that resists copper oxidation. Also, this finish offers great solderability. The PCB manufacturer needs to understand some crucial parameters like dip soldering time, welding temperature, and lifting speed.
Hot air leveling has gained popularity over the years. Also, it is commonly used in SMT process. When leveling PCB with hot air, there are some important points to know. First of all, the manufacturer should immerse the PCB in molten solder. Also, the wind knife must blow the solder before it becomes solid. The wind knife can reduce the solder meniscus on the copper surface.
There are two types of HASL. These are lead free and lead-based HASLs. The lead free HASL comprises tin, but has no lead. On the other hand, the lead-based HASL comprises both lead and tin. The lead free HASL is environmentally friendly. Hence, more used in printed circuit boards.
HASL is a cheap alternative to other finishes. This is because hot air solder leveling uses tin or lead to form a protective coating on the PCB. Manufacturers dip the PCB in molten solder and take it out. After that, the manufacturer uses hot air bursts to remove too much tin or lead from the PCB surface. HASL isn’t ideal for wire bonding and capacitive touch switches.
Electroless Nickel Immersion Gold (ENIG)
ENIG is one of the most popular surface finishes in PCB manufacturing. This surface finish offers more benefits. Also, ENIG treatment features two metallic layers. Here, the manufacturer places the nickel on the copper layer already coated in gold atoms. Also, the nickel serves as a barrier to copper while the gold prevents any form of corrosion.
In addition, gold features a very low contact resistance. Immersion gold offers great flatness. The gold used in this finish offers a great solderability. However, ENIG surface finish is very expensive since it involves the use of gold. The gold used in this finish prevents nickel oxidation during storage.
Electroless nickel immersion gold (ENIG) meets a lot of standards in the PCB industry. Also, this surface finish is ideal for plated through hole. In addition, Immersion gold is durable and ideal for aluminum wire bonding as it doesn’t give uneven surfaces.
Immersion Tin
Immersion tin is ideal for planar fine pitch. It is a metallic finish deposit. Immersion tin surface finish process involves immersing the PCBs in several chemical baths. This helps to create a good adhesion for the tin. Also, the immersion tin offers perfect flatness. The tin offers the best protection for the underlying copper.
Immersion tin is cost-effective. However, this surface finish can experience tin whiskering. This is because of the strong bond between the copper and the tin. Also, it has a long shelf life and it is very reworkable. Immersion tin is the least expensive type of immersion finishing. Also, immersion tin is ideal for horizontal production line and fine geometry processing.
Furthermore, immersion tin features great solderability. This finish is lead-free. Therefore, it is ideal for fine pitch components and flat surface requirements. Immersion tin is a great alternative to lead-based surface finish. Also, PCB manufacturers prefer immersion tin because of its sustainability.
Regardless of the benefits it offers, this surface finish has its drawbacks. It is very sensitive to handling and very aggressive to solder masks. Also, it isn’t ideal for contact switches.
Organic solderability preservatives (OSP)
OSP is one of the most reliable PCB finishes. This finishing produces a thin protective layer on the copper surface of the circuit board. Also, this protective layer shields the circuitry from any form of oxidation during assembly and storage. PCB manufacturers apply OSP over copper pads to shield the pad before soldering.
Furthermore, OSP offers an organometallic layer to shield the copper before soldering begins. Also, this helps to preserve the copper surface from oxidation. Organic solderability is a water-based surface finish. Therefore, it is environmentally friendly. Also, it doesn’t require much equipment maintenance.
OSP process is straightforward. The first step in this process is cleaning the organic contaminants. This will prevent the copper foil from being contaminated. After this, the manufacturer applies micro etching to get rid of the oxidation produced on the copper foil. Also, micro etching helps to improve the bonding forces between the OSP solution and the copper foil.
This surface finish is very easy to control. Also, OSP performs well in terms of coplanarity and solderability. However, there are some downsides to using this type of surface finish. OSP is not ideal for PTH and crimping technology. Also, PCB manufacturers can’t rework OSP more than twice.
Immersion silver
This surface finish uses a non-electrolytic chemical finish. It involves the application of this finish by dipper the copper PCB in a tank filled with silver ions. Immersion silver is an ideal choice for printed circuit boards with EMI shielding. Also, it is widely used for wire bonding and dome contacts.
Immersion silver is a more suitable option than ENIG and HASL in terms of environmental safety. Also, it is widely used in PCB manufacturing due to its low cost. Silver is different from tin as it doesn’t react with copper. However, it tarnishes when you subject it to air. Therefore, immersion silver requires anti-tarnish packaging for its storage.
Furthermore, immersion silver is ideal for membrane switches and fine traces. This PCB surface finish features a good shelf life of 12 months. However, it is quite sensitive to contaminants in the air. Also, it is not ideal to utilize peelable masks when using this surface finish.
Benefits of PCB Surface Finishes
PCB surface finishes offer a lot of benefits when used in circuit boards. Sometimes, PCBs are subjected to harsh environment. Therefore, it is important to protect them from contaminants from these environments.
Prevents corrosion
Corrosion of the PCB surface is one of the challenges PCB manufacturers are trying to fight. Surface corrosion occurs on metallic connections and PCB surfaces. The surface and wire leads are vulnerable to corrosion. PCB finishes prevent any form of corrosion on PCB surface. Hence, there is a significant need for PCB finishes.
Prevents oxidation
One of the major benefits of PCB finishes is the prevention of oxidation. Oxidation occurs when copper molecules interact with the oxygen available in moisture and air. Also, the surface finish serves as a protective film against copper oxidation and green layer formation.
Generates solderable surface
PCB surface finish forms a layer for soldering. The PCB manufacturer mounts the components on the surface finish coating.
How to Choose the Right PCB Surface Finish
There are different types of PCB surface finishes. It is important to know what works well for your PCB project. However, you will need to consider some factors before choosing the most suitable surface finish.
Operational parameters
You need to consider operational parameters such as the rust-proofing, number of layers, and high temperature resistance. Also, surface finishes may exhibit different compatibility with PCB operational parameters. Therefore, it is a crucial consideration
Operational environment
The operational environment of the PCB will also determine the type of finishing to use. PCBs subjected to very harsh environments will require some surface finishes. For instance, immersion gold is ideal for PCBs subjected to extreme temperatures. Also, the type of surface finish to use will depend on the contaminants in the intended operational environment.
Shelf life
PCB finishes have different shelf lives. The shelf life indicates the duration of storage of the surface finished boards without tarnishing. Some finishes have longer shelf lives than others. For instance, ENIG has a long shelf life.
Cost
This is an important factor you need to seriously consider when choosing PCB surface finish. Some surface finishes are cheaper. For instance, immersion tin is very cheap. On the other hand, immersion gold is very expensive. Your budget will determine if that finish is ideal for your project. However, you should never undermine the quality of your board due to the cost of PCB finish.
Shock and drop factor
Shock and drop happens to some electronic devices. For instance, when you drop a smartphone, there is a component fracture risk in the PCB. In cases like this, you won’t use ENIG. This is because it has a tin-nickel bond. It is advisable to use a tin-copper bond.
What is the Difference Between ENIG and ENEPIG?
ENIG means electroless nickel immersion gold. This PCB finish features electroless nickel coated with a thin layer of immersion gold. ENEPIG refers to electroless nickel electroless palladium immersion gold. ENIG is different from ENEPIG. This is because ENEPIG involves an application of a palladium layer. Also, the palladium serves as the resistance layer that prevents the nickel from oxidizing. This layer prevents nickel from diffusing to the copper layer.
Both ENEPIG and ENIG offer the highest solderability. Also, ENEPIG finish comprises four metal layers which are copper, nickel, palladium and gold. On the other hand ENIG comprise three metal layers which are copper, nickel, and gold.
The major difference between ENIG and ENEPIG is the extra layer of palladium. Also, the palladium shields the nickel layer form corrosion. Therefore, this helps to prevent black pad. Black pad is commonly associated with immersion gold.
Furthermore, electroless nickel electroless palladium immersion gold is more affordable. The addition of palladium to this PCB finish reduced its cost. Also, the palladium layer minimizes the required thickness of the gold layer. ENIG is more expensive than ENEPIG. ENEPIG is durable and can resist tarnishing. In terms of solderability and durability, ENEPIG and EPIG are a great option.
What is the Difference Between Hard Gold and Soft Gold?
Gold is a common metal used for surface finish in a printed circuit board. It is important to understand the different gold used in PCBs. PCB manufacturers use different type of gold in circuit boards. There is a difference between hard gold and soft gold used in PCBs.
Hard gold also refers to a plating alloy. Also, hard gold features other elements. Therefore, this makes it harder. Soft gold is pure gold. Hard gold plating is ideal for use in applications where friction is necessary. Also, you will find hard gold in keypad and gold finger.
On the other hand, soft gold is widely used gold or aluminum wiring on chip on board. As a result of the addition of nickel matter, the surface of soft gold is not bright. Hard gold has a smooth and hard surface. Also, the surface of hard gold appears brighter. Soft gold is widely used for wire bonding.
Hard gold is not applicable to solderable areas due to its poor solderability and high cost. Also, hard gold comprises a gold layer plated over a nickel barrier. It is commonly used in high-wear areas.
Conclusion
Understanding the PCB surface finish types helps to choose the appropriate finish for your PCB project. In this article, we have provided detailed information about the surface finishes used in PCBs.
