Surface finishes of printed circuit boards - complete guide
Immersion Silver (ImAg)
This is a method of immersion plating copper with silver. This means that copper atoms are replaced by silver atoms. The process of this method involves immersing the PCB in a solution containing silver ions, which are reduced to elemental silver on the surface of the copper.
Advantages of immersion silver plating of copper:
+ ImAg provides excellent solderability.
+ Bonding wires, for example in medical devices, is no problem for ImAg. On the contrary, this method ensures good performance when joining wires using this method.
+ No hazardous substances or heavy metals. ImAg contains none of these, making it more environmentally friendly.
+ The high surface energy ensures that the components adhere to the PCB surface.
+ If you need to save money, this method is for you. This is because ImAg is relatively inexpensive compared to other surface treatments.
Disadvantages of copper immersion plating with silver:
- The surface finish is susceptible to the presence of sulphur. This causes darkening of the plates and solderability problems.
- Since ImAg can degrade over time, it has limited shelf life.
- ImAg requires careful handling during the manufacturing process to avoid contamination.
- A thin layer of this coating may not provide sufficient protection against certain types of corrosion or wear.
- For applications that require greater thickness or a different level of protection, this coating may not be suitable.
Immersion Gold (ENIG)
After applying a thin layer of gold, the circuit board is immersed in a solution containing nickel ions. The nickel chemically displaces the gold, creating an intermetallic compound of nickel and gold.
Advantages of immersion plating copper with gold:
+ Due to the protective nickel layer, ENIG exhibits excellent corrosion resistance.
+ ENIG provides a uniform layer thickness, which is important for SMT surface mount technology.
+ If you need to bond some conductors, ENIG is the right choice because it provides a highly conductive surface that is easy to bond to.
+ ENIG provides a flat surface that is suitable for chip packaging (CSP).
+ If you need to re-melt your board multiple times, ENIG is the right choice.
Disadvantages of gold immersion plating:
- Cost: ENIG is more expensive than other finishes.
- ENIG requires careful handling during the manufacturing process. At the same time, nickel is susceptible to oxidation.
- Beware of thermal shock. Heat shocks can cause cracking and flaking of the nickel layer in the ENIG method.
- The ENIG nickel layer is difficult to visually inspect.
- Avoid high temperatures: The intermetallic compound of nickel and gold can disintegrate at elevated temperatures.
Immersion tin (ImSn)
This is a method of immersion plating copper with tin. This means that copper atoms are replaced by tin atoms. The process of this method involves immersing the PCB in a solution containing tin ions, which are reduced to elemental tin on the surface of the copper.
Advantages of tin immersion plating:
+ Suitable for SMT applications due to the flat surface that tin creates.
+ ImSn is well solderable.
+ ImSn has a long shelf life.
+ Price: ImSn is relatively cost-effective compared to other surface treatments.
Disadvantages of tin immersion plating:
- ImSn is not suitable for applications where wire splicing occurs due to its brittleness.
- ImSn has limited heat resistance.
- ImSn is susceptible to tin whiskers formation, which can lead to short circuits.
- Handling Caution: ImSn is sensitive to handling and requires careful handling.
- Harsh environments are not conducive to this finish. In fact, ImSn provides only limited corrosion resistance.
Organic Soldering Preservatives (OSP)
An environmentally friendly surface treatment that applies a layer of organic material to copper plates. OSP is a water-based organic compound that forms a protective layer on copper.
Benefits of OSP coating
+ The absence of heavy metals makes this coating environmentally friendly.
+ OSP coating is relatively simple to apply.
+ The cost of this coating is very affordable.
+ OSP provides a flat finish on the circuit board.
+ No extra thickness due to the OSP method.
Disadvantages of OSP surface treatment
- Over time, the surface finish will begin to degrade.
- It is easily damaged by handling and can be affected by contaminants.
- Avoid high temperature applications: OSP can break down and lose its protective properties.
- OSP is difficult to reprocess because the organic layer can be damaged during reprocessing.
- OSP provides only limited protection against corrosion.
LF-HASL surface treatment
Full name Lead-Free Hot Air Solder Leveling. Exposed copper surfaces are coated with a thin layer of molten solder, which is then leveled with hot air knives. LF-HASL is a lead-free alternative to the traditional HASL process.
Advantages of LF-HASL
+ Relatively low cost makes this method an attractive choice.
+ LF-HASL offers good solderability due to its thin solder layer.
+ LF-HASL can be stored for a long time
+ LF-HASL is suitable for the use of fine-pitch components
+ The hot air leveling process produces a uniform surface which helps prevent component placement problems.
Disadvantages of LF-HASL
- High frequency applications can be a problem for the LF-HASL method. This is due to the thick layer of solder that is used in the LF-HASL method.
- The thickness of the solder layer can vary and cause deviations from design specifications.
- Despite the absence of lead, the LF-HASL method is not environmentally friendly. It still involves the use of hazardous chemicals such as flux that can be harmful to the environment.
Surface finish HASL (tin-lead)
Hot Air Solder Leveling (HASL) is a common surface treatment used on printed circuit boards. Coating the copper surface with a layer of tin-lead alloy and then leveling with hot air creates a flat and uniform surface for soldering.
Benefits of HASL treatment:
+ Need a strong and reliable solder joint? HASL surface treatment is a good candidate for this application.
+ HASL is a relatively inexpensive finish compared to other alternatives.
+ HASL has a wide range of applications.
+ Long shelf life due to the fact that HASL retains its properties for a longer period of time.
Disadvantages of HASL treatment:
- The lead contained in HASL is hazardous to the environment and human health.
- HASL is not suitable for fine pitch components and for SMT technology.
- The thickness of the HASL surface may vary depending on the size and shape of the PCB.
- The cost of HASL can be higher than for OSP and ENIG treatment.
- The use of HASL treatment is not RoHS compliant.
Each of these methods has advantages and disadvantages. When selecting a finish, it is important to carefully consider factors such as solderability, wire bonding, corrosion resistance, durability, and cost.
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