Surface Finish in PCB refers to the coating or treatment applied to the copper surface of PCB. It protects them from oxidation like a solder mask on PCB and also improves their solderability. These two are the significant functions of PCB surface finish. In PCB manufacturing, PCB finish is one of the most critical factors. It influences the overall performance of the circuit board, including reliability and solderability.
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The PCB surface finish also protects the PCB from environmental factors. Those factors can degrade the copper surface over some time. The selection of surface finish depends on the specific requirements and the manufacturing process. PCB finish is used for various reasons, including:
The #surfacefinish is a coating that covers the outer surface of the PCB after the copper has been etched to protect the exposed copper circuitry on the PCB and improve solder ability. Choosing the right surface treatment can help our circuit boards to avoid damage from the outside of the environment, extend the shelf life of the board, improve the durability of the board, and better achieve the needs of our final product. So do you know how to choose the right PCB surface treatment process? In this article, I will introduce you to nine common surface treatment processes to help you understand how to choose the correct PCB surface treatment process.
PCB Surface Finish
The following 9 common surface treatment processes
&#;HASL (Hot Air Solder Leveling)
#HASL involves wetting the PCB with flux and then immersing the PCB in a tin/lead alloy furnace to form a thin protective layer on the surface of the PCB. Finally, the PCB is removed after the protective layer is formed, and the excess solder (lead or tin) is removed by blowing hot air across the surface of the board with an "air knife".
HASL
Advantages of HASL
&#;Low cost
&#;Can be reworked for repair
&#;Long shelf life
&#;Good corrosion resistance and test ability
&#;Exposing the PCB to temperatures up to 265°C allows for better identification of potential delamination problems in the assembly of components onto the board
Disadvantages of HASL
&#;Poor flatness or coplanarity of the HASL surface compared to other surface treatments such as submerged gold
&#;Not suitable for SMD and BGA with <20 mil
&#;Contains hazardous substances lead, not environmentally friendly and not RoHS compliant
&#;Exposing PCBs to high temperatures may expose them to thermal shock and affect other materials in the PCB
&#;May block PTH (Plated Through Hole)
&#;Poor wettability
&#;Not suitable for capacitive touch switches
&#;LF HASL (Lead-Free Hot Air Solder Leveling)
#LFHASL works similarly to HASL, but LF HASL immerses the PCB in molten lead-free solder (e.g. Sn/Ag/Cu (SAC), Sn/Cu/Co, Sn/Cu/Ni/Ge) and then blows off the residue on the PCB surface with a hot air knife.
LF HASL PCB
Advantages of LF HASL
&#;Environmentally friendly, complies with ROHS standard
&#;Durable
&#;Affordable price
&#;Suitable for optical inspection and electrical testing
&#;Reprocessable
&#;Very good bonding to copper with excellent solderability
Disadvantages of LF HASL
&#;Inconsistent coating thickness
&#;Solder bridge formation
&#;Not suitable for very dense, high pin count, closely spaced assemblies
&#;Not suitable for HDI (High-Density Interconnect) PCBs
&#;Can form tin whiskers, a fine conductive filament that can cause short circuits and various other problems
&#;OSP (Organic Solderable Preservative)
#OSP is a technique involving the immersion or spraying of water-based organic compounds that selectively bind to copper and provide an organometallic, non-conductive coating with a protective effect. Simply put, OSP is the formation of a thin, uniform protective layer on the copper surface of the PCB to prevent oxidation of the board during storage and assembly operations.
OSP PCB
Advantages of OSP
&#;Protects copper solder with resistance to oxidation, thermal shock, and moisture
&#;Low cost
&#;Lead-free, environmentally friendly
&#;Can be reworked for repair
&#;Very flat surface, ideal for fine-pitch component assembly
Disadvantages of OSP
&#;Thermal exposure can damage OSP films, which reduces the solderability of through-holes
&#;Not conducive to electrical testing
&#;Short shelf life, typically only 6 months
&#;ENIG (Electroless Nickel Immersion Gold)
#ENIG is a two-layer metallic coating of 2-8 µin of gold over 120-240 µin of nickel. The process of electroless gold deposition is to use the autocatalytic formation of nickel deposits as a barrier to copper surfaces and component soldering surfaces, followed by deposition by immersion to form a very thin protective layer of gold to avoid corrosion of nickel.
ENIG PCB
Advantages of ENIG
&#;Protects copper from passivation and oxidation
&#;Good wettability
&#;No hazardous substances, RoHS compliant
&#;Durable, long shelf life
&#;Very good solderability
&#;Flat surface
&#;Ideal for PTH (Plated Through Hole)
&#;Suitable for complex surface mount components (e.g. fine pitch BGAs and flip chips)
Disadvantages of ENIG
&#;Cannot be reworked for repair
&#;May suffer from black pad syndrome (phosphorus buildup between gold and nickel layers) resulting in PCB surface breakage and connection errors
&#;High Cost
&#;May cause signal loss for signal integrity applications
&#;Not suitable for touch contact boards
&#;The process is complex and difficult to control
&#;ENEPIG (Electroless Nickel-Electroless Palladium-Immersion Gold)
#ENEPIG is an upgrade of ENIG, which is a three-layer metal coating consisting of nickel, palladium, and gold. On the copper substrate is a chemically plated nickel layer, followed by a chemically plated palladium layer, and the final layer is an immersion gold layer. The thickness of the palladium is in the range of 3-6um, and its function is to prevent nickel from passing through the top gold layer, that is, to prevent nickel from being oxidized and corroded.
ENEPIG VS ENIG
Advantages of ENEPIG
&#;Provides a flat pad surface, allowing more areas to ground the wire
&#;Ideal alternative to soft-bonded gold
&#;Ideal for multiple reflow cycles
&#;No black pads
&#;Long shelf life
&#;Compatible with a variety of lead-free solders
&#;Supports alternative connections such as lead bonding, edge card connections, and push buttons
&#;Ideal for in-circuit testing and crimping connectors
&#;RoHS compliant
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&#;Ideal for high-frequency applications with limited pitch
Disadvantages of ENEPIG
&#;Reduced solderability and poor wettability with thick palladium layers
&#;A complex process, not widely used
&#;High Cost
&#;Gold wire bonding may not be as reliable as soft gold compared to soft gold
&#;ISn (Immersion Tin)
#ISn is the deposition of the tin onto the copper surface through a chemical replacement reaction, forming a thin layer of tin (between approximately 1um-40um thick) that protects the underlying copper from oxidation.
Isn
Advantages of ISn
&#;Uniform plating, flat and smooth surface
&#;Multiple remelting and rework repairs are possible
&#;Ideal for HDI PCBs and fine pitch surface mount components
&#;Excellent protection of the underlying copper from oxidation
&#;Suitable for fine processing, lead-free assembly
&#;Suitable for backplanes and press-fit connectors
Disadvantages of ISn
&#;The material contains thiourea, which is carcinogenic
&#;Easily oxidizes and forms tin whiskers, which can lead to PCB short circuits
&#;Not suitable for PTH (Plated Through Hole)
&#;Short shelf life, only 3 to 6 months, not suitable for long term storage
&#;Difficult to test electrically,, requires special setup (soft probe landing)
&#;IAg(Immersion Silver)
#Immersionsilver is a PCB immersed in a silver ion bath to obtain a thin layer of lead-free silver on the surface of the PCB through a chemical replacement reaction, which is used to protect copper traces from corrosion. Immersion silver is 5-12 µin thick and is mainly used for lead bonding, EMI shielding and membrane switches.
IAg PCB
Advantages of Immersion Silver
&#;Excellent surface flatness
&#;High stability (stable chemical properties)
&#;Suitable for fine-pitch components
&#;Low loss for signal integrity applications
&#;Can be reworked for repair
&#;Excellent solderability
Disadvantages of Immersion Silver
&#;High coefficient of friction/not suitable for flexible pins
&#;Stringent storage requirements
&#;Sensitive to contaminants in the air or on the PCB surface, must be soldered the same day the PCB is removed from storage
&#;Tends to lose gloss when mishandled
&#;Not suitable for micro-through holes with 1:1 aspect ratio
&#;May form silver whiskers (hair-like metal bumps on the surface of the board), leading to board breakage or short circuits
&#;Difficult to perform electrical testing
&#;Hard Gold
#Hardgold is first plated on the PCB copper surface with a layer of nickel, and then gold is plated on the nickel coating
The gold purity of hard gold is 99.6%, mostly used for PCB side contacts (usually called gold fingers) and keyboards, etc. The plating process of hard gold: acid cleaning &#; electroplating nickel &#; pre-plating gold &#; electroplating gold-nickel or gold-cobalt alloy
Hard Gold PCB
Advantages of hard gold
&#;Hard and wear resistant
&#;Lead-free, RoHS compliant
&#;Long shelf life
&#;Suitable for electrical testing
Disadvantages of hard gold
&#;High cost
&#;Requires additional conductive traces
&#;Poor solderability, not solderable above 17 µin thick
&#;Soft Gold
#Softgold is also composed of a layer of gold plated in a nickel coating. The purity of soft gold is 99.9%, while the hardness of pure gold is softer, so this surface treatment process is called soft gold. The plating process of soft gold: acid washing &#; electroplating nickel &#; electroplating pure gold
Soft Gold PCB
Advantages of soft gold
&#;Suitable for lead bonding
&#;RoHS compliant
Disadvantages of soft gold
&#;Poor solderability
&#;How to choose the right surface treatment?
Each surface treatment has its advantages and disadvantages, and we recommend that you compare the characteristics of various surface treatments before making your choice. The following are the main factors to consider when choosing a surface treatment
PCB Surface Finish Comparison
1) RoHS compliance
2) The expected yield of the project
3) The possibility of high frequency
4) Budget cost
5) The environment in which the end product will be used, is 6) the PCB susceptible to contamination and damage?
7) Fineness of component pitch, type of assembly
8) Reworkability
9) Shelf life
10) Impact and drop resistance
11) Whether lead build-up or wave soldering is required
12) Solderability
13) Thermal shock
14) PCB board surface aesthetics
Conclusion
In our experience, you usually only need to list the most important requirements for your PCB project and choose one of the surface treatment processes that meet the requirements to ensure that the manufactured boards will meet your requirements to the greatest extent possible. Of course, no matter which surface treatment process you choose, quality is our primary concern.
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