With the continuous improvement of human requirements for the living environment, the environmental issues involved in the PCB production process have also attracted more and more attention.
Especially the topic of lead and bromine is the most popular during PCB surface finishes.
The most basic purpose of PCB surface finish is to ensure good solderability or electrical properties.
Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as original copper for a long time, so other treatments are needed for copper.
There are many PCB surface finish processes, the common ones are Hot Air Solder Leveling(HASL), Organic Solderability Preservative (OSP), Full-board Nickel-plated Gold, Immersion Gold(ENIG), Immersion Tin, Immersion Silver, Chemical Nickel-Palladium-Gold(ENEPIG), and Electroplated Hard Gold, the following will introduce one by one:
The general flow of the Hot Air Leveling process is Micro-Etching→Preheating→Coating flux→HAL→Cleaning.
Hot Air Leveling is also known as Hot Air Solder leveling (commonly known as HASL).
It is a process of coating molten tin (lead) solder on the surface of the PCB and flattening (blowing) it with heated compressed air to form a layer that is resistant to copper oxidation.
It can also provide a coating layer with good solderability. During Hot Air Leveling, the solder and copper form a copper-tin intermetallic compound at the joint.
When the PCB is leveled with hot air, it must be submerged in the molten solder; the air knife blows the liquid solder before the solder solidifies; the air knife can minimize the meniscus of the solder on the copper surface and prevent solder bridging.
There are two types of hot air leveling: vertical and horizontal.
It is generally considered that the horizontal type is better.
The main reason is that the horizontal hot air leveling is more uniform and can realize automated production.
HASL Surface finish
- Long storage time;
- Suitable for lead-free soldering;
- Mature technology, low cost, suitable for visual inspection and electrical testing
- It’s not suitable for wire binding;
- Due to surface flatness, there are limitations on SMT;
- It’s not suitable for contact switch design. Copper will dissolve when HAL and the board will withstand a high temperature. Especially thick or thin boards have limitations in spraying tin, making the production and operation inconvenient.
The general process is Degreasing–>Micro-etching–>Acid Pickling–>Pure Water Cleaning–>Organic Coating–>Cleaning.
Process control is easier than other processes.
OSP is a process for the surface treatment of printed circuit board (PCB) copper foil in compliance with the RoHS directive.
OSP is The abbreviation of Organic Solderability Preservatives, the Chinese translation is organic solder mask, also known as the copper protector, and English also known as Preflux.
OSP is to chemically grow a layer of organic film on a clean bare copper surface.
This layer of the film has anti-oxidation, thermal shock resistance, and moisture resistance, and is used to protect the copper surface from rusting (oxidation or sulfide, etc.) in a normal environment; but in the subsequent welding high temperature, this protective film must be very easy to be quickly removed by the flux so that the exposed copper surface can be immediately combined with the molten solder into a strong solder joint in a very short time.
OSP Surface finish
- The process is simple, the surface is very flat and it’s suitable for lead-free soldering and SMT. Easy to rework, convenient production and operation, suitable for horizontal line operation.
- The board is suitable for the coexistence of multiple processing (for example OSP+ENIG).
- Low cost and environmentally friendly.
- The limit of the number of reflow soldering (multiple soldering thick, the film will be destroyed, basically there is no problem with 2 times).
- Not suitable for crimping technology and wire binding.
- Visual inspection and electrical measurement are not convenient.
- N2 gas protection is required for SMT.
- SMT rework is not suitable.
- High storage conditions are required.
3, Soft Gold Plating and Hard Gold Plating(Gold Fingers)
Plate nickel-gold is to plate a layer of nickel and then a layer of gold on the PCB surface conductor.
Nickel plating mainly prevents the diffusion between gold and copper.
There are two types of electroplated nickel gold: soft gold plating (pure gold, the gold surface does not look bright) and hard gold plating (the surface is smooth and hard, wear-resistant, contains cobalt and other elements, and the gold surface looks brighter).
Soft gold is mainly used for gold wire during chip packaging; hard gold is mainly used for electrical interconnection in non-welded areas.
Gold Finger Surface finish
- A longer storage time is more than 12 months.
- Suitable for contact switch design and gold wire binding.
- Suitable for electrical testing
- Higher cost, thicker gold.
- When electroplating gold fingers, additional design lines are required to conduct electricity. Because the thickness of gold is not the same, it may be too thick to cause embrittlement of the solder joints and affect the strength when used in welding.
- The uniformity of the plating surface.
- The electroplated nickel gold does not cover the edges of the wire.
- Not suitable for aluminum wire bonding.
4, Immersion Gold(ENIG)
The general process is: De-pickling cleaning–>Micro-etching–>Pre-dipping–>Activation–>Electroless nickel plating–>Chemical immersion gold; there are 6 chemical tanks in the process, involving nearly one hundred chemical products, the process is more complicated.
Immersion gold is a thick, good electrical nickel-gold alloy on the copper surface, which can protect the PCBs for a long time; in addition, it also has the tolerance to the environment that other surface treatment processes do not have.
In addition, immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly.
Immersion Gold Surface finish
- It is not easy to oxidize, can be stored for a long time, and the surface is flat, suitable for welding small gap pins and components with small solder joints.
- The first choice of PCB board with keypads(such as mobile phone board).
- Reflow soldering can be repeated many times without reducing its solderability.
- It can be used as a substrate for COB (Chip On Board) wire bonding.
- The cost is higher, and the welding strength is poor.
- Because the electroless nickel plating process is used, the problem of black PAD disk is prone to occur.
- The nickel layer will oxidize over time, and long-term reliability is a problem.
Since all current solders are tin-based, the tin layer can match any type of solder.
The immersion Tin process can form a flat copper-tin intermetallic compound.
This feature makes the immersion Tin have the same good solderability as hot air leveling but without the flatness problem of hot air leveling; the immersion Tin can not be stored for a long time, The assembly must be carried out according to the order of immersion tin.
Immersion Tin Surface finish
- Suitable for horizontal production.
- Suitable for complicated line processing.
- Suitable for lead-free soldering, especially suitable for crimping technology.
- Very good flatness, suitable for SMT.
- Good storage conditions are required, preferably no longer than 6 months.
- Not suitable for contact switch design.
- The production process has relatively high requirements for the solder mask film process, otherwise, it will cause the solder mask film to fall off.
- N2 gas protection is best when welding multiple times.
- Electrical testing is also a problem.
The immersion silver process is between OSP and electroless nickel/immersion gold.
The process is relatively simple and fast; even if exposed to heat, humidity, and pollution, silver can still maintain good solderability but will lose its luster.
Immersion silver does not have the good physical strength of electroless nickel/immersion gold because there is no nickel under the silver layer.
Immersion Silver Surface finish
- Simple process, suitable for lead-free soldering and
- The surface is very flat, low cost, suitable for very complicated
- High storage requirements, easy to contaminate
- Welding strength is prone to problems.
- Electrical measurement is also a problem
7, Chemical Nickel Palladium Gold(ENEPIG)
Compared with immersion gold, chemical nickel palladium gold has an extra layer of palladium between nickel and gold.
Palladium can prevent corrosion caused by substitution reaction and make full preparations for immersion gold.
Gold is tightly covered on palladium, providing a good contact surface.
ENEPIG Surface finish
- Suitable for lead-free soldering.
- The surface is very flat and suitable for SMT.
- Through holes can also be coated with nickel and gold.
- Long storage time, storage conditions are not harsh.
- Suitable for electrical testing.
- Suitable for switch contact design.
- Suitable for aluminum wire bonding.
- Suitable for thick boards, and strong resistance to environmental attacks.
PCBMay could provide these kinds of surface finishes as above, when you have the RFQ, please send it to email@example.com.