Common PCB surface treatment

Here, "surface" refers to PCB for the electronic components or other systems to the PCB circuit to provide electrical connections between the connection point, such as contact pad or connection point connection.
Bare copper itself is very good solderability, but is easily exposed to air oxidation, and susceptible to contamination. This is also the PCB surface treatment must be the reason.
1, HASL
The dominant device in the perforation of the occasion, wave soldering is the best way. Using hot air leveling (HASL, Hot-air solder leveling) surface treatment sufficient to meet the wave of technological requirements, of course, the node strength (especially the contact link) requires high, more than electroplated nickel / gold method . HASL is a major application in the world of surface treatment technology, but there are three main motivation driving the electronics industry have to consider the HASL alternative technologies: cost, demand for new technology and lead-free needs.
From a cost point of view, many electronic devices such as mobile communications and personal computers are becoming the civilian population of consumer goods. Cost or lower sales prices to the fierce competition in an invincible position.
After the development of the SMT assembly, PCB pads in the assembly process requires screen printing and reflow soldering process. Occasions in the SMA, PCB surface treatment process is still followed the HASL technology initially, but as SMT devices continue to shrink, pad and stencil openings are along smaller, HASL technical problems gradually exposed. Technical treated HASL pad not smooth, coplanarity can not meet the technical requirements of fine pitch pads.
Environmental concerns usually focus on the potential environmental impact of lead.
2, organic solderability protective coating (OSP)
OSP protection mechanism
Name suggests Italian, organic solderability protective coating (OSP, Organic solderability preservative) is an organic coating, used to prevent oxidation of copper in the welding before, that is to protect PCB pad solderability from destruction. OSP is widely used belong to two nitrogen compounds, that even the three-indolizine (Benzotriazoles) and imidazole organic crystalline base (Imidazoles). They are very good adhesion to the bare copper surface, and very specific --- a soft spot in the copper only, and not adsorbed on insulating coating, such as solder mask.
With three indolizine in copper film formed on the surface layer of molecules in the assembly process, when the reach a certain temperature, this layer of film will be melting, especially in the reflow process, OSP evaporates more easily. Imidazole base in organic crystals formed on the surface protection film of copper with three indolizine than the thicker, the assembly can withstand more heat cycle impact.
OSP Coated Technology
Cleaning: Prior to the OSP, the first preparation to do is to clean the copper surface. The main purpose is to remove the copper surface of the organic or inorganic residues, to ensure uniform etching.
Micro-etching (Microetch): corrosion of copper by the surface of fresh copper will be exposed to bright, and this combination will help with the OSP. Can be etched with an appropriate corrosive, such as excessive sodium sulfide (sodium persulphate), sulfuric acid peroxide (peroxide / sulfuric acid) and so on.
Conditioner: optional steps, depending on the circumstances or requirements to decide whether to carry out these processing.
OSP: OSP coating solution and then the specific temperature and time depending on the equipment, solution properties and request.
Cleaning residues: the completion of the above chemical treatment after each step, the excess must be cleansed, the chemical residues or other unwanted ingredients. General cleaning once or twice a foot Yi. Extremes meet, the rather, caused by excessive cleaning products such as oxidation or loss of luster, it is we do not want to see.
The entire process must operate in strict accordance with process requirements, such as strict control of time, temperature and flow processes.
OSP applications
Deal with the OSP PCB surface after the copper surface in a thin layer of organic compounds, thereby protecting copper will be oxidized. Benzotriazoles type OSP of the thickness of 100A °, and Imidazoles type OSP thickness to thicker, typically 400 A °. OSP film is transparent to the naked eye is not easy to identify the existence, detection difficult.
In the assembly process (reflow), OSP is easy to melt into the solder paste or acidic Flux inside, while showing strong activity of the copper surface, the final component and the pads in between the Sn / Cu intermetallic compound, Therefore, OSP is used to deal with welding has a very good surface characteristics.
OSP does not exist the problem of lead pollution, so green.
OSP limitations
1, transparent and colorless as OSP, it is more difficult to check, it is difficult to distinguish whether the PCB coated OSP.
2, OSP itself is insulated, it is not conductive. Benzotriazoles class OSP thin, may not affect the electrical test, but Imidazoles class OSP, the formation of relatively thick protective film will affect the electrical test. OSP also can not be used as a handle electrical contact surface, such as keys of the keyboard surface.
3, OSP in the welding process, the need for more robust Flux, or remove protective film can not, causing welding defects.
4, in the storage process, OSP surface can not come into contact with acid, the temperature not too high, or OSP will evaporate.
With technology innovation, OSP has gone through several generations of improvement, its heat resistance and storage life, and the Flux of compatibility has been greatly improved.
3, Nickel Immersion Gold (ENIG)
Protective mechanism of ENIG
The copper surface by chemical plating Ni / Au. Ni deposition thickness of the inner layer is generally 120 ~ 240μin (about 3 ~ 6μm), the deposition thickness of the outer Au thin, usually 2 ~ 4μinch (0.05 ~ 0.1μm). Ni and Cu in the solder layer between the barrier. Welding, outside Au in the solder which melts rapidly, forming Ni solder with Ni / Sn intermetallic compounds. Outside the gold-plated to prevent Ni oxidation during storage or passive, it should be enough gold plating density, the thickness can not be too thin.
ENIG Process
Cleaning: The purpose of cleaning the same with the OSP process, clear the surface of organic or inorganic copper residue, ready for the etching and catalysis.
Etching (Microetch): ... ... with the OSP process
Catalyst: the role of this step is in the copper films deposited on the surface layer of the catalyst, which reduces the activity of the energy of copper, so relatively easy to deposit Ni on the copper surface. Palladium, ruthenium catalysts are available.
Nickel Plating: This will not detail the specific process of the. Nickel deposits contain 6 to 11% of the phosphorus, according to the actual specific purpose may be used for welding of nickel surface or as a contact surface, but no matter what the need to ensure there is sufficient thickness of nickel, in order to protect the role of copper.
Leaching of gold: In the process, the aim is to deposit a thin layer of gold and a continuous protective layer, the thickness of the main gold can not be too thick, otherwise the solder will become very brittle and seriously affect the reliability of electric welding. And nickel, as the high temperature leaching of gold, a very long time. In the immersion process, the replacement reaction occurs --- the surface of nickel, gold replacement of nickel, but when the replacement to a certain extent, the displacement reaction will automatically stop. Gold high strength, abrasion, high temperature, oxidation is not easy, so you can prevent oxidation or passivation of nickel and strength requirements for working in high places.
Cleaning residues: the completion of the above chemical treatment after each step, the excess must be cleansed, the chemical residues or other unwanted ingredients. General cleaning once or twice a foot Yi. Extremes meet, the rather, caused by excessive cleaning products such as oxidation or loss of luster, it is we do not want to see.
Application ENIG
ENIG PCB surface treated very flat, very good coplanarity for the key contact surface Harnoncourt. Secondly, ENIG excellent solderability, gold solder will quickly melt into the inside, thus exposing fresh Ni.
The limitations of ENIG
ENIG process more complex process, but if you want to achieve good results, must be strictly controlled process parameters. The most trouble is, ENIG treated in the ENIG PCB surface or the welding process can easily lead to the black wall effect (Black pad), and thus to the reliability of solder joints have a devastating impact. Black disk produces a complex mechanism, which occurred in the Ni and gold interface, and direct the performance of over-oxidation to Ni. Gold too, brittle solder joints will affect the reliability.
4, nickel Palladium Immersion Gold (ENEPIG)
Nickel Palladium Immersion Gold Principle
ENEPIG compared with ENIG, the nickel and gold over a layer of palladium between. Ni deposition thickness of 120 ~ 240μin (about 3 ~ 6μm), palladium thickness of 4 ~ 20μin (about 0.1 ~ 0.5μm), gold thickness of 1 ~ 4μin (about 0.02 ~ 0.1μm). Palladium can prevent corrosion caused by replacement reaction, well prepared for the leaching of gold. Gold in the palladium coverage is close above provide a good contact surface.
Nickel palladium gold leaching process steps
Nickel palladium gold leaching process steps shown in Table 3
Nickel Palladium Immersion Gold Application
Nickel palladium leaching of gold is widely used, can replace the nickel leaching. In the welding process, palladium and gold to the molten solder will melt inside, thus the formation of Ni / Sn intermetallic compounds.
Nickel Palladium Immersion Gold limitations
Nickel Palladium Immersion Gold Although there are many advantages, but the price of palladium is very expensive, while palladium is a shortage of resources, mainly published in the former Soviet Union. At the same time as leaching of gold and nickel, the process control is very strict.
5, Immersion Silver (Immersion silver)
Immersion Silver Works
Immersion Silver Process through the thin (5 ~ 15μin, about 0.1 ~ 0.4μm) and dense to provide a layer of silver deposited organic protective film, the surface of the silver seal of copper, the greatly extended life. Immersion Silver surface is very flat and very good weldability.
Immersion Silver steps to
Immersion Silver steps to table 4.
Other steps will not go into here, prepreg main purpose is to prevent the introduction of pollutants. Immersion Silver process is also a silver in the copper surface to reflect the process of replacing copper replacement. Silver sediments also contain organic additives and organic surface active agents, organic additives to ensure the formation of silver leaching, and organic surface active agents can protect the PCB in the absorption of moisture during storage of silver.
Application of Immersion Silver
Silver solder dip very good solderability, silver in the welding process to melt the solder paste will melt inside, and the HASL and OSP surface, like welding Cu / Sn intermetallic compounds. Immersion Silver surface coplanarity well, while there is not as conductive OSP obstacles, but as the contact surfaces (such as the button face), its strength is not money well.
Immersion Silver limitations
Immersion Silver a people can not ignore the problem is that the electronic migration of silver. When exposed to humid environment, the Bank's role in the voltage generated under electron transfer. By adding organic components within the Bank can reduce the occurrence of electron transfer.
6, Soldering
Principle Soldering
Only used for two reasons dip tin process: first, the surface is very flat dip tin, coplanarity good; the other is lead-free solder dip. But in the dip tin process prone to Cu / Sn intermetallic compound, Cu / Sn intermetallic poor solderability.
If you use dip tin process, must overcome two obstacles: the particle size and the Cu / Sn intermetallic compound formation. Soldering particles must be small enough, but to no holes. Tin deposition thickness of not less than 40μin (1.0μm) is more reasonable, so as to provide a pure tin surface to meet the solderability requirements.
Soldering process steps similar to silver impregnated, not repeat them here.
Soldering applications and limitations
Soldering biggest weakness is the short life span, especially stored in hot and humid environment when, Cu / Sn intermetallic compounds will continue to grow until the loss of solderability.
7, compare:
Since each have their own unique surface treatment technology, application is not quite the same. However, nickel Palladium Immersion Gold (ENEPIG) is a universal approach, it can meet the requirements of the various assembly occasions.