PCB Surface Finish&#;What is PCB Surface Finish?
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A PCB surface finish is the coating between the bare printed circuit board and a component. PCBs with copper finish are susceptible to oxidization and deterioration if left unprotected. To this end, it becomes necessary to apply finishing for two crucial purposes:
&#; To protect the copper surface and circuitry
&#; To ready the surface for soldering associated components during assembly
Now&#;s take a look at the specific surface treatments and the differences between them.
PCB Surface Finish&#; 11 Types of PCB Surface Finish & Comparison
There are seven types of PCB surface finishes, and each comes with its advantages and shortcomings:
PCB Surface Finish Type: Hot Air Solder Leveling (HASL)
HASL is the most common surface finish. It is also the most affordable. The application process involves dipping the PCB in a solution of molten solder and then blowing off the residue using a hot air knife. The resolution of molten solder is made of lead and tin alloys.
The most significant advantage of HASL surface finishing is that it is economical. Other advantages include:
As mentioned, this surface finish involves dipping the board in a solution of molten solder made of lead and tin. It makes it unsuitable for panels that demand RoHS compliance. This shortcoming is tied to other disadvantages that include:
PCB Surface Finish Type: Lead-Free HASL
Lead-free HASL is the most popular surface finish used on PCBs today. It is applied by immersing the PCB in a molten or tin alloy lead, ensuring that the entire surface is covered. Air knives are then used to blow off the residue for an even coating.
One of the most significant advantages of lead-free HASL is that it exposes potential delamination issues by exposing the PCB to temperatures of about C. Other advantages include:
PCB Surface Finish Type: Immersion Tin (ISN)
Immersion Tin is the ideal surface finish for fine pitch products, planar, backplanes, and press fit. ISN is applied by initiating a chemical displacement reaction with thPCB&#;s&#;s copper surface.
Immersion tin comes with a wide range of advantages that far outweigh its shortcomings. They include:
Tin and copper have a strong affinity with each other. It means that the diffusion of one of these metals into the other is inevitable. The result is a shorter shelf life when compared to most other PCB surface finishes. It may also limit the PCB performance. It is its most significant shortcoming. Other shortcomings include:
PCB Surface Finish Type: Immersion Silver (IAG)
Immersion Silver has been widely popular since the WEEE and RoHS directive was passed. It is considered an excellent alternative to ENIG for several reasons, chief among them being that it is ideal for fine pitch. Immersion silver is mainly used for aluminum wire bonding, membrane switches, and EMI shielding.
One of the main advantages of immersion silver is that it contains OSP, which prevents tarnishing. However, this also makes it necessary for the PCB to be packaged immediately after application, as OSP is sensitive to contaminants on the board. Advantages of immersion silver include:
It is compliant with RoHS directives and requirements
As mentioned earlier, the presence of OSP makes immersion silver susceptible to tarnishing, which is its most significant shortcoming. Other shortcomings include:
PCB Surface Finish Type: Electroless Nickel Immersion Gold (ENIG)
One Person Answering Questions About Electronics
Electroless Nickel Immersion Gold is fast becoming popular in the PCB industry as it overcomes many of the significant shortcomings associated with other surface finishes. ENIG has a two-part application process. First, a nickel layer is applied to serve as both a barrier to the copper and a suitable surface on which components can be soldered. Another layer of gold is then used to protect the nickel layer for the time during which the board is in storage.
The most significant advantage of ENIG is that it is ideal for the new and upcoming generation of complex surface components, including flip chips and BGAs; other surface finishes have shortcomings that limit their application in these new boards. Other advantages include:
Although ENIG has a long shelf-life, it has also been associated with the &#;black pad, &#;a common problem that leads to the buildup of phosphorus between the layers of nickel and gold, thus resulting in fractures as well as faulty board connections. The only other major disadvantage is that it is not ideal for reworking.
Green Print Circuit Board Enig Gold
PCB Surface Finish Type: Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) PCB surface finish is an upgrade of ENIG. In ENIG, immersion gold has been shown to destroy the layer of nickel. ENEPIG introduces a layer of plating palladium between the layers of gold and nickel.
ENEPIG has been dubbed the&#; universal finish&#; because it applies to many modern, highly advanced boards with multiple surface packages. It comes with a range of advantages that include:
PCB Surface Finish Type: Organic Solderability Preservative (OSP)
OSP is an organic, water-based surface finish. People sometimes refer to It as selective in its bonding with copper, and it also makes the PCB solderable.
OSP surface finishing is notable for its environmental-friendliness since it is organic and water-based. It also makes it easy to apply, and the process is relatively short and straightforward compared to other surface finishes. Its advantages include:
OSP is organic and water-based. This makes it susceptible to damage during handling. Because of this, OSP has several other shortcomings:
PCB Surface Finish Type: Hard Gold (Plating)
Hard gold is the most expensive type of surface finish. It consists of a gold alloy layer bound to a coat of nickel, cobalt or iron. Manufacturers typically reserve it for components that will experience heavy use.
As such, manufacturers commonly use gold finish for edge connectors, test boards, interconnected carrier boards, keypads and contacts.
The thickness of the hard gold finish/plating can range from 25 to 100 micrometers depending on what applications the PCB will be used for.
For instance, electronics for industrial and military use cases require PCBs with thicker, hard gold plating.
PCB Surface Finish Type: PB-Free HASL
Lead-free HASL is a more environmentally friendly version of the Hot Air soldier leveling treatment process. The technique uses solder that consists of lead-free alloy.
PCB Surface Finish Type: Carbon Ink
Carbon ink finishing involves applying carbon ink to PCBs using a silk screen-like process. Thickness can range from 10 to 50 micrometers. Manufacturers apply carbon ink to form protective contact surfaces for components such as switches, LCDs, and jumpers.
PCB Surface Finish Type: Bare Copper
The term&#; &#;bare copper fini &#;h&#; is slightly misleading. In truth, bare copper means that the manufacturer has not added any additional surface treatments or finishes to the PCB, leaving the traces and pads bare.
The Purpose of PCB Surface Finishes
The surface does not exclusively exist for aesthetic value. It has some very important functions for PCBs. The most obvious is its prevention of oxidation.
Conductors, particularly those consisting of copper, are highly susceptible to degradation from oxidation. Oxidation occurs when these components suffer exposure to oxygen, which can typically be found in the air. Thus, the surface finish provides the PCB with a protective layer, ensuring the reliability of the PCB in the long run.
Surface finishes also promote solderability. In fact, this is their primary function. They provide a foundation or base that makes it easier to attach components to the PCB. The absence of a surface finish increases the risk of solder defects such as bridging, tombstoning, and generally poor connections.
Soldering Electronic Circuit Board Electronic Components
In addition to improving thboard&#;s&#;s solderability, many surface finishes promote wire bonding. Not all components are attached to the PCB via solder. Semiconductor packages in particular, use wire bonding to connect to the PCB. Surface finishes can enhance this process.
PCB Surface Finish&#;How to Choose PCB Surface Finishes
The differences in these types of PCB surface finishes make them suitable and unsuitable for specific uses. To this end, anyone looking to apply the ideal surface finish should consider several factors:
&#; PCB Surface Finish&#;Pad Flatness
As mentioned severally, some surface finishes result in uneven surfaces that may impede performance, solderability, and other factors. If flatness is an important factor, consider surface finishes that have thin, even layers. Suitable options, in this case, include ENIG, ENEPIG, and OSP.
&#; Solderability and Wettability
Solderability is always a crucial factor when working with PCBs. Specific surface finishes such as OSP and ENEPIG have been shown to hinder solderability, while others such as HASL are ideal for it.
&#; Gold or Aluminum Wire Bonding
If your PCB requires gold or aluminum wire bonding, your options may be limited to ENIG and ENEPIG.
&#; Storage Conditions
As mentioned, some surface finishes, such as OSP, make PCBs fragile when handling, while others improve durability. It should be considered beforehand when taking into consideration storage and handling requirements. Only when risk-free storage and handling requirements can be met, Surface finishes that make PCBs delicate should be applied.
&#; Solder Cycles
How many times will the PCB be soldered and reworked? As shown, many surface finishes are ideal for reworking. Others, such as immersion tin, are not ideal.
&#; PCB Surface Finish&#;RoHS Compliance
RoHS compliance is crucial when determining the surface finish to go with. Usually, all surface finishes that utilize lead are not suitable for RoHS compliance and should be avoided.
FAQs
What is the cheapest PCB surface finish?
Many consider bare copper to be the least expensive type of surface finish. This is because the bare copper finish process adds no additional surface treatments to the PCB. If you&#;re looking for a surface finish that prevents oxidation and can increase, the next cheapest option is hot air solder leveling (HASL).
How thick is a PCB surface finish?
The thickness of the PCB surface finish depends on the type. For instance, the thickness of an HASL-based surface finish can be as much as 50 um or as thin as 0.2 um, and gold plating can be as thick as 100 um.
Want more information on pcb surface finish comparison? Feel free to contact us.
What is the difference between gold plating and immersion gold?
Gold plating tends to be thicker than immersion gold. As such, gold plating enables more full-bodied joints and is more suitable for harsh environments. Immersion gold is slightly less expensive, and it offers a more consistent coatingIt&#;s&#;s also ideal for fine-pitch SMT components.
Conclusion
As outlined, each type of surface finish has unique elements that set it apart from the rest. These elements make these finishes both suitable and unsuitable for specific applications, and the ideal option ultimately depends on the PCB&#;s makeup.
At WellPCB, we have vast experience with PCB boards and surface finishes. Get in touch to learn more about the different types of PCB surfaces and more! We would love to help you come up with the ideal surface finishing for your PCB.
PCB (Printed Circuit Board) is responsible for loading and connecting electronic components in electronic equipment in order to implement electronic circuit function. For PCBA (Printed Circuit Board Assembly), electric connection between components and circuit on PCB mainly lies in tin plating or wire bonding and rarely in plug-in and fastening connection. In order to assemble connection in PCB design, pad or land is designed. To easily and reliably implement electric connection between pad or land and components, surface finish has to be laid on the surface of pad or land in the PCB manufacturing process , which aims to stop copper from oxidation or deterioration and to provide surface that's convenient for tin plating bonding and wire bonding.
Confronted with so many types of surface finish on PCB pad or land, how should we select the best type of surface finish for our PCBs? This article is a comprehensive guideline for you to refer to.
Up to now, diversity of application market of PCB leads to diversity of surface finish requirement and suitable surface finish and requirement should be selected based on different applications.
Improvement stage. Advent of 21 st century has witnessed new requirement on surface finish owing to the development of PCBs. In order to conform to RoHS regulations and implement lead-free PCB Assembly , electroplated tin lead alloy has to be totally eliminated and HASL containing lead has been reduced increasingly, replaced by lead-free HASL or other types of surface finish. With the development of PCB towards high density and fine spacing, pad space has to be fine and surface finish be smooth so as to adjust micro mounting component soldering assembly and chip metal line assembly so that application of lead-free HASL is being reduced. Performance of existing OSP and ENIG is improving as well and ENEPIG becomes pervasive. Requirement of high density and high frequency of PCB leads to advent of new nickel-free coating.
Maturity stage. At the beginning of s, HASL started to be applied to take place of electroplated soldering and hot melting tin soldering. Meanwhile, OSP took place to substitute passivation treatment stopping copper surface oxidation and flux treatment with rosin coating. In the middle of s, ENIG took off and ImAg and ImSn became mature as well.
Primary stage. Since s, PCB depended on flux to protect its circuit and passivation treatment was applied to stop copper oxidation by surface finish on pad, which aims to stop copper surface from oxidation. Solderability, however, was insured by second-time flux coating in the process of assembly. Then, some other types of surface finish came into being such as chemical gold plating, chemical tin plating, electroplated soldering and hot melting tin soldering and some products with high reliability applied electronickelling and electrogilding.
Preliminary stage. Now, boards with no components assembled on the surface of board are called bare boards. More than four decades ago, soldermask didn't exist and copper lines were exposed to the air. Put in another way, this type of finished PCB is actually an exposed bare board. In this stage, components were assembled on PCB through lead plug-in soldering. Pad of PCB is a copper surface and solder wire with rosin core is applied to be soldered by soldering iron. Later, flux is coated on the whole PCB surface in order to protect circuit and solder pad and the coating method is mostly through flux of rosin and resin spraying. Of course, chemical silver plating is applied sometimes.
&#; OSP
OSP, short for Organic Solderability Preservative, refers to a thin film that is formed at the surface of bare copper in a chemical method. This film features oxidation resistance, thermal shock resistance and anti-wettability, more suitable for developing requirement of SMT in electronic industry.
The main content of OSP is organic substance containing heterocyclic nitrogen such as alkylbenzimidazole, BTA (benzotriazole), benzimidazole etc. An organic film is coated on clean copper surface of PCB pad and through-hole via through complexation and cross-linking reaction. The key ingredient of OSP solution determines solderability and thermal resistance of PCB that can be illustrated from color changing state due to heat and film decomposition temperature, which is extremely significant to the soldering performance of surface mounting,
Thickness of OSP film should be in the range from 0.2μm to 0.5μm, neither too thick nor too thin. Otherwise, it fails to stop copper surface from oxidation if it is too thin while flux fails to totally dissolve OSP plane on copper surface in the process of assembly, which will lead to bad soldering instead.
The responsibility of OSP is to cut off humidity in order to stop oxidation of copper surface, to beat against high temperature and maintain activation and it can be easily melted by flux so as to keep excellent tin plating capacity. Furthermore, different from physical coating, OSP features such excellent selectivity that contaminant won't be generated on soldermask, carbon film or gold side.
Here are advantages of OSP surface finish:
1). Suitable for PCB with SMT and fine line space;
2). Capable of stopping copper surface from oxidation, suffering repeated
heat shock and compatible with multiple solder and easy for soldering;
3). Beneficial to smoothness of PCB;
4). Environment friendly;
5). Contributive for low cost;
Of course, OSP film is so thin that it's easy to be scratched and engineers have to be very careful since once film is broken, solderability won't be insured. Furthermore, PCB with application of OSP as surface finish has to be stored in the environment at dry room temperature without aggressive gas (usually 15°C to 25°C, &#;60%RH). Moreover, storage span won't be more than three months.
&#; HASL
HASL, short for Hot Air Solder Leveling, refers to dipping PCB into melting solder bath and then redundant soldering tin is blown away from PCB surface or metalized via through hot wind so that a smooth, uniform and bright soldering tin coating can be obtained. As users' requirement on PCB with flux coating on bare copper increases, HASL receives fast development.
Boards have to meet the following requirement after HASL:
1). All solder coating layers have to be smooth, uniform and bright with no defects such as knot or exposing copper. In addition, soldermask can never suffer from bubbles, falling off or color changing and copper under soldermask should never be oxidized or change color. There shouldn't be foreign matters on the surface of PCB or in via and soldering tin shouldn't be there either.
2). Thickness of soldering tin layer should be in the range from 3μm to 8μm with overall covering soldering tin layer and solderability as principles.
3). Traditional soldering tin layer containing lead is increasingly be cancelled and lead-free soldering tin with tin as core, coupled with infinitesimal copper or nickel. The key is to maintain solderability. Melting point of lead-free solder tin is higher than that with lead by 30°C so heat resistance of substrate material and reliability of through-hole vias on the board will be challenged.
The essential advantage of HASL lies in solderability. This type of solder tin is basically the same as assembly solder and they are united through mutual melting when soldering. However, disadvantages of HASL are so obvious that coating features bad smoothness and temperature is so high that PCB performance will be influenced. With the escalation of high density circuit design, micro component assembly requires a smooth surface and otherwise, connection reliability will be affected. Moreover, PCB with high density is designed to be so thin that material fails to suffer from being dipped into high temperature melting material. Therefore, HASL is on a difficult way.
&#; ENIG
ENIG, short for Electroless Nickel and Immersion Gold, refers to first deposition of nickel layer and then gold layer on copper surface of PCB through chemical electroplating. Chemical nickel player and gold layer have been massively applied owing to their advantages including good dispersibility, solderability, wiring bond performance, soldering performance and their compatibility with all kinds of flux. Compared with OSP and HASL, ENIG is capable of meeting multiple assembly requirements with functions of solderability, connection, wiring and heat dissipation. Moreover, both board surface and SMD pad are smooth, suitable for fine lines.
Chemical nickel plating layer is actually a Ni-P alloy containing phosphorus whose content is in the range from 7% to 9%. PH value and stabilizer in solution play a significant role in determining content of phosphorus in plating layer so it's necessary to control PH value at about 5.1. Thickness of nickel layer is required to be more than 3μm, determining reliability of ENIG.
Immersion gold on nickel surface is actually a replacement reaction. In principle, when nickel surface is totally covered by copper, gold precipitation will stop. However, as a result of too many holes on the surface of gold layer, nickel under gold with multiple holes will still be dissolved and gold will still continue to be precipitated on nickel with an increasingly low speed until it stops. Immersion gold layer features thickness in the range from 0.03μm to 0.1μm. Copper only plays a role in protecting nickel layer from oxidation and dialysis. However, thickness of copper can't be too high, since otherwise, breakdown will take place in terms of brittleness and insecurity.
ENIG is simple to operate without need of technical leads. But black pad will possibly take place in the process of soldering, leading to reliability issue. The reason for black pad lies in the fact that immersion gold is a replacement reaction and nickel layer is partially dissolved and eroded by gold solution. Then metal compound is generated between nickel and gold layer and contaminated on this layer. Long storage span of PCB will cause color change of gold layer or black pad as a result of heating.
Now that black pad is occurred as a result of nickel layer corrosion, how to reduce nickel corrosion lies in control of phosphorus content. Besides, gold content has to be controlled in immersion gold solution, which helps to decrease nickel corrosion. In addition, change of additive and some operation parameters is also capable of helping to decrease black pad.
Up to now, immersion gold solution contains cyanide in it. Since it is an extremely toxic substance that greatly influences environment and people's health, it is suggested that cyanide will be applied less or won't be applied at all.
At present, ENIG is still an important type of surface finish and new nickel coating solution has been developed, which is capable of effectively controlling stability of solution and decreasing brittleness of nickel layer for more flexibility. PH value of new type of immersion gold solution tends to be neutral with gold content greatly decreased, leading to obvious reduction of cost and corrosion.
&#; ENEPIG
ENEPIG, short for Electroless Nickel Electroless Palladium and Immersion Gold, completely aims to prohibit the production of metallic compound between nickel and gold layer through participation of a type of stable mental layer, palladium, between nickel and gold layers. Palladium layer totally stops nickel migration and production of new compound, effectively avoiding black pad that occurs in ENIG.
ENEPIG, as a type of surface finish, integrates advantages of other types of surface finish such as solderability, wirability, smoothness, oxidation resistance, heat resistance and long-term reliability, so it is regarded as "versatile" surface finish by the industry.
In the process of application of ENEPIG as a surface finish, palladium is added to original nickel plating immersion gold, calling for a palladium plating tank in the original production line. As a result, cost can be saved. From the perspective of cost of metal material, reduction of gold layer thickness leads to lower cost of nickel/palladium/gold coating than that of nickel/gold coating.
Progress from ENIG to ENEPIG surface finish is beneficial to ensure reliability after component assembly. Based on analysis on palladium layer in ENEPIG, it can be demonstrated that palladium layer is composed by pure palladium and palladium-phosphor alloy with different hardness. Therefore, different palladium layers should be selected in accordance with requirement of wiring bonding or plating. What's more, thickness of palladium should be just right because existence of micro palladium will increase copper-tin production thickness while too much palladium will strengthen brittleness of palladium-tin alloy, which then will decrease soldering intensity.
&#; Nickel/Gold plating
As a traditional technique in PCB, nickel/gold plating is majorly applied in surface nickel/gold plating of plug at the side of PCB or switch contact, playing a role in improvement of abrasion resistance and conductivity or applied in nickel/gold plating at the surface of circuit and pad, playing a role in protecting copper layer and improving reliability of plating or wiring connection.
Advantages of nickel/gold plating are displayed as simplicity in terms of control and quality insurance. However, the foremost disadvantage is that it has to depend on technical leads to ensure certain plating connection between terminal points and nickel/gold. Technical leads adding and elimination increases amount of work and isn't suitable for PCB with high density. Therefore, this type of surface finish receives increasingly fewer applications.
&#; ImAg and ImSn
Both ImAg (Immersion Silver) and ImSn (Immersion Tin) are traditional techniques. At the beginning of their development, they receive few applications due to their bad stability and reliability. At present, with the progress and improvement of ImAg and ImSn, both surface finishes maintain being applied in PCB. Put in another way, both techniques are constantly improving with their own attributes.
a. ImAg
Silver is good conductor with excellent conductivity and surface of silver is smooth and solderable, beneficial to signal transmission integrity. However, silver is so sensitive to environment that it tends to change into yellow through chemical reaction and when oxidation layer becomes black, solderability will definitely be influenced. In order to avoid such situations, on one hand, storage environment should be improved and storage times be reduced. On the other hand, micro amount of organic matters should be contained in silver layer so that oxidation will be stopped.
Here are attributes of ImAg surface finish:
1). Excellent solderability, relatively high wetting ability and capable of
meeting requirement of multiple reflow
;
2). Suitable for wiring bonding and pressure contact technology;
3). Even coating and high smoothness of surface, suitable for fine space
assembly
;
4). Excellent conducting performance and reliable bonding;
5). Low operation temperature, suitable for thin boards;
6). Relatively low cost;
b. ImSn
As a major element of solder, tin coating features smooth surface and excellent solderability. The foremost disadvantage of ImSn lies in tin whisker generation as a result of migration between copper and tin, influencing reliability of PCB and prohibiting the development. However, with the increasing mature technology of tin, special additive has been developed to stop tin whisker from being produced.
In the process of manufacturing, tin surface tends to become black because tin surface tends to be oxided so easily that high acidity of water cleaning after immersion tin leads to black tin surface with oxidation. Moreover, contaminant of acid solution on transmission roller also tends to become black as a result of oxidation. Tin thickness is greatly correlated with temperature instead of tin concentration, acidity, thiourea concentration and additive.
Here are attributes of ImSn surface finish:
1). Excellent coplanarity of coating;
2). Even coating thickness distribution;
3). Lead-free
;
4). Capable of suffering from multiple soldering;
5). Low operation temperature;
6). Relatively low cost;
&#; New types of surface finish
With PCBs developing towards high density and high performance, some present surface finishes fail to meet all requirements. Take fine space circuit diagram as an example. When space is 25μm between adjacent two pads and conventional ENIG or ENEPIG are applied, space will be decreased to about 15μm. When reduction reaches 40%, circuit accuracy and insulativity will be influenced. This can be displayed in the following figure.
Furthermore, nickel features bad conductivity that is about one third of that of copper and nickel features ferromagnetism that obviously leads to increasing of signal loss. However, OSP, ImAg and ImSn have no evident influence on signal loss.
In accordance to the problems occurred by ENIG and ENEPIG, here are some solutions.
a. EPIG or EPAG
Due to the bad influence of nickel layer on fine circuit and high-frequency signals and nickel's complete inadequacy, thin ENEPIG is applied as a solution to disadvantages of ENIG and ENEPIG. Chemical palladium/gold plating is applied to replace ENIG and ENEPIG. Without the participation of nickel, surface finish becomes so thin that deformation won't be generated to circuit. Moreover, high-speed signal transmission loss is decreased as well.
A variation of ENIG is EPIG (Electroless Palladium/Immersion Gold) whose palladium layer is 0.1μm thick and copper layer 0.1μm thick. Through experiment and test, EPIG is concluded to be with good reliability and expansibility.
Another variation is EPAG (Electroless Palladium/Autocatalytic Gold) with the thickness of palladium layer 0.15μm and copper 0.1μm, suitable for wiring bonding of gold lines and copper lines. Advantages of EPAG include better high-frequency attribute, suitable for fine space circuit as a result of thin coating, reliable soldering or wiring bonding and reduction in procedure and cost.
b. Electroless nickel ImAg (Ni/Ag)
In spite of low cost and excellent solderability of ImAg, it tends to be easily oxidized owing to its bad anti-corrosion capability. As a result, some improvement has been done with Ni/Ag introduced. Nickel features good conductivity and solderability while nickel has anti-corrosion capability.
With similar technology with ENIG and ImAg, Ni/Ag is developed from practical perspectives. Ni/Ag features palladium thickness in the range from 2μm to 6μm and that of silver 0.1μm.
c. HASLEN
HASLEN is the combination between HASL and electroless nickel. Usually, nickel is difficult for wetting and plating as a result of oxidation when it's exposed to air, so liquid flux is developed based on DES. Compared with HASL, the advantages of HASLEN include: a nearly unlimited life span as a result of higher anti-oxidation reliability; higher reliability owing to its high temperature resistance; avoidance of black pad; low cost.
d. SENIG/OSP
Electronic package arouses a lot of requirements on PCB surface finish. Miniature and lead-free trend of electronic assembly put higher requirements on surface finish, which is why SENIG/OSP occurs. This type of technology is complicated in that after the formation of PCB pattern and soldermask pattern, imaging is implemented to protect copper surface for OSP and expose copper surface for ENIG after which photo-induced resistant layer is eliminated to complete OSP. The key point of these technologies lies in corrosion resistance of electroless nickel layer.
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