Comparison between ENIG and ENEPIG

Among technologies applied in PCB (Printed Circuit Board) fabrication process, those contributing to surface finishes play a crucial role in PCB assembly and application of electronic products with circuit boards applied in them. Copper layer on PCB tends to be oxidized in the air so that copper oxidation tends to be generated, which will seriously lower soldering quality. Surface finish, however, is capable of stopping copper pad from oxidizing so that excellent solderability and corresponding electrical performance can be guaranteed. The uprising market demands on miniaturization, higher functionality and reliability for electronic devices push PCBs towards thinness, light weight, high density and higher speed of signal transmission. Accordingly, surface finishes have to embrace upcoming challenges in terms of stability and reliability to be compatible with development requirement mentioned above. Moreover, based on rising consciousness of environment-friendly sustainable development, environmental pollution issues concerning PCB surface finishes are attracting increasingly more attention from the globe. The implementation of legislations of RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electrical and Electronic Equipment) laid by EU aims to eliminate the hazardous substances such as lead and mercury from electronic products, calling for green or lead-free manufacturing of PCB surface finish. As a type of surface finish, ENIG (electroless nickel immersion gold) and ENEPIG (electroless nickel electroless palladium immersion gold) can not only meet the technological requirement called by PCB market, but are adjustable for lead-free solder tendency as well, with far development potential.

Nevertheless, it's a little difficult for people to tell differences between ENIG and ENEPIG, let alone to be aware when to depend on which. The following content in this article will provide definitions of ENIG and ENEPIG and their manufacturing processes, discuss advantages and disadvantages of them and aim to supply a guide on when to use each finish in specific situations.

Surface Finish Selection Considerations

Up to now, the prevailing surface finishes largely accepted are HASL (hot air solder leveling), OSP (organic soldering preservatives), Immersion Tin, Immersion Gold, ENIG and ENEPIG. Confronted with different surface finishes with their own advantages and disadvantages, have you suffered from severe pain when picking up one type compatible with your products? In fact, no matter what type of your PCB product or what requirement it has to be met, your selection of surface finish must be based on considerations among cost, application environment for end products, fine pitch components, lead or lead free, RF applications (high frequency probability), shelf life, shock and drop resistance, thermal resistance, volume and throughput.
 

Therefore, consideration elements mentioned above can be available as one of your references to your final decision of surface finish for your PCBs. Naturally, these items can NEVER be evenly important with the same degree of significance. Then, degrees of significance for each item should be clarified before you're ready to rely on this list with considerations of your specific product situations.

Advent of ENIG and ENEPIG

As early as 1990s, due to PCB development towards finer lines and micro vias plus protruding disadvantages of HASL and OSP, like flatness issues of the former and flux elimination issues of the latter, ENIG started to be used as another alternative for surface finish in PCB fabrication.

In order to defeat black nickel board, the leading weakness of ENIG, ENEPIG comes out as an upgrading version of ENIG. With plating palladium added between electroless nickel and immersion gold, ENEPIG results in containing a thin layer for resistance whose thickness usually falls in the range from 0.05μm to 0.1μm. Palladium layer plays a role in stopping immersion gold technology from corroding nickel layer. As a result, ENEPIG is capable of defeating the defect of black pad held by ENIG. Furthermore, ENEPIG features highly-reliable wire bonding capability, excellent multiple reflow soldering capability and contains switch contact surface, which makes it able to meet stringent requirement of PCBs with high density and multiple surface packages simultaneously. Based on those advantages, ENEPIG is also called universal finish.

Advantages and Disadvantages of ENIG and ENEPIG

In the 1990s, with the development trend of PCB fine lines and micro-via and flatness problems of HASL (hot air solder leveling) and solder elimination problem of OSP (organic solderability preservatives), ENIG technology began to be massively used in PCB fabrication.

Compared with ENIG, ENEPIG technology was applied in PCB fabrication as early as 1980s. However, ENEPIG wasn't been massively used and popularized as a result of its high cost and products' low requirement in terms of surface finish. At present, the requirements of miniaturization, thinness and multiple functions give more opportunities to ENEPIG.

The advantages of ENIG and ENEPIG are displayed in the following table.

ENEPIG technology develops based on ENIG technology with palladium layer added so that its performance has been highly improved. The reasons are:
a. Palladium layer with dense membrane structure totally covers on nickel layer and phosphorus content in palladium layer is less than ordinary content in nickel layer so that the generation conditions of black nickel is avoided and black pad possibility vanishes.
b. The fusion point of palladium is at 1,554°C that is higher than the fusion point of gold (1,063°C). Therefore, the fusion speed of palladium at a high temperature is relatively slow with sufficient time for the generation of resistance layer to protect nickel layer.
c. Palladium has higher hardness than gold, which results in the improvement of solder reliability, wire bonding capacity and antifriction.
d. Tin-Palladium alloy has the strongest anticorrosion capacity that is capable of stopping the creep corrosion caused by primary cell corrosion so that lifespan can be increased.
e. The usage of palladium is capable of reducing the thickness of gold layer, which downsizes the cost by 60% compared with ENIG.

Each coin has two sides. Apart from advantages, ENIG and ENEPIG have some disadvantages as well.

Measures to Cost-Effective Surface Finish

According to advantages and disadvantages of ENIG and ENEPIG, it's natural to select ENEPIG as a better solution when reliability is first considered. However, its higher cost stops some companies sacrificing some revenue.

Black pad is born with the advent of ENIG. During the process of immersion gold in ENIG, black pad tends to be caused due to nickel corrosion under bad operation. Excessive nickel corrosion will dramatically reduce wettability and bonding performance of soldering and solder will have to withstand larger stress when it is bonded with corroded nickel surface. Finally, the contact layer for contact between solder and nickel will break with black nickel surface generated, which is called black pad.

Since ENIG contains an electroless gold layer, it's quite difficult to summarize whether a black pad exists or not. Nickel won't be exposed until gold is peeled off from the surface through a chemical method. Plus, a P-rich nickel layer will be formed at contact of nickel and gold (before soldering) and contact of solder and nickel (after soldering). That is actually a natural phenomenon and features no correlation with black pad.

Leading causes for black pad come in two aspects. First, technology implementation goes through such bad control so that crystal particles grow with nonuniformity and lots of cracks take place among crystal particles with nickel film with low quality generated. Second, it takes such a long time to implement gold immersion that corrosion tends to be created on nickel surface with cracks generated.

Among all the elements affecting electroless nickel, solder mask stands out with the following reasons:
Reason#1: Solder mask suffers from such insufficient cross bonding and rigidity that contaminate tends to be left on copper surface, stopping activating reaction from taking place. In hot electroless nickel solution, solder mask monomer is released out as hydrogen is generated. Then, it prohibits electroless nickel from reaction and breaks chemical balance.
Reason#2: Surface with bad solder mask leads to degraded pad surface.
Reason#3: Solder mask filled in micro via tends to go through electrochemical reaction so that uniform catalytic surface will be stopped from being created.

To successfully solve the problem of black pad, three measures can be taken:
Measure#1: Value of pH should be controlled of electroless nickel solution.
Meaaure#2: Stabilizer content of electroless nickel solution has to be analyzed.
Measure#3: Nickel surface corrosion should be stopped during immersion gold.

Up to now, improvement in terms of immersion gold technology brings forward good effect. Newly-developed immersion gold technology not only reduces corrosion on nickel surface, but helps decreasing cost as well. Compared with immersion gold solution (pH=4.5-5.5) of last generation, new-generation immersion gold solution is close to be neutral with a pH value in the range from 7.0 to 7.2. Neutral liquid performs best in stopping hydrogen ion corroding nickel surface. Moreover, new-generation immersion gold technology can be implemented in lower gold solution, which makes the cost of initial raw materials go down by 50% to 80% and leaves little influence on nickel in the bottom layer.

When it comes to surface finish for flexible PCBs, if current ENIG is directly applied to flexible circuit boards, nickel film with layers will suffer from cracks as substrate is being bent, which will further lead to cracks of copper in bottom layer. To be compatible with demand of surface finish for flexible boards, newly-developed electroless nickel technology is capable of generating nickel film with a columnar structure. Only micro crack can be formed at the surface as substrate is bent and cracks won't be dissipated into copper in bottom layer.
 

All the analysis and measures listed above only work for ENIG while ENEPIG doesn't need them as an upgrading version of ENIG.
 

Application Comparison between ENIG and ENEPIG

ENIG and ENEPIG application fields are different based on their distinctive advantages. ENIG is suitable for lead-free soldering, SMT (surface mounted technology), BGA (ball grid array) package etc. The industries and products ENIG is capable of serving include data/telecom, high end consumer, aerospace, military and high performance devices and medical industries. Furthermore, ENIG is especially used in flex market owing to its high reliability.

ENEPIG is capable of meeting more stringent requirements of multiple types of packages including THT (through-hole technology), SMT, BGA, wire bonding, press fit etc. What's better, ENEPIG is also suitable for PCBs with different packaging technologies. As a result, application fields ENEPIG can serve include aerospace, military and high performance devices and medical industries with higher requirement of density and reliability.

Actually, it's PCB manufacturers' job to provide top-quality products to their customers. As a significant step during PCB manufacturing, high quality of surface finish definitely decides high quality of circuit boards. Therefore, PCB manufacturers have to ensure that surface finish is capable of meeting requirement called by circuit boards and end products they'll serve.

Technologies and Manufacturing Processes

To get aware of technologies and manufacturing processes of ENIG and ENEPIG may be a little dull but it is able to let you exactly know what happens in those two surface finishes.

1) Technology and Manufacturing Process of ENIG

Three layers of metal structure are concerned in ENIG including copper, nickel and gold. The process mainly includes: copper activation, ENP (electroless nickel plating) and immersion gold.
 

• Copper Activation

Copper activation is the privilege of selective deposition which take place in ENP. Displacement reaction is needed so that a thin layer of palladium can be generated on copper layer acting as a catalytic surface. During PCB manufacturing, PdSO₄ and PdCl₂ are often used as activator with below reaction formula:

Cu + Pd²⁺ → Cu²⁺ + Pd

• ENP

In ENIG technology, nickel layer has two functions. As a barrier layer, it can stop the mutual inter-diffusion of copper and gold. On the other hand, it will react with tin, with the generation of excellent IMC (intermetallic compound) Ni₃Sn₄ so that good assembly solderability can be ensured. Under the action of catalytic surface, ENP leads to the deposition of nickel layer through redox reaction with NaH₂PO₂ as reducing agent. As soon as the nickel layer is totally covered with palladium catalytic surface, the elementary substance nickel makes nickel deposition carry on as the catalyst of ENP.

It's important to point out that atomic state active hydrogen emitted by the hydrolysis of the reducing agent NaH₂PO₂ makes Ni²⁺ reduced to the elementary substance of nickel while H₂PO₂^- to the elementary substance of phosphorus. Therefore, ENP layer in ENIG technology is actually nickel-phosphorus alloy layer. The reaction formula of this step is shown as the follows:

H₂PO₂^- + H₂O → H⁺ + HPO₃^2- + 2H

Ni²⁺ + 2H → Ni↓ + 2H⁺

H₂PO₂^- + H → P↓ + OH^- + H₂O

H₂PO₂^- + H₂O → H₂↑ + H⁺ + HPO₃^2-

• Immersion Gold

In the ENIG technology, gold layer has advantages of low contact resistance, few chances of oxidation, high strength and antifriction, which is capable of meeting circuit conductivity requirements and protecting copper layer and nickel layer from oxidation so that solderability of nickel layer can be guaranteed. Immersion gold refers to the generation of gold layer on nickel layer surface through displacement reaction that won't stop until the generated gold layer is totally covered with nickel layer. That's why gold layer is relatively thin. The reaction formula indicating this step is shown as the follows:

2Au(CN)₂^- + Ni → 2Au + Ni²⁺ + 4CN^-

2) ENEPIG Technology and Manufacturing Process

Different from ENIG, ENEPIG takes four layers of metal structure including copper, nickel, palladium and gold. The process of ENEPIG is the same as that of ENIG except that electroless palladium plating is added between ENP and immersion gold.

Palladium layer is added to ENEPIG technology as a barrier layer, stopping corrosion of the nickel layer caused by solution in the process of gold deposition and diffusion from nickel layer to gold layer. Meanwhile, palladium layer can be treated as anti-oxidation layer and anti-corrosion layer because of its feature of compact to increase the solderability. Similar to electroless nickel plating, electroless palladium plating leads to the deposition of palladium layer through redox reaction with NaH₂PO₂ as reducing agent. The reaction formula indicating this step is shown as the follows:

H₂PO₂^- + H₂O → H⁺ + HPO₃^2- + 2H

Pd²⁺ + 2H → Pd↓ + 2H⁺

H₂PO₂^- + H → P↓ + OH^- + H₂O

H₂PO₂^- + H₂O → H₂↑ + H⁺ + HPO₃^2-