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FPC Research Blog
Preparation of FPC based on ultrasonic spraying method_4_Experimental Results
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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.


Surface Finish Option Selection Consideration | FPCway


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.


Advantages of ENIG


 • Easy process mechanism

• Flat surface

• Good oxidation resistance

• Good electric performance

• High temperature resistance

• Good thermal diffusion

• Long shelf life

• No skin effect

• Available for untreated contact surfaces

• Lead free


Advantages of ENEPIG


• Excellent multiple reflow cycles

• Capable of ensuring good solderability

• Highly reliable wire bonding capacity

• With a surface as key contact

• High compatibility with Sn-Ag-Cu solder

• Available for multiple packages, especially for PCBs with multiple types of packages

• Black pad free


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.


Disadvantages of ENIG


• Affected by plating conditions & control over whole process

• Affected by thickness of electroless nickel and gold

• Plating is affected by metal area size in the plating bath

• Relatively low wettability

• Black pad tends to be easily caused

• Greatly reducing solder joint reliability

• No skin effect


Disadvantages of ENEPIG


• Solderability performance is reduced due to too thick palladium layer

• Slower to be wet

• Costly


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. But, you're totally allowed to gain optimal balance between quality and cost in FPCway since we have measures to defeat black pad issue with ENIG applied.


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.

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