Printed Circuit Board Surface Method
Anyone who is involved in the printed circuit board (PCB) business knows that PCBs feature copper coatings on their surface. If they’re not protected and exposed to the elements, the copper will begin to oxidize and degrade which renders the circuit board inoperable. The surface finish is an important connection between the components and PCB. The finish serves two crucial roles, to shield the exposed copper circuitry as well as to create a solderable surface for making (soldering) parts to the circuit board.
When you are creating surface finishes that are suitable for printed circuit boards (PCBs) you are able to pick between metallic or organic materials. For instance, lead-free, tin-lead, and various gold finishes and silver.
Furthermore, knowing what kinds of finishes are available is not difficult, however, how do you decide determine which the best option for the PCB you’re designing? They are similar, but each one has distinct advantages, disadvantages, and considerations for technical reasons.
We’ve created a comprehensive listing of nine different finishes for surfaces. We’ll go over the function and the common use of each one as well as the advantages and disadvantages, to help you determine the most suitable choice to complete your project.
The Nine Different PCB Surface Finishes
1. Hot Air Solder Leveling (HASL)
HASL Finish is the main “leaded” surface finish that is used in the industry.
HASL is a shorthand in the term Hot Air Solder Leveling. These boards should be immersed in a tin/lead alloy to achieve this finish. Air knives are then used to remove all solder left by blowing warm air over the board’s surface.
HASL Finish offers many benefits with the printing circuit assemblies (PCA) procedure. It is among the most affordable surface finishes and the finish is solderable after multiple reflows, wash cycles, or storage cycles. When conducting an in-circuit test (ICT) the HASL Finish covers all test pads and through holes using solder. However, the smoothness or coplanarity of the surface is not as good contrasted with other options. The extremely bumpy surface isn’t just an aesthetic issue but could cause problems when you send board assemblies. However, its resistance to corrosion and testability are top-quality.
With HASL your printed circuit board will be held in place vertically using clamps, and then immersed in an aqueous bath before being placed in the hot vat of solder that has molten. The surface, which is made up of lead and tin, is leveled by hot air knives. Then, that printed circuit board is removed from the solder vat that is molten. This creates uniform thickness across the entire board. It was one of the most popular finishes, however, it is currently used mainly for aerospace and military applications.
Advantages：
● Provides superior wetting during component soldering.
● Prevents corrosion of copper.
● Allows for a longer processing time.
● Widely used in boards with no RoHS restriction.
Disadvantages：
● Thickness differs between small and large pads.
● Not suitable for <20mil pitch BGA or SMD..
● Bridging on the pitch is fine.
● Not recommended for High-density Interface (HDI) products.
2. Lead-Free (HASL)
Lead-free HASL Finish is a fantastic alternative to led HASL Finish. The coating planarity of many HASL finishes that are lead-free, reportedly superior to the leaded HASL Finish the concerns of the dissolution of copper and damage caused by heat to circuit boards have largely been largely overcome with different soldering alloys, such as SnCuNi, SnAgCuNi or SnCuCo Although lead-free HASL Finish isn’t the ideal coating for projects with a tiny gap between components due its tendency to bridge across the gap in the process of heating, it is being applied to products that have a component pitch as small as 0.5 mm.
The main benefit that comes with using HASL Surface Finish regardless of whether it is lead or lead-free it’s its excellent solderability.
The properties and applications that this finishing has are exactly the same as HASL, except there is almost zero lead content in this finish. The alloy is composed of copper and tin as well as nickel and tin, or nickel, tin, copper as well as germanium alloy. The finish is not commonly utilized.
Advantages:
● Contains no Lead
● RoHS certified
Disadvantages:
● It is not a valid format to create HDI applications.
● It might not meet the requirements of aerospace and military.
● The possibility of the creation of “tin whiskers” (thin conductive filaments that could cause short circuits as well as a myriad of other problems)
3. Immersion Tin
As per IPC, the Association Connecting Electronics Industry, Immersion Tin (ISn) is an electroplated finish that is created by a chemical displacement reaction which is directly layered on the metal that forms the basis that is the base metal of the circuit board which is copper. In its intended shelf life, ISn protects copper from oxidation.
Immersion Tin is RoHS-compatible and a more affordable alternative to immersion silver and ENIG. Similar to immersion silver this surface finish can be directly placed on the metal base of the board by chemical displacement. This finish creates an intermetallic compound that is flat copper-tin that is able to be soldered without intermetallic diffusion issues or flatness issues. It is a preferred option for press-fitting applications.
Advantages:
● Cost Effective.
● Substitute for solder that has been reflowed.
● High Reliability.
● Flat Surface.
● No Pb.
● Re-workable.
● Top Choice for Press Fit Pin Insertion.
Disadvantages:
● Handling damage is easy to cause.
● It’s not good for PTH.
● Rework is limited.
● It could damage the solder mask
● Dealing with concerns.
● Using a carcinogen (thiourea) in the process.
● On final assembly, exposed tin can corrode.
● Tin Whiskers.
● It is not recommended for multiple reflows and assemblies Processes.
● Thickness is difficult to measure.
4. Electroless Nickel Immersion Gold (ENIG)
The most appealing feature of ENIG Surface Finishing is the smooth surface and outstanding solderability. The Electroless Nickel process deposits Nickel on a Palladium-catalyzed Copper surface through an auto-catalytic reaction.
Immersion Gold is replacement chemistry. By replacing Nickel atoms with Gold atoms, it attaches to Nickel. The suggested Gold thickness is between 2-4 µin. The intention behind the immersion gold layer is to shield it from damage to the Nickel surface and ensure its solderability.
In contrast, Nickel acts to act as a protective layer for Copper. It will eventually be able to diffuse onto its surface Gold and trigger the same problem of solderability (although however, it happens at a lower rate than Copper).
The typical ENIG specifications are set out in IPC 4552, which is the Specification of Electroless Nickel and Immersion gold. The thickness of the Nickel must be between 3 to 6 µm. This will be enough to ensure that it is not able to penetrate into copper’s base.
ENIG is rapidly becoming the most sought-after surface finish in the market. It’s a metallic coating with two layers that acts as a barrier for the copper as well as a surface where components can be soldered. Gold helps protect the nickel while it is stored. ENIG can be a solution to major industry trends, such as lead-free standards and the increasing use of complex surfaces (especially Flip chips and BGAs) that need flat surfaces. However, ENIG is expensive and can sometimes cause what’s called “black pad syndrome,” the buildup of phosphorous between the nickel and gold layers, which can lead to broken surfaces and faulty connections.
Advantages:
● Good for PTH.
● Long Shelf Life.
● Flat Surface.
● Suitable for PTH (Plated Through Holes).
● Strong.
● Lead-free.
Disadvantages:
● Complicated Process.
● Black Pad / Black Nickel.
● Signal Loss (RF).
● Expensive.
● Not Re-workable.
● Damage caused by ET.
● It is not suitable for reworking.
5. Gold – Hard Gold
Hard Electrolytic Gold is consists of an layer of gold that is plated over a barrier layer of nickel. It is extremely durable and is often applied to areas that are prone to wear like edge connector fingers as well as keypads.
As opposed to ENIG, the thickness of the finger can be altered by adjusting the length of the plating process however, the most common minimum values to be used for finger thickness are 30 μin gold over 100-μin nickel in Class 1 and Class 2. 50 μin gold above 100 μin nickel for Class 3.
Hard gold isn’t typically utilized on solderable surfaces due to its cost and its limited solderability. The maximum thickness IPC considers solderable is 17.8 μin, therefore should this type of gold is required on the surfaces that are to be soldered on, the nominal thickness recommended should be 5-10 μin.
Advantages:
● Long Shelf Life.
● Hard, Durable Surface.
● No Pb.
Disadvantages:
● Use of Resist / Tape.
● Plating/Bus Bars are required.
● Very Expensive.
● Extra Processing / Labor Intensive.
Demarcation
● Other Surface Finishes Difficulties.
● Slivering/flaking can result from an etching undercut.
● Not Solderable Above 17 μin.
● The finish does not fully encapsulate the trace sidewalls, except for the finger areas.
6. Soft Bondable Gold
This PCB finish is used for many years and is the same wire bonding procedure as ENEPIG. The pad is a crown-like surface, which meaning it has less surface area for wires to land wires.
Advantages:
● It is suitable for bonding wires
● RoHS conform
Disadvantages:
● Difficult solderability
7. Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
ENEPIG is a relative newcomer to the world of circuit board finishes, first appearing on the scene in the latter part of the 90s. This three-layer metallic layer composed of palladium, nickel, and gold offers a solution unlike other options as it can be bondable. ENEPIG’s first crack at a printed circuit board surface treatment fizzled with manufacturing due to the extremely expensive palladium coating and the lower demand for its usage.
The necessity for an entirely separate manufacturing line has not been accepted due to these reasons. In recent times, ENEPIG has made a return due to its ability to meet the requirements of reliability, packaging requirements, as well as RoHS standards is a benefit when you have this type of finish. It is perfect for high-frequency use where space is very limited.
In comparison to the other four top finishing options: ENIG as well as Lead Free-HASL OSP, and immersion silver, ENEPIG has the edge on the level of corrosion after assembly.
Advantages:
● No Black Pad Ris.
● Wire Bondable.
● Multi-Cycle Assembly.
● No Corrosion Risks.
● Shelf life of 12 months or Greater
It is suitable for various types of boards, hence the term “universal finish”.It is suitable for There are many surface packages and highly advanced boards.
● Easier to process.
● Lead-free, which is why it’s RoHS complaint.
● Suitable for multiple reflows cycles.
● Highly compatible with Sn-Ag-Cu solders.
Disadvantages Of ENEPIG
● Frequently leads to the presence of the black pad.
● Reduces the durability of solder joints.
● It does not support soldering the palladium layer due to its thickness.
● Longer time needed to be wet when contrasted with other finishes.
● The efficiency can be affected by the plating conditions.
● More expensive than other finishes for surfaces.
● Re-workable subject to certain Restrictions.
● Processing Limits.
8. Immersion Silver(IAg)
Immersion Silver is among the most recent updates to the available surface finishes available. Traditionally, it has been used in Asia but is now gaining popularity in North America and Europe as well.
Immersion Silver is the preferable surface finish for those who are concerned with solderability as well as being able to directly probe the surface when using ICT. When soldering is completed the silver layer disintegrates within the solder joint leaving a (6-12 µin) Tin/Lead/Silver/Silver alloy on Copper which creates extremely secure solder joints to BGA packages. Another advantage of making use of Immersion Silver is the color contrast that makes it easier to check.
Its popularity increased after the Underwriters Laboratory performed temperature/humidity/bias tests, which showed no electromechanical migration.
However, when it was scaled to larger quantities for electronic production in commercial settings Immersion Silver revealed a range of flaws. This includes a tendency to create micro-voids, discoloring any exposed silver until it is becoming black, and “creep corrosion” when used in a climate with high levels of air-borne acid and high humidity.
In recent times However the issue of micro-voids is now gone because of improved plating techniques. Furthermore, the tarnishing issue doesn’t necessarily lead to board failures. It’s typically an issue of perception of poor quality by consumers based solely on aesthetics.
Immersion Silver is a good surface finish when one is certain that the product is not exposed to sulfur during transportation or use for the item. It’s a good surface finish for for most other attributes.
Advantages:
● Nickel-free.
● Higher signal speed than Tin.
● RoHS-certified and eco friendly.
● Planar.
● Fine pitch.
● Cost-effective.
● Excellent alternative to ENIG.
● High stability.
Disadvantages:
● It is easily contaminated.
● Not recommended for two-sided assembly.
● More expensive.
● Requires the use of sulfur-free paper.
● May tarnish when exposed to air.
● Not suitable for storage that lasts for a long time.
● Tarnishes.
● Silver Whiskering
● Certain systems cannot be used to calculate aspect ratios for microvias that exceed 1:1.
● High friction coefficient/not suitable for interconnection with compliant pins (Ni-Au Pins).
9. Organic Solder Preservative (OSP)
Organic Solder Preservative (also called OSP Finish, is the most popular surface finish with a low cost. It’s designed to create an extremely thin, uniform protection layer over the copper PCB’s surface that protects the circuitry from oxidation during storage as well as assembly processes. Although OSP Finish is in use for a while, however, it’s only recently gained traction because of the increasing demand for lead-free and fine-pitch alternatives.
In terms of coplanarity and solderability, OSP is superior to traditional HASL for PCB assembly. It does however require an important change in the process, depending on how much flux is used and the number of heat cycles required. Additionally being careful with handling is vital due to the degrading effect fingerprints’ acid has on the OSP and can leave the copper vulnerable to the oxidation process.
OSP is an organic coat applied using a wet in-line panel process. It is among the most commonly used finishes and is a great option for smaller assembly projects. However, it is not as effective in the event that wave soldering is needed to make the double-sided board. This is due to the surface mount thermal exposures to the surface of the board can cause the film to break down and cause oxidation of Copper in the barrels which reduces the solderability of the vias through-hole.
This finish also has some difficulties during circuit testing. Because it is non-conductive, probing through the coating isn’t advised.
OSP Finish is the ideal for fine pitch assembly since its smooth surface allows the stencil to be firmly pressed against the surface of copper pads. It’s an excellent option for volume orders at a low price.
Advantages:
● Repairable.
● Flat Surface.
● Simple process.
● No Pb.
● Simple Process.
● Re-workable.
● Cost Effective.
Disadvantages:
● Exposed Cu on Final Assembly.
● Sensitive.
● Can Cause ICT Issues.
● Handling Sensitive.
● No Way to Measure Thickness.
● Not Good for PTH (Plated Through Holes).
● Short Shelf Life.
● Not recommended for PTH.
Surface Finish for PCB Assembly
Selecting the best PCB surface finish is crucial to determine the price production process, quality, and reliability that any circuit board printed. Every surface finish has its own strengths and weaknesses that you should consider when preparing for PCB assembly. One method to ensure you pick the right surface finish for your boards would be to identify the issues that need to be addressed first and ensure that they are met. For instance, soldering circuit boards require the proper surface finish as opposed to other PCB assembly methods.
Factors That Affect the PCB Surface Finish You Use
Many factors can influence the PCB finish you choose to choose. As a whole, all of these factors need to be considered. While they all play a role in the process, your final PCB might not come out exactly as you intended if you prefer one feature over the other. As an example, you might select lead-free HASL due to its low cost but then realize that it lacks co-planarity in comparison to other finishing techniques.
The following are some crucial components that should be kept in mind when researching PCB surface finishes.
Cost
The amount you pay for surface finishing will depend on the type of finish you pick. Costs are also affected by finish quality. Finishes from HASL are cheaper than ENIG finishes, however, they might not meet the exact quality standards you are looking for.
The cost of fabricating the board will also have an impact. After the initial design will you have enough money to consider more sophisticated finishing techniques like ENIG? PCBs used in industries such as consumer electronics are generally more affordable to make and can permit you to use more expensive finishing methods for superior results.
Volume
Production volume has a major impact on what type of surface finish you choose to use. For instance, the surface finish made of tin infused into the surface will begin to tarnish shortly after it is deposited on the PCB made of copper. However, having a larger number of PCBs will aid in avoiding getting tarnished. If you’re only dealing with only a few PCBs then it might be best to choose a finishing such as the one used in immersion silver.
Cosmetics
Processes such as HASL and OSP are more likely to provide more aesthetic appeal than ENIG which is a different aspect that may affect which sort you pick. Do you want a glossy surface that’s less prone to whiskers, creep corrosion and other similar problems? You might want to avoid using materials such as silver and tin, and instead, choose alternatives that are less likely to cause major corrosion.
Contact Jarnistech for More Information on PCB Surface Finishes
With so many options to pick from, it’s difficult to decide the best one for your specific application. Luckily, JarnisTech will assist you through the entire process.
JarnisTech is a manufacturer of printed circuit boards offering small-batch and large-volume PCBs to people all over the world. As an ISO-9001 certified PCB manufacturer, We have years of experience with printed circuit boards as well as their intricate details. Since PCBs and PCB components are what we specialize in and we are able to answer any you might have about this type of technology.
If you’d like to know more about PCB’s surface finish and to explore the possibilities, contact us today by completing our form or by calling us at 0086-755-23034656
