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You may not know who Forrest Mars or Bruce Murrie is, but if you combine their last names&#; initials, they may sound familiar. In Mars and Murrie formed a company that produced small chocolates surrounded by a candy shell, a treat we know today as M&Ms. Their product was initially sold exclusively to the U.S. Army, who saw the value in soldiers being able to carry chocolates in hot climates without melting. Not only was the hardened sugar candy shell tasty in itself, but it also protected the more vulnerable chocolate inside.
Printed circuit boards are also coated or plated for their protection, and often this is done with a surface finish of solder or some other material. Not only does this help to protect the exposed metal on the board from corrosion, but it helps with component soldering. These aren&#;t the only plating processes used to fabricate a circuit board. Here is a more in-depth look at the different PCB plating processes used during circuit board manufacturing.
Plating Copper During Printed Circuit Board Fabrication
A printed circuit board comprises a combination of conductive and non-conductive materials arranged in multiple layers to provide electrical connectivity throughout the board. The conductive material, which is usually copper, will be etched away from the inner layer pairs leaving only the circuitry&#;s patterns. Once these inner layers are completed, they will all be laminated together into one composited board. The next step is drilling all of the holes needed for components and vias, and the circuit board will then be ready for copper plating of its exterior layers and thru-holes.
The first PCB plating process will be an electroless procedure that will chemically coat a thin layer of copper to the board&#;s surface areas, including the drilled holes&#; interior. This plating operation aims to create a thin metal base on the board and in the holes that will serve as the foundation for the electroplating process. Next, the board will be prepared for electroplating:
The holes are cleaned of contaminants and residues from drilling.
The holes&#; interior is prepped with a micro etching to help the copper binding.
Both the top and the bottom of the board are covered with a photoresist material.
The photoresist is exposed to ultraviolet light except for where the metal circuitry patterns are to be plated.
The pliable photoresist covering the circuitry patterns is washed off, leaving the rest of the board surface areas masked by the hardened photoresist.
The board is now ready for the copper&#;s bulk to be electroplated onto the external layers and into the holes. It will be connected to the negative charge, or cathode, of an electrical current and immersed in a chemical bath for plating.
Dissolved copper will be drawn through the solution to the negative charge and deposited onto the circuit board&#;s exposed copper circuity. The electroplating process includes dipping the board alternatively into the plating and cleaning baths under controlled conditions to ensure that the copper is evenly applied. Once the copper plating is completed, tin will be plated onto the copper to protect the circuitry while the photoresist is removed and the non-circuitry copper is etched away. Finally, the tin will be removed, leaving only the bare copper circuity.
While this concludes the circuit board&#;s copper plating, another vital plating process is necessary to complete the circuit board&#;s fabrication.
The PCB Plating Process for Applying Surface Finishes
As we have seen, printed circuit boards are fabricated with copper features for electrical connectivity. Although the traces and area fills will usually be covered and protected with a solder mask, the pads and holes must be left out in the open for soldering. This exposure will pose a problem because copper left unprotected for any length of time will begin to oxidize and deteriorate, making the circuit board unusable. For this reason, a surface finish is applied or plated onto the exposed copper for its protection.
Different types of surface finishes can be used on a circuit board depending on what kind of protection the board needs:
HASL (Hot Air Solder Leveling)
Up until recently, HASL has been the most widely used surface finish in the industry. The circuit board would be dipped into a molten pool of solder and then run through hot blasts of air to remove the excess. HASL is a low-cost operation available almost anywhere. It has a long shelf life and is easy to use. The downside of HASL is that it leaves uneven surfaces that can cause solder bridging and are not detailed enough for fine pitch components.
ENIG (Electroless Nickel Immersion Gold)
ENIG has become one of the most popular surface finishes currently being applied to circuit boards. It is composed of two layers of metal coatings, with the first being a layer of nickel that is chemically plated to the board. The nickel provides a barrier of protection for the copper and then, in turn, is protected from oxidation by a thin layer of gold. ENIG has excellent surface planarity making it ideal for soldering fine pitch components. It complies with the Restriction of Hazardous Substances (RoHS) requirements and is durable with a long shelf life. On the other hand, it is more expensive than other finishes, such as HASL.
Immersion Silver
This finish is often used in high-speed circuit boards as the silver has low losses in high-frequency applications. Immersion silver is deposited onto the copper using a chemical process and has excellent surface planarity for fine pitch parts&#; solderability. Immersion silver is also RoHS compliant. The downside is that this finish is sensitive to contaminants in the air or on the board and must be kept in protective packaging to prevent it from becoming tarnished. Even with special packaging, its shelf life is roughly 12 months and must be used quickly after air exposure.
There are many other types of surface finishes in the PCB plating process, including; lead-free HASL, immersion tin, organic solderability preservative (OSP), and electroless nickel electroless palladium immersion gold (ENEPIG), and hard gold. While these all have their uses, HASL, ENIG, and immersion silver are the most commonly used surface finishes for circuit boards. The question then is, which of these different plating processes will be best to use on your circuit board?
How Your Local PCB Contract Manufacturer can Help with PCB Plating Questions
Weighing all of the board&#;s parameters and requirements to come up with the correct copper weight, plating types, and thicknesses are essential for copper plating. Data will include the current calculations, copper distribution (balancing), the board&#;s size, and its PCB technology type. When it comes to surface finishes, considerations must include how many boards will be built, how long the expected shelf life is, and what components will be soldered to it.
At VSE, we can give you the help you need to make plating and surface finish decisions for your board. Our engineers understand how the board&#;s power requirements will dictate the copper weight required and the fabrication vendor&#;s materials to build to those weights. Our years of experience working with different PCB finishes and can help you make the best choices for your board.
For a PCB CM that understands how circuit board fabrication will impact assembly so that your board is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.
Once your board passes through the standard PCB fabrication process, the bare copper in your PCB will be ready for the application of a surface finish. PCB plating is applied to protect any copper in your PCB that would be exposed through the solder mask, whether it&#;s a pad, via, or other conductive element. Designers will often default to something like tin-lead (SnPb) plating, but other plating options may be better for your board&#;s application.
In this article, I&#;ll run over the different PCB plating material options and their advantages in your PCB. There are several options to choose from, and depending on your reliability or application needs, you may need to check that your fabricator can apply the plating you need in your design. We&#;ll look at these options as well as a brief discussion of how plating affects losses.
PCB plating materials come in several varieties. I&#;ve compiled the popular materials designers should know and understand in the sections below. I&#;ve never seen a manufacturer that doesn&#;t offer all of these options. If your intended manufacturer doesn&#;t explicitly state they offer one of the options in the list below, you can always them to get a list of their capabilities, including their PCB plating material options.
This PCB surface finish is probably the cheapest option, but it will not comply with RoHS due to use of lead in the plating finish. Immersion tin is a lead-free alternative that can be used in entry-level boards.
Advantages
Disadvantages
Very flat surface
Not good for multiple assembly processing passes or rework
Inexpensive
Forms tin whiskers over time
Compatible with standard solders
Can experience damage from handling
Addition of Pb suppresses whisker formation
Sn diffusion into Cu can reduce shelf like, depending on intermetallic content
Could damage solder mask during the plating process
HASL was historically a very popular surface finish choice, but it is not as reliable as other plating materials. It is inexpensive and is available in a lead-free option, so it can be used as an entry-level plating option.
Advantages
Disadvantages
Inexpensive
Uneven surface makes it less useful for small SMD parts
Can be repaired
Can be damaged from thermal shock
Can be difficult to solder due to poor wetting
Given the disadvantages of SnPb and immersion tin, ENIG is now arguably the most popular surface finish in the industry. In this plating material, nickel acts as a barrier layer between copper and the thin gold surface layer where components will be soldered.
Advantages
Disadvantages
Very flat surface
Not good for multiple assembly processing passes or rework
Easily plates PTH holes
Can be expensive
Widely available
JYN supply professional and honest service.
Can experience phosphorous infiltration between gold and nickel layers, known as black pad syndrome
Easily solderable
Rough interface creates signal losses at high frequencies
Suitable for fine-pitch components
Highly reliable against mechanical damage
Wire-bondable (Al)
This organic water-based surface finish selectively bonds to copper to provide a highly planar surface finish. As an organic material, it is sensitive to handling and contaminants, although the application process is simpler than for other PCB plating materials. It also has very low loss at high frequencies.
Advantages
Disadvantages
Very flat surface
Easily damaged
Repairable after application
Short shelf life
Simple application process
Very low loss in high frequency interconnects
Wire-bondable (Al)
This is my PCB plating material of choice for high-frequency applications. It forms a smooth interface against bare copper, so it does not add as much conductor loss as other PCB surface finishes. The main drawback is tarnishing on bare boards, so it should be soldered and packaged ASAP after fabrication.
Advantages
Disadvantages
Easily solderable and wire-bondable for aluminum
Silver whiskering can occur over time
Very flat surface
Exposed (un-soldered) conductor can tarnish over time, although added OSP helps prevent this
Suitable for fine pitch
Can be difficult to plate into small-diameter vias
Preferable for high frequency interconnects in high reliability systems
Wire-bondable (Al)
This plating material has a copper-nickel-palladium-gold layer structure that is wire-bondable directly to the plating. The final layer of gold is very thin, just as is the case in ENIG. The gold layer is soft, just as in ENIG, so excessive mechanical damage or deep scratches might expose the palladium layer.
Advantages
Disadvantages
Easily solderable and wire-bondable
Expensive
Very flat surface
Palladium layer can make the material more difficult to wet and solder
Suitable for fine pitch
May require separate processing line
Lowest corrosion level among commercially available PCB plating materials
Wire-bondable (Al and Au)
This plating material is essentially ENIG but with a very thick gold outer layer, thus it is among the most expensive PCB plating materials. The gold layer provides a hard surface that can be damaged, but its thickness makes it difficult to totally expose the nickel layer.
Advantages
Disadvantages
Wire-bondable (Al and Au)
Very expensive
Very durable surface
Not suitable for solderable areas
Requires additional processing steps to selectively apply
Can experience slivering
Among all of the above options, ENIG is arguably the best balance of cost durability, and range of application. For most low-frequency analog systems or digital systems that don't always run at fast edge rates (e.g., SPI or I2C), ENIG will often be the plating of choice, including in high reliability systems that need to reach IPC Class 3 compliance. It&#;s also suitable for pads on dense BGAs or QFN packages. Once we look at alternative plating materials shown above, we see some other applications that are more ideal: immersion silver or OSP are best for RF systems, while immersion tin is probably fine for throwaway (Class 1) products that just need lead-free compliance. In more specialized applications like very high speed digital and RF, the thickness is very important, as I&#;ll detail below.
Typical PCB plating thickness values are somewhere around 100 micro-inches. For immersion silver and OSP, the typical thickness can be as low as approximately 10 micro-inches. Specifying the type and thickness of PCB plating is easy: you include it in your fabrication notes (see the example below). If you&#;re producing a prototype and the manufacturer has a standard quote form, you&#;ll have an opportunity to specify the plating type in their form. In these forms, they might not ask you for the thickness, so make sure you specify this if you need a specific thickness. Once you&#;ve specified the required plating value, it&#;s up to your fabricator to ensure the plating can be reliably deposited to the required thickness.
Example fabrication note specifying PCB plating. Here the thickness of the plating specifically is un-specified and is instead lumped in with the finished copper weight. Read this blog to find a link to download the full fabrication notes.Why should the thickness of the plating material matter? There are two reasons for this. First, the IPC-A standard specifies a minimum plating thickness for each of the IPC product classes (see Table 4.3, you can download a copy of this standard from my site at this link). If you want your product to be compliant with any of the standard IPC product classes, then you&#;ll want to ensure your plating thickness meets their spec. Normally, if you specify a product class as you would typically do in your fabrication notes, then the minimum plating thickness is implied. Just make sure you don&#;t contradict yourself, otherwise your fabricator will you asking about the plating note.
The other reason to worry about PCB plating thickness is the effect it has on losses. At low frequencies, you probably won&#;t notice any effects on frequency, so low-speed digital signals and sub-GHz radios won&#;t need to worry so much about PCB plating thickness. I&#;ve done custom printed emitters operating at 5.8 GHz WiFi with ENIG (not the best for high frequency) that swamped the receiver in our test setup, so you can even get away with most platings at these frequencies if your circuit is designed correctly.
The issue with losses arises at mmWave frequencies, like short range radar (24 GHz) and higher. At these frequencies, copper roughness becomes a very noticeable contributor to losses, especially on low-loss RF substrates like Rogers. The plating thickness will determine the amount of roughness experienced by signals as they propagate, which will manifest itself in the skin effect resistance. For some example results, look at the results from John Coonrod in this article, specifically the set of graphs showing insertion loss. As can be seen, larger amounts of rough plating can increase losses. For convenience, I&#;ve reproduced one graph below for microstrips.
Insertion loss per unit length for bare copper and ENIG-plated copper with two thicknesses. Thicker ENIG plating produces more loss. [Source]Once you&#;ve determined the PCB plating you need in your design and you&#;re ready to specify your fabrication requirements, you can create your documentation with the easy-to-use manufacturing tools in Altium Designer®. Once your design is ready for a thorough design review and manufacturing, your team can share and collaborate in real time through the Altium 365&#; platform. Design teams can use Altium 365 to share manufacturing data and fabrication requirements through a secure cloud platform.
We have only scratched the surface of what&#;s possible with Altium Designer on Altium 365. Start your free trial of Altium Designer + Altium 365 today.
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