You must have seen someone plating a hair and you wondered why it is important to do so. While one of the major reasons is to change the appearance, one core reason is to help the hair grow faster and have a good texture. A similar perspective can be used to describe the plating of PCBs.
This sometimes appears to be a confusing concept, considering that it is often intertwined to mean surface finishing. And while the two can coexist, they do have a couple of differences. In this article, we explain in-depth, what PCB plating means.
What Does It Mean when a PCB is Plated?
A Printed Circuit Board (PCB) is plated for different reasons, but one of the most outstanding is that the process helps to facilitate the flow of current within and around the board. As you know, a circuit board thrives on current and the distribution of the same to the components in the electronics.
The Plating Process Simplified
To simplify how the process works, we will go this way: it is an electromechanical process. The process involves the deposition or insertion of a metal (usually copper) to the PCB’s surface areas. A similar deposit is made into the plated through-holes or the holes drilled on the board.
When is PCB Plating Required?
In recent years and with the advancement of the modern circuit board processing technologies, plating of PCBs has become a trend when working with multilayer circuit boards. The goal is to facilitate the interconnection of the different layers and components that go into the making of such PCBs.
The Two-Way PCB Plating Process
This is where there is a bit of confusion and you need to pay an attention to how it works. Commonly, circuit boards are plated through the via process, where copper is used.
For the via plating, the Plated Through Hole (PTH) process commonly called vias is used. The process entails using a metal (copper) to fill the drilled holes on the board as a way of channeling the current to move from the board’s surface to the inner layer. The current’s pathway can also be made in a way that it moves from one surface to the other or between two internal layers on the PCB.
Surface finishing is the name of the other plating process for PCBs. It is usually regarded as the second step of the process and rightly so because of the designation for covering the surface copper traces. Doing so helps to protect those traces from overt exposure to adverse environmental risks, such as contamination, oxidation and moisture.
The Processes for PCB Plating
To start plating your PCB, it has to begin with an electroless process where the metal (copper) is to be used. Before we continue, let us highlight a couple of reasons why copper is the most preferred for this via plating process:
- Robust Connection: due to the reliability of the material and the internal properties, it is better to use copper for via plating, as it provides a robust connection. Such connection is needed to facilitate the flow of current from one layer to the other, as well as from one PCB surface to the next.
- Pad and Conductor Thickness: the use of copper helps to increase the copper thickness of the conductors and the pads.
With that being noted, let us now go deeper into the copper plating process for PCBs, which is an electroless process.
1. Chemical Coating
A chemical coating process is used to place or deposit a thin copper layer to the PCB’s surface. A similar process is used to make a similar deposit in the interior of the drilled holes in the circuit board.
The goal of the chemical coating process is to create a thin metal base both in the drilled holes and on the circuit board’s surface. That, in turn, lays the foundation for the electroplating process to commence in earnest.
2. Board Preparation
You will now prep the circuit board for the electroplating process by implementing a couple of strategies, such as:
- Hole Cleaning: the drilled holes are to be cleaned, as there is a possibility of having getting filled in some places by the contaminants and residues generated while the drilling process was on.
- Micro Etching: the interior of the drilled holes is now to be subjected to micro etching. As the name signifies, it is a minimal etching process – and one that aims at improve the copper binding possibilities.
3. Photoresist Material Application
The electroplating process begins. The step here is to add the photoresist material to both the bottom and top of the PCB. Once these parts are covered with the material, the photoresist material is to be exposed to Ultraviolet (UV) light.
While doing this, make sure that the exposed parts are limited to the areas on the PCB that don’t have patterns. That is, those areas or surfaces where the plating of the metal circuitry patterns are to be done.
4. Wash off the Photoresist
The photoresist material can now be washed off the PCB’s surface (i.e., top and bottom). Note that it is only the pliable photoresist material that gets washed off. That is, the part that covers the circuitry patterns.
Once this is done, what would be left is the remaining parts of the circuit board that are masked or well-covered by the hardened photoresist material.
5. Copper-to-External Layer Plating
It is now the time to make an electroplating of the most parts of the copper to the external layers, as well as into the drilled holes. The processes to achieving this include:
- The circuit board would be connected to a negative charge, also called the cathode. The charge has to be that of an electrical current.
- Step 2 is to immerse or dip the copper into a chemical bath for plating.
- The third step will see to the drawing of the dissolved copper through the solution to the negative charge. It would then be deposited onto the exposed copper circuitry.
- An even or uniform copper layer has to be produced and that can be done via the dipping of the circuit board into the plating and cleaning solutions/baths. This has to be done under strict, controlled conditions to get the best results.
- It is now the time to plate the tin onto the copper. This is a measure implemented to bolster the circuitry’s protection.
- The next step is to remove both the photoresist (material) and the non-circuitry copper. The process used here is etching.
- The removal of the tin that was previously plated to the copper is the last step. When this is done, you will be left with the bare copper circuitry.
Comparing the Electroplating and the Electroless Plating Processes
From the above discussions, you could see an interchange between the electroplating and the electroless plating methods for plating circuit boards. They are quite different and we want to quickly point out some of the differences.
Electroplating PCB Plating Method
This refers to the PCB copper plating method that doesn’t rely on the use of external electric power. Rather, it uses a chemical reaction, which, in turn, facilitates the reduction of metal atom.
It is important to mention that the particles or metal ions in the reducing agent solution getting in contact with the catalytic metal leads to the transformation to a metal solid. This, in turn, triggers the required reaction that brings forth the required atom reduction of the metal (copper).
Electroless Plating
When a PCB is subjected to electroless plating, it means that it doesn’t use either plating baths or an external power.
The attributes include usability with different materials, such as copper, gold plating and silver. You can also take advantage of the electroless PCB plating method to derive improved solderability, reflectivity, and changing conductivity. It is also reliable for the availability in different sizes, and the support for different materials.
Despite being an affordable variant to the electroplating method, the electroless method has a couple of downsides, such as:
- Regulation difficulties
- It is slower process than the faster electroplating method.
- It is almost impossible to derive or produce thick plates using the electroless plating method.
The Major PCB Plating Methods You Need to Know
There are four (4) major methods or ways you can plate a Printed Circuit Board (PCB). These methods include the popular PCB Through Hole plating process, down to the brush plating, reel linkage plating and finger-plated plating.
Each of these methods offer distinct processes and following these to the latter assures of the best results.
With that being said, let us now extensively talk about the different methods available for you to pick from.
1. Finger-Plated PCB Plating
Another name for this is protruding partial plating. It includes the use of three (3) major items to achieve excellent circuit board plating. These are the items and how they all make a difference:
- Rare Metals: these metals are to be plated on any of the following places on the PCB: edge joints of the PCB, the board’s edge connectors and at the gold fingers. The primary goal of the plated rare metals is to reduce the circuit board’s contact resistance. It also helps to bolster or increase the PCB’s abrasive resistance.
- Gold Finger: this uses either the manual or automatic plated technology to bolster the plating.
- Gold: this is another item and it is often plated on the inner nickel coating of the circuit board edges connector’s protruding contact head.
2. Brush Plating
This is also one of the popular plating methods for PCBs. As the name suggests, it has to do with brushing – but it goes deeper than that. Here are some of the things you need to know about brush plating for PCBs:
- Part-Plating: not all the parts of the circuit board are to be immersed or dipped into the electrolyte. Only a limited area is dipped into the same during the PCB plating process.
- Plating Process: for the best results and to prevent an overall plating, a chemically-reactive anode, such as graphite has to be wrapped in a cotton stick or any other absorbent material. It is then used to bring the electrolyte (solution) to where the plating is most required on the PCB.
3. The Plated Through Hole PCB Plating Method
This is the most used process for plating circuit boards, especially the multilayer PCBs. It is prioritized due to the extensive features it offers. Sometimes, it is compared to the non-plated through hole process.
First, let us understand what the Through Hole Technology or THT is all about. According to Wikipedia, this has to do with “the manufacturing scheme in which the leads on components are inserted through holes drilled in Printed Circuit Boards (PCBs).”
Thus, the process leverages on the already-drilled or existing holes in the PCB to integrate the leads on the circuit board components.
Also, the Through Hole Technology (THT) thrives on the soldering of these holes to the pads on the opposite side. The soldering process can either be by the use of automated pick-and-placed machines or manually done, by hand.
Here are some quick facts about the plated through hole process:
- It uses the electroless copper plating method.
- Coverage: the Plated Through Hole (PTH) plated method is considered to be through hole, only if it is able to go through the entire PCB. This is important because this plating method is done to enhance the flow of current throughout the circuit board.
The following attributes define the process of using PTH for your circuit board:
- Compact PCB Size: it is possible to achieve a smaller footprint for your circuit board, using the Plated Through Hole (PTH) method. It is attained due to the vertical-stacking of the circuit board via the integration of the Plated Through Hole (PTH).
- Current Flow: the plated through hole process is designed to facilitate an all-around flow of current within the PCB. It is expected that current would flow to the following places: to the other side of the PCB, through the circuit board’s non-conductive substrate, and through the components on the PCB’s side.
Plated Through Hole (PTH) Vs. Non-Plated Through Hole (NPTH)
If a circuit board can be plated, what is also the possibility of not having it plated? This brings us to the discussion on some notable differences between the Plated Through Hole (PTH) and the Non-Plated Through Hole (NPTH) plating methods for PCBs.
First, understand that a circuit is subjected to the through-hole plating method because of the need to facilitate current flow to different parts of the board. It is also a measure aimed at plating a thin copper layer onto the walls or holes drilled in the PCB. In addition to boosting the electrical connectivity of the surfaces, it is also a means to improving the mechanical stability and lowering the resistance between the copper tracks and components’ leads.
On the other hand, the non-plated through hole plating method doesn’t have the most important ingredient that the PTH has – copper plating. To this end, it doesn’t provide or support the electrical properties that the PTH supports.
These are some of the additional differences between the PTH and NPTH plating methods for PCBs:
- With the PTH method, copper is plated onto the walls of the holes drilled on the PCB, while the same is not obtainable with the NPTH method.
- The electroless plating process enhances the conductive properties of the inner walls of the holes. For the non-plated through hole plating, no electrical or conductivity properties are obtained, due to the absence of copper plated onto the holes’ walls. This, in turn, prevents the barrel of the hole from holding any electrical function.
4. Reel Linkage Selective Plating
This can be a manual or automated plating process and which is based on the reel-to-reel plating method. It is to be noted that the following attributes make the plating process unique:
- Selective Deposition: it supports the selective or handpicked deposition of metals, such as contact pins and pins. These are to be deposited on the following: connectors, flexible PCBs, transistors and Integrated Circuits (ICs).
- Plating Process: both the manual and automated process can be used. However, for the best results and to facilitate bulk plating, it is best to go for the automated process called batch welding.
- Plating Procedure: using reel linkage selective plating on PCBs involves the coating of a portion of the metal copper coil plate. The coating must be done with a resist film. Note also that the plating procedure must only be performed on the selected portion of the copper foil.
The Processes for Surface Plating of PCBs
This is the second procedure involved in the plating of Printed Circuit Boards (PCBs). It has to do with the “surface preparation” of the PCB to have a defined outlook. For this purpose, a wide range of methods are used.
The definition of surface plating can be looked at from different perspectives:
1. Suitable Surface
One of the reasons for finishing or smoothening the surface of PCBs is to make it readied for the solderable components to be able to pass through. This comes in handy during the Printed Circuit Board Assembly (PCBA) process.
2. Environmental Protection
Also called surface finishing, the surface plating method for PCB refers to the processes taken to protect the board’s surface from overt exposure to the harsh environments. This is important due to the fact that the drilled holes and pads are to be left open after the initial plating process. Leaving these open exposes them to potential hazards, such as deterioration and oxidation of the copper.
To this end, making a surface finishing is a measure aimed at covering/plating the exposed copper for the protection from the aforementioned hazards.
Surface Finishing Options for PCBs
Now, your Printed Circuit Board (PCB)’s surface can be finished or plated with any of the following methods:
1. Electroless Nickel Immersion Gold (ENIG)
Also called immersion gold plating, ENIG refers to the two-layered metal coating, whereby one of the layers of nickels is chemically-plated to the circuit board. The other layer of nickel serves as a source of protection for the exposed copper. It is also protected from oxidation by using a thin layer of gold.
As an electroless PCB plating method, ENIG plating doesn’t require electric. It may also align with immersion to create a formidable metallic barrier.
2. Hot Air Solder Leveling
Also known as HASL, it refers to the circuit board surface finishing method that requires the dipping or immersing of the circuit board into hot solder. An air knife will be used to level-off the excess solder after the plating.
Despite being an affordable surface finishing process for PCBs, the HASL method tends to leave behind lots of uneven surfaces, despite the passage through hot blasts of air for leveling. To this end, there might be solder bridging issues arising from this.
3. Immersion Tin
Also called immersion tin plating, it has to do with the plating process that requires the use of tin. Some of the advantages of using immersion tin plating for your PCB surface finishing process include:
- No lead is required.
- The absence of lead and the associated process also means that the process is RoHS-compliant.
On the flipside, you may not want to go ahead with the immersion tin plating due to the creation or leaving behind of tin whiskers. This, in turn, creates connections issues between the components.
Wrapping Up: Why Plating is Important in the PCB Production Process
As Printed Circuit Boards (PCBs) form a bond between the components and the overall functionalities of the electronics; it makes sense to have them designed the right way.
The benefits of plating a PCB include but are not limited to providing a clean platform for soldering to take place. You also want to plate the circuit board, because of the aesthetic appeal it brings, the uniformity of the surfaces, and the protection of the components from contamination, moisture and oxidation.
At Hillman Curtis, we help recommend the best plating method to choose from (between PTH and NPTH). We also help you choose the best surface finish for your PCB.
