It is often said that there is a time for everything. This can be interpreted in a number of ways, one of which is that there is or will be a time when everything is possible. This is an optimistic view, to be sure, but it is theoretically possible that it is, or will be, valid at some point in time. A more pragmatic perspective is that everything has its proper time, and this time is distinct from the proper time for other things. This viewpoint logically leads to questioning how the right time can be known.
View of copper vias inside a PCB
The PCB construction process is the implementation of a well-defined paradigm wherein each step has its specific time or place to be performed. In order to understand this ordering of operations, it is important to know why each step is completed and its overall importance for your board development. The plating of your board’s surface traces and vias is an example of a situation where an understanding of its purpose(s) enables you to make decisions that aid your contract manufacturer (CM) in building the best board for your design. Before discussing the surface PCB plating process options available to you, let’s clearly define what PCB plating is.
What is PCB Plating?
When the topic of circuit board plating is discussed, it is immediately assumed that a multilayer PCB, which may have various via types, is the focus. Although it certainly could be, this is too narrow a perspective. A more comprehensive definition of PCB plating is given below.
PCB plating may be defined as one or both of the following:
- Via plating - the process of filling a drilled hole with copper to provide a path for current from a surface of the board to an inner layer, between two inner layers or from one surface to the other. These plated through holes (PTHs) are better known as vias.
- Surface plating or finishing - the process of covering surface copper traces to protect against the environment, oxidation, moisture, and contamination, and to provide a more suitable surface for soldering components during printed circuit board assembly (PCBA).
Both of the processes listed above may be referred to as PCB plating. And although the primary objective of both is to help facilitate good current flow along board circuit paths, there are differences in the materials used.
Types of Materials used for PCB Plating
PCB plating material is typically a metallic alloy, mostly composed of the following metals:
- Via plating
- Surface finishing
As you probably suspect, your choice of PCB surface finish matters, as there are advantages and disadvantages to each type. Similarly, there are PCB plating process options and design selections should be based on their attributes, as discussed in the next section.
Surface PCB Plating Process Options
The PCB plating options available for your board’s surface are listed below.
Options for Surface PCB Plating Process
- Hot Air Solder Leveling (HASL)
For this process, the board is dipped into hot solder, after which an air knife is used to level it off. This method has been around for a long time and is probably the most economical.
This method involves using a DC current to initiate a chemical process for the deposition of the metal plate over the copper. It is used for surface traces and through-hole vias. The metal may be gold, nickel, or tin.
This process is usually performed with tin. A key advantage is that no lead is required, which means it is RoHS compliant. However, tin whiskers may form, creating undesirable solder bridges between components. As these whiskers are microscopic, PCB optical inspection by your CM is necessary to detect them.
This process, which does not require electricity, may be used with immersion to create a metallic barrier. The process uses a catalyst to dissolve a copper rod. The copper is then distributed on the surface.
Similar to immersion, this process is considered eco-friendly, since no metal-to-metal contact is necessary. However, the shelf-life is short; therefore, good PCB storage guidelines would need to be in place.
With an understanding of these options, you are in a great position to work with your CM and select the best plating option for your design. Research is continually being done to increase the number of PCB plating options available and their effectiveness; such as described in , where a method to improve the throwing power (TP) or more evenly distribute the copper from the surface through the drill hole is presented.
|Tempo's Custom PCB Manufacturing Service
The PCB plating process is an important step of the board fabrication process to ensure the best surface for PCBA is available and that your traces and vias are reliable and durable. At Tempo Automation’s smart manufacturing facility, we provide various options for the plating of your boards that meet or exceed IPC class 1, 2 and 3 standards and your expected quality.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
 Maria Nikolova, “Via Fill and Through Hole Plating Process with Enhanced TH Microdistribution,” PCB007 Magazine, November 18, 2018, http://iconnect007.uberflip.com/i/1050827-pcb007-nov2018/77?m4=.