How to choose the plating and thickness of the PCB board?

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Once the board has passed through the standard PCB manufacturing process, the bare copper in the PCB is ready for surface treatment.PCB plating is used to protect any copper in the PCB that would be exposed through the soldermask, whether it be pads, vias, or other conductive components. Designers usually default to tin-lead plating, but other plating options may be better suited for your board application.

In this article, I will cover the different PCB plating material options and their advantages in PCBs. There are a variety of options available and depending on your reliability or application needs, you may want to check if the manufacturer can apply the plating you need in your design. We will look at these options and briefly discuss how plating affects loss.

Types of PCB Plating

There are a variety of PCB plating materials. I have summarized the popular materials that designers should know and understand in the following sections. I have never met a manufacturer that does not offer all of these options. If your target manufacturer does not explicitly state that they offer one of the options in the list below, you can always email them for a list of their capabilities, including their PCB plating material options.

Tin Lead and Immersion Tin Plating
This PCB finish may be the least expensive option, but it is not RoHS compliant due to the use of lead in the plated finish.Dip tin is a lead-free alternative for entry-level boards.

Advantage:
▶ Ultra-flat surface
▶ Inexpensive
▶ Compatible with standard solder

Disadvantages:
▶ Not conducive to multiple assembly processes or rework
▶ Forms tin whiskers over time
▶ Tin diffusion into copper may reduce shelf life depending on intermetallic compound content
▶ May damage soldermask during plating process

 

Hot Air Solder Leveling (HASL) and Lead Free HASL
HASL was once a very popular finishing option, but it is not as reliable as other plating materials. It is inexpensive and has a lead-free option, so it can be used as an entry-level plating option.
Advantage:
▶ Inexpensive
▶ Can be repaired
▶ Due to poor wettability

Disadvantages:
▶Uneven surface makes it less useful for small SMD devices
▶ May be damaged by thermal shock
▶May be difficult to solder

 

Electroless Nickel Immersion Gold Plating (ENIG)
Considering the drawbacks of SnPb and immersion tin plating, ENIG is now arguably the most popular surface treatment in the industry. In this plating material, nickel acts as a barrier between the copper and the thin gold surface layer of the component to be soldered.

Advantage:
▶ Ultra-flat surface
▶ PTH holes can be easily plated
▶ Widely available
▶Easy soldering
▶Suitable for fine-pitch components
▶Highly reliable against mechanical damage
▶Wire bondable (aluminum)

Disadvantages
▶ Not favorable for multiple assembly processes or rework
▶ May experience phosphorus penetration between gold and nickel layers, known as black pad syndrome
▶ Rough interfaces can cause signal loss at high frequencies

 

Organic Solderability Preservative (OSP)
This organic, water-based finish selectively binds to copper to provide a highly flat surface finish. As an organic material, it is sensitive to handling and contaminants, although the application process is simpler than other PCB plating materials. It also has very low losses at high frequencies.

Advantage:
▶ Ultra-flat surface
▶ Repairable after application
▶Simple application process
▶Very low interconnection loss at high frequencies
▶Wire bondable (aluminum)

Disadvantages:
▶Easy to damage
▶Short shelf life

 

Immersion Silver Plating
This is my preferred PCB plating material for high frequency applications. It forms a smooth interface with the bare copper and therefore does not increase conductor losses as other PCB finishes do. The main disadvantage is that it loses its luster on the bare board, so it should be soldered and encapsulated as soon as possible after fabrication.

Advantage:
▶ Easy aluminum soldering and wire bonding
▶ Ultra-flat surface
▶ Suitable for fine pitch
▶ Better suited for high-frequency interconnections in high-reliability systems
▶Wire bondable (aluminum)

Disadvantages:
▶ Silver whiskering over time
▶Exposed (unsoldered) conductors lose luster over time, although added OSP helps prevent this ▶May be difficult to plate into small-diameter vias

 

Hard Gold
This plating material is essentially ENIG, but has a very thick outer layer of gold, making it one of the most expensive PCB plating materials. The gold layer creates a hard surface that can be damaged, but its thickness makes it difficult to fully expose the nickel layer.

Advantage:
▶ Wire bondable (aluminum and gold)
▶ Very durable surface

Disadvantages.
▶ Very expensive
▶ Not applicable to solderable areas
▶ Requires additional process steps for selective applications
▶ May experience brittle cracking

 

How to specify PCB plating material and thickness

Typical PCB plating thickness values are about 100 micro inches. For immersion silver and OSP, typical thicknesses can be as low as about 10 micro inches. If you are producing a prototype and the manufacturer has a standard quotation, you will have the opportunity to specify the type of plating on their forms. On these forms, they may not ask you to provide the thickness, so be sure to specify it if you need a specific thickness. After specifying the desired plating value, your manufacturer will need to ensure that the plating can be reliably deposited to the desired thickness.

Why is the thickness of the plated material important? There are two reasons. First, the IPC-2221A standard specifies minimum plating thicknesses for each IPC product category (see Table 4.3). If you want your product to comply with any of the standard IPC product categories, then you need to make sure that the plating thickness meets its specifications. Normally, if you specify the product category, as you normally do in your manufacturing notes, the minimum plating thickness will be implied. Just make sure you don’t contradict yourself, otherwise the manufacturer will email you asking for plating comments.

Another reason to worry about PCB plating thickness is its effect on losses. At low frequencies, you probably won’t notice any effect on frequency, so low-speed digital signals and sub-GHz radios don’t need to worry much about PCB plating thickness. I’ve completed custom printed transmitters running at 5.8GHz WiFi with ENIG (not the best choice for high frequencies) that swamped the receiver in our test setup, so if your circuit design is correct, you can even bypass most plating at these frequencies.

The loss problem arises at millimeter-wave frequencies, such as short-range radar (24 GHz) and higher. At these frequencies, the roughness of the copper becomes a very noticeable factor in loss, especially on low loss RF substrates like Rogers. The thickness of the plating will determine the roughness experienced by the signal as it propagates, and this will be reflected in the skin effect resistance.