PCB surface finishes

PCB Design for High Frequencies: Start with the Finish

Dave Burrel - Senior Consultant, Product Design

By: Dave Burrell
Senior Consultant, Product Design

20th December 2017

Home » PCB Design

A Printed Circuit Board (PCB) surface finish is a coating between a component and a bare board PCB. It is applied for two basic reasons: to ensure solderability, and to protect exposed copper circuitry.

Since the early days of Tin/Lead Hot Air Solder Levelling (HASL) finish, there have been many PCB finishes over the years, each with their own advantages and limitations. Cost, technology requirements and legislative demands are only some of the reasons for this growth in choice.

The current, common finishes like Electroless Nickel Immersion Gold (ENIG), Immersion Silver and organics like Organic Solderability Preservative (OSP) provide much better planarity and smoothness for finer pitch devices. An example of such devices would be a Ball Grid Array (BGA), a Quad-Flat No-Leads package (QFN) or a Land Grid Array (LGA).

Changes in RoHS regulations (Restriction of Hazardous Substances) have also made these common finishes more mainstream, making them more accessible over their cheaper counterparts, like OSP and Silver, which tend to be susceptible to shelf life issues.

This difference can be seen more clearly when RF frequencies are introduced. At low RF frequencies, current will typically pass through the copper track of a PCB surface very efficiently. However, as the frequency increases, current tends to pass more on the outer surface/skin of the track, so the plating and its conductive loss becomes of greater significance.

Copper, gold and silver all provide very low resistance and insertion loss; however bare copper is, of course, not suitable as a finish as it will degrade, similarly (but to a lesser extent) to silver.

This leaves us with gold as the most suitable top plating but this has its own unique setback. Gold cannot be put directly onto copper; it needs a barrier layer, provided either by the nickel in ENIG, the silver in ISIG (Immersion Silver/Immersion Gold) or by Palladium in ENIPIG (Electroless Nickel Immersion Palladium Immersion Gold).

At this point, most engineers would opt for nickel in ENIG (the most common solution), but it is very resistive to RF and, as frequency increases, preference moves towards ISIG or ENIPIG. Both of which provide a highly conductive outer skin and, therefore, a better signal path.

As RF frequencies increase to 60 GHz – 80 GHz, the PCB finish has a greater significance to the efficiency and performance of the PCB, becoming a crucial part of the overall design functionality.

In addition, with technologies pushing the boundaries of RF frequencies further in sensors and radar, I predict that these more exotic PCB finishes are going to become more prolific in the future.

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A Printed Circuit Board (PCB) surface finish is a coating between a component and a bare board PCB. It is applied for two basic reasons: to ensure solderability, and to protect exposed copper circuitry.

Since the early days of Tin/Lead Hot Air Solder Levelling (HASL) finish, there have been many PCB finishes over the years, each with their own advantages and limitations. Cost, technology requirements and legislative demands are only some of the reasons for this growth in choice.

The current, common finishes like Electroless Nickel Immersion Gold (ENIG), Immersion Silver and organics like Organic Solderability Preservative (OSP) provide much better planarity and smoothness for finer pitch devices. An example of such devices would be a Ball Grid Array (BGA), a Quad-Flat No-Leads package (QFN) or a Land Grid Array (LGA).

Changes in RoHS regulations (Restriction of Hazardous Substances) have also made these common finishes more mainstream, making them more accessible over their cheaper counterparts, like OSP and Silver, which tend to be susceptible to shelf life issues.

This difference can be seen more clearly when RF frequencies are introduced. At low RF frequencies, current will typically pass through the copper track of a PCB surface very efficiently. However, as the frequency increases, current tends to pass more on the outer surface/skin of the track, so the plating and its conductive loss becomes of greater significance.

Copper, gold and silver all provide very low resistance and insertion loss; however bare copper is, of course, not suitable as a finish as it will degrade, similarly (but to a lesser extent) to silver.

This leaves us with gold as the most suitable top plating but this has its own unique setback. Gold cannot be put directly onto copper; it needs a barrier layer, provided either by the nickel in ENIG, the silver in ISIG (Immersion Silver/Immersion Gold) or by Palladium in ENIPIG (Electroless Nickel Immersion Palladium Immersion Gold).

At this point, most engineers would opt for nickel in ENIG (the most common solution), but it is very resistive to RF and, as frequency increases, preference moves towards ISIG or ENIPIG. Both of which provide a highly conductive outer skin and, therefore, a better signal path.

As RF frequencies increase to 60 GHz – 80 GHz, the PCB finish has a greater significance to the efficiency and performance of the PCB, becoming a crucial part of the overall design functionality.

In addition, with technologies pushing the boundaries of RF frequencies further in sensors and radar, I predict that these more exotic PCB finishes are going to become more prolific in the future.

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Further Reading

Give Yourself the Best Chance of Getting Your PCB Design Right First Time!

Give Yourself the Best Chance of Getting Your PCB Design Right First Time!

Chris Martens - Senior PCB Designer, Product Design

By: Chris Mertens
Senior PCB Designer, Product Design

4th October 2017

Home » PCB Design

In all my years as a PCB Design Engineer, I have not come across two people who have the same design style. We all have our own individual ways of designing boards and there is no right or wrong way of achieving the end result. Many design engineers see their designs as ‘works of art’ and I can add myself to that category, seeing your fully assembled board which bursts into life the first time you apply power to it can be a very proud moment as well as the feeling of relief that the circuit works as intended.

For me, having a strategy is important and much like when I play golf, I plot my way around the course by first identifying all the dangers and pitfalls I will need to anticipate. And by doing this, I give myself the best chance of avoiding them and completing the course in as few strokes as possible. The strategy I am about to briefly outline here has worked well for me over the years and will give you the best chance of getting your PCB design right first time.

Circuit Schematic and Board Outline

Most designs will start with a circuit (schematic) and the engineer will create a netlist from this that can be imported into your PCB design software tool. When your netlist is imported, this will pick the components from your library that are required and automatically add the net connections between the components. Your design may also need input from the mechanical design engineer and a board outline. Other positions for connectors, LED’s, sensors or various other interface parts can also be imported.

In addition to this, a design brief to outline critical parts of the layout and some assembly and manufacturing guidelines can all help to build a picture for your design. Your design software tool should allow you to link the schematic to the PCB layout tool, this helps to select groups of components on the schematic which are automatically highlighted within the layout tool. I tend to place these around the periphery of the board outline in readiness for my next task, which in my opinion is the most important. I have shown in Figure 1 an example of how your design could look at this early stage.

Figure 1 - Pre-placement
Figure 1 – Pre-placement



Placement of Components

Placement of components or groups of components is so important and needs to be right and reviewed before you start tracking the board, even if this takes a little longer to achieve. All PCB software tools have the ability to highlight nets so whether the board is for RF purposes, where the ‘RF path’ is important, digital signal processing, where impedance track length is critical, or indeed any net which requires attention, I will always highlight these nets.

Colour coding your ground and power nets is also a good idea and can certainly help to minimise the power planes by identifying where on the board these connections are. In Figure 2, you can see the highlighted nets which indicate the ‘RF Path’, screened walls can be added and possible fixing points can be shown all in a very short space of time. This is a good basis for a placement review so the engineers can get a good idea of the flow of the circuit and can make any comments before tracking will start.

Figure 2 - Placement
Figure 2 – Placement


Placement Review

During the placement review process is a good time to contact your PCB manufacturer and discuss the PCB build. They will want to know the following:

the number of PCB layers

the proposed order of signal and plane layers

which layers require impedance and the type (i.e. single ended or differential pairs)

track widths and gaps

sizes and types of copper weights

Having all this kind of information to hand helps the manufacturer when supplying a build.

In the review, try and give a strong view of the PCB design requirements and explain how and why you have arrived at your provisional placement layout, this will help the engineer understand your issues and how they can fit in with their circuit design. After your review and when you are happy with your PCB build, we can now start to track the board. This is the part which most layout designers would say is the most fun and can be really stimulating for the brain to achieve your ‘work of art’.

The good news in the example I have shown in Figure 2 is that because you have thought about your placement and reviewed it, this board could almost route itself and should be finished relatively quickly.

Routing the PCB

In the finished layout (Figure 3) I have shown below, you will see that the tracking is pretty much along the same lines as the highlighted nets on your placement board. I will track the most critical nets first so that these can be protected and avoided by other nets. After all the other nets are tracked, I would then create the ground and power planes so that the board is fully connected.

Design rule checks can then be run and any errors rectified before finishing the design with a detailed silkscreen if required. Finally, the manufacturing information can be created which would include the Gerber data / ODB++ file, drill data and a readme file for documentation purposes.

Figure 3 - Routed
Figure 3 – Routed


This strategy can work in so many different ways. For example, if you have a mix of RF, Digital and Analogue circuits for a design, you could group all the different circuits around the periphery of the board, highlight your critical nets and then start bringing them onto the board to find the best placement to keep all disciplines happy. There are occasions where you could create more than one placement layout for the engineers to review before deciding on the one that will work well for layout and this process can often show where you may need to compromise.

Imagine if you have no placement review and you plough on regardless to meet your deadline only to be told at the Layout review that you need to re-arrange your layout for the design to work as intended. This approach could set you back weeks or even months but by concentrating on getting your placement right and then holding a review that all disciplines can approve before starting to track the board gives you the best chance of ensuring your PCB design is right first time.

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In all my years as a PCB Design Engineer, I have not come across two people who have the same design style. We all have our own individual ways of designing boards and there is no right or wrong way of achieving the end result. Many design engineers see their designs as ‘works of art’ and I can add myself to that category, seeing your fully assembled board which bursts into life the first time you apply power to it can be a very proud moment as well as the feeling of relief that the circuit works as intended.

For me, having a strategy is important and much like when I play golf, I plot my way around the course by first identifying all the dangers and pitfalls I will need to anticipate. And by doing this, I give myself the best chance of avoiding them and completing the course in as few strokes as possible. The strategy I am about to briefly outline here has worked well for me over the years and will give you the best chance of getting your PCB design right first time.

Circuit Schematic and Board Outline

Most designs will start with a circuit (schematic) and the engineer will create a netlist from this that can be imported into your PCB design software tool. When your netlist is imported, this will pick the components from your library that are required and automatically add the net connections between the components. Your design may also need input from the mechanical design engineer and a board outline. Other positions for connectors, LED’s, sensors or various other interface parts can also be imported.

In addition to this, a design brief to outline critical parts of the layout and some assembly and manufacturing guidelines can all help to build a picture for your design. Your design software tool should allow you to link the schematic to the PCB layout tool, this helps to select groups of components on the schematic which are automatically highlighted within the layout tool. I tend to place these around the periphery of the board outline in readiness for my next task, which in my opinion is the most important. I have shown in Figure 1 an example of how your design could look at this early stage.

Figure 1 - Pre-placement
Figure 1 – Pre-placement



Placement of Components

Placement of components or groups of components is so important and needs to be right and reviewed before you start tracking the board, even if this takes a little longer to achieve. All PCB software tools have the ability to highlight nets so whether the board is for RF purposes, where the ‘RF path’ is important, digital signal processing, where impedance track length is critical, or indeed any net which requires attention, I will always highlight these nets.

Colour coding your ground and power nets is also a good idea and can certainly help to minimise the power planes by identifying where on the board these connections are. In Figure 2, you can see the highlighted nets which indicate the ‘RF Path’, screened walls can be added and possible fixing points can be shown all in a very short space of time. This is a good basis for a placement review so the engineers can get a good idea of the flow of the circuit and can make any comments before tracking will start.

Figure 2 - Placement
Figure 2 – Placement


Placement Review

During the placement review process is a good time to contact your PCB manufacturer and discuss the PCB build. They will want to know the following:

the number of PCB layers

the proposed order of signal and plane layers

which layers require impedance and the type (i.e. single ended or differential pairs)

track widths and gaps

sizes and types of copper weights

Having all this kind of information to hand helps the manufacturer when supplying a build.

In the review, try and give a strong view of the PCB design requirements and explain how and why you have arrived at your provisional placement layout, this will help the engineer understand your issues and how they can fit in with their circuit design. After your review and when you are happy with your PCB build, we can now start to track the board. This is the part which most layout designers would say is the most fun and can be really stimulating for the brain to achieve your ‘work of art’.

The good news in the example I have shown in Figure 2 is that because you have thought about your placement and reviewed it, this board could almost route itself and should be finished relatively quickly.

Routing the PCB

In the finished layout (Figure 3) I have shown below, you will see that the tracking is pretty much along the same lines as the highlighted nets on your placement board. I will track the most critical nets first so that these can be protected and avoided by other nets. After all the other nets are tracked, I would then create the ground and power planes so that the board is fully connected.

Design rule checks can then be run and any errors rectified before finishing the design with a detailed silkscreen if required. Finally, the manufacturing information can be created which would include the Gerber data / ODB++ file, drill data and a readme file for documentation purposes.

Figure 3 - Routed
Figure 3 – Routed


This strategy can work in so many different ways. For example, if you have a mix of RF, Digital and Analogue circuits for a design, you could group all the different circuits around the periphery of the board, highlight your critical nets and then start bringing them onto the board to find the best placement to keep all disciplines happy. There are occasions where you could create more than one placement layout for the engineers to review before deciding on the one that will work well for layout and this process can often show where you may need to compromise.

Imagine if you have no placement review and you plough on regardless to meet your deadline only to be told at the Layout review that you need to re-arrange your layout for the design to work as intended. This approach could set you back weeks or even months but by concentrating on getting your placement right and then holding a review that all disciplines can approve before starting to track the board gives you the best chance of ensuring your PCB design is right first time.

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Further Reading

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