What Is Thieving In PCBs And Why Is It Done?
In our high-speed design courses, we pass around sample PCBs so that the students can see the physical representation of the things that we describe in class. One question that we are frequently asked relates to thieving and the benefits it delivers. This article describes thieving, to what sections of the board it is applied; at what point in the fabrication is it done, how it relates to plating operations; who is responsible for adding thieving to the outer layer artwork and its shortcomings and benefits.
Descriptions and Depictions
The questions that arise relative to thieving include:
- What is thieving?
- Why is it added to the outer layers?
- Why do the thieving dots vary in size?
- Who is responsible for putting the dots on the PCB outer layer artwork?
The answers to these questions are provided in the following commentary. First, thieving is the addition of “dummy” pads across the surface that are plated along with the features designed onto the outer layers. The purpose of the process is to provide a uniform distribution of copper across the outer layers to make plating current and plating in the holes more uniform. These dummy pads are connected together along with all of the other features that will be plated by the outer layer copper laminated onto the PCB stackup. After plating, the copper between the dummy pads is etched away, leaving them isolated from each other along with other features on the surface layers.
Figures 1 and 2 show two examples of thieving. As can be seen in Figure 1, there are round dots around a BGA. Here, thieving has to be done to make the copper the same height across the board and the component. Figure 2 shows thieving around a press-fit connector. This ensures that the plating in the holes is uniform in thickness.
To better understand thieving, it’s useful to review the plating operations as described below. Note: Thieving is not required on inner layers as they are only exposed to etching and require no plating.
Outer Layer Plating
Outer layer plating is done to deposit copper in the vias and component lead holes so that connections are formed from one side of the PCB to the other. It’s also done to make the connections to signals and planes on inner layers. The two outside layers of a PCB are kept as solid copper until the drilling and plating processes have been completed to provide a path for the plating current needed to plate the copper in the holes.
In the early days of PCB manufacturing, the entire panel on which a PCB was formed would be immersed in the plating bath after the holes had been drilled and the burs and debris were cleaned away. This is referred to as panel plating. The challenge with panel plating is that copper is plated all over the surface even in those areas where no traces or pads will be placed after the etching process. This results in two issues:
- The etching of the unwanted copper has to go through the copper plated onto the outer surface as well as the original copper on the outer layer. This requires the use of more chemicals and also takes more time.
- As the etching goes down through the copper it also etches sideways resulting in less accurate trace width control. This led to the pattern plating process described below.
Pattern plating is a process where copper is plated up only on those features that will remain behind after the outer layers are finally etched. Usually, the features included are all of the plated through holes, the traces and the component mounting pads. This process is accomplished by applying a plating resist to the two outside layers after drilling has been completed. This plating resist is photosensitive and is exposed to a light that solidifies the resist in areas that are not to be plated. This unexposed resist is washed away, and the areas that must be plated are left exposed. Plating is the next step that takes place.
The problem with pattern plating is that if the features on the outside layers are not evenly distributed across the surface. The plating currents will also not be evenly distributed and some features, such as vias and connector holes, will plate less heavily or uniformly than the others that are spaced farther apart. When there is a need for very uniform plating, such as in the holes of a press-fit connector, as shown in Figure 2, something must be done to spread the plating current evenly across the entire surface. This is where thieving comes into play.
Who adds Thieving to Outer Layer Artwork and why do the Dots Vary in Shape and Size?
Each fabricator has algorithms that examine the distribution of copper in the artwork as supplied by the customer. The manufacturing engineering staff at the fabricator will determine how much copper must be added to achieve uniform plating. This extra copper is added in the form of dots or squares. As can be seen in the figures included with this article, each fabricator has a different shape and size for the thieving features.
A Caveat About Thieving
If there are traces on the layers just below where thieving has been added, the impedance of these traces may be adversely affected if thieving is applied over them. To ensure this does not happen, a note must be added to the fabrication drawing instructing whether thieving is allowed and where it is permitted. The following is one way to specify this on a fabrication drawing:
- “Thieving is allowed on outer layers to ensure uniform plating. Thieving shall be no closer than 0.100” (2.5 mm) from any other copper feature on the outer layers. It shall not be within 0.100” (2.5 mm) of traces on the first buried signal layer beneath the outer layers. The thieving pattern shall be at the supplier’s discretion and not consist of solid copper.”
The Consequences of not Adding Thieving to Outer Layers
If thieving has not been added to the outer layer of PCBs that have an uneven distribution of features to be plated such as controlled impedance traces, holes for press-fit connectors, and component mounting pads, the features that are isolated will plate up with far more copper than those that are densely packed.
The controlling parameter for plating outside layers of a PCB is to ensure that there is adequate copper plated into the vias and component mounting holes to ensure robust connections. In most cases, the most critical holes are those that will receive press-fit pins of connectors. The control on these is usually +/- 2 mils or 0.05 mm. Platers will allow plating to continue until this specification has been satisfied. Without thieving, most other features will be plated with excessive amounts of copper such that the finished hole size is too small or trace impedance is impacted.
Thieving is a method used by fabricators to ensure there is uniform plating of copper in the holes drilled through a PCB. To accomplish this, the plating current must be uniformly distributed over the PCB outer layer surfaces. Thieving ensures that the copper will be evenly dispersed when the distribution of the holes that are to be plated are not uniform.
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