Topic

Build information on a PCB

One of the frustrations of the “reverse engineer” is the relative lack of information about a board that is available to a casual observer. Ideally, one would have both the layout information and the build specification, from which a duplicate board could potentially be made. In practice, such information is never available, but some data can be gleaned from close scrutiny of the board.

Layer count

Single-sided boards can be differentiated from double-sided types, but it is often difficult to be certain that a structure is a multi-layer, let alone know how many layers are inside the board, except of course by sectioning or X-ray. Even here sectioning can give misleading information in cases where annular rings have been removed from internal layers as part of the clean-up process. And X-rays can fail to show all the layers, unless one uses a sophisticated sectioning technique.

In some cases, however, designers include a “layer table”, on the lines of that shown in Figure 1. Here the number corresponding to each layer occurs only on that layer. With a strong enough light one could in theory be able to see all the figures, but in practice many of the laminates used are comparatively opaque, so the technique works best with X-rays, offering a simple way of verifying that all the internal layers have been correctly fitted.

Figure 1: Design for a layer table for a 12-layer board, as seen on the Gerber viewer

Design for a layer table for a 12-layer board, as seen on the Gerber viewer

 

Even so, the design of the more typical layer table shown in Figure 2 indicates from the outside of the board how many layers the design has, though only the topmost will be visible.

Figure 2: A typical layer table, as seen on a real 8-layer board

A typical layer table, as seen on a real 8-layer board

 

Sometimes these layer tables are placed within the periphery of the board, so are part of the shipped product, but unfortunately it is more common practice to put such information in the waste material that is removed during singulation.

Other information

There is, however, other information on most boards, as indicated in Figure 3.

Figure 3: The layer table on a 6-layer board, seen in the context of other board markings

The layer table on a 6-layer board, seen in the context of other board markings

 

Here the other markings all have some purpose:

Figure 4: Close-up of the information area on the board in Figure 3

Close-up of the information area on the board in Figure 3

 

Identifying individual boards

Where it is necessary for reasons of traceability to mark boards with a unique identifier, rather than a batch code, a number of techniques are available. Increasingly this will be done as part of a direct patterning process, but traditionally individualisation has been carried out using bar codes. These may be the conventional linear bar codes with which we are so familiar from the supermarket, but there are also a number of two-dimensional patterns that serve a similar purpose, but contain more information in less space.

More information about 2D bar codes, and a number of excellent examples, at the Unibar 2-Dimensional bar code page. You can even create your own bar codes: a link at the ID Automation Internet & web page barcode implementation tutorial created the bar code below, which contains the information “Welcome to AMI4945, Bolton University’s module “Design for Product Build”, tutored by Martin Tarr”!


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