For correct production of your printed circuit board, it must have a label in the copper of the top and bottom layer, for multilayer additionally on the inner layers.

In German, it was once common practice to label the Bestückungsseite (assembly side) with BS, and the Lötseite (solder side) with LS. With SMD assembly this is no longer appropriate, and even worse, in a technical environment where English is more common, the abbreviation “BS” may be misunderstood as “bottom side”! We therefore recommend a uniform inscription using TOP and BOT (for ALL languages!), to indicate the upper and lower sides. 

Please NOTE: missing or incorrect labelling is the most frequent cause of problems, since many Layout programs permit mirrored data output! For this reason, please label your circuit boards always with TOP and BOT in the copper! Correct labelling with TOP and BOT in the circuit board’s copper provides information about the layer definition, and also for the layer orientation

layer definition = unique labelling of top and bottom layers.
layer orientation
= labelling to avoid mirrored layers.

Problems with layer orientation

Problems with layer orientation

Even when your data uniquely defines which layer the top or bottom layer is, the layer orientation cannot be determined without an inscription in the copper, since nearly every CAD program permits a mirrored output of the data (this is even true for single-sided circuit boards).

Note: This is not a special case just for Multi CB, it applies worldwide for every printed circuit board manufacturer.

This is the TOP layer of a circuit board. Which (layer) orientation is correct (right or left)?

 

Correct layer orientation

Correct layer orientation

In your layout program, the label should appear in the copper as shown in the diagram. As seen from above, the word “TOP” should be legible, the label “BOT” for the bottom layer should be mirrored.

Important: do not mirror the data output! 

BOT will be legible once the circuit board is complete.

 

Layer definitions for multilayer

Layer definitions for multilayer

For multilayers, we recommend a continuous label for the layers, as shown in the diagram. Once again, here it is important that the label for the bottom layer appears mirrored when viewed from above. 

The captions "layer 1" to "layer 4" are shown only for clarity.