what does pcb theiving actually mean and why do we need it?

[akshatanayak]

what does pcb theiving actually mean? as in from what i have read uptil now it is meant for copper balance!!! but how does this help and why is this needed?why would we want an extra fabrication step and increase the cost of manufacturing? and how does this help to increase the yield of manufacture? i am a novice in this novice in this subject so please give me a simple but detailed explanation. thanks.

 

[godfreyl]

From here: https://electronics.stackexchange.com/questions/17203/what-should-i-put-on-my-almost-empty-pcb-layer -

Thieving consists of covering large expanses of unused outer layers with a pattern of shapes, usually diamonds or squares, disconnected from one another. Thse shapes are kept away from other features, such as holes, board edges, or traces. The sole purpose of thieving is to improve manufacturability by ensuring a constant PCB thickness given any particular area on the board, say, half a square inch.

Without thieving, the rollers that are used to laminate the layers together will not exert as much force on the copper-starved areas, which could lead to delamination (looks like light spots inside the board).

How is this different from an (unconnected) copper pour?

The difference is that thieving is a bunch of small pieces of copper not connected to each other. This prevents eddy currents from forming, generating heat and sapping power, and prevents a source of parasitic capacitance. (Everything that plane is near is connected to it with stray capacitance, creating a crosstalk path. This is not a problem for a ground fill because the latter just looks like stray capacitance to ground with no significant crosstalk.)