A mechanical layer is one of the many layers used in the production of Printed Circuit Boards (PCBs). In this article, you will learn about the function and some of the core considerations for the usage.
What is a PCB Layer?
You don’t build a house without laying the foundation. Likewise, we cannot break down the concept of mechanical layer if we don’t understand how the layers work in a PCB.
Layers are the levels of conducting signals throughout the circuit board. As the baseline, they provide the medium for the board’s signals to be transmitted and in the right directions.
What is a Mechanical Layer?
It is a type of PCB layer used to highlight the PCB’s physical boundaries. The idea is to use the mechanical layer to point out the following:
- The layer highlights the circuit board’s physical dimensions.
- It is also used to show where “indicative information” is to be placed in the board.
- The mechanical layer also shows you the relevant pieces of information about the PCB’s assembly and printing. Examples of the data shown are the board’s assembly instructions, the dimension lines of the board, vias information, dimension markings and the datasheets.
When to Use a Mechanical Layer
The mechanical layer is commonly used when manufacturing a multi-layered circuit board. It is also possible for the layer to be more than one. Despite the increase in the number, you must prioritize using just one layer for these reasons:
- The basic layer, also called the Mechanical Layer 1, is used to facilitate the circuit board, by outlining the physical dimensions.
- The PCB fabricator also relies on the mechanical layer to cut-out the circuit board from its stock material.
What is Contained in the Mechanical Layer of a PCB?
The mechanical layer contains relevant pieces of information that helps you fabricate, assemble and print the circuit board. The data includes the dimension markings, information about the vias and the physical dimension lines of the board.
The mechanical layer also includes the following:
- The specific tolerances required for the PCB.
- The dimensions and overall profiles of the circuit board’s contour.
- The mechanical layer data file also includes the sizes and positions of the cut-outs and internal slots of the board.
- The special build requirements and drill map can also be included.
- The reference hole and layer sequence are also added.
- Customer panel and;
- Drill position symbols
The Altium Mechanical Layers
If you are using the Altium Computer-Aided-Design (CAD), there are certain processes to adding the mechanical layers.
Classifications of Altium Mechanical Layers
Altium supports two (2) types of mechanical layers. These are the pair of layers and individual mechanical layers.
On the one hand, the pair of layers are used for such tasks, such as component courtyard. These layers are primarily used for component-related details, and apply to one side of the PCB.
On the other hand, the individual mechanical layers are the ones used for tasks that do not directly to a specific PCB side. This is why you can use these layers for the circuit board outline.
The Individual Mechanical Layers
Different kinds of properties are included in the Altium’s individual mechanical layers. These range from the shape of the PCB to the route tool path and the assembly notes.
Here is a breakdown of the different properties you can find inside the individual mechanical layers on Altium Designer:
- Sheet: this mechanical layer is used for defining the outer document drawing template border.
- Assembly Notes: these are the layers used for highlighting the relevant instructions for assembling the circuit board. The assembly notes can also be used to define the components’ load.
- V Cut: this defines the “V Cut” details, which are used to divide the PCB. For this to happen, this layer is used to make a “V-like groove” at the board’s bottom and top. Afterwards, a minimum amount of material is left in place to hold the PCB panels together.
- Board and Board Shape: while the “board” refers to the highlighting the board-related instructions; the “board shape” highlights the overall presentation or outline of the circuit board.
- Dimensions: this mechanical layer defines the dimensional details required for the PCB’s production.
- Route Tool Path: this mechanical layer contains details for the circuit board’s mechanical routing.
- Fab Notes: this layer contains important pieces of information about the board’s fabrications.
Component Layer Pairs Mechanical Layer
This refers to the pair of layers used for component-related details. The following are some of the properties:
- Courtyard: this layer defines the space required for placing the component on a PCB. In addition to specifying the space for the component placement, the courtyard also specifies the collision detection, component selection and the component’s area.
- 3D Model: this layer is used for making a Three-Dimensional (3D) representation/outline of the circuit board’s component.
- Designator: it is used in the assembly drawings requiring component designators. This layer is primarily used for placing the .Designator special string on the PCB.
- Gold Plating: this component layer highlights the requirements for making a selective gold plating.
- Coating: this layer highlights the PCB’s component areas that require protective coating.
- Component Outline: this is an Altium board outline mechanical layer used for representing the mass that the electronic component occupies on the circuit board.
- Assembly: this layer is used to draw the assembly data for the PCB component.
- Dimensions: used for defining the components’ dimensional details.
Differences between the Mechanical Layer and the Keep-Out Layer
Have someone ever tried to cross your boundary and you are forced to install a “keep away from here” sign? This is a good way to establish a boundary and something similar can be seen in the PCB industry.
There are a couple of differences between the mechanical layer and the keep-out layer and the major one has to do with boundaries.
1. Boundary Specifications
The mechanical layer is used to specify the physical boundaries or physical dimensions of a circuit board. However, the keep-out layer is used to restrict the layers within a specific (working) area.
2. Who Uses the Layers?
Since the mechanical layer specifies the board’s physical dimensions, it is given to the fabricator. However, the fabricator doesn’t always use the keep-out layer. Rather, the keep-out layer is “fed” into the PCB design software. The goal is to help the circuit board designer to follow the existing dimensions and never have to step out of the “boundaries” when designing the board.
3. For Mechanical Clearance Purposes
The keep-out layer can also be used to enhance the mechanical layers functions. For example, it can be used to identify the internal segments of the circuit board that need to be cleared off the path for mechanical reasons.
Other Types of PCB Layers
Besides the mechanical layer, there are a couple of other layers that enhance the performance of a Printed Circuit Board (PCB).
Routing Layers
These are both internal and external PCB layers, because they can be found both inside and outside the circuit board. The routing layers are used to make an interconnection of the board’s components.
Solder Paste Layers
Primarily used with the Surface-Mount Devices (SMDs); the solder paste layers are used for solidifying the bonding of the solder paste with the PCB components.
The solder paste layers are also used because of the way they increase the flow of solder in the board.
Solder Mask Layers
These refer to the layers containing the protective coating or mask on a PCB. The mask is called the solder mask.
Typically green in color, the solder mask layers help to protect the top and bottom of the circuit board from contamination and damage.
Silkscreen Layers
These include the top and bottom overlays and are used for placing the texts of the PCB components. With the silkscreen layers, you will be able to identify the profile of the circuit board’s components, as well as the number of components on the board.
The Ground and Power Planes
These layers are usually connected to an onboard voltage point and are used to boost the PCB’s performance, under specific conditions.
The ground and power planes also oversee the distribution of power and ground across the components, as well as been positioned within the bottom, top and internal parts of the circuit board.
Best Practices for Mechanical Layers in a PCB
To be on a safe side, here are some of the best ways to maximize the use of mechanical layers in your circuit board:
- It is better to make an outline of the board by using a small line that measures 0.5mm wide. The center of the line represents the circuit board’s outline.
- The actual milling layers of the circuit board’s contour should not be provided. Those of the cut-outs or internal slots should not be provided either.
- The mechanical layer must not be rotated or mirrored.
- The mechanical layer must always be a 1:1 and always represent the PCB’s actual dimensions.
Final Words
The mechanical layer provides the guideline for producing the PCB in the right dimensions. Always ensure to use the 1:1 ratio, never scale the layer, and create the outline on time.
