Understanding Gerber Layers for PCBs and Stencils


This is the second installment in a three-part article series in which we will:

Explain the Gerber file format, file names, and standards revisions.
Help you understand the Gerber layers used for PCB and stencil manufacturing.
Explain the importance of Design for Manufacturing (DFM) checks and introduce free resources.

Through this series, we will demonstrate why it is important to check your Gerber file before sending it to the PCB manufacturer, help you identify and avoid common problems, and describe Gerber file checks that you should be performing.

If you read part 1 of this article series, you have a background on the Gerber format and file names. This article will provide a layer-by-layer explanation of the name, abbreviation, and purpose for each layer in a basic PCB.

 

Gerber Layer Orientation

Gerber files are viewed as if you were looking through the PCB from the top to the bottom so the vias, cut-outs, and traces line up. That means the bottom layers will all appear reversed.

 

Figure 1. How Gerber layers combine to produce different features on a PCB board.

 

A Detailed Look at Gerber PCB Layers

In the following subsections, we will take a detailed look at these Gerber file layers:

Top silkscreen
Top solder mask
Top copper
Bottom copper
Bottom solder mask
Bottom silkscreen
Outline / mechanical
Drill
Inner copper

 

Top Silkscreen (GTO)

Purpose: The top silkscreen Gerber file contains data to be printed, typically the white or black text on the PCB board, such as part designators, component outlines, logos, polarity indicators, etc.

Notes: This layer is optional. It is entirely OK to have a PCB without silkscreen if that is what you want.

 

Gerber top silkscreen layer.

Figure 2. Gerber top silkscreen layer.

 

Top Solder Mask (GTS)

Purpose: Contains data for the protective solder mask which protects the copper. This is typically a green film but can come in a variety of colors.

Notes: Solder mask layers are negative—meaning filled areas correspond to openings in the solder mask where no solder mask will be applied. These filled areas in the Gerber solder mask layer will leave copper exposed in the final product. This exposed copper is used for component attach and test access points.

 

Gerber top solder mask layer

Figure 3. Gerber top solder mask layer.

 

Top Copper Layer (GTL)

Purpose: Contains data for all areas requiring copper, including traces, pads, copper pour, vias, plated through holes, etc.

Notes: All filled areas correspond to areas with copper plating.

 

Gerber top copper mask layer

Figure 4. Gerber top copper mask layer.

 

Bottom Copper Layer (GBL)

Purpose: Identical to top copper, but flipped horizontally around the y-axis.

Notes: All filled areas correspond to areas with copper plating.

 

Gerber bottom copper mask layer

Figure 5. Gerber bottom copper mask layer.

 

Bottom Solder Mask (GBS)

Purpose: Identical to the top solder mask, but flipped horizontally around the y-axis.

Notes: Solder mask layers are negative—meaning filled areas correspond to openings in the solder mask where no solder mask will be applied. These filled areas in the Gerber solder mask layer will leave copper exposed in the final product. This exposed copper is used for component attach and test access points.

 

Gerber bottom solder mask layer.

Figure 6. Gerber bottom solder mask layer.

 

Bottom Silkscreen (GBO)

Purpose: Identical to the top silkscreen mask, but flipped horizontally around the y-axis.

Notes: This layer is optional. Many single-sided PCBs will not have any silkscreen on the bottom side.

 

Gerber bottom silkscreen mask layer.

Figure 7. Gerber bottom silkscreen mask layer.

 

Outline (GKO) and Mechanical Layer (GML) 

Purpose: Contains the board outline, any cut-outs, v-cuts, panel outline, tabs, etc. Without this layer, the manufacturer does not know how to cut out the individual PCBs from the larger panels typically used in manufacturing.

Notes: Mechanical elements should be included in a single outline layer to prevent some features from being missed unless it makes the outline unclear.

 

Gerber outline layer

Figure 8. Gerber outline layer.

 

Drill Layer (DRL)

Purpose: Contains all the drill locations and hole and via sizes. Drill files have numerous file extensions, including .txt or .drl. This layer file must not be confused with the drill drawing or drill map files. Technically not a Gerber file.

Notes: Some EDA software will export separate files for plated and non-plated holes (vias). This has little meaning, and whether the holes are plated or not will depend on the copper layers.

 

Drill layer

Figure 9. Drill layer.

 

Inner Copper Layer (GT#)

Purpose: Multilayer boards may contain additional copper layers for signals or planes (ground or power) between the top and bottom copper layers.

Notes: Inner plane layers may be in negative format, meaning the filled areas are areas of no copper.

 

Negative view of Gerber inner copper plane layer.

Figure 10. Negative view of Gerber inner copper plane layer.

 

PCB Stencil Gerber Files

Gerber files are also used in the manufacturing of the stencils that are used during the application of solder paste prior to component attachment. Figure 11 highlights a portion of a solder stencil.

 

A portion of a stencil for applying solder paste
Figure 11. A portion of a stencil for applying solder paste.

 

The solder stencil is aligned to the PCB and placed in direct content. The solder stencil openings are aligned to exposed copper on the surface of the PCB. The solder paste is forced through the openings by a squeegee blade on the stencil printer.

 

Top Paste (GTP) and Bottom Paste (GBP)

Purpose: Contains data to make PCB stencils for the boards. Similar to solder mask layers, the data typically contains openings for surface mount parts only.

Notes: Openings may be slightly smaller than the solder mask openings or partitioned to reduce the volume of solder paste that is applied to the pads. The bottom paste layer is identical in function to the top paste layer but is flipped horizontally around the y-axis.

 

Orange indicates the desired location of the solder paste

Figure 12. Orange indicates the desired location of the solder paste.
 

Gerber bottom paste layer for the stencil

Figure 13. Gerber bottom paste layer for the stencil.

 

Viewing Gerber Files and DFM Checks

Hopefully, understanding Gerber layers will improve your designs, help you avoid some common mistakes, and make it easier to communicate with your PCB manufacturer. If you didn’t read the first article in this series, you can go learn Gerber file fundamentals. In the third and final installment of this article series, we will discuss Design for Manufacturing (DFM) checks for Gerber files.

HQDFM from HQ NextPCB is a free Gerber tool for engineers to check for common DFM problems in their PCB Gerber files. Designed by the manufacturer, the tool is based on real problems CAM engineers encounter on a daily basis. Engineers can also use it as a general-purpose Gerber Viewer to visualize their Gerber data. Download the HQDFM desktop suite to check for over 20 different DFM problems and get actionable insights, or try the online Gerber Viewer version with a free DFM report. For additional support with Gerber files and your PCB manufacturing, contact HQ NextPCB.

 

Featured image background used courtesy of Adobe

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