A Brief Introduction To PCB Gerber Files

What is a Gerber File?

Gerber is also called “photoplotters”, usually only represents a format such as RS-274, 274D, 274X, etc., serving as the medium for converting the designed graphics data into PCB manufacturing, that is, a CAD-CAM data conversion format specification.

The important use is to draw the PCB layout, and finally, the PCB manufacturer completes the production of the PCB.

No matter which kind of CAD system, the external CAD database must be converted into a GERBER format file.

In this process, the Aperture table depicts the size, shape and position of the plotter’s lens.

The conversion between the two is usually tangible, and once Gerber occurs, the plotter can begin the task. The plotter is relatively expensive but very accurate equipment, the accuracy can be less than 1mil.

There are Two Important Standard Gerber Formats:

1) RS-274-D is derived from EIA’s RS-274-D specification code in 1985, and its material content includes word address materials and parameter files, and control codes of plotters.

Gerber in this format must include an Aperture file, which means that Gerber File and Aperture file are separate files.

RS-274-D has been in use for countless ten years.

Because the evolution of electronic products has long exceeded current needs, the original RS-274-D format is gradually inadequate, and an enhanced version has been derived from it and Replaced by RS-274-X.

2) RS-274-X occurred in 1992, which is the most popular format today. It is an extended version of RS-274-D, based on RS-274-D, only RS-274-X format Aperture is integrated into Gerber File, that is, “with D code”.

In addition to the above situation, there are a few data format specifications that are being used or developed, such as MDA/Fire9000 and Barco DPF. Since they are not very commonly used, they will not be explained in detail here.

Several Common Formats of Gerber:

Gerber Format is a common format in the electronics industry.

It is connected with designing and manufacturing PCBs, just like the DXF or HPGL format input by the AutoCad software commonly used in the civil engineering or mechanical hardware industry.

It is a designer that connects the documents generated by the completion of the artwork design with other systems.

This was finally developed by the American Gerber company for the optical plotters consumed by the company, therefore, called Gerber Data.

However, because the format can meet the needs of the electronics industry, it has been widely used overnight and evolved into an industry-standard format.

Later, because the format was widely accepted by the electronics industry, it gradually became known as the Gerber Format.

Description of Aperture File:

The Aperture file mainly describes the shape and size of the lens used in the Gerber File

Aperture File + Gerber File = Good PCB Layout graphics

Common fields: D_CODE:

D code: Aperture number;

SHAPE: Aperture shape;

SIZE: Aperture size;

Drilling and it’s meaning:

PTH—Plated through-hole, the hole wall is plated with metal to connect the conductive pattern of the two layers or the outer layer;

NPTH—Non plated through-hole, the whole wall is not plated with metal and is used for mechanical devices or mechanical fixing components;

VIA—through holes, used for electrical connection between conductive patterns in different layers of the printed board (such as buried holes, blind holes, etc.),but can not insert component leads or other plated through holes that strengthen materials;

Blind vias—only extend to an external via hole of the printed board;

Buried vias—vias that do not extend to the surface of the printed board.

Gerber File Polarity Description:

Positive: Gerber depicts the circuit layer, and the depicted figure is mainly part of copper, or Gerber depicts the solder resist layer, and the depicted figure is mainly the solder mask part (that is, the part covered with ink);

Negative: Gerber depicts the circuit layer, and the depicted figure is mainly without copper part, or Gerber depicts the solder mask, and the depicted figure is mainly without solder mask (that is, the part is not covered with ink);

Compostive: The layers described by Gerber have different polar layer decompositions. Usually, the layer is dug and the positive polarity is superimposed. The excavation layer has a polarity of c, which is important for line protection or additional materials.

How Gerber Files Are Used In PCB Manufacturing

The PCB manufacturing technology used today has come a long way over the past decades.

Previously, vector photoplotters were used to create the tooling film used in the PCB manufacturing process.

A focused light conducted through an aperture was used to expose the film to create the flashes and draws for each individual pad and trace.

There was only a minimal set of apertures available and designers had to be creative in restricting their flashes and lines to only the apertures that were available to them.

These older vector machines have now been replaced by a newer breed that uses a raster laser process to expose the film.

For larger pieces of film that had a lot of line drawing on it, a vector photoplotter could take many hours.

This time has been cut down to a matter of minutes with a laser plotter.

Gerber file history can be traced back to the needs of the original vector photoplotters.

In order to give the plotter its instructions, a Gerber file contained minimal plotter configuration information, and X/Y coordinates followed by a flash or drawing command and which aperture position to use.

The Gerber data has increased in functionality over the year; it now includes additional configuration information as well as macro and aperture definitions.

The laser plotters used today still use the same Gerber information, but the aperture restrictions of the older vector plotters no longer apply.

The laser plotters convert the Gerber coordinates into a raster file, and that information instructs the laser plotter on how, where, and what is to be created on the films.

For example, the aperture definitions convey the thickness and sizes of the traces and pads while the drawing commands define whether lines, polygon fills, or flashes are to be created. Then the laser sweeps across the film exposing the image as it goes.

Time does not sit still though, and the world of Gerber files continues to change.

Circuit board manufacturers have now begun using direct laser imaging to create PCB images directly onto the copper and bypassing the need for film.

There are also new database formats that are being used to create PCB images that contain much more intelligent board design data such as net connectivity data. But it’s safe to say that Gerber files will continue to remain in their current form for some time to come, and you need to be aware of the part they play in PCB manufacturing.

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