Essential Components of a PCB Fabrication Drawing
In the world of electronics, a PCB (Printed Circuit Board) fabrication drawing plays a crucial role in transforming design files into physical circuit boards. This article will delve into the key components of a PCB fabrication drawing, ensuring that all engineering, mechanical, and manufacturing parameters are communicated clearly to the manufacturer.
A PCB fabrication drawing, often referred to as a fab drawing, serves as a master specification guiding the fabricator. It provides comprehensive information about the circuit board's dimensions and outline, layer stack-up, copper thickness, solder mask and silkscreen details, drill drawings, finish and plating specifications, board thickness and tolerances, fabrication notes, reference to manufacturing files, material specifications, component placement, and height restrictions.
The layer stack-up section of the fabrication drawing provides vital information about layer order, dielectric spacing, copper weight, material type, and the total thickness of the stack-up. A fab drawing also specifies details such as the board outline, layer stack-up, drill chart, and more required for PCB manufacturing.
Critical mechanical specifications related to the hole position should also be included in the fab drawing. For instance, a drill chart on a fab drawing demonstrates all the particular symbols assigned to the drilled holes in the board outline.
The title block of a fab drawing includes essential information such as the customer name, part number, revisions, fabrication number, and measurement units.
In terms of manufacturing notes, specific requirements such as minimum trace width/spacing, via types, and special instructions are crucial. Fabrication notes clarify manufacturer options such as plating, soldermask color, copper weight, and tolerances that might be queried otherwise in the order process.
The board outline on a fab drawing defines the shape and dimensions of the circuit board. Solder mask and associated specifications, such as color and finish, should also be specified. Plating details should be specified in the fab drawing, and controlled impedance requirements may necessitate varying trace thickness, trace width, trace distance from reference planes, and dielectric constant to achieve the desired controlled impedance.
Presence of via-in-pad/plugged via, as well as the implementation of blind/buried via, should be noted in the fab drawing. Gold fingers, if present, can be optionally mentioned in the fab drawing.
The fab drawing, along with the assembly drawing, are the reference documents required to build and assemble a circuit board. In KiCad, the fab drawing is created by adding the title block, adding text and logo, adding board dimensions, adding dimensions between mounting holes, generating the stack-up, and adding fab details. In Altium Designer, the fab drawing is created by using a blank drawing format, adding fab notes and drill charts, and linking all files to the design database.
In Eagle EDA, the fab drawing is generated by going to CAM Processor > GerberAuto, opening Layout editor > Options/Set/Drill, and saving the file in Excellon format.
In conclusion, a PCB fabrication drawing must comprehensively communicate all engineering, mechanical, and manufacturing parameters needed to transform the design files into an actual physical PCB without ambiguity. Including a Bill of Materials (BOM) and assembly drawings are usually separate but also part of the overall PCB documentation for a complete production package.
A stackup designer is essential in determining the layer order, dielectric spacing, copper weight, material type, and total thickness of the printed circuit board (PCB), as detailed in the PCB fabrication drawing. An impedance calculator might be used to ensure controlled impedance requirements are achieved by adjusting trace thickness, trace width, trace distance from reference planes, and dielectric constant, as specified in the PCB technology section of the fabrication drawing.
