Guidelines in Creating PCB Fabrication Instructions

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PCBs have a wide range of applications, especially in today’s world full of electronic products. From cellular phones, tablets, high-speed computers and laptops, to household appliances and into more sensitive applications in aerospace industries, weapons, military, medicine and scientific research, PCBs are at the heart of all of these.

Pcb fabrication

PCB fabrication process starts with the creation of the PCB design. Along with the design are the instructions for the fabrication process. This section is very important because the entire assembly process depends on it.

Relevant information like the following should be in the printed circuit board fabrication instruction:

1.  Hole sizes (drill holes where components are later mounted) – should contain information on the tolerances and plating requirements
2.  Dimensions of the edges, such as holes or other features
3.  Details of the outline of the board including all dimension and tolerance requirements
4.  Type of preferred surface finish
5.  Finished Copper Weights
6. Solder mask requirements
7. Technical contact information
8. Silkscreen requirements
9. Finished Board thickness, including tolerances, and measurement criteria
10.  Any additional requirements such as controlled dielectrics, impedance, plugged vias, etc.

The PCB fabrication instructions can be found in the following files:

BOM (Bill of Materials)

This file contains a complete description of the different parts and electronic components and where these are placed on the printed circuit board. The BOM should have the following information:

1.  Line / Item #
2. Reference Designator
3. Part Description
4. Part Number (Digi-Key part numbers are ok)
5. Quantity Per (# of instances of a single part number)
6. Type (SMT, through-hole, fine-pitch, or BGA / leadless)
7. Package (QFN32, SOIC, 0805, etc.)

Gerber Files

Gerbers are also required by manufacturers, which are generated by CAD program using the RS-274X program. The minimum requirements include trace, silkscreen and solder paste layers.

In simple terms, a Gerber file is like a PDF file. This is the core component that is very crucial in the supply chain of the electronics manufacturing field. This is the standard file format used in the electronics industry, which communicates critical design information for the different types of PCB before PCB fabrication gets under way.

The above mentioned RS-274X program is the newer file format. It contains all the information for a single layer of printed circuit board in just a single file.

Gerber files contain the following information:

1. Configuration parameters

This controls several factors in the assembly such as the rendering process, geometry (black drawn on white or white drawn on black)

2.  Macro and aperture definitions (AD and AM parameters)

Macros or AM parameters refer to the complex shapes that will be used for registration marks, lettering, logo and other special geometry for the design. Apertures determine the thickness of the copper traces and the shape and size of the pads.

3.  Drawing commands

These refer to the geometric shapes, lines arcs and flashes drawn on the circuit board hat refer to the traces, components and pads.

4.  X and Y coordinate

This refers to the orientation of the components in relation to the width and length of the board.

Centroid

Centroid refers to the XY data or the pick-and-place or insertion data. This will determine the specific part and its position on the board. This file is in the ASCII Text format, which includes the Theta, X and Y orientations. The X axis refers to the length of the board and the Y-axis refers to the width of the printed circuit board. It also includes which side of the board the components go and the reference designator.

Assembly Instructions, drawings and photos

These are not generally asked for in the PCB fabrication process but these will greatly help in the easy and accurate manufacture and carrying out of instructions for special PCB assembly. These are strongly recommended when the assembly requires non-standard processes or if ambiguous placements are required.