Getting through the development stage is always cause for celebration. You just cleared through the last PCB project hurdles, whether it was securing quantities for new components, finalizing the environmental testing, or getting that green light from your EMC (Electromagnetic Compliance) engineer. Your board is now in the “released” status with all approval signatures in place, and it’s time to hand it over to production. This path of transitioning and translating your design specifications into a set of workable files for your PCB manufacturer should be straightforward, but it never is.
It doesn’t take much to bring the production process to a halt with one missing piece of documentation that can easily blur the line of communication between what your specs are and what your documentation depicts. The saying “the devil is in the details” couldn’t be more true when it comes to your PCB fabrication and assembly documentation. With a single source of truth that communicates what has transpired for the period of the project involving multiple stakeholders, the more details, the better. With all the tools at your disposal to navigate this post-design process and output of the necessary fabrication files, your goal is to ensure that your manufacturer avoids any guesswork that could lead to quoting you a higher price due to discrepancies, delaying the production process, or worse - building a nonconforming PCB. Translating your design intent from digital bits into a physical good requires a better understanding of what a complete documentation package looks like, from your manufacturer’s perspective.
The Basics of Manufacturing Files
From a manufacturer perspective, it’s a major error when your package is missing one of the documentation pieces or sets referenced in your design files. An incomplete production documentation will most likely delay your project due to the time wasted clarifying and mitigating the issue. So here is the list of the output file categories you need to be familiar with:
Not only should your production documentation package be complete, but one must keep in mind that your manufacturer requires that your package come in a format that is generally accepted by the PCB industry.
The most popular file formats accepted for PCB designs are: Gerber (either the standard RS-274-D or extended RS-274X), ODB++, and Excellon. The good news is most of the CAD systems can generate or export your design data in one of these formats. And it’s always a good idea to confirm with your manufacturer exactly what manufacturing files they require and in what format before sending them anything. However, one thing for sure to avoid, although there are exceptions, is sending your design to the manufacturer in your internal CAD format. This single file format is often not readable and can’t be used to manufacture your board.
What you should include
So what should you include at a minimum? While this might change from one manufacture house to another, the minimum documents they often need from their customer to ensure they are on the same page are:
PCB data that includes:
PCB Fabrication instructions that include:
Slot and Hole Sizes
Finished Copper Weight
Finished PCB Thickness
Bill of Materials (BOM) that includes:
Manufacturer Part Number
With this information, your manufacturer can give you an accurate enough quote with an estimated delivery date, and more importantly, understand your design intent and build a PCB that conforms to your specific requirements.
Complete Documentation Data Package (Output Job Files)
What Does your Manufacturer Do with the Data Anyway?
Boards are typically fabricated in about 25 digitally controlled steps using production tools. Some designers think that the PCB fabrication data they submit, such as the Gerber files, directly drive aspects like the photoplotter, or that the drill files will directly be used by the drilling machine. However, that is not the case, and panelization is clear evidence of that. Let’s go over some of these factors.
The Layer Stack
The first step your manufacturer performs with the data set you send them is load it into their CAM system to recreate your PCB’s model. This is the model that will actually drive the manufacturing process as the system converts the image files. That’s why your data has to clearly specify information that defines the function of each file on the stack, so it’s clear which is the top layer, the bottom layer, etc.
The Drill Files
This file specifies geometric information that indicates the PCB material that should be removed and where it needs to be plated. The drilling machine must be able to read the geometric data that should be standard and specified by their diameter, start and end layer, and whether they are plated or not.
The PCB Outline
Without the PCB outline and knowing what area is the PCB and what isn’t, the board simply cannot be built. Your PCB outline should specify a closed solid contour without holes. The reason you don’t want to intentionally include holes is to avoid any confusing duplication since your drill file will contain a well-defined drill hole pattern.
While every manufacturer relies on your set of digital data as input to their production tooling system, actual technical drawings, such as mechanical, should be included. And while these drawings are not needed to process your PCB digitally, they are meant to be manually checked by a technician when needed. Also, one thing to remember is that drawings will never be used as a substitute for your digital data, whether it is for copper layers, solder masks, legends, drill patterns, or whatever other pattern your design includes.
Always include a netlist with your documentation package. Why? Well, a netlist will help ensure your design is correctly transferred into the CAM system. After inputting your fabrication data into the CAM system, the first thing it generates is a netlist from the image, which they call a reference netlist. Due to potential errors, either from the software or the operator, this reference netlist is often compared against the job data by the CAM engineer to make sure any possible mistakes are caught.
Avoid Any Guess Work
If you’re still using a PCB design tool that requires you to manually generate output documentation, pay close attention to the files you are outputting and keep them organized in a single repository. To put it simply - a complete PCB documentation package should contain all the files required by your manufacturer, organized in a file format and structure that is easily interpreted without any guesswork. Any redundant or errant files found by your manufacturer will only add delays to your manufacturing process, and that’s the last thing you want to deal with.
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About the AuthorMore Content by David Marrakchi