As PCB assemblers, manufacturing is all about taking data and delivering good working circuit boards. Well, it can be just data, as in full turn-key, data plus some parts, or a partial turn-key or a kitted job. Regardless of whether you're sending parts and boards or having us buy everything, PCB assemblers need good data and a lot of it.
Data is the difference between the working PCBs you want and need and a random jumble of expensive paperweights. We need a bill of materials (BOM)—the job specifications that you give us by ordering and describing any special instructions—and the CAD design files.
The BOM is a list of all of the components to be placed on the PCB. The file typically includes an index number, the number of times a specific component will be used on the board, the reference designator from the schematic, the component manufacturer, and the manufacturers’ part number.
If a specific component is used more than once—such as a common bypass capacitor—it will still only take up one line in the BOM. One field in the BOM will list the number of times the component is used, and another field will list all of the reference designators for that part number.
You may also want to include alternate parts for components that are likely to go out of stock. Passives, such as capacitors and resistors, are notorious for going out of stock without notice. Invariably, though, there will be a half dozen nearly identical parts that will fit the bill just as well. Create a list of alternatives so your purchasing folks or manufacturer won’t get stuck not knowing if a substitute is valid or not.
Fabrication and assembly drawings are always a good idea too. A little extra time spent on the files you send us leads to reduced risk, which is good for everyone.
The CAD design files include Gerber files, and Centroid (i.e., pick-and-place or XYRLS file) and intelligent CAD files, such as ODB++ or IPC-2581. In some cases, such as EAGLE CAD, we can use the native CAD board file. The ODB++ and IPC-2581 file formats are the future of electronics manufacturing. They come with more data and more accurate data than Gerber files. If you can send either of these two, please do so. Even if you have those, you should still send the Gerber files. Gerber files are the lowest common denominator and provide a base that assemblers and PCB fabrication houses can work from.
Gerber files are a set used to create the various layers of the PCB. Each layer requires an individual file, so a six-layer board with six copper layers will typically require at least 13 distinct files including:
- One file for each copper layer
- Top solder mask
- Bottom solder mask
- Top silkscreen
- Bottom silkscreen
- Drills holes
- Solder paste for the top and bottom if the board has SMT parts on both sides
The drill file is combined with the Gerber files to line up the via and through-hole component holes with the appropriate spots in the PCB. Then, the pick-and-place file will tell the assembly house where to put each component, what angle to place it at, and which side of the board it goes in.
Fabrication drawings hold a human-readable description of the board—often in PDF format—and any special instructions needed by the fabrication house. The assembly drawing would be the same but for the assembly house.
Sometimes the parts are too densely packed for the reference designators and polarity marks to show up on the actual board, or for aesthetic reasons, the designer doesn’t want them on the board. In such cases, all of that information would be put into a set of assembly drawings—PDF files showing all of the necessary reference information.
The ODB++ and IPC-2581 file formats still aren’t universally accepted but are becoming more so all the time. Using and promoting these new intelligent CAD output file formats help reduce the number of manual steps and human interpretations, and will eventually lead to better quality and faster manufacturing times.
Duane Benson is chief technology champion at Screaming Circuits. To read past columns or contact Benson, click here.