On June 22nd, 1893, the Royal Navy’s battleship HMS Victoria collided with her sister ship the HMS Camperdown and quickly sank, killing 358 of her crew. In a confusing order, fleet commander Vice Admiral Sir George Tryon called for the two columns of ships led by the battleships to turn into each other to reverse the fleet’s direction. The ships’ crews knew that they were too close for this maneuver, but they followed his orders until it was too late. Lost along with his crew, Admiral Tryon’s last words reportedly were: “It was all my fault.”
What exactly Admiral Tryon was expecting to accomplish that day will never be known, but a horrible accident occurred because he didn’t make his intentions clear. That is a good reminder for all of us in many situations, including in PCB design. It’s important that printed circuit board designers clearly communicate the manufacturing intentions of a project are so that there aren’t any problems in building the board. To do that, let’s take a look at some ways to make sure that data in your PCB design documentation is as clear as possible.
Clear PCB Design Documentation Starts with Manufacturing Drawings
Most printed circuit board designers have created drawings to go along with their designs–after all, it is part of the job. The problem is that, sometimes, PCB designers don’t know everything that a contract manufacturer needs in those drawings to successfully build the board. Creating a simple drawing using PCB design CAD tools is pretty easy, but without all of the necessary manufacturing data, a simple drawing won’t be enough. You can expect your CM to be spending a lot of time with you on the phone to fill in the missing details.
Here are some of those details that you should include when you create your manufacturing drawings.
The fabrication drawing displays all of the drilled holes in the board and is usually auto-generated by the CAD system. Make sure to dimension features such as the board outline and mounting holes, and include specific notes to instruct the fabrication house on the details of the board’s construction. The following should also be included:
- A complete stackup view specifying layer dimensions and board materials. Some of the detailed board materials are often included in the drawing notes, as well.
- Complete drill location information with different drill symbols for each size. There should also be a separate drill chart that associates the finished hole size to the symbol.
- Non-standard board features, such as slots or cut-outs, must be fully dimensioned. These often aren’t included in the NC drill file and should be included in the drawing.
- Processing requirements, such as solder mask color or controlled impedance trace routing, should be included in the drawing notes.
Like the fabrication drawing, the assembly drawing is also typically auto-generated from the PCB design CAD tools. This drawing shows all of the components on the board in their assembled locations, along with their corresponding reference designators for identification. You will also want to include some notes to instruct the manufacturer on how the board is to be assembled. Depending on the complexity of the board though, there may be a lot more that needs to be included. Consider the following when you are creating your next assembly drawing:
- Critical components, like connectors or heat sinks that need specialized mounting instructions, should be mentioned in the assembly notes on the drawing.
- Additional hardware that is to be mounted onto the board, such as stiffener bars, handles, or ejectors, should be shown with their specific instructions included in the drawing notes.
- Critical areas of assembly should have an exploded view or “detail” shown that is enlarged and pulled away from the rest of the board image.
- The location of assembly stickers and labels should also be shown on the drawing.
By making sure to include these important details in your manufacturing drawings, you can help speed your circuit board through fabrication and assembly processes. There’s more to talk about though because an even larger problem can exist in the bill of materials.
Potential Problem Areas in the Bill of Materials File
Like the manufacturing drawings, bill of materials files are usually auto-generated from the PCB design CAD tools. As such, they should be complete. However, CAD tools give you a lot of flexibility, and it can be easy to leave important information out of the final bill of materials (BOM) report.
Additionally, the BOM generator will report exactly what is built into the design database. If there are part errors, they will get reported in the BOM. Here are some examples of items you should double-check to ensure your BOM file is complete and correct:
- Description mismatch: When the description of the part in the BOM doesn’t match the description of the part numbers, it slows PCB assembly until the BOM descriptions can be verified.
- Missing reference designators: Without reference designators on listed parts in the BOM, the manufacturer doesn’t know which parts get installed to which footprint on the board.
- Incorrect or partial manufacturing part numbers: Sometimes, trailing dash numbers are omitted on the BOM, or the part number is incorrect. This will force the PCB manufacturer to delay the build until they can check with you.
- Incomplete information: In some cases, entire parts may be left out of the BOM. This can easily happen if the PCB designer forgets to update the BOM after a design change as gone through. This may not even show up in the assembly process until it is clear that there are missing components on the board.
Three Commonly Forgotten Files Your Manufacturer Needs
By going through your documentation and making sure that it is complete with all of the necessary data, you can help your circuit board go through the manufacturing process without any problems. There are three additional files, though, that are important to mention because these three often get left out of the documentation package and can cause problems for the contract manufacturer:
- IPC netlist: This data file often doesn’t get sent to the CM because it isn’t a typical Gerber, drill, or BOM file. However, the CM will use this netlist to develop the test fixtures and procedures, as well as to identify board fabrication details that need special attention.
- Schematic database: Like the IPC netlist, this database file is often left out of the manufacturing deliverables. For layouts that need to be compared to the schematic to resolve design issues, the schematic database is essential to extract the netlist from.
- List of critical components: Include a list in your documentation package of those components that have special fabrication, assembly, or test requirements, or are unique in that they are special-order or obsolete parts. This allows the CM to know up front which components need to be looked at right away instead of finding out later when they do their pre-manufacturing review.