I was relieved the installation instructions for my new Bluetooth touchscreen stereo were so simple. It should only take a matter of minutes. If you’re not laughing, it may be because you’ve never experienced the horror of installing a new car radio yourself.
Not only have I carved three tons of plastic and metal out of my car using a Dremel tool, pliers, and a fair amount of anger, but I have also purchased additional brackets, adapters, wiring harnesses, and soldering supplies. I even had to return the original stereo unit for a more expensive one that was supposed to fit better.
Like the knights of the round table, I have sworn a solemn oath that from now on, my goal is to create PCB assembly documentation that is much more informative than those stereo installation instructions. I vow to provide all manufacturers’ data to build a high-quality printed circuit board. To that end, here are the details that I will be sure to include in my next PCB assembly drawing, and hopefully, these details can help you as well.
What Information Do We Need to Communicate?
Building a printed circuit board requires a lot of data and information for your manufacturers. A circuit board is first fabricated, where the layers of dielectric material and metal interconnects are laminated together and then assembled where the components are soldered into place. Finally, the board will be fully tested before being shipped back to the customer who ordered it.
Each of these three areas of PCB manufacturing requires different types of data and information:
- Fabrication: Circuit image files, drill data, and fabrication drawings and instructions.
- Assembly: Mechanical drawings of chassis or sub-assemblies, bill of materials (BOM), component XY locations, and assembly drawings and instructions.
- Test: Testpoint XY locations, schematics, and netlists.
As you can see, drawings provide build instructions for PCB fabrication and assembly operations. These drawings must be made accurately, which isn’t a problem for modern PCB CAD systems that generate drawings automatically. However, including all of the necessary elements of the drawing is still the designer’s responsibility, and we’ll look at the list of those required elements in an assembly drawing next.
What Elements are Needed for a Good PCB Assembly Drawing?
A good assembly drawing needs to include several elements to show how to assemble the circuit board correctly. Here are the elements designers need to include in their drawings:
- A drawing format or outline that includes text boxes for document information
- Current document information, including part numbers, versions, revisions, dates, corporate contact information, board name, project name, and sign-off boxes
- The board outline
- Individual components outlined in their to-be assembled locations
- Reference designators to identify each component outline
- Assembly instructions and notes
- Mounted hardware on the board such as ejectors or stiffener bars
- Detailed views are enlarged areas of the circuit board pulled out to the side of the main image to clarify specific assembly instructions
- Side views of the board to clarify hardware mounting
- Back-side view of the board if parts are placed on both sides. This view is often included on an additional sheet of the drawing
- Specialized mounting instructions for unique parts such as heat sinks, switches, and connectors
- Locations for labels and manufacturing stickers or other required printing
These required elements will vary with different boards. For instance, a board that doesn’t have any parts placed on the back probably won’t require a back-side view. However, the designer must include all pertinent information to the board in these elements. Here are some other concerns that designers need to be careful about:
- Ensure that the reference designators are readable and designated on the drawing. This may require enlarged detailed views, using a range of reference designators (R101-116), or pulling the designators off the drawing and using pointers to locate them to their parts. Many assembly drawings are shown with the board view enlarged at 2X, 4X, or more to identify the components and their reference designators clearly.
- The assembly notes need to cover all aspects of the PCB assembly, including board materials, soldering processes, and solder paste requirements. You should also add references to industry standards & specifications, locations for special features, testing instructions, and parts lists if requested by your manufacturer.
- Make sure to update the drawing and all of the documentation with each board revision. Sometimes this step is missed during PCB design and can create a lot of confusion for the manufacturer.
Many designers feel that the level of work required to create a complete and comprehensive assembly drawing like this is not worth their time, especially if they are already working with a manufacturer during PCB prototyping. But it is important to remember that circuit boards that a manufacturer produces for prototyping may end up going out to multiple manufacturers for full production. These other manufacturers won’t have the same development history to fall back on and will need detailed assembly instructions. Next, we’ll look at how your manufacturer can help you with your PCB assembly drawing.
Who to Turn to for More Information on Manufacturing Documentation
There are many reasons to work closely with a PCB contract manufacturer. The CM can help you choose the highest quality parts readily available at the best prices. They can help you with engineering input for your circuit design and give you layout recommendations to make the board more manufacturable. They can also help you create a PCB assembly drawing that will be usable not just by them but also by any circuit board assembly shop.
At VSE, we have a 35-year reputation for helping our customers generate manufacturing data that will help with all aspects of PCB fabrication and assembly. And if your design needs revisions to go through manufacturing cleanly, we also have the resources to update the assembly drawing for you. Our goal is to ensure that you end up with a fully buildable design that will give you high quality and performance.