Instructions Are Easy When You Write Them: The PCB Assembly Drawing
“Some assembly required.” Those words can strike terror into anyone looking forward to the new item that they’ve purchased. I, too, know this terror; mine was a foosball table that we gave to our children one Christmas. Due to the size of the thing, we left it in its box hidden away in the garage until late Christmas Eve. “It couldn’t be that difficult,” I reasoned, as I started to work on it after the kids had gone to bed. After many cups of caffeine, bruised knuckles, lost parts, and a few words that should have landed me on the naughty list, I finally got to bed myself shortly before sunrise. The family photos from that Christmas morning show very happy children playing foosball, and one zombie-like dad clutching yet another cup of caffeine.
There is always going to be something that needs assembling, and that is certainly true in the world of printed circuit boards. We as designers don’t always think about that while we are placing parts and routing traces, but the simple truth of the matter is that someone is going to have to build what we design. That is where the PCB assembly drawing or fabrication drawing comes in: to tell the manufacturer how to assemble the board that we design.
If you haven’t created an assembly drawing before, this will be a good introduction for you to the process. I promise, this won’t be a thirty-page manual with design sketches that look like my children’s kindergarten drawings. Knowing the different elements of an fabrication drawing, and how your PCB layout tools can help you in creating this drawing are the only tools you’ll need here. And the best part? All of this information is yours for the reading, no assembly required.
The PCB Assembly Drawing Table of Contents
An assembly drawing can be made in several forms so as to be the most convenient, and accurate for the manufacturer you work with. Make sure that during your printed circuit board design process you are communicating with your manufacturer to know what they need in their assembly drawings. But while it’s important to keep in mind who you’re working with, here are some basic elements that are common to most printed circuit board fabrication drawings to become familiar with:
Drawing format: Some CAD systems will generate the drawing format for you while others require you to create it as a separate library shape. Whichever method that you use, you will combine that format with your design database to build your drawing.
Board outline: As with a fabrication drawing, you will want to display the board outline of your design. Depending on the size of the board, the image can be scaled down to fit the drawing format, or scaled up to show greater detail.
Part shapes: Within the board outline you will want to display all the part shapes and their reference designators that are going to be soldered onto the board.
Mechanical parts: You will also want to display any mechanical parts that will be attached to the board with mounting hardware. Since these parts may not be a regular PCB footprint, you may have to add or draw these shapes separately. An example of this kind of mechanical part is an ejector handle. This is a non-electrical part and may not be represented in the schematic, but it will need to be included on the assembly drawing and in the bill of materials.
Assembly notes: These are a list of instructions that include basic assembly details, references to industry standards & specifications, and locations for special features.
Identification label locations: Any identification labels such as barcodes or assembly tags will need to be located with a drawing pointer and a reference to the specific label in the assembly notes.
Additional views: PCBs that have parts on both sides of the board will usually need a view of the back side of the board in addition to the front. If the board is small enough, both views are sometimes placed on the same drawing sheet. For larger boards additional drawing sheets can be used. You may also need to include a side view of the board to show how certain mechanical parts need to be attached.
Expanded “cut-away” views: When an assembly area needs to be shown in greater detail such as the ejector handle, you will want to place a cut-away view of that area off to the side of the main board view. This cut-away view can be scaled up for clarity, and you will want to add a drawing pointer showing where this view is on the main board.
Parts list: Your manufacturer may request that you display the bill of materials on the assembly drawing itself. This is more common with smaller boards.
How Your Layout Tools Can Help You to Create the Drawing
Many years ago I had to use a separate drafting tool to create assembly drawings. This was because the older PCB layout systems that I used at that time didn’t have the ability to create the detailed drawings that we needed. Today though, layout tools have the functionality that you need to create your assembly drawings with perfect component placement accuracy across the assembly layer or layers. Board outlines with part information can be imported, scaled in size, rotated, and mirrored as needed for the drawing. Assembly instructions can be composed or imported from an external file, and added to the drawing. Detailed cut-away views of sensitive areas of the assembly can be pulled out and expanded. And once completed, your drawings can be converted into electronic format and included with the rest of your manufacturing output files.
Unlike blurry-eyed parents frustrated by confusing assembly instructions at the eleventh hour, you can help your manufacturer by creating a clear and informative PCB assembly drawing. With this drawing you can assure your manufacturer that your printed circuit board will be built correctly and without delay.
Do your CAD tools give you the power to create PCB assembly drawings quickly and easily? Altium Designer is PCB design software with an advanced drawing creation utility for this purpose. Draftsman will help you to create a clear and informative assembly drawing that will be a joy for your manufacturer to work with. Talk to an expert at Altium Designerto find out more.