Everything is packed and I am ready to leave for my camping trip. In fact, as soon as I finish writing this we will be heading out of town. There is one thing that happens every time when we go on a trip like this though, when we are setting up camp tonight we will discover to our annoyance which items we’ve forgotten and left at home. Normally it is something simple like a toothbrush, which can easily be replaced with a trip to the store. But on those trips where important items have been left behind – like my pillow – that’s when it can get uncomfortable. You would think that after all of these years that I would finally put a checklist in place to avoid this yearly annoyance, but no, I am a creature of habit.
I will definitely make it through the next week without my favorite pillow (albeit with a lot of whining and complaining), but your circuit board will not do so well if you neglect any of the details needed for its manufacturing. With the different amounts of documentation needed for the fabrication and assembly of your PCB, it can be easy to forget an important detail. For this reason you should develop a routine along with a checklist to make sure that you don’t forget anything. Here are some of those items that you should remember to include when creating your PCB fabrication and assembly drawings.
The Basics of the Fabrication and Assembly Drawing
Your fabrication and assembly drawings will communicate different information to the vendors that they are intended for. The fabrication drawing will include information on how the raw printed circuit board is to be built, while the assembly drawing will include details on how the components will be attached to that raw PCB. In either case however, you will start from the same place with these drawings; in your PCB design tools. As such, there are a few generic items for both drawings that you will want to make sure that you include:
Drawing Format: While some CAD systems will require you to use a drawing format that is a library part, other systems will auto-generate the format for you. Whichever method that your CAD system uses, you will combine that format with your PCB layout to build your drawing.
Board Outline: This is the outline of your circuit board design from your layout database. Usually it will include slots and cutouts in it that will be dimensioned on the fabrication drawing and referenced with components on the assembly drawing.
Identification Information: Usually your drawing format will have one or more areas on it for you to add the drawing identification number, revision level, board name, corporate address and contact data, and the creation date.
These are the basic elements needed for any drawing. Now let’s take a look at the specific elements needed for each.
For complicated layouts like this, you need powerful drawing tools in your CAD system
Key Elements for Both Drawings
Starting off with the fabrication drawing, here are some of the features that you will want to include on your drawing:
Drill Locations: Every hole in your board for parts or vias needs to shown in this drawing. These are usually auto-generated by your PCB design CAD system. Although your fabricator will use the NC drill file that you send them for the actual hole locations, the drill symbols here are important as a reference.
Hole Chart: This table which is also known as a “drill schedule”, will associate each finished hole size with a unique drill symbol and quantity of the holes.
Board Layer Stackup: This is a cut-away view of the board that displays its layer structure. Pointers are usually used to detail the configuration and width of the conductive layers of the board including their associated prepreg and core layers.
Dimensions and Notes: Dimensions will show the overall size of the board as well as the location of different features around the board outline and within it. The notes will give the fabricator specific instructions not included elsewhere.
Next, let’s take a look at what you need to include on your assembly drawing:
Component Outlines: You will want to display all of the component shapes as well as their reference designators that are going to be soldered onto the board. This will also include mechanical parts that will be press fit in or attached with mounting hardware. Sometimes these parts may not be accurately represented in your footprint library requiring a little extra drafting effort on your part to include them in the drawing.
Additional views: Double-sided boards will usually need a view of the back side of the board in addition to the front. Both of these views can be included on one assembly drawing sheet if the board is small enough to allow for that, otherwise, you will need additional sheets in your drawing. You may also need to include expanded views to detail the attachment of mechanical parts at a larger scale for clarity.
Manufacturing Labels: Any manufacturing labels such as barcodes or assembly tags will need to be identified with a pointer and referenced in the notes.
Assembly Notes: These are a list of manufacturing instructions that include assembly details, industry standards & specifications, and special feature locations. They can also include a parts list if that is requested by the manufacturer.
Any of these drawing elements can be changed or modified as needed, and in some cases omitted entirely if appropriate. The important thing is to find out from your contract manufacturer what they need to build your board, and include those details on your drawings. Don’t forget that creating a checklist so that nothing gets left out is a great idea too.
With advanced PCB design tools like OrCAD Designer, you can create drawings like this
How Your PCB Design Tools Can Help
The older PCB design systems used to not have much in the way of drafting capabilities. Many of the items that I listed above had to be created manually each time you needed them. In some cases we used third party CAD drafting applications simply for ease. Those days are thankfully over, and today’s PCB design systems have the power and utilities to create any drawing you need with impressive levels of automation. If you are still using an older PCB design system that requires a lot of your time to create drawings, do yourself a favor and check out the help you can get from today’s powerful design tools.
The PCB design systems from Cadence will give you powerful utilities and automation to create precise fabrication and assembly drawings quickly and efficiently. With its manufacturing drawing creation tools and other powerful utilities, OrCAD PCB Designer has the capabilities that you are looking for.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.