If you look at a PCB layout, you might ask yourself a simple question: why is it so common to see components rotated at only multiples of a 90 degree angle? Typically, you will see components with orthogonal rotations in a PCB layout, where component axes are oriented only parallel or perpendicular. There are a few interesting reasons for this, some of which remain valid and others that have to do with the operation of placement tools in CAD software.
There are also standards that specify packaging requirements for components, and these standards should be used when specifying creating your footprints. At minimum, it will aid SMT line programming and quality control for your assembly orders. This will also help prevent assembly defects by enforcing consistent rotation values without the use of a large number of local fiducials.
What Rotations Should Be Used?
In general, it’s a good idea to stick with the orthogonal orientations used in most PCB design software. There are a few reasons for this:
- PCB layout tools have hotkeys for 90 degree rotations, mirroring, and flipping layers; these tools help you stay productive and get to a useful placement quickly.
- Defining rotations in 90 degree angles with respect to a board edge eliminates the need for fixtures on an SMT line when soldering odd-shaped boards.
- Making rotations consistent with pin-1 orientation in packaging helps prevent programming errors on an SMT line.
Where should these various rotations be enforced? Is it the designer’s responsibility to enforce orientation with a design rule or in a PCB library? What about in the PCB layout and during assembly? The answer is in a combination of these areas.
In the PCB Layout
Technically, you can place any arbitrary rotation you like in a PCB layout. While it may be less common, it is possible to place components at 45 degree angles and at arbitrary angles with respect to your main blocks of orthogonal components. For example, look at LED boards used in lighting products; these will commonly use rings of LEDs with odd rotations applied in order to create uniform illumination.
Although your PCB editor may snap to rotations in 90 degree increments that align with orthogonal board edges, they are not required. For example, odd-shaped boards and curved boards can have component orientations that involve odd rotations where the components do not align with any board edge. While it is possible to assemble these boards, they require some extra programming effort and potential for error prior to assembly (see below).
Rotation in Footprints
Components that are supplied on reels from manufacturers will have a specific orientation that is specified in IPC/JEDEC joint standards. If you are designing your own footprints, it is generally a good idea to ensure that 0 degree rotation in the footprint corresponds to the feed direction for the component packaged in its reel. This will speed up checking of pick-and-place data against the footprint orientation in the PCB layout and the reel, and it will eliminate the need to manually change pick-and-place data to match part orientation in a reel.
Component orientation matching the feed direction on a reel. [Image adapted from JLCPCB]
Rotation During Assembly
In the past, placement at arbitrary angles was problematic due to the lacking capabilities of pick-and-place machines. Today’s machines can do much more and can place components at just about any arbitrary angle. As long as the angular rotation is specified correctly in a pick-and-place file, then it can be placed in a board during PCB assembly. A manufacturing engineer might complain about this when programming the SMT line due to the need to add up angles with respect to components on reels, but that is just part of the job.
If your board has an odd shape or multiple curves, the board might require a fixture in order to set a clear reference for rotating components in the pick-and-place machine. Consider the example shown below; if the board has an odd shape and orientation in the layout tool, then the orientation in the pick-and-place machine may not match the reference edge used to fixate the board. Using a fixture in these cases is very important for assembly, but all of the pick-and-place data will need to be changed if a straight edge used for fixation does not match the edge in the PCB layout data.
Here we have two odd angles applied in the PCB layout and in the board shape. The board will have to be rotated so that the long edge is horizontal or vertical.
A preferred method is shown below, where the reference edge is horizontal. This allows you to more quickly place components at orthogonal angles.
If you are using large BGAs or QFNs, consider placing local fiducials along the diagonal axis of components to ensure highly accurate placement and rotation. Local fiducials are sometimes placed diagonally along larger chips to provide a localized angular measurement during component placement. Before placing these, ask your assembler if they are needed on larger components to avoid rotation errors. Sometimes, your assembler will add these for you as long as space is available on the board.
No matter what components you want to assemble on your PCB, you can create cutting-edge products with the complete set of design features in Allegro PCB Designer. Allegro is the industry’s best PCB design and analysis software from Cadence, offering a range of product design features with a complete set of management and version control capabilities. Allegro users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.