Have you ever been driving through unfamiliar territory, when you suddenly realize you are going the wrong way on a one-way road? I have—and I am still amazed at how the feeling of fear that should have been dominant was actually surpassed by a sense of embarrassment. The sad thing is that there were plenty of warning signs to let me know that my orientation on the street was completely backward, but I missed them all. Aside from a little damage to my ego however, there, fortunately, weren’t any negative consequences of my mistake.
The same can’t be said when placing components on our printed circuit boards, however. If we have components that aren’t aligned with the correct orientation, there could be a lot of damage when it comes to getting the board assembled. Fortunately, there are some warning signs here that can help, starting with some standard design for manufacturability (DFM) guidelines. Here is some more information on those guidelines and how you can ensure you have the correct component orientation on PCB designs.
Manufacturing Considerations for Component Orientation on PCBs
There are three primary goals that your PCB contract manufacturer will focus on when they review your design for manufacturability:
- Standardization: By making sure that your PCB design will fit into a standard assembly process, your CM can reduce the labor and time involved, which lowers your cost.
- Assembly problems: By eliminating known potential assembly issues, your CM will improve production yields and once again pass on cost savings to you.
- Quality: By identifying and replacing parts or materials that could be potentially defective from the assembly process, your CM can once again improve yields.
During the DFM review, one of the details of your PCB that will be looked at to achieve these goals is component orientation. The rotation of parts on a PCB can have a great effect on its manufacturability, starting with the soldering process.
For boards that are waved soldered, components that are misaligned could cause poor or bad solder joints, leading to intermittent electrical contact, no contact, or even direct shorts. Here are some component orientation considerations to keep in mind for wave soldering:
- Surface-mount passive components such as resistors, capacitors, and inductors, should be aligned perpendicular to the direction of the board going through the wave so that both pins get soldered equally.
- Larger SMT components should not precede smaller components through the wave. This could cause “shadowing” of the smaller components and reduce the wave’s ability to produce an effective solder joint on the trailing smaller components.
- SMT dual inline parts, such as small outline integrated circuits (SOICs) should be placed with their long axis aligned with the wave. Traveling perpendicular to the wave could also produce a shadowing effect on the trailing pins, and cause incomplete or bad solder joints.
- Multi-pin parts, such as large through-hole connectors, should be placed so that they travel perpendicular to the wave. This will reduce their chance of creating bad solder joints on the trailing pins.
There can also be component orientation problems for circuit boards that go through solder reflow as well. In some cases, component placement schemes will group passive parts together and orient the parts so that all of the common pins will connect to a single power bus. If the orientation of these components cannot be changed, then their connection to the power bus should be through a thinner connecting trace to avoid large areas of metal flooding the pin. Passive parts with unbalanced metal connections between their two pins run the risk of floating on their pads or standing up in an effect known as “tombstoning” during solder reflow.
Another concern is the orientation of parts near the edge of the PCB. When boards are prepared for separation from their manufacturing panels using V-groves scoring, the location and orientation of the parts must be positioned to give the scoring tool room to operate. This will prevent potential board defects or damage to the components by the scoring process.
In addition to manufacturing the board, you also need to consider the functional aspect of how your components are oriented as well.
Don’t Forget the Functional Implications of Component Orientation
The parts that you place on your printed circuit board still needs to provide a function. As much as it is important to make your board manufacturable, it is also important that your board be functional. The best scenario, of course, is to place your parts where you can satisfy the needs of both functionality and manufacturability.
In cases in which those two objectives conflict, however, it is possible to get your contract manufacturer to sign off on less desirable component orientations and outright DFM violations. Just remember though, each assembly deviation can potentially add up in additional manufacturing costs.
Here are some considerations to keep in mind from a functional perspective when it comes to placing and orienting your PCB components:
- Human interface considerations: Many parts on the board need to be accessible for human interfacing. Connectors will need to be locked or unlocked, switches may need to be set, or cables and harnesses will need to interface to other system ports. These parts must be placed and oriented so that they can easily be worked with. Having to squeeze into a tight area just to plug a connector in not only could leave you with pinched and bleeding fingers, but it may also damage other components on the board.
- Marking orientation: Assuming that the component is oriented correctly to begin with, you can create a huge problem if your silkscreen marking is incorrect. Please make sure that your polarity indicators are marking the correct pins.
- Quality validation: Along with polarity markings, it is very helpful to arrange groups of components so that they are all aligned in the same direction or in a consistent pattern. This reduces the possibility of a rework or a test technician missing the one diode in a group of parts that is rotated 180 degrees differently than the rest.
- Thermal performance: Some parts need to be orientated in a certain direction to maximize their cooling with the airflow across the board.
Work Ahead with Your Assembler to Get it Right
The key to creating the best component orientation and placement on your printed circuit board is to understand all of the different DFM and functionality requirements before you start. Here is where working with an experienced PCB assembler can help. They are in the business of building quality circuit boards for their customers, and they’ve worked with many different types of circuit board technologies.
