In the 1995 movie Apollo 13, there is a short sequence where the flight controllers are shown going through their go/no-go for launch checklist. To launch, all the different spacecraft systems and flight parameters had to align with their specified criteria. This narrow margin was the tolerance that was defined for a safe lift-off, and if those tolerances couldn’t be met, the launch would be scrubbed. In circuit board manufacturing, we also deal with different tolerances that ultimately define how the board will be assembled, including tolerances for the surface mount components placed on the board.
For a successful assembly of a circuit board, the components must be the correct size and shape and placed in the right locations. Not only does this mean that the design of the board must adhere to tolerances governing the creation of the SMT footprints and their placement, but the physical parts must be correct before assembly can even begin. We’re going to look at these SMT component placement tolerances here, and how those tolerances impact the way a PCB is designed.
Component Manufacturing Tolerances
In order for an electronic component to be successfully assembled onto a printed circuit board, it must conform to its specified tolerance range. The leads of the component must be the correct length, while the overall package size cannot exceed its published dimensions. The height of the component can also have a direct effect on the manufacturing process, and those examples are just for starters. Another potential problem with surface mount components is how their leads are configured. In order to be successfully soldered, the leads must all be planar to each other. In other words, all of the pins of the component must sit flat on their associated metal pads of the circuit board for a good solder joint to be formed.
To help ensure compliance, the PCB contract manufacturer that assembles the circuit board will conduct various inspections of the components. Samples of incoming parts will be examined using a low power microscope, and those components that do not pass the inspection will be corrected or returned. After the board has been manufactured, the parts will be examined again using a combination of manual and automated inspection processes. This is to verify that the parts have been successfully assembled to the board with good solder joints.
While great care is taken to ensure that the PCB components match their published specifications, it is equally important that the CAD footprints meet those same requirements.
For manufacturing success, it is essential that the leads of a component all sit flat on the board
PCB Design Techniques for Accurate SMT Component Placement Tolerances
The PCB CAD footprints that will be used to design the circuit board must be carefully matched to the specifications of the parts that they will host. If not, there may be some serious problems when it comes to manufacturing the board. Parts that have incorrect pad sizes, or pads that are incorrectly located, may not form good strong solder joints during automated assembly. In some cases, this can create a thermal imbalance when the part is being soldered that may cause smaller discrete surface mount parts to stand up on end like a tombstone. In other cases, footprints that are not built to the correct size may be too small for their components to fit on, or so large that they end up wasting precious board space.
To avoid these problems, here are some general rules of thumb to keep in mind when preparing the PCB footprints in your CAD tools:
- Component footprints outlines should be built to their maximum specified material width.
- Surface mount (SMT) pads or land patterns should be built to allow for the minimum to maximum pin dimensions specified on the datasheet. At the same time, the pads should be large enough to promote the creation of good solder fillets during assembly while not so large that the part is able to float off the pad.
- Thru-hole pins in SMT parts must be built with drill holes large enough for easy component insertion, while not being so big that the solder wicks up and through the hole.
- Make sure to include complete component data in the footprint such as height information for 3D checking and mechanical design.
Another important design technique is to fully set up and use the design rules in your PCB design CAD system. These will keep your component spacing in compliance with the manufacturing rules as well as prevent shorts between metal objects on your design. A well-developed set of design rules in conjunction with component footprints built to exacting specifications is the foundation of building a manufacturable design.
To build good PCB CAD footprints however takes good information, and next, we’ll look at where that information can be found.
Resources for More Information on Component Tolerances
There are many places where you can find component information, including the different footprint generators and wizards that are available in many design tools today. For the most accurate component specifications and tolerances, however, consider these resources:
- Manufacturers datasheets:These are the people that built the component that you are working with in the first place, and their data will be exact. In some cases, they will even provide you with a recommended land pattern saving you the task of defining your own pad sizes and locations.
- Industry standards and specifications: There are many publications out there that will provide general rules for land pattern dimensions. One such specification is IPC-7351, the Generic Requirements for Surface Mount Design and Land Pattern Standard.
- PCB contract manufacturers: Your PCB CM has been working with components for a long time and has a vast amount of experience and information as to exactly what kind of land pattern will work the best for the type of board that you are manufacturing.
At VSE, we have dedicated teams of component specialists and manufacturing engineers that will make sure that your board will be assembled with the best components possible. At the same time, we use some of the latest equipment in our automated assembly lines to ensure that the components on your board are precisely placed.