Before we look at the different tolerances of how components are placed on a circuit board, we need to first start with the part’s tolerances.
Each component footprint in the CAD system must be built according to its manufacturer’s specifications. If a CAD footprint is built incorrectly, the actual physical part may not fit on the metal land patterns of the circuit board. Problems like these can cause the board to not be manufacturable.
Building the component footprint within the PCB design system consists of two steps:
Create the metal land patterns that the leads of the part will solder to. Land pattern dimensions can sometimes be found in the manufacturer’s datasheets or they may already exist as an industry-standard land pattern, such as a SOIC-14.
Build the physical shape or outline of the component according to the specifications in the manufacturer’s datasheet.
Building PCB CAD footprints is a much more important part of PCB layout than most people realize, and it is important to understand why. Take a standard bypass capacitor in an 0603 package, for example. Depending on its density, your design may require hundreds of different bypass capacitors that come in the same 0603 package. You will only need one 0603 footprint in the design, and the CAD system will replicate it for each instance of a capacitor throughout the board. However, if it is incorrectly built, then all of those parts will be wrong. If the footprint is built too small, the actual parts may not even fit on the board, while a larger footprint may affect how the capacitors are soldered during manufacturing.
To avoid PCB component placement tolerance problems, keep these guidelines in mind while you are working with PCB footprints for your layout:
Surface mount (SMT) land patterns need to be large enough to promote good solder fillets during PCB assembly. While these land patterns are usually specified in industry standards, there may be occasions when you have to create your own land patterns for special needs parts. In that case, make sure to build the land pattern to occupy both the minimum and maximum pin dimensions of the part.
Thru-hole pads should be built with a drill hole large enough for component insertion yet small enough to prevent solder from wicking up through the hole.
Component outlines should be constructed to the maximum material width specified in the manufacturer’s datasheet.
Also, keep in mind that many land patterns will have to be used for the same part from different manufacturers. In that case, you will want to work with multiple data sheets to ensure that you allow for the maximum material widths for all parts.
At this point, your design should have good PCB footprints to work with, so let’s look at placement tolerances between parts for the best electrical performance of the board.
Placement Tolerances for Electrical Design Performance
When placing components in a PCB quote layout, one of the considerations to keep in mind is the component’s function in other parts. For instance, power supply components have strict placement requirements to ensure they produce as little noise and EMI as possible. These parts certainly must adhere to good design for manufacturability (DFM) requirements in their placement, but they also must be arranged for their best functionality. Here are some general placement rules to ensure maximum performance of your circuitry:
Power supply components should be kept as close together as possible to minimize their trace lengths. Shorter and wider traces in power supplies lower the inductance and reduce electromagnetic interference (EMI).
Bypass capacitors for large pin-count high-speed devices need to be placed as close as possible to the device’s power pins. This proximity will help to reduce current spikes and the resulting signal integrity problem of ground bounce.
High-speed components connected through a signal path must be placed close together to prevent the signal path from wandering around on the board. A closely placed signal path will help ensure lower impedance in high-speed transmission lines.
Keep components of functional partitions together except for where signals cross between the functional areas. This will prevent analog noise from contaminating digital circuitry and vice versa.
For thermal management, keep hot-running components like processors and memory chips closer to the center of the board and separate power supply circuitry from each other.
These PCB component placement tolerance recommendations are designed to help your circuit board perform at its best. However, the circuit board still needs to be manufacturable, requiring a whole different set of placement tolerances.
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