The Importance of Specifying Keep-Out Areas in Mechanical Outline Drawings in Printed Circuit Board Design

In designs where the fit of the PCB or its interaction with heat sinks or other mechanical features in the overall design is negligible, the Mechanical Outline

Drawing (MOD) and its impact on manufacturability and product reliability is often overlooked. Here are a few examples of items that need to be considered:

1. Placement of Multi-Layered Ceramic Capacitors (MLCCs) need to be away from any area where the PCB assembly will experience significant mechanical stress during assembly, test, normal handling, and product use. Obvious examples would be adequate keep outs away from mounting holes or underneath components that may experience stress during assembly (Connectors, Press Fit components and components that have mechanical secondary operations such as the application of thermal pads.) This is particularly important since a common failure mode from a damaged MLCC may not present itself immediately and the effect of micro-cracking of the internal ceramic layers will likely eventually lead to a dead short and a field failure.
2. Placement of leadless components such as Ball Grid Arrays and Land Grid Arrays away from areas of mechanical stress. As with MLCCs, BGAs and LGAs need adequate distance from features that may flex during the manufacturing process or product use. A courtyard around BGAs and LGAs that allows the access to the device by standard rework equipment is also important to avoid excessive scrap in the manufacturing process.
3. Placement of components on a flex circuit needs to have the bend area and radius of the bend taken into consideration to avoid mechanically stressing components or solder joints. The thickness of the copper foil must also be taken into consideration.
4. The proximity of surface mount devices to the leads of plated trough hole components needs to be adequate to allow the use of selective soldering or wave soldering with a wave solder fixture. This is especially important to achieve product cost goals if the product in to be manufactured domestically.
5. How is the PCB to be manufactured; is it going to be a one-up or panelized? If it is going to be panelized, there will be different considerations based on the retention method (Route and Retain or V-Score) and the method of depanelization at the factory.

Every design is unique and often tradeoffs must be made to achieve a design with the available placement area for components. Whenever possible it is best to get input from the manufacturer of the materials, components, and the assembler of your product to make certain that risks to the manufacturability and long-term reliability have been mitigated. Freedom CAD offers DFM services along with PCB Layout to help make certain you avoid any potential problems with your design. Contact us today.