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PCB Manufacturing Tolerances: Adding Fabrication Tolerance Options to Your Manufacturer Deliverables

Created: February 1, 2019
Updated: September 25, 2020

Precision PCB measurement system

Much as we would like, no manufacturing process is perfect. Every process produces slight errors in part sizes or placement, and PCBs are no different. Since some PCB applications have very tight tolerances, you’ll need to specify your tolerance requirements for your manufacturer. Let alone maintaining secure understandings of solder mask and solder placement, material thickness specifications, or any surface mount components - designating your printed circuit for production can be a hassle. 

So how do you communicate this critical information to your manufacturer? Having a clear fabrication drawing helps the assembly of your printed circuit, especially for a more standard PCB, but a PCB designer may find themselves in need of more. This is where your bill of materials comes in. Your bill of materials contains a number of pieces of information, and your manufacturer will need a number of deliverables in addition to this document.

Component Tolerances

Every component has tolerances on its specifications, usually expressed as a percentage of some nominal value. Components with tighter tolerances are generally more expensive as they require their own precise manufacturing and testing process. Different electronics applications require that various components have specific tolerance levels.

A bill of materials for your new board that is intended for production might refer to a manufacturer’s part number by default rather than an internal part number. Some manufacturers want complete component data included in the bill of materials instead of just a general part description. This component data includes electrical parameters, footprint, and of course, tolerances in these values.

This information doesn’t just help designers and engineers ensure that the board can meet its technical requirements. Sourcing information can change quickly, and the parts you choose for your PCB might be unsourceable by the time production begins. This information helps manufacturers locate parts and choose suitable replacements if necessary. Always consult your manufacturer regarding their information requirements before preparing your bill of materials.

A PCB with many components

Component information can be difficult to track and manage when working with several boards

Feature Tolerances

Specifying tolerance values in your PCB for your manufacturer goes beyond component electrical specifications. Some important aspects that must be specified are trace width, pad size, and hole size tolerances for plated-through-hole components and vias. Your design software should make it simple to include this information for your manufacture in an easy-to-read format.

Plated-through-hole components require their own tight tolerances in order to ensure that these components will sit properly on the board and make reliable electrical contacts with conductors. Vias also require tight tolerances to ensure they do not span into the wrong layer by mistake. These hole tolerance specifications for your vias and pads can be communicated to your fabricator in a drill table.

Drill depth tolerances for vias are extremely important in multilayer PCBs. As layer counts increase, depth tolerances are critical to ensuring that your vias only span across the correct layers in your PCB. Unless you are using through hole vias or you are stacking blind vias across multiple layers, your drill tolerance should be set to a level that is much smaller than your layer thickness in order to prevent drilling through the wrong layer. Always check your manufacturer’s capabilities when designing vias and specifying drill depth tolerances.

Engineer drafting a mechanical system

Being clear with your manufacturers enables less pain upon production

Using An Integrated Bill of Materials Tool

Using PCB design software that integrates your component libraries with your design and bill of materials tools ensures that you can always find the right components with the right tolerances for your specific application. An integrated library feature with search capabilities makes it easy to find components with the tolerances you need for your circuit board.

The best integrated component library tools help you manage components in your device throughout their lifecycle, helping you avoid problems due to component obsolescence. This type of library accesses component sourcing information, giving you a live view of your component options. You’ll be able to avoid costly and time-consuming redesigns that can arise from including unsourceable components in your PCB.

If your favorite or preferred components are not available within your schedule, a great integrated component library gives you the ability to choose your own suitable replacement components that meet your tolerance requirements. You can then include this sourcing information in your bill of materials for your manufacturer. This prevents manufacturing delays that can result from old or incorrect sourcing information.

Access to current component sourcing information also saves a huge amount of time while preparing for manufacturing. You won’t have to browse through component manufacturer or distributor websites just to locate the components you need for your PCB. All of this information should be accessible within your design software, allowing you to instantly add a suitable replacement to your layout that meets your tolerance requirements.

Unifying your schematic, layout, component data, and manufacturer deliverable tools in a single interface gives you the power to build the latest and greatest PCBs and smoothly bring them to production. The information you need for your bill of materials and other deliverables is all in one location, and you can remain confident that your manufacturer can take your PCB to production successfully.

Great PCB design software like Altium Designer 18.1 is essential for building modern PCBs and generating deliverables for your manufacturer. The ActiveBOM tool, component libraries, and other design features in Altium Designer let you create high quality layouts, generate a thorough bill of materials, and manage your components.

To learn more about how Altium Designer can help you with product management and bring your next device to production, talk to an Altium expert today.

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