What is a Fabrication Drawing?

Not clearly specifying information may result in errors along the manufacturing supply chain. For example, one of my customers had an old legacy design they needed to have built. The fabrication drawing called for FR4 as the material the board was to be built on. The original designer specified a high volume resistivity requirement in an email sent to the previous manufacturer. The requirement was never added to the the fabrication drawing. The company that used to build the pcb went out of business. The original designer retired. My company built the design. A contract manufacturer assembled the boards. The customer installed it into their rack. None of the assemblies worked.

To make a long story short, a copy of the email was never sent to my company. The manufactured pcbs developed cross-talk between unlike conductors on different layers. There are dozens of FR4 manufacturers. They all manufacture different grades of FR4 with different dielectric properties. The boards we built were made with a grade of FR4 that did not meet the volume resistivity requirement intended by the original designer. Hence the cross-talk problem.

Since the fabrication drawing only specified FR4 my company was not at fault. However, the customer had assemblies they couldn’t use and needed to pay to have the pcbs rebuilt on suitable material. The total cost to my customer was over $75,000. This included the cost of the scrapped assemblies, the cost of rebuilding everything and the impact on their own production cycle. This cost could have been avoided if the original designer took the time to specify the volume resistivity requirement on the fabrication drawing and not in a misplaced email. In this age of downsizing and outsourcing can you survive a $75K mistake?

This brings us to the main topic of this post. What is a fabrication drawing?

The fabrication drawing is the master control document for the printed circuit board. It should specify all critical starting and finished requirements. It should also reference any critical company or industry specifications. In the event of conflicts between the fabrication drawing and other supplied information, the fabrication drawing takes precedence. Other supplied information refers to artwork, CNC drill/rout data and company/industry specifications.

Common characteristics of the fabrication drawing include the following items…

  • Fabrication notes.
  • A dimensioned mechanical detail that specifies the part size and profile specifics.
  • The mechanical detail should provide a drilled hole to edge dimension.
  • A drill detail/legend that differentiates between plated and non-plated holes.
  • Side details or geometric tolerances that clearly define special milling operations.
  • A title block that clearly defines company ownership, part identification, revision history and default tolerances.

Printed circuit board fabricators and contract manufacturers rely on the fabrication drawing to be their blueprint. This document is used in the setup phase, manufacturing phase and inspection phase of the manufacturing process. A detailed fabrication drawing ensures that all present and future manufactures build the part consistently and accurately. It limits if not eliminates the variances produced by multiple suppliers.

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