Understanding Tolerances in Flexible Circuit Design

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The challenge with cumulative tolerances is meeting the dimensional requirements for items dimensioned on a drawing or specification for a flexible or rigid-flex circuit. It is critical to understand the fabrication processes and how features are defined when creating your tolerance requirements.

Common Features to Keep in Mind

Copper features

These are almost always photo-defined, either through an additive or subtractive (etching) process with subtractive being more common. Copper thickness is the primary factor impacting tolerancing on an etched feature. The thicker the copper, the more difficult it is to control the width or the size of the etched feature. For instance, you may be able to control the width of a conductor to ±12 μm on copper that is less than 18 μm (half-ounce) thick, while attempting this tolerance on one- or two-ounce copper would be much more challenging. Remember, when you have two or more layers, you must factor in plating, as it adds to the copper thickness. Button or pads-only plating can help reduce the impact of this. As copper gets thicker, the ability to control these dimensions diminishes. A realistic tolerance on 35 μm (1-ounce) copper is typically greater than ±25 μm. The tolerances go up from there as copper thickness increases.

Holes

Drilled holes: Mechanical drill bits are generally available in 25 μm or less increments through the range of common bit sizes of 100 μm to ~ 6.5 mm. Common tolerances for mechanical drill operations are ±25 μm for a non-plated hole and ±75 μm for a plated hole. Remember, any flexible circuit that is two or more layers will typically require plating.

To continue reading this article, which originally appeared in the March 2026 of I-Connect007 Magazine, click here.