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26 I-CONNECT007 MAGAZINE I MARCH 2026 feature. This eliminates the etching tolerance. To dimension from an edge of an etched fea- ture to a trim edge, you have the cumulative tolerances of the etching itself, the definition of the edge, and the registration of the two. • Precut lasered openings would be reserved for odd shapes when a polyimide cover is re- quired. After the laser, the material is typically accurate to within ±12 µm. • Laser-ablated openings are the most expen- sive option, as the laser must raster back and forth to define the opening rather than cutting a shape. • Laser time is more expensive. This method also requires the copper feature to be larger than the opening being laser ablated. For a cover opening, when a polyimide cover is laminated, there will be adhesive flow or squeeze out. Thicker copper requires a thicker adhesive to ensure complete encapsulation of conductors. For instance, with a 12 μm thick adhesive, you could expect up to 75 μm of adhesive flow. This is not always consistent, as it depends on whether a copper feature can act as a dam, or if the cover layer is on top of the copper feature. Now that we have covered common feature creation techniques, let's discuss how tolerances can accumulate. First, understand the physical properties of flex- ible circuit materials, as they can stretch and shrink by up to 25 µm per 25 mm due to temperature and humidity. Factor this in if you are dimensioning from one feature to another. This happens before any processing stresses or variations induced during fabrication are added. Some common processing variations are: • Cover layer registration: When considering a cover opening, the predefined opening (drilled or lasered) must be registered to the etched copper layer to which it is applied; ±125 μm positional tolerance is about as tight as should be expected, and this should be in isolated areas only. Laser-ablated and photo- defined openings (solder mask) offer an ad- vantage, as they are defined after the material is laminated or bonded to the etched panel. • When dimensioning an etched feature to a part edge, it is typically better to dimension from the center of the etched feature to the part edge, rather than the edge of the etched Figures 2 and 3: Pad-defined solder mask opening.