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44 The PCB Magazine • March 2017 volves two very different processing effects as flexible substrates will dimensionally change during the fabrication process. Most often these changes are not easily predictable. Dimensional changes might vary slightly due to raw material variation from batch to batch. Changes may also occur as a function of constructions (thin materials are less stable) and processing conditions. Mate- rial thicknesses, ambient humidity, copper elec- troplating densities, and percentage of copper etched can all be contributing factors. As the circuitry panel is processed, it is ex- posed to a variety of chemistries, temperatures, pressures, electroplating, etching, and will un- dergo small dimensional changes. Etching cop- per releases stress and is sometimes called "etch shrink." This term mistakenly becomes a catch phrase for all the dimensional changes oc- curring during processing of a flexible circuit. Compensating for these changes may be con- sidered by the fabricator when setting up a new flexible circuit part number, but often empiri- cal data from actually producing parts is needed to accurately predict feature movements. The consequences of misalignment are illustrated in Figures 1 and 2. One possible way to deal with dimensional changes is to minimize the panel size, which may be done when tolerances are extremely tight. The smaller the panel size, the less dimen- sional stability issues will affect registration and alignment. Handling damage is also minimized with smaller panels. The downside of smaller panel sizes is they are less efficient to process vs. larger panels, as many costs in a circuit "noo- dle" factory are panel-driven. There are many ways to compensate for di- mensional changes while balancing panel size for cost-effective production. The following are often adopted as methods to adjust for the di- mensional changes that may occur during cir- cuit fabrication: • Scaling factors can be applied to secondary layers or tooling based on the predicted dimen- sional change of the material. Scaling factors can also be dynamically calculated based on in-process measurements for a given lot. One example could be a solder paste stencil that is created based on the measured scaling factor of a panel ready for component population. Di- mensionally compensating a final drill program in a multilayer is another example. • Software-controlled operations use align- ment systems to detect dimensional shifts and compensate by using optical fiducials. Many fabrication machines can perform a dimension- UNDERSTANDING DIMENSIONAL STABILITY IN FLEXIBLE CIRCUITS Figure 1: Material stability can create a violation of the minimal annular ring requirement or it can possibly cause full break-out of the hole-to- pad alignment (as shown). When the material change is predictable, then either the drill pattern or the conductor layout can be adjusted to re-center the plated-through-hole in the pad. Figure 2: Example showing misalignment of coverlay.

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