IPC International Community magazine an association member publication
Issue link: https://iconnect007.uberflip.com/i/1543955
28 I-CONNECT007 MAGAZINE I MARCH 2026 • Flexible or rigid-flex circuits constructed with three or more copper layers will require two or more copper-clad substrates. Layer-to-layer registration tolerances accumulate as layers are added. Each layer will shrink and stretch independently before lamination, based on copper weight, polyimide thickness, and the percentage of copper removed. Drilled and plated vias will almost always connect the lay- ers. Size the via pads and drilled holes ade- quately to allow the manufacturer to meet the annular ring requirements of IPC-6013 and the performance class you define on your print. • Drilled holes and the final part outline are typi- cally defined in two different operations. A cu- mulative tolerance comes into play. If you are dimensioning a hole center relative to a part edge, this needs to be considered. Your fabricator has options to compensate for some of these items: • Scaling: Applied at various steps, including etching and drilling. Often, this can be pre- defined based on known characteristics of the materials and particular stackups. • Targeting bias: If a set of features, drilled holes, or a particular etched layer is more critical, targeting schemes can be adjusted to compensate for this. • Compensation: Similar to scaling. If, after pro- cessing, you know a feature will be slightly larger or smaller than nominal, apply compen- sation. Final Thoughts Don't overdimension your flex circuits, limit dimen- sions to a single process defining them, and avoid interactions when possible. Flex circuits, by nature, compensate for a lot of variation. They connect things electrically and slight variations can be ab- sorbed during installation. Keep in mind these are not machined metal parts, with all features defined by a single machining process. Work with your fabricator, as they know the limitations of the materials and equipment. Also, get your supplier involved early. Provide preliminary prints and data, and determine whether what you are asking for in terms of dimensioning and tolerances is achievable. Excessive or unreasonable tolerances will limit the number of suppliers willing to manufacture your product. Suppliers that attempt to build your product will experience lower yields, longer inspec- tion times, and likely delivery interruptions due to lot-to-lot variation. Any dimension that you include on your print carries a cost. Each dimension must be verified by the manufacturer for each lot of shipped parts. When dimensioning your drawing, ask yourself whether each dimension is truly critical to the part's function. If the answer is yes, include them. If not, leave them off because they will only add unnecessary cost. I-CONNECT007 Chris Clark is a senior principal applications engineer at Flexible Circuit Technologies. Figure 4: Dimensioning from the center of an etched feature to the part edge (right) eliminates etch tolerance from the dimension chain and reduces cumulative variation.