Understanding Tolerances in Flexible Circuit Design

In the world of cumulative tolerances, it can really be a challenge to meet the items being 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.  

Here are some common features and how they are defined. It is very important to keep these in mind as you create a print or specification.

• Copper features- These are almost always photo-defined, either through an additive or subtractive (etching) process with subtractive being more common. The primary factor that impacts tolerancing on an etched feature is copper thickness. 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 plus or minus 12um on copper that is less than 18um ( ½ oz) thick, while attempting this tolerance on 1 oz or 2 oz copper would be much more challenging. Remember, when you have two or more layers you must factor in plating as well, as this 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 35um(1 oz) copper is typically greater than +/- 25um. The tolerances go up from there as copper thickness increases
• Holes:
  • Drilled holes: Mechanical drill bits are generally available in 25 um or less increments through the range of common bit sizes 100um – ~ 6.5 mm. Common tolerances for mechanical drill operations are +/- 25um for a non-plated hole and +/- 75um for a plated hole. Remember any flexible circuit that is two or more layers will typically require plating.

• Lasered holes: The tolerances on a lasered hole feature are a little tighter and typically fall in the +/- 12um range. But be aware that lasering is more expensive than mechanically drilling. Mechanical drilling is typically performed to a stack of materials, which in some cases can be up to 10 or more sheets or panels high. Additionally, today’s high-speed drills may have up to 6 spindles which can all run simultaneously.

• Edge or trim line features:
  • Soft tooling – Chemically milled dies, or steel rule dies. These options typically offer profile tolerances of ~ +/- .4 mm.
  • Laser trimmed outlines – The actual laser pattern typically is defined to a tolerance of +/- 50um.
  • Hard tool punch and dies- The tolerances on this method of excising a part are very similar to that of laser trimmed outlines. Usually in the +/- 50um range.
• Cover openings: If we lump together both Soldermask (photoimageable coverlay) and traditional polyimide covers, they can be defined a few different ways.
  • Soldermask openings are typically imaged with an LDI (laser direct imaging) or high intensity printer and an artwork, with LDI becoming more and more common. In both cases the size of a typical SMT opening is typically held to +/- 12um.
  • Drilled cover openings would follow the same guideline as the drilled holes above. +/-12um as they come off the drill.
  • Precut lasered openings would be reserved for odd shapes when a polyimide cover is required. Coming off the laser the material is typically accurate within ~+/- 12um.
  • Laser ablated openings are the most expensive option, as rather than cutting a shape, the laser must raster back and forth to define the opening.
  • As mentioned above, laser time is more expensive. This method also requires the copper feature to be larger than the opening being laser ablated.

When thinking in terms of a cover opening, it is important to remember that when a polyimide cover is laminated there will be adhesive flow or squeeze out. Thicker copper will require thicker adhesive to ensure complete encapsulation of conductors. For instance, with a 12um thick adhesive you could expect up to 75um of adhesive flow. This is not always consistent, as it depends whether a copper feature can act as a dam, or if the cover layer is on top of the copper feature.

So now that we have covered common feature creation techniques, let’s discuss how tolerances can accumulate.

First and foremost, it is critical to understand the physical properties of flexible circuit materials. It is very possible to see stretch and shrink up to 25um per 25 mm due to temperature and humidity. So, if you are dimensioning from one feature to another, it is important to factor this in. This comes into play before adding in any processing stresses or variations induced during fabrication.

Some common processing variations are:

• Coverlayer registration. When considering a cover opening, you must keep in mind that the predefined opening (drilled or lasered) has to be registered to the etched copper layer it is being applied to. +/- 125um positional tolerance is about as tight as should be expected, and this should be in isolated areas only. Laser ablated and photo defined openings (soldermask) offer an advantage here, 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, as opposed to the edge of the etched feature. This eliminates the tolerance of the etching itself. If you were to dimension from an edge of an etched feature to a trim edge, you have the cumulative tolerances of the etching itself, the definition of the edge, and the registration of the two.

• Flexible or rigid flex circuits constructed with three or more copper layers will require 2 or more copper clad substrates. Layer to layer registration tolerances accumulate as you start adding layers. Each layer will shrink and stretch independently prior to lamination based on factors such as copper weight, polyimide thickness and % of copper being removed. Almost always there will be drilled and plated vias connecting the layers. It is important to size the via pads and drilled holes adequately 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 final part outline are typically defined in two different operations. There is a cumulative tolerance that comes into play. If you are dimensioning a hole center relative to a part edge this needs to be considered.

The fabricator you choose does have a few tools in their toolbox they can use to compensate for some of these items.

• Scaling: This can be applied at various steps including etching and drilling. Often this can be predefined based on known characteristics of the materials and particular stack ups.
• 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: This is similar to scaling. If after processing it is known that a feature will be slightly larger or smaller than nominal, compensation can be applied.

Final thoughts:

• Don’t over dimension your flex circuits. It is always best to limit dimensions to a single process defining them. Avoid interactions when possible.
• Flex circuits by nature compensate for a lot of variation. They connect things electrically and slight variations can be absorbed during installation.
• Keep in mind these are not machined metal parts created with all features being defined by a single machining process.
• Work with your fabricator. They know the limitations of the materials and equipment.
• Get your supplier involved early. It is best to provide preliminary prints and data and see if what you are asking for in the way of dimensioning and tolerances are 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, increased inspection times and likely interruptions in delivery because of lot-to-lot variation. Any dimension that you include on your print carries a cost.  Each dimension will have to be verified by the manufacturer on each lot of parts shipped.  When dimensioning your drawing, ask yourself if each of those dimensions are truly critical to the function of the part. If the answer is yes, then include them. If not, leave them off because they will only add unnecessary cost.

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At Flexible Circuit Technologies, our engineering team has supported a wide range of applications—from everyday electronics to highly specialized designs. We work with you early in the process to help determine the right specifications and avoid costly missteps. For more information, view our free Design Guide or reach out to schedule an early design consultation.  

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