The flex PCB stackup documentation is an important component of the data set of a flexible printed circuit board design. It consists of a description of a flex or rigid-flex circuit board that defines in detail the specific material requirements and construction of the design.
The material stackup is required by the supplier to ensure that the finished part is manufactured according to all customer specifications and meets both the mechanical bend requirements and electrical requirements of the design.
Material Stackup Format
Flex PCB material stackups are a graphical, side view profile of the flexible printed circuit board which is typically embedded in the mechanical drawing in either PDF, DXF, or Gerber format.
Example of a 2-Layer Flex Circuit Stackup with ZIF Contacts
Example of a 4-Layer Rigid-Flex Circuit Stackup
Stackup Material Information
As seen in the previous examples, the stackup contains a significant amount of information. It defines the specific material types to be used, their locations within the construction, the individual material layer thicknesses, material part numbers if required, and the total thickness of the various areas of the design.
Most flex and rigid-flex circuit designs have multiple areas with differing constructions resulting in multiple thickness requirements. Some area thicknesses may be critical to the performance of the part, either to meet a specific bend requirement, impedance value, or connector specification (ZIF connectors, for example).
For rigid-flex PCB designs, the stackup also defines the key construction elements required to meet IPC 2223C design guidelines and ensure the reliability of the finished parts.
Specific elements include:
- Adhesiveless flex core(s)
- Selective coverlays applied to the exposed flex areas only and not within the rigid areas
- No-flow pre-preg for rigid layer lamination and high TG FR4 rigid cores
Common Material Details
Along with the material stackup information, below are some additional details that should also be included.
Flex Cores:
- Type (Adhesive or Adhesiveless)
- Thickness, Copper weights
- Copper Type (ED or RA)
- Lamination Adhesive Thickness
Coverlays / Soldermask:
- Thickness
- Adhesive Thickness
- Adhesive Type (Epoxy, Acrylic etc.)
- Color
Stiffeners:
- Material Type (FR4, Polyimide, Aluminum, Stainless etc.)
- Thickness
- Attachment Adhesive (PSA or thermally bonded)
Additional Materials:
- PSA (Pressure Sensitive Adhesive)
- EMI Shielding Films
- Epoxy Strain Reliefs
Flexible circuit board designs typically have a finished thickness tolerance of +/- 30um to 50um or larger depending upon the layer count. The thickness of a flex circuit will vary depending upon where the part is measured. It will also vary due to the adhesives having a reduced thickness after lamination due to a percentage of the material being consumed by filling in the spaces between traces.
Complex Flex and Rigid-Flex PCB Designs
Complex flex designs that have multiple extensions may require the material stackup to be divided into regions to allow for a full and clearly defined description of the design. Trying to define the part in one stackup results in areas of differing construction overlapping and distorting the graphical representation of the design. The specific regions are then identified in the part outline.
Example of Advanced Multiple Area Material Stackup
Example of Flex Circuit Drawing with Multiple Area Material Stackup
Example of Advanced Rigid-Flex Circuit Material Stackup
Flex Board Material Specifications
We recommend that the flexible and rigid materials (if required) be defined by IPC standards as part of the drawing notes. This allows for the use of equivalent material brands that meet the same IPC specifications. It’s not practical for a supplier to stock all the available material brands and configurations. In addition, not all material brands are readily available or cost effective in different geographical areas.
The exception to using these specification standards is for designs that require specialty materials for which there are either no existing IPC standards or have specific performance requirements. This most commonly applies to high performance flex materials, PSAs, shielding films, and epoxy strain reliefs. If known equivalents are available, the drawing notes should state “or equivalent”.
Examples of specialty materials:
- PSA(s): 3M9077, 3M467MP, etc.
- EMI Shielding Films: Tatsuta SF-PC6000
- Flex materials: DuPont TK, DuPont HT
- Epoxy Strain Relief: Eccobond 45/15 Flexible Formula
Summary
All flex or rigid-flex PCB designs require a complete and accurate PCB stackup as part of their included documentation to ensure the supplied parts meet all the design requirements for the customer. Ensuring that this information is correct helps eliminate unnecessary technical questions from popping up and avoiding delays in the delivery time of your parts.
