Polyimide Flex Core Material Types: Adhesive Based & Adhesive-less

 By Paul Tome, Product Manager Flex & Rigid Flex - Epec Engineered Technologies

Two distinct types of flex core material constructions are utilized in today’s flex circuit manufacturing.
The difference is in the method used to physically attach the copper layers to the polyimide core. This results in a different set of material properties for each of the material types which must be factored in to a specific application to ensure the design meets all the requirements and is cost effective.

“Adhesive Based” flex cores utilize a layer of flexible adhesive to bond, under heat and pressure, each copper layer to the polyimide core. The adhesive is typically either epoxy or acrylic based and 0.0005” to 0.001” in thickness.
           

Adhesive Based Flex Cores

   
“Adhesiveless” flex cores have the copper directly attached to the polyimide core without the use of any adhesives.

Adhesiveless Flex Cores

 Adhesive Based Flex Core Benefits:  

• Reduced Material Costs
• Higher Copper Peel Strengths (Dependent Upon Adhesive Type Used)           

 

Adhesive-Less Flex Core Benefits:

• Reduced Flex Thickness
  • Due to Elimination of Adhesive Layers
• Improved Flexibility
  • Due to Reduced Finished Flex Thickness
• Tighter Possible Minimum Bend Radius
  • Due to Reduced Finished Flex Thickness
• Improved Plated Hole Reliability in Higher Layer Count Flex Designs (i.e. 3- 7 layer designs)
  • Reduces Number of Layers of Adhesive Within the Flex Construction
  • High Co-Efficient of Thermal Expansion of Flex Adhesives can Place Plated Holes Under an Excessive Z-Axis Stress During Assembly/Operation Which can Lead to Hole Plating Cracks
• Higher Potential Temperature Ratings
  • Flex Adhesives have a Substantially Lower Max. Operating Temperature than Polyimide
• Improved Controlled Impedance Signal Characteristics
  • Homogenous Construction of only Polyimide Between Signal Layers and Reference Planes.
• Required to meet IPC 2223C Rigid Flex Design Guidelines
  • Eliminates and or Reduces the Use of Flex Adhesive within Rigid Areas
  • Improved Plated Hole Reliability
• Better Suited to Harsh Environment Applications

Both material types are available with a range of Polyimide core thicknesses and copper weights. 0.001”and 0.002” are the most commonly used and cost effective core thicknesses. Thicker and thinner cores are available to meet unique specific design requirements. Thicker cores, greater than 0.002”, may be required for impedance controlled designs etc., but have a negative impact on flexibility, reliability and cost. Thinner cores, less than 0.001”, are available for very thin designs that require maximum flexibility but negatively impacts cost.

Both material types are also available with a range of copper thicknesses with ½ oz and 1 oz being the most common and cost effective. Thicker and thinner copper weights are again available to meet specific requirements. Thicker copper, greater than 1 oz, for higher current carrying requirements and thinner copper, less than ½ oz, for high circuit density and more flexible designs. Both negatively impact cost as compared to the ½ and 1 oz standards.

Which Flex core material type is best suited for an application? 

This depends upon many elements of a specific design. A full electrical and mechanical design review is recommended to ensure the design is optimized.

Please feel free to contact us if you have any questions or need assistance in selecting the correct material type for your design.