High Frequency Flex

Flexible printed circuits (FPCs) provide a practical option for interconnection of printed circuit boards and electronic modules, particularly where space is constrained or weight is a factor. Moreover FPC construction offers greater levels of impedance control compared with co-axial and other wired connections. However with FPCs increasingly being used in high speed digital applications, understanding their electrical performance at high frequencies becomes a growing challenge. This project is characterizing the effect on signal integrity for flexible printed circuit boards operating at high end digital transmission frequencies (up to 20 GHz) with regards to different design features, specifically cross hatched ground planes.

 

Project stage: 
Project type: 
Lead company: 
DuPont

If you are interested in participating in this project:

Members - Go to the "Subscribe Here" section to the right and select the "Subscribe to space" key.

Non Members - Go to the "Contact" section to the right and select the "Contact project facilitator" key.

Idea Information

Background: 


Impedance models do not always match the actual product results, especially when it comes to utilizing a non-contiguous ground plane in flexible products.  The design knowledge for these non-contiguous ground planes often resides within a company and not much is published in the literature. The team decided to investigate signal integrity for cross hatched ground planes of various design and copper opening.

Problem: 


 Flexible printed circuits (FPCs) provide a practical option for interconnection of printed circuit boards and electronic modules, particularly where space is constrained or weight is a factor. Moreover FPC construction offers greater levels of impedance control compared with co-axial and other wired connections. However with FPCs increasingly being used in high speed digital applications, understanding their electrical performance at high frequencies becomes a growing challenge. Several issues arise from using them for this purpose.

  1. Controlling the critical geometries of flexible materials as they differ from rigid PWBs.
  2. The differing properties of the materials (Dk, Df etc.) involved and the fact that some dielectrics and adhesives are very hygroscopic.
  3. Cross hatch GND planes. Simulating the effect of the non-contiguous plane and the effect of various shapes. Cross hatch GND Planes are used for Flexibility & Impedance control

Definition Information

Goals / Benefits: 

Of the three areas of concern the team decided to conduct a study to characterize the effect on signal integrity for flexible printed circuit boards operating at high end digital transmission frequencies with regards to:

  • Different design features: Various Crosshatched ground plane designs
  • Material choice: Polyimide (DuPont/Panasonic)
  • Laminate thickness:  100 um (stripline); 50 um (microstrip)
  • Frequency ranges up to 20 Ghz
  • S-parameters, Short Pulse Propagation (SPP), and Radiated Emissions (RE) testing of FPC test vehicles

Evaluate how the cross hatch designs effects impedance and S parameters (insertion and return loss) on flexible laminates at high frequencies up to 20 GHz.

·        Evaluate Cross hatching (non-contiguous planes) vs. solid planes to understand how the cross hatch designs effects impedance and S parameters (insertion and return loss)

·         Build test vehicles with the various features (line widths, line spacing, and total copper in plane), measure the S parameters/Impedance up to 20 GHz and compare this data to present day models (2D and 3D simulations).

 

Example of the 50% copper cross hatch showing both the diamond and circle opening

Benefits of Project:

·     Provide the industry with the foundation knowledge for the mechanical and electrical properties differences between flex and rigid. This will lead to a better understanding and more usefulness of higher frequency flex products.

·        Provide better methods for designing with flex.

·      Increase understanding of why the present simulation models do not match the measured performance of real products.

·         Understand the effects of crosshatching designs and how to improve designs with crosshatch planes.

High level objectives: 

   Objectives:

  •  Design a test vehicle using a Solid Plane as the reference feature and various cross hatch designs.
  • Run with a series of frequencies up to 20 GHz and see the effect on impedance/loss.
  • Evaluate how the cross hatch designs effects impedance and S parameters (insertion and return loss)

Deliverables:

  • Publish a comparison of measured SI data to present model predictions (if models are available)
  • Synthesize engineering trade-offs on crosshatch design findings
  • Publish final report to members
  • Present finding at industry conference (EIPC/IPC APEX)
  • Recommendation for the next phase of work
Approach: 

 

This project is characterizing the effect on signal integrity for flexible printed circuit boards operating at high end digital transmission frequencies with regards to different cross hatched ground plane design features and materials by building a set of TV and measuring the SI performance in comparison to a full ground plane.

Of the three areas of concern the team decided to conduct a study to characterize the effect on signal integrity for flexible printed circuit boards operating at high end digital transmission frequencies with regards to:

  • Different design features: Various Crosshatched ground plane designs
  • Material choice: Polyimide (DuPont/Panasonic)
  • Laminate thickness:  100 um (stripline); 50 um (microstrip)
  • Frequency ranges up to 20 Ghz
  • S-parameters, Short Pulse Propagation (SPP), and Radiated Emissions (RE) testing of FPC test vehicles

Schedule:

Flow Diagram:

Key Participants: 
Ciena
Cisco
DuPont
Flex
H3C Technologies
Hitachi Chemical
IBM
Introbotix
Juniper
Nokia
Panasonic
Rogers Corporation
Shengyi
TTM Technologies
Ventec
WUS
Public