| Created: 5/21/95 |
Updated: 4/18/97 |
Flexible Printed Circuit Design Guidelines
Objectives
FPC design guidelines are needed for improved
reliability in terms of usable ranges of material
properties and geometry, rather, than specific
materials and architecture, so as to not exclude
alternate or future materials and concepts.
Accurate life-estimation models that can answer
fundamental design questions based upon the
physics of failure are required.
Background
The two primary failure modes of FPCs are
cracking of plated through-hole (PTH) barrels
and lands and fatigue of the copper circuit
traces. PTH failures are principally driven by
thermomechanical stresses due to the CTE
mismatch of the various layer materials. The
copper circuit trace failures are driven both by
thermomechanical stresses as well as the
flexural stresses in the FPC. Very little
fundamental research has been conducted on
FPC design with respect to understanding and
modeling active failure mechanisms.
Approach
The research effort this past year has been
devoted to measuring and understanding the
basic mechanical properties of the constituent
materials in a typical FPC layup. Extensive
testing was conducted on actual FPC samples,
measuring the CTE, elastic modulus, and creep
response of the material. Using this
information, stress analysis modeling efforts
will be started.
Work Accomplished
- Analytic and FEA modeling have
identified the out-of-plane elastic modulus
of the rigid flex circuit card as one of the
most critical design parameters affecting
the PTH barrel stress. It was also found
that too little was known about the out-of-
plane modulus to accurately model the
fatigue response.
- A new series of experiments were
conducted to measure the out-of-plane
modulus of rigid flex circuit cards at
different temperatures. Different analytic
schemes were evaluated on their
capability to accurately estimate the
modulus.
- PTH stresses in typical rigid flex have
been examined with analytic and FEA
simulations. A parametric study was
conducted to determine the influence of
the geometric layup structure and material
property variations on predicted fatigue
life. A closed-form empirical equation for
fatigue life was developed analogous to
the equation for PTH stresses.
- Flexural fatigue models are
critically examined and improved.
- FPC design guidelines are
developed, based upon modeling and
analysis of the active failure mechanism.
The guidelines account for raw
materials and manufacturing limitations
are documented based upon the EPRC effort
and data obtained from outside sources.
