Browse Prior Art Database

Composite FR-4, Polyimide Printed Circuit Cards

IP.com Disclosure Number: IPCOM000108061D
Original Publication Date: 1992-Apr-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 144K

Publishing Venue

IBM

Related People

Snyder, CH: AUTHOR

Abstract

Two factors driving the complexity of printed circuit cards are the area(s) of highest wiring density and the necessity for interconnections with specific, tightly controlled signal transmission characteristics.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Composite FR-4, Polyimide Printed Circuit Cards

       Two factors driving the complexity of printed circuit
cards are the area(s) of highest wiring density and the necessity for
interconnections with specific, tightly controlled signal
transmission characteristics.

      At this time, commodity cards are constructed with uniform
cross- sections which must support the required wiring density and/or
transmission characteristics over the entire card area.

      One method which can be used to address these factors is to
group those components requiring locally dense wiring and/or critical
signal transmission properties onto a smaller subassembly with a high
performance cross-section. However, this introduces the necessity for
a connector between the subassembly and the remainder of the system,
which can increase the cost and be detrimental to signal transmission
properties.

      This article presents a composite FR-4/polyimide structure
which is used to supply locally dense wiring and/or improved
electrical performance.

      In regard to maximum wiring density, it frequently occurs that
the area of maximum density does not extend over the entire card area
and may involve a relatively small number of the total components on
the card, such as a processor and several associated modules.  For
such cases it would be advantageous to use a simpler, less expensive
card cross-section augmented by a polyimide structure to supply the
additional wiring density only where necessary.

      In regard to interconnections requiring specific signal
transmission characteristics, it frequently occurs that such signals
(system clocks, asynchronous control nets) are relatively few
compared to the total number of signals on the card.  Again, a less
complex card cross- section combined with a polyimide structure
containing the critical signal wiring would be beneficial.

      It is noted that for both cases, the polyimide structure and
the circuit card can be tested separately prior to assembly.  This is
in contrast to present card testing, where a defect in an assembled,
complex FR-4 card requires either rework or that the card be
scrapped.

      As the dielectric constant of polyimide is lower than that of
FR-4 (3.3 and 4.4, respectively), the signal time delay
characteristics of copper conductors in a polyimide structure will be
improved as compared to the same conductors in a FR-4 structure with
the same physical dimensions. This can be quantified by using the
equation for time delay, where time delay is given by the square root
of the product of the inductance and the capacitance of the
conductor.  Comparing the two materials gives a speed improvement for
polyimide of approximately 15%.

      One factor which may...