MANUFACTURING FLEXIBLE CIRCUIT by controlled collapse chip connections (C4). The flexible
BOARD ASSEMBLIES WITH COMMON circuit board is folded around a stiff heat spreader of
HEAT SPREADERS copper-INVAR-copper (INVAR is a trademark of Creusot
Loire for a nickel-iron alloy) with the chips on the inside of
This is a Divisional application under 37 CFR §1.60, of 5 the fold. The exposed surfaces of each row of chips are
pending prior application Ser. No. 08/071,630 filed in Jun. 3, bonded with a thermally conductive adhesive to a respective
1993 of L. CUTTING ET AL for FLEXIBLE CIRCUIT major surface of the spreader.
BOARD AND COMMON HEAT SPREADER ASSEM- U.S. Pat. No. 5,179,501 to Ocken, and U.S. Pat. No.
BLY. 5,159,751 to Cottingham disclose bonding flexible circuit
... 10 boards directly to one side of an aluminum plate. In Ocken,
FIELD OF INVENTION heat Is conducted through the circuit board to a first heat sink
The present invention relates to multi-layer substrates for plate then to a second heat sink plate bonded to the first
flexible circuit boards and flexible circuit board assemblies plate. "Concept for Forming Multilayer Structures for Elec
and their methods of manufacture. More particularly this tronic Packaging" (Curtis), IBM Technical Disclosure Bul
invention relates to a multi-layer flexible circuit board 15 letin Vol. 30, August 1987, pp. 1353-1356 and U.S. Pat. No.
substrate for attaching components including chips and heat 4,811.165 to Currier, disclose folding a flexible circuit board
spreaders to form a three-dimensional circuit board assem- around a heat sink plate and bonding the board to both sides
bly. of the plate. In both citations, before bonding the board to
the plate, components are connected to the side of the board
BACKGROUND 20 wnich is not connected to the plate. 'Integral Heatsink
All levels of packaging, (chip, electronic circuit assembly Printed Circuit Board" (Askalv), IBM Technical Disclosure
and systems) are becoming more miniaturized. A major Bulletin Vol. 25, December 1982 p. 3606, discloses a flexible
technical challenge is thermal dissipation as power density circuit board adhesively bonded to a heat sink of steel or
increases. Typically, heat spreaders or heat sinks are bonded aluminum.
to microelectronic components and chips, individually. 25 U.S. Pat. No. 5,168,430, discloses a portion of a flexible
Occasionally, a common heat sink may be bonded to mul- circuit board applied to a heat sink plate and provided with
tiple chips on co-planer top surfaces. In many applications, a cutout in which a hybrid circuit structure is cemented to the
space limitations and cost do not allow the use of heat sinks plate. The hybrid circuit structure is wirebonded to the
or co-planer electronic assemblies. flexible circuit board.
Flexible circuit boards offer advantages of light weight, U.S. Pat. No. 4,834,660 to Cottiat, FIG. 9, shows a circuit
thinness, three dimensional configuration, and flexibility. board interconnection system in which layers of a flexible
Space and height constraints for circuit packaging may circuit board are removed in part of a bend to expose a
dictate the use of flexible circuits over rigid laminates and selected wiring layer for connection. The flexible circuit
very low profile chips and packages over high profile chip 35 board is of copper clad polyimide.
modules. All the above citations are hereby incorporated by refer
U.S. Pat. No. 3,781,596 discloses a single layer intercon- ence. nection structure of metallic conductors on a polyimide film
(e.g. KAPTONTM by E. L by DuPont de Numers). U.S. Pat OBJECTS OF THE INVENTION
No. 3,868,724 discloses metallic conductors sandwiched 40 A , ,. x . it_ . ^ . ^ ..
between polyimide film which project through the film. . A &enff °b->ect of, *e TMTt" P
., , , ..,.„„„ „ improved flexible circuit board substrate.
Rigid-Flex circuit boards are described in U.S. Pat. No. , ,. . , . . . ., ,
5,121,297 to Haas and U.S. Pat. No. 5.144,742 to Lucas. In u obJect f to uwentm is10, P10^ enhanced
those patents single layer flex circuit boards are integrally thermal operation of flexible circuit boards with components
connected between rigid circuit boards. In both patents 45 mounted t0 both sldes
multiple rigid circuit board layers are laminated to a flexible Another object of the invention is to provide a method and
circuit board substrate and components are attached only apparatus to produce a flexible circuit board with compo
onto the rigid circuit board sections. nents mounted on screened paste on both sides of the circuit
Connecting components directly to one side of a single- board- Components include wire bond, flip chips, SM
layer flexible circuit board is described by McBride, "Multi- 50 (surface mount) active discreates, and PIH (pin-in-hole).
function Plug for IC Package", IBM Technical Disclosure A further object of the invention is to provide an improved
Bulletin Vol. 21, February 1979, pp. 3594-3595. I/O termi- heat sink structure for flexible circuit boards, nals on the bottom of a chip are soldered to pads on top of
a thin polyimide flexible deed. Also, a depression in a heat BRIEF DESCRIPnON OF THE DRAWINGS
sink cover is bonded onto the top of the chip. Connecting 55 nG ^ is ess flow ^ of a fflc embodiment
lower power chips to a bottom side of a multi-layer flexible of ^ gs for formin a board ... of the
circuit board and high power chips to the top side of the invention
flexible circuit board in order to connect the high power ', . . .„ ...
chips to the module cap is suggested by McBride, "Multi- mG-* •18 a. thematic cross section of a specific embodi
layer Flexible Film Module", IBM Technical Disclosure 60 ment of me cffcmt board substrate of the mvention showing
Bulletin Vol. 26, May 1984, p. 6637. In that article I/O pins *e of stdfer we3S and thumer' more
connect the flexible film to a metalized ceramic substrate, exl e 31035'
and smaller pins interconnect the layers of the film. FIG. 3 is a schematic cross section of a part of the
Schrottke, "Removal of Heat from Direct Chip Attach embodiment of FIG. 2 showing the plated through holes and
Circuitry", IBM Technical Disclosure Bulletin Vol. 32, Sep- 65 windows in the stiffer areas;
tember 1989, pp. 346-348 describes a flexible circuit board FIG. 4 is a process flow diagram of a specific embodiment
with two rows of Direct Chip Attach (DCA) chips attached of the initial process for mounting components to the front