EMI protection and CTE control of three-dimensional circuitized substrates

A circuit board includes a substantially non-conductive substrate and first and second rigid sheets. The first sheet forms a grid pattern substantially encapsulated by the substrate, and a portion of the first sheet extends beyond a boundary of the substrate to form a first interconnection terminal. The second sheet is also substantially encapsulated by the substrate and has a portion which extends beyond the boundary of the substrate to form a second interconnection terminal. The second sheet acts as an electromagnetic interference shield, and also has a coefficient of thermal expansion less than a coefficient of thermal expansion of the substrate.

TECHNICAL FIELD 
This invention relates to molded circuit boards and, more particularly, to 
a three-dimensional molded circuit board having an in-molded 
electromagnetic interference shield. 
BACKGROUND ART 
Conventional planar printed wiring boards (PWBs) have been replaced by 
three-dimensional molded circuit boards in many applications, such as in 
cellular telephones, pagers and computers. The three-dimensional circuit 
boards act as a substrate for the metallized circuitry, plated 
through-holes and electronic components which are subsequently mounted 
onto the circuitry. 
These molded circuit boards are normally comprised of polymeric materials 
such as polyetherimide (PEI), acrylonitrile-butadiene-styrene (ABS) and 
polypropylene (PP), which have large coefficients of thermal expansion 
(CTE) relative to the metallization and the electronic components. This 
difference in CTE can degrade the long term reliability of the final 
electronic assembly when exposed to repetitive thermal variations. The 
molded circuit board is also susceptible to electromagnetic interference 
(EMI) which affects the function of the circuitry. 
SUMMARY OF THE INVENTION 
The present invention is a circuit board comprising a substrate, electrical 
circuitry disposed on a surface of the substrate, and a rigid sheet 
substantially encapsulated by the substrate. The rigid sheet shields the 
electrical circuitry from electromagnetic interference generated on a side 
of the sheet opposite from the electrical circuitry. Optionally, a portion 
of the sheet extends beyond a boundary of the substrate to form an 
interconnection terminal. 
In another embodiment of the invention, the circuit board comprises a 
substrate and first and second rigid sheets. The first and second rigid 
sheets are substantially encapsulated by the substrate. A portion of the 
first rigid sheet extends beyond a boundary of the substrate to form an 
interconnection terminal. 
Accordingly, it is an object of the present invention to provide a circuit 
board of the type described above which protects the electronics on or 
near the circuit board when exposed to repetitive thermal variations. 
Another object of the present invention is to provide a circuit board of 
the type described above which protects the electronics on or near the 
circuit board from electromagnetic interference. 
Another object of the present invention is to provide a circuit board of 
the type described above for use in environments such as motor vehicles 
where EMI protection is particularly desirable. 
These and other objects, features, and advantages of the present invention 
are readily apparent from the following detailed description of the best 
mode for carrying out the invention when taken in conjunction with the 
accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to the drawings, the preferred embodiments of the present 
invention will be described. FIGS. 1 and 2 show a molded circuit board 10 
according to the present invention. The circuit board 10 includes a 
three-dimensional substrate 12 and first, second and third sheets 14, 16 
and 18, respectively. 
The substrate 12 is substantially non-conductive, preferably formed from a 
plastic such as PEI, ABS or PP. The first sheet 14 is preferably a rigid, 
conductive material such as a relatively heavy gauge metallic sheet stock 
mesh stamping or etching. The first sheet 14 is substantially encapsulated 
by the substrate 12, except for an edge connector portion or pin 20 that 
extends beyond the lateral boundary of the substrate defined by the side 
wall 22. Like the first sheet 14, the third sheet 18 is preferably a 
rigid, conductive material such as a heavy gauge metallic mesh. The sheets 
16 and 18 are also substantially encapsulated by the substrate 12, except 
for respective edge connector portions or pins 24 and 26 that extends 
beyond the side wall 22 of the substrate. The sheets 14, 16 and 18 are 
preferably molded into the substrate 12 during the injection molding, 
thermoforming, blow molding, lamination or other molding process. 
The sheets 14, 16 and 18 are formed in similar, although not necessarily 
identical, grid patterns. Through-holes 28 drilled or otherwise formed in 
the substrate 12 and plated with a conductive material electrically 
connect a plurality of locations 30, 32 and 34 on the first sheet 14 with 
a first or upper surface 36 of the substrate. Various electrical surface 
circuitry 38 mounted on the upper surface 36 communicates via the 
through-holes 28 with the first sheet 14 at the locations 30, 32 and 34. 
Locations 40, 42, 44 and 46 of the third sheet 18 are likewise in 
electrical communication with electrical components 48 on a lower surface 
50 of the substrate. Optionally, electrical components such as an small 
outline integrated circuit package 52 bonded with a thermal adhesive 54 or 
otherwise mounted on the upper surface 36 of the substrate also 
communicate with the sheet 18. 
The pins 20, 24 and 26 are integrally formed with respective layers 14, 16 
and 18, and are disposed within a connector shroud or housing 56 molded in 
the substrate 12. The pins 20, 24 and 26 are adapted to form terminals for 
physical and electrical interconnection with an external connector. For 
example, the sheet 14 may be designed as a power or signal plane, and the 
sheet 18 may be designed as a ground plane. Thus, a wide variety of 
three-dimensional circuit paths are available on the circuit board 10, 
either alone or in conjunction with other circuits mating with the board 
10 through the pins 20 and 26. For example, specific through-holes 28 are 
only joined with the ground plane 18, while other through-holes 
interconnect with the power layer 14. The layers 14 and 18 can be designed 
prior to fabrication, or a generic mesh layer pattern can be machined via 
drilling to define the circuit layout after molding to provide defined 
circuit paths between components, through-holes and terminal pins. 
The unconnected sheet 16 acts as a shield to inhibit the EMI susceptibility 
of the circuit board 10, and particularly the sheets 14 and 18 and the 
surface circuitry 38. To achieve this effect, the shield 16 can be either 
a metal stamping, an etched metal sheet or a woven metallic screen of a 
given thickness, patterning and three-dimensional shape. The shield 16 can 
be sized to protect the entire board 10 or specific areas of a board. 
To provide the further advantage of controlling the CTE of the circuit 
board 10, the shield 16 is preferably comprised of a material having high 
EMI attenuating characteristics with a low CTE relative to the substrate 
material. In addition, the material thickness of both the metal layers and 
the dielectric substrate material between layers is controlled to achieve 
the desired degree of CTE control. Copper, a copper-beryllium alloy or a 
carbon fiber composite are materials suitable for this purpose. For 
example, ABS normally has a CTE ranging from 6-13.times.10 -5 ppm/C, while 
copper normally has a CTE of approximately 17.6.times.10 -6 ppm/C. 
Alternatively, the EMI shield 16 can be formed by molding a plated mylar 
film with a grid or mesh pattern in situ in the substrate 12. Prior to the 
in-molding, wire mesh is ultrasonically embedded in the film to form a 
laminated film stock. 
As an alternative to molding the shield 16 in situ, a first 
three-dimensional molded plastic substrate is plated with an EMI mesh, and 
then the mesh is over molded in a second molding operation to produce a 
final part having the plated EMI shield completely embedded in bulk of the 
substrate 12. The part can then be drilled, photoimaged and plated to 
create circuitry and plated through-holes in the substrate. 
Integrated into the bulk of the molded substrate, the metal shield 16 acts 
to constrain the CTE of the plastic and provide EMI protection to the 
circuitry and components located on the side of the shield opposite of the 
EMI source. Controlling the difference in CTEs between the substrate 12 
and the metallization and the electrical components improves the 
reliability of the circuit board 10. 
It should be understood that while the forms of the invention herein shown 
and described constitute preferred embodiments of the invention, they are 
not intended to illustrate all possible forms thereof. For example, 
additional molded-in shields or layers with independent interconnect pins 
to the EMI shield, signal and power planes can be provided. It should also 
be understood that the words used are words of description rather than 
limitation, and various changes may be made without departing from the 
spirit and scope of the invention disclosed.