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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to printed wiring boards and more particularly, to
printed wiring boards utilizing metal substrates.
2. Description of the Prior Art
In both additive and subtractive techniques of printed circuit manufacture,
a great variety of base materials have been employed as an insulating
support. There is great interest in using thin metal blanks, e.g., 1 to 7
mils in thickness, which are coated with dielectric material, for flexible
printed wiring boards. In the past, thicker, non-flexible metal blanks,
e.g., 16 to 32 mils in thickness, have been coated with dielectric
materials using a pre-heated substrate and a fluidized bed powder coating
process or an electrostatic coating process. Both of these techniques have
a disadvantage where thin (1 to 7 mils thick), flexible metal blanks are
contemplated which contain through-holes. If such a thin metal blank is
coated using known techniques, the dielectric coat obtained either does
not provide adequate through-hole edge coverage, or, if it does provide
adequate edge coverage, the resultant coating is too thick or the surface
topography thereof is too rough and not useable, for practical purposes.
If poor dielectric edge coverage is obtained, then a short will likely
take place in the resultant printed wiring board between the metal blank
or substrate and the conductive circuit pattern formed on the dielectric
coat. If the dielectric coat is too thick, the flexibility of the
resultant circuit suffers as well as results in an increase in material
costs. Also, where the topography of the surface is rough and uneven, it
is very difficult to print or stencil either a conductive pattern or a
resist pattern thereon. Also, upon metallization of such a rough surface,
the metal deposit obtained will have inherent mechanical stress therein
resulting from rough topography.
A process which yields a flexible metal printed wiring board having a
dielectric coated surface having good edge coverage as well as desirable
topography properties is needed and is desired.
SUMMARY OF THE INVENTION
This invention relates to printed wiring boards and more particularly, to
printed wiring boards utilizing metal substrates.
The method comprises forming a metal substrate having a through-hole. The
metal substrate is coated with a dielectric powder having a suitable flow
length value to form a dielectric coat thereon having a sufficient
through-hole edge coverage.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more readily understood by reference to the
following drawing taken in conjunction with the detailed description,
wherein:
FIG. 1 is a cross-sectional view of a portion of a printed wiring board
having a dielectric coating and a conductive layer thereon;
FIG. 2 is a graph plotting glass plate flow length in centimeters with
temperature in degrees centigrade;
FIG. 3 is a block diagram of an assembly line for manufacturing printed
wiring boards in accordance with the invention;
FIG. 4 is a plot of edge coverage in percent in relation to substrate
thickness in mils; and
FIG. 5 is a fragmentary view of metal substrates for printed wiring boards
as blanked.
DETAILED DESCRIPTION
Referring now to FIG. 1, there is shown a cross-sectional view of a portion
of a flexible printed wiring board 50 fabricated using conventional
techniques, such as a fluidized powder bed or electrostatic coating
technique. Board 50 has a metal substrate 51, typically ranging from about
1 to about 7 mils in thickness, with a circuit or through-hole 52. A
dielectric coat 53 is formed on substrate 51. Deposited on coat 53 is a
conductive layer 54 representing a portion of a circuit pattern. In
fabricating board 50, using conventional techniques (fluidized bed,
electrostatic powder coating), it has been found that the edge coverage
53(a) of coat 53 (from an edge of through hole 52 to conductive layer 54)
is unsatisfactory in that it is too thin thereby leading to dielectric
breakdown and electrical short circuiting of board 50. It has been found
that dielectric coat 53 should have an edge coverage 53(a) having a
thickness of at least about 40 percent of the principal thickness of coat
53. By "principal thickness" is meant the average thickness of dielectric
coat 53 covering a principal surface 55 of metal substrate 51. Such an
edge coverage is achieved by initially coating a metal substrate with a
suitable dielectric powder having specific liquid flow or melt flow
properties upon a melting thereof. Specifically, an adequate edge coverage
is achieved by coating a metal substrate, as by an electrostatic
technique, with a thermosetting dielectric powder having a suitable flow
length value at a particular temperature, e.g., a glass plate flow length
value of from about 0.1 cm. to about 0.65 cm. at 150.degree. C. The glass
plate flow length value is the value obtained with a procedure where 0.2
gm. of the particular dielectric powder is pressed into a 0.6 cm. diameter
circular pellet (typically 8.+-.1 mm. high) at a pressure of 2,000 pounds
for ten minutes using a conventional pelleting apparatus. A typical
conventional pelleting apparatus includes a chrome-plated steel
cylindrical body (about 50 mm. in length) having a 0.6 cm. diameter round
central aperture passing therethrough. Capping the lower end of the
aperture is a base having a cylindrical steel rod (0.6 cm. in diameter by
about 12.7 mm. in height) which is within the central aperture. The powder
to be pelletized is maintained within the capped central aperture and a
cylindrical top pressure or tamping rod (having a diameter of 0.6 cm. and
a length of about 35 mm.) is inserted, under pressure, into the top of the
powder loaded aperture to yield the desired pellet. The pellet is then
placed at a 45.degree. angle on a hospital grade glass microscope plate
(as sold by Fisher Company and designated as "Fisher Brand Microslide"),
maintained in an oven at a particular temperature, e.g., 150.degree. C.
The pellet at first melts and tends to flow until it sets again. The flow
length is the total length of flow observed for the pellet plus the
remaining pellets diameter minus the pellet's initial diameter (0.6 cm.)
The flow length value is determined at any temperature between the melting
point and the decomposition temperature of the thermosetting powder.
Reference in this regard is made to FIG. 2 which illustrates the range of
suitable glass plate flow length values which the thermosetting dielectric
powder should have, e.g., 0.05 to about 0.35 cm. (100.degree. C.); about
0.05 to about 0.50 cm. (125.degree. C.); about 0.1 to about 0.65 cm.
(150.degree. C.); about 0.175 to about 0.70 cm. (180.degree. C.); about
0.30 to about 0.80 cm. (225.degree. C.); about 0.35 to about 0.90 cm.
(250.degree. C.). Suitable glass plate flow lengths for any temperature
can be determined from the plot of FIG. 2, the ranges thereof being
bounded by lines A and B thereof.
Referring to FIG. 3, there is shown a continuous in-line fabrication line
60 having a coil of sheet metal 61 suitable to use as a substrate for a
metal printed wiring board. Some suitable sheet metals include sheet
steel, copper, aluminum and alloys of the foregoing. For the fabrication
of flexible printed circuits the thickness of sheet metal ranges from 1 to
7 mils. If the sheet metal is less than 1 mil, there is a mechanical
difficulty in handling the sheet metal leading to creasing thereof, as
well as difficulty in defining the printed wiring substrates such as by
stamping. Also, the resultant printed wiring substrate which would be a
poor support for the ultimate dielectric coating and metallization, as
well as being of poor dimensional stability. If the sheet metal is greater
than 7 mils, then it would not be practical for a flexible printed circuit
board since the flexibility is lost or greatly diminished with boards
having a metal substrate in excess of 7 mils in thickness. It is to be
noted hereat that metal substrates of up to 7 mils in thickness inherently
have poor edge coverage of their through-holes unless the subject
invention is employed. Reference is made to FIG. 4 which is a plot of
various sheet steel (C1010 steel) substrates which have been
electrostatically coated with a thermosetting epoxy powder which did not
have the requisite viscosity or glass plate flow length value. The powder
had a glass plate flow length value of 0.7 cm. at 150.degree. C. As can be
seen from FIG. 4, a metal substrate up to 10 mils in thickness does not
get adequate edge coverage. This inadequacy is due to the fact that in
relatively thin metal substrates (1 to 10 mils thick), the surface tension
effect dominates upon the melting of the coating powder. If the powder
does not have the desired fluid characteristics, the surface tension
effects at the sharp edges of the through-holes predominates and causes
the melt (molten powder) to pull away therefrom, thereby leading to a poor
edge coverage.
Referring back to FIG. 3, the coil of sheet metal 61 is initially fed into
any coventional straightener, illustratively a roller type stock
straightener 62. The rolls of the metal straightener 62 pull sheet 61 from
the coil and after straightening, sheet 61 passes into any conventional
apparatus 63, illustratively an electron discharge machining apparatus,
for forming a plurality of metal substrates having a desired pattern of
circuit or through-holes. Other apparatus 63 which may be employed include
drilling, punch press, chemical milling and electrochemical machining
apparatus. Referring to FIG. 5, apparatus 63 (FIG. 3) shapes sheet 61 into
a plurality of desired substrate configurations 64, while leaving
interconnecting tabs 65 between the individually configured substrates 64
to retain the substrates 64 in sheet or tape form. Apparatus 63 (FIG. 3)
also forms a pattern of apertures or through-holes 70 for through-hole
connections.
Referring back to FIG. 3, the blanked metal sheet 61 is passed through a
conventional degreaser 66. Degreaser 66 may be of the spray type which
includes a compartment for a vapor type degreaser such as, for example,
trichloroethylene; a tank containing a rust removal chemical such as, for
example, a solution of hydrochloric acid, a first water rinse compartment
for rinsing off hydrochloric acid; a second tank containing an alkaline
solution such as, for example, that sold under the trademark Oakite 190; a
second water rinse compartment for rinsing away the alkaline solution; a
third tank containing a zinc-phosphate solution which may be, for example,
that sold under the trademark Phosdip R2; and a second degreaser
compartment where the blanked metal sheet is degreased and vapor dried
with, for example, trichloroethlyene. The blanked sheet metal is subjected
to the degreasing vapor for about 5 to 10 minutes, the rust removal
chemical until the rust or metal oxide, if any, is removed, the alkaline
solution for about 3 to 5 minutes, the zinc phosphate solution for about 6
minutes, and the drying vapor until dry.
The blanked sheet metal 61 is next prepared for electrostatic powder
coating by drawing the metal sheet 61 through a cleaning station 67 where
it is cleaned, as for example with aqueous phosphoric acid, washed and
then dried in an oven 68 which may be an infrared type oven, typically
maintained at 150.degree. C. The dried blanked sheet metal 61 is then
inserted into a powder booth or bed 69. The booth or bed 69 is equipped
with either conventional manually operated or automatic powder coating
guns for electrostatically powder coating the blanked sheet metal 61 to
obtain a dielectric coat 71 thereon. Some typical conventional
electrostatic powder spraying apparatus and techniques are described in
Fundamentals of Powder Coating, Society of Manufacturing Engineers, 1974.
It is again to be pointed out and stressed hereat that unless a suitable
thermoset powder is employed in the electrostatic coating process an
unsuitable dielectric coat through-hole edge coverage, such as that shown
at 53(a) in FIG. 1, will be obtained. Such a suitable powder is one having
critical fluid characteristics upon melting as reflected by a suitable
glass plate flow length value, e.g., 0.1 cm. to 0.65 cm. at 150.degree. C.
If the powder employed has, at a particular temperature, a flow length
value greater than the suitable flow length value as indicated in FIG. 2,
little or no through-hole edge coverage is obtained. If on the other hand
the powder employed has a flow length value less than the suitable flow
length value within the bounded area of FIG. 2 (at the particular
temperature), adequate edge coverage is obtained, but the resultant
dielectric surface is much too rough and irregular for purposes of a
printed wiring board.
Any thermosetting powder can be employed which exhibits the required glass
plate flow value. Some suitable dielectric powders which may be employed
are epoxy resin powders, polyesters, acrylics, etc.
Dielectric coat 71 has a thickness sufficient to impart the degree of
insulation desired without imparting a rigidity which destroys the
flexibility. For flexible printed wiring boards, having a metal substrate
of 1 to 7 mils, a total dielectric coat having a thickness of up to 7 mils
per principal surface of the metal substrate can be accommodated before
there is a rigidity obtained which destroys the use of such a composite as
a flexible printed wiring board. For flexible printed wiring boards, a
total dielectric coat, comprising a single layer or a plurality of
discrete layers, having a thickness on each principal surface in excess of
7 mils imparts an undesirable rigidity and an additional undesirable cost.
Typically, dielectric coat or layer 71 has a thickness of about 3 to 4
mils.
After powder coating, the dielectric coated blanked sheet metal 61 is
heated in an oven 72 at a temperature and for a time period sufficient to
fuse the resultant dielectric coating 71 on the metal sheet 61. For a
thermosetting resin, such as an epoxy resin, the sheet 61 and dielectric
coat 71 are heated in oven 72 at a temperature sufficient to fuse the
powder and cure the resin to the "B" stage. Typically, for an epoxy resin,
sheet metal 61 and coat 71 are heated at 240.degree. C. for one minute.
The resultant fused and partially cured dielectric coat 71 insulates the
metal substrates of sheet metal 61 from the electrical circuitry destined
to be formed on the top surface of the resultant printed wiring board.
Where a relatively smoother dielectric surface is required, the sheet metal
61 having dielectric layer 71 thereon is further treated with a second
powder having a melt flow characteristic to impart the desired smoothness
to the dielectric coat 71. A suitable second powder is one having for
example a glass plate flow length value at 150.degree. C. of about 1.2 cm.
to about 1.5 cm. Typically the second powder has a glass plate flow length
in excess of the ranges illustrated in FIG. 2. In this regard, it is to be
pointed out that the second powder may comprise a resin composition which
is the same as the first powder, but which has been treated in a different
fashion, e.g., it has not been aged or it has been cured to a lesser
extent, or the second powder may comprise a different resin composition
(thermosetting or thermoplastic) than the first powder. It is of course to
be understood that suitable second powders are easily ascertained by one
skilled in the that art to impart the smoothness desired or required.
The dielectric coated sheet metal 61 is directly passed from oven 72 into a
second electrostatic powder booth or bed 73. A smooth topcoat 74,
comprising the second powder material, is obtained on dielectric coat 71.
The thickness of topcoat 74 is only enough to impart the required
smoothness to dielectric layer or coat 71. Typically, for a 1 to 7 mils
thick steel substrate having a 3 to 4 mils thick dielectric layer on each
principal surface, the topcoat is about 1 to 2 mils thick on each
principal surface.
The resultant top coated sheet metal 61 is then passed into an oven 76
where the top coat 74 is fused to form a continuous coating and to obtain
a full cure of any thermosetting resin which may be employed in coat 71
and/or 74. Typically, where an epoxy resin is employed in forming coat 71
and/or 74, the top coated sheet metal 61 is heated in oven 76 at a
temperature of 240.degree. C. for 3 to 5 minutes. By a full cure is meant
that the resultant cured resin has been optimized, to the extent possible,
with respect to electrical properties, mechanical properties and chemical
resistance, i.e., with respect to criteria which are well known in the art
and are easily ascertainable experimentally by one skilled in the art.
Where an epoxy resin is cured, a full cure typically means that the epoxy
or oxirane groups initially present have been consumed during the curing
and the degree of crosslinking provides optimum physical properties for a
desired application.
The resultant insulated sheet metal 61 is then passed through standard
metallization processes to achieve selective metallization in the form of
a desired electrical circuit pattern. In this regard, it is to be noted
that an electroless metal catalyzing species, e.g., palladium metal, may
be incorporated into topcoat 74 to form a catalytic layer which can be
metallized. However, any conventional metallization technique may be used
such as vapor plating, electroless plating, vacuum plating, etc.
Typically, the surface of topcoat 74 is subjected to a conventional
electroless metal deposition sequence in which a catalytic species is
deposited thereon which catalyzes the reduction and deposition of a metal
from an electroless metal deposition solution. The surface of topcoat 74
may be blanket metallized followed by subtractive patterning thereof or
may be selectively metallized to achieve the desired conductive circuit.
Some typical suitable processes are disclosed by I. B. Goldman, Plating,
January 1974, pages 47 through 52, incorporated hereinto by reference.
Another metallizing technique involves depositing a conductive ink or
adhesive on the surface of topcoat 74 followed by photoresist masking and
electroless metallization thereof. U.S. Pat. No. 3,934,334, incorporated
hereinto by reference, discloses one such process.
EXAMPLE I
A. A powder composition was prepared comprising (1) 49.5 weight percent of
a diglycidyl ether of bisphenol A having an epoxide equivalent weight of
750, a Durran's softening point of 92.degree. C. ("EPON 2001" obtained
from Shell Chemical Company); (2) 7.6 weight percent of a diglycidyl ether
of bisphenol A having an epoxide equivalent weight of 2000 to 2500, and a
Durran's softening point of 125.degree. to 135.degree. C. ("EPON 1007"
obtained from Shell Chemical Company); (3) 7.6 weight percent of a
brominated diglycidyl ether of bisphenol A containing about 21.+-.2
percent bromine, having an epoxide equivalent weight of 455 to 500 and a
Durran's softening point of 70.degree. to 80.degree. C. ("ARALDITE 8011"
obtained from Ciba Products Company); (4) 11.4 weight percent of a
brominated carboxyl terminated acrylonitrilebutadiene copolymer having a
Brookfield viscosity of 550 to 750,000 cps at 27.degree. C. and 26 to 28
weight percent acrylonitrile ("HYCAR CTBN 1300X13" obtained from B. F.
Goodrich Co.); (5) 1.0 weight percent red iron oxide; (6) 5.0 weight
percent antimony oxide; (7) 14.0 weight percent titanium oxide; (8) 2.3
weight percent of an amine curing agent ("D.E.H.-40" obtained from the Dow
Chemical Company; and (9) 1.5 weight percent of dicyandiamide.
A 3 inch by 6 inch by 1 mil thick steel (C1010 steel) foil having electron
discharge machined through holes therein was degreased by immersion for 3
minutes in a 1,1,1-trichloroethane bath maintained at 71.degree. C. The
degreased foil substrate was then cleaned in a 8.2 weight percent aqueous
H.sub.3 PO.sub.4 solution maintained at 71.degree. C. for 3 minutes. The
substrate was then dried in a convection oven maintained at 150.degree. C.
for 3 to 5 minutes.
The prepared powder was aged at 55.degree. C. for about 8 hours to yield a
powder having a glass plate flow length value of about 0.4 cm. at
150.degree. C. The dried substrate was then powder coated with the
prepared and aged powder composition using a conventional manual powder
coating apparatus (GEMA Type 710 spray apparatus) to yield a 6 to 10 mils
thick, unfused powder coat. The powder coated substrate was heated in an
infrared oven at 240.degree. C. for one minute whereby a 3 to 4 mils thick
fused powder coating was obtained on each principal substrate surface. The
resultant coating gave an adequate edge coverage of the through hole as
evidenced by microscopically examining the cross-sectional area thereof at
a magnification of 100. Pictures of the cross-sectional area of the coated
through hole were also taken and the edge coverage measured therefrom was
at least 40% of the principal thickness of the coating, that is of the
principal surface coating thickness.
B. The procedure of Example I-A was repeated except that a topcoat was
applied to the coated surface. A powder formulation was prepared which
comprised (1) 25 weight percent of a diglycidyl ether of bisphenol A
having an epoxide equivalent weight of 750 and a Durran's softening point
of 92.degree. C. ("DRH 201" obtained from Shell Chemical Company; (2) 25.2
weight percent of a brominated epoxy resin having an epoxide equivalent to
600 to 750, having a Durran's softening point of 90.degree. to 100.degree.
C. and containing 42% bromine ("EPI-REZ 5183" obtained from Celanese
Corporation); (3) 12.6 percent of "EPON 1007" obtained from Shell Chemical
Company; (4) 12.6 weight percent of a diglycidyl ether of bisphenol A
having an epoxide equivalent weight of 2500 to 4000 and a Durran's
softening point of 145.degree. to 155.degree. C. ("EPON 1009" obtained
from Shell Chemical Company); (5) 10.1 weight percent of a brominated
carboxyl terminated acrylonitrile-butadiene copolymer having a Brookfield
viscosity of 550 to 750,000 cps at 27.degree. C. and 26 to 28 weight
percent acrylonitrile ("HYCAR CTBN 1300X8" obtained from B. F. Goodrich
Company); (6) one weight percent of red iron oxide; (7) 10 weight percent
of antimony oxide; (8) 1.8 weight percent of an amine curing agent
("D.E.H. 40" obtained from the Dow Chemical Company); (9) 1.2 weight
percent of dicyandiamide; and (10) a 0.05 weight percent of Resimix P flow
aid obtained from Mohawk Industries. The prepared powder had a glass plate
flow length of about 1.2 cm. at 150.degree. C. The prepared powder was
applied with the apparatus of Example I-A to form an unfused topcoat 3 to
5 mils thick on each principal substrate surface. The resultant composite
was then fired in an infrared oven at 240.degree. C. for 3 to 5 minutes to
fully cure both powder coatings and to yield a smooth topcoat having a
thickness of 1 to 2 mils on each principal surface.
EXAMPLE II
The procedure of Example I-A was repeated except that the first powder
coating was obtained using a powder formulation comprising (1) 21 weight
percent of "EPON 2001" obtained from Shell Chemical Company; (2) 7.6
weight percent of "EPON 1007" obtained from Shell Chemical Company; (3)
15.2 weight percent of "EPON 1009" obtained from Shell Chemical Company;
(4) 21 weight percent of "EPI-REZ 5183" obtained from Celanese
Corporation; (5) 11.4 weight percent of a brominated carboxyl terminated
acrylonitrile-butadiene copolymer ("HYCAR CTBN 1300X9" obtained from B. F.
Goodrich Company); (6) 1.82 weight percent of an amine curing agent
("D.E.H. 40" obtained from the Dow Chemical Company); (7) 1.22 weight
percent of dicyandiamide; (8) 5 weight percent of antimony oxide; (9) 14
weight percent of titanium dioxide; and (10) 2.0 weight percent of an
inorganic pigment ("10334 Brite Blue" obtained from Drakenfeld/Hercules
Incorporated). The powder was aged at 55.degree. C. for about 12 hours to
obtain an aged powder having a glass plate flow length of about 0.1 cm. at
150.degree. C.
A 3 to 4 mil fused coating of the powder was obtained on the principal
surfaces of the substrate. A second powder comprising the unaged formula
was then applied by the same apparatus to the coated surface and heated at
240.degree. C. for 3 to 5 minutes to fully cure both powder coatings and
to obtain a smooth topcoat having a thickness of 1 to 2 mils. Again
microscopic and photographic examination of the cross section of the
through holes indicated that an edge coverage of at least 40% of the
principal surface coating thickness was obtained.
EXAMPLE III
The procedure of Example I-A was repeated except that the dielectric coat
was obtained from a powder formulation comprising (1) 20.1 weight percent
of "EPON 2001" obtained from Shell Chemical Company; (2) 3.6 weight
percent of "EPON 1007" obtained from Shell Chemical Company; (3) 7.3
weight percent of "EPON 1009" obtained from Shell Chemical Company; (4)
5.5 weight percent of "HYCAR CTBN 1300X13" obtained from B. F. Goodrich
Company; (5) 2 weight percent of red iron oxide; (6) 30 weight percent of
alumina trihydrate (Alcoa Hydral 705); (7) 30 weight percent of alumina
trihydrate (Alcoa Hydral 710); and (8) 1.5 weight percent of an
accelerated amine type curing agent which has an amine-nitrogen content of
54.5 to 57.5 percent by weight ("P-108" obtained from Shell Chemical
Company). The mixture had a glass plate flow length of 0.15 cm. at
150.degree. C. Essentially the same edge coverage results as in Example
I-A were obtained.
It is to be understood that the above-described embodiments are simply
illustrative of the principles of the invention. Various other
modifications and changes may be made by those skilled in the art which
will embody the principles of the invention and fall within the spirit and
scope thereof.
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