Laminated synthetic mica articles

Laminated synthetic mica articles are disclosed that have particular applicability as substrates in electronic components. The articles comprise a core of stacked and consolidated, synthetic mica sheets that are impregnated with resin, and a copper foil sealed to a face on the article. Adherence of the copper foil is enhanced by use of a micaceous facing layer and bonding sheet, both being resin-impregnated and the resin cure being advanced beyond the normal B-stage level before assembly. Insulation resistance is enhanced by providing an advanced resin coating on the copper foil before assembly. Use of a bismaleimide triazine and epoxy mixed resin is also disclosed whereby a transformation temperature (Tg) of about 200.degree. C. is obtained.

BACKGROUND 
This invention relates to a laminated article comprising a core portion 
formed from sheets of mica paper. The sheets are composed primarily of 
synthetic mica platelets, and are impregnated with an organic resin. A 
plurality of such impregnated paper sheets are stacked, and the stack 
consolidated by pressing, to form the core. 
Synthetic mica materials, the processing of such materials to form gels, 
and the use of such gels to produce films, papers, boards, and the like, 
are disclosed in U.S. Pat. No. 4,239,519 (Beall et al.) 
U.S. Pat. No. 4,559,264 (Hoda et al.) discloses a substrate in which a 
plurality of synthetic mica papers are laminated together. Each paper is 
composed of overlapping, ion-exchanged platelets of a synthetic, lithium 
and/or sodium, water-swelling mica, and is impregnated with an organic 
resin. A laminated stack of such papers has a planar, isotropic 
coefficient of thermal expansion in the x-y plane which may be less than 
100.times.10.sup.-7 /.degree.C. over a range of 25.degree.-300.degree. C. 
When the substrate is used in forming an electronic component, it may be 
provided with conductive connections by subtractive or additive circuitry 
processes. For this purpose, a conductive metal foil, usually copper, is 
attached over a face on the core. It is a prime requisite that such copper 
foil strongly adhere to the core. This property, known as copper peel 
strength, is measured in lbs/inch, and is the force required to separate 
the copper foil from the core face. 
Another property of considerable interest is insulation resistance. This is 
measured in ohms and is the resistance of the core surface to passage of 
electric current. Moisture pickup is also a concern. While many tests are 
available, one commonly used is the weight gain in percent of a body after 
two hours immersion in boiling water. Dieletric constant and loss tangent 
are also commonly specified properties. 
A very low copper peel strength was observed when a copper foil was 
assembled directly against the face of a synthetic mica sheet stack and 
the assembly consolidated. To correct this situation, a glass fiber mat or 
fabric was interposed between the metal and the stack to act as a facing. 
This does tend to improve peel strength but, obviously, introduces an 
added step and a non-micaceous material. Both of these would desirably be 
avoided. 
There is a constant desire to increase the potential or maximum operating 
temperature of a component. Accordingly, a transition temperature (Tg) as 
high as 200.degree. C. may be specified for a core. Such temperatures were 
very difficult to attain with previously used epoxy resins. 
PURPOSES 
It is a basic purpose then to provide an assembly adapted to electronic 
component production and use, and including a laminated core composed of a 
stack of resin-impregnated, ion-exchanged, synthetic, sodium and/or 
lithium mica sheets. 
A specific purpose is to provide such an assembly wherein a copper foil is 
attached to a face on such a core and demonstrates a copper peel strength 
of at least 7 lbs/inch. 
Another purpose is to dispense with the need for glass fabric facings 
between the core and the copper foil in a laminated component. 
Another specific purpose is to provide a laminated board composed of 
resin-impregnated, synthetic mica sheets and having a transition 
temperature of at least 200.degree. C. 
A further purpose is to provide the transition temperature of at least 
200.degree. C. while maintaining other requisites such as flame 
resistance. 
Another specific purpose is to insure an insulation resistance of at least 
10.sup.8 ohms in a laminated board composed of resin-impregnated, 
synthetic mica sheets. 
SUMMARY OF THE INVENTION 
My invention contemplates a laminated article comprising a core, at least 
one copper foil covering a face on the core and a facing layer interposed 
between the copper foil and the core, the core being formed from a 
plurality of stacked mica paper sheets, each sheet being composed of 
overlapping synthetic mica platelets and being impregnated with an organic 
resin, and the facing layer being composed of a synthetic mica paper 
impregnated with resin, containing at least 50% chopped glass fibers and 
cured to a degree greater than ordinary B-staging but less than a complete 
cure. 
In one preferred embodiment, a bonding layer, composed of a 
resin-impregnated sheet of synthetic mica paper wherein the resin is 
advanced beyond normal B-staging, is assembled between the core and the 
facing. In another, the impregnating resin is a mixture where 50-70% of 
the solids content is a bismaleimide triazine resin and 30-50% is one or 
more brominated epoxies. 
The invention also comprehends a laminated article comprising a core, at 
least one copper foil covering a face on the core, the core being formed 
from a plurality of stacked mica paper sheets, each sheet being composed 
of overlapping synthetic mica platelets, and being impregnated with an 
organic resin, and the copper foil having its inside surface coated with a 
thin, relatively advanced resin layer. 
The invention further contemplates a method of improving the copper peel 
strength between a laminated core and a copper foil attached to a face on 
the core, the core being formed from sheets of mica paper, the mica sheets 
being composed of overlapping synthetic mica platelets impregnated with an 
organic resin, the method comprising forming a facing layer composed of a 
synthetic mica paper containing at least 50% chopped glass fibers and 
impregnated with resin, heating this facing layer at a temperature and for 
a time to advance the resin cure beyond the normal B-stage but short of a 
full cure, assembling the laminated core and the copper foil with the 
facing layer interposed between the foil and a face on the core, and 
applying heat and pressure to the assembly to effect consolidation. 
GENERAL DESCRIPTION 
The present invention is concerned with an article comprising a core 
member, such as a laminated board or substrate, composed of sheets of mica 
paper impregnated with an organic resin. The makeup and manner of forming 
such laminated bodies are disclosed in detail in the Hoda et al. and Beall 
et al. U.S. patents mentioned earlier. The entire disclosures of those 
patents are incorporated herein by reference to avoid duplication. 
Briefly, a selected mica is gelled, flocculated, washed and formed as thin 
sheets in conventional paper-making manner. The sheets are dried and 
impregnated with a resin material which is then cured. The impregnated 
papers are stacked and cut to form prepregs. These may be placed in a die 
and hot pressed at elevated pressures to form a consolidated body. 
Reference is made to the Hoda et al. patent for resin impregnating 
procedure. In general, a solution of resin in a solvent, such as acetone, 
is employed. The dried synthetic mica sheets are immersed in the solution 
and soaked for a time that may be up to an hour. The sheets are then 
removed, B-staged and laminated by pressing at a somewhat raised 
temperature. 
It is customary to cure the resin in the impregnated paper in two stages. 
In the first stage, referred to as B-stage, the resin is cured 
sufficiently to be stiff at ambient. However, it is still capable of flow 
under elevated temperature and pressure. In the final stage, C-stage, the 
resin becomes essentially rigid and resistant to flow. 
It is also common practice to adhere a thin foil of copper over the surface 
or face of the laminate to permit subsequent production of circuitry by 
either additive or subtractive processes. A tendency for the copper foil 
to peel away from the surface has created an adherence problem. This 
problem is a basic concern of the present invention. 
The copper peel tendency may be substantially corrected by insertion of a 
glass cloth facing between the copper and the laminate. However, this 
tends to introduce variable directional expansion coefficients, a 
condition wherein the expansion coefficients in the x-y axial planes are 
substantially different. This has an adverse effect on dimensional 
stability, and hence is undesirable. 
I have now found that the adherence of copper foil to a face on a 
laminated, synthetic mica core can be substantially enhanced by assembling 
a micaceous facing layer between the core and the copper foil before 
laminating. The facing layer is composed of synthetic mica paper 
containing at least 50% by weight chopped glass fibers and impregnated 
with organic resin. A key feature is curing of the resin to a relatively 
advanced stage prior to assembly of the article elements for lamination. 
"Relatively advanced resin" means the resin is cured to a degree greater 
than that obtained by the normal B-stage, but still short of the C-stage 
where it becomes rigid and non-flowing. Thus, it is cured to a point 
intermediate the normal B- and C-stages. 
The degree of cure is dependent on both time and temperature as well as on 
catalyst level. The normal B-stage schedule depends on the resin employed. 
A typical schedule is 4 minutes at 150.degree. C. In accordance with the 
present invention, the B-stage, partial cure may be advanced by increasing 
either time, temperature, or both. Thus, the time may be held constant at 
4 minutes while temperature may range up to 200.degree. C. with a 
bismaleimide-triazine resin. In contrast, the temperature may be held at 
150.degree. C. while the time is extended, e.g. to 8 minutes or longer. 
Optimum conditions will vary with the resins employed, and the usual 
experimentation will be necessary to determine them. In general, the 
time-temperature cycle of the advanced B-staging must provide a 
substantially improved copper peel strength. 
For optimum resistance to copper peeling, a further bonding layer may be 
interposed between the facing and the papers making up the core. It was 
found that separation could occur at the interface between the facing 
layer and the paper core rather than the interface between the copper and 
the facing layer. This is corrected by the bonding layer which is a sheet 
of impregnated mica paper wherein the resin has been advanced by a heat 
treatment beyond B-staging as described for the facing layer. 
Recently, a new organic resin family, known as bismaleimide triazine 
resins, has become available. This family has the advantage that it 
provides service temperatures on the order of 200.degree. C. Also, the 
resins have a low dielectric constant of about 3, and a z-direction 
coefficient of thermal expansion not over about 50.times.10.sup.-6 
/.degree.C. up to 200.degree. C. 
The bismaleimide triazine resins, hereafter termed "BT resins", are 
described in considerable detail in U.S. Pat. No. 4,456,712 and in the 
technical bulletins of Mitsubishi Gas Chemical Company cited therein, the 
Mitsubishi Company being the source of the resin. Reference is made to 
this literature to avoid duplication. 
It was readily apparent that the higher temperature capabilities of the new 
BT resins would, as disclosed in the patent, render them highly desirable 
in component production, providing other requirements could be satisfied. 
One particularly important property was flame resistance. This had 
previously been obtained by using brominated epoxy resins. However, these 
resins generally provided markedly lower temperature capabilities. 
If have now found that the high temperature characteristics of the BT 
resins in micaceous materials can be retained while providing flame 
resistance. This combination of properties is achieved with a mixed resin 
containing in weight percent on a solids basis, 50-70% BT resin and 30-50% 
of one or more brominated epoxy resins, the total bromine content being at 
least 12% by weight. One particularly effective mixture for my purposes 
employs, on a solids basis in weight percent, about 60% of a BT resin 
designated as BT 2170, about 22% of a brominated epoxy resin having a 20% 
bromine content, and about 18% of a second brominated epoxy resin having a 
bromine content of 48%. This mixture can be combined in an acetone 
solvent. 
I have further found that, where a relatively low insulation resistance 
occurs, this can be substantially improved by pretreatment of the copper 
foil before assembling it for consolidation. Specifically, I have found 
that an acceptable level of resistance can be provided by applying a resin 
varnish to the interior, or core-contacting, surface of the foil, and 
advancing such resin by a modified B-staging treatment as described for 
the facing layer.

In the drawing, numeral 10 generally designates the component assembly. The 
assembly includes copper foils 12 shown as applied over both an upper and 
lower face. Impregnated mica papers 14 will constitute the core after 
lamination. In accordance with the present invention, facing layers 16 are 
interposed between the copper foils 12 and the papers 14. In a preferred 
form, a bonding sheet 18 may be placed between each facing layer 16 and 
the papers 14. 
SPECIFIC DESCRIPTION 
The invention is further described with reference to specific examples, 
including comparisons. In these examples, pre-pregs of synthetic mica 
papers impregnated with resin were employed. 
The mica papers were prepared from synthetic mica glass-ceramics having the 
following compositions: 
______________________________________ 
A B 
______________________________________ 
SiO.sub.2 57.5 59.5 
MgO 25.0 23.4 
Na.sub.2 O 6.8 -- 
Li.sub.2 O -- 6.1 
F 10.7 10.9 
______________________________________ 
The papers were impregnated with a mixed resin. The mixture was composed of 
60% by weight of a bismaleimide-triazine resin available from Mitsubishi 
International Corp. under the designation BT2170, 22% of a 20% brominated 
epoxy resin available from Shell under the designation EPON 1123-A-80 and 
18% of a 48% brominated resin available from Dow Chemical Co. under the 
designation DER-542. This mixed resin was cured with 0.1 phr dicumyl 
peroxide and 0.02 phr zinc octoate. 
The mica papers were prepared and impregnated in accordance with procedures 
described in the Beall et al. and Hoda et al. patents mentioned earlier. 
EXAMPLE 1 
Synthetic mica papers prepared from a glass-ceramic of composition A, and 
having a 20% content of chopped glass fibers, were employed to produce a 
stacked paper core. A facing was prepared based on the B composition mica 
and containing 70% chopped glass fibers. This was assembled intermediate 
the core papers and the copper foil. 
The resin used for impregnation was the 60/22/18 mixed resin described 
earlier. All of the papers were subjected to a pretreatment with 
guanidinium hydrochloride prior to use. The copper sheets applied over the 
facings were varnished with resin as described in later examples. 
The facings on top and bottom of the stack were given an advanced B-staging 
at 150.degree. C. by heating the top facing for 16 minutes and the bottom 
for 8 minutes. After 428 hours service the Insulation Resistance was 
measured in ohms as Top: 1.times.10.sup.8 ; 1.7.times.10.sup.9 (2 
samples). Bottom 1.5.times.10.sup.8 ; 2.2.times.10.sup.8. 
These samples equalled or exceeded the standard requirement of 10.sup.8 
ohms. Copper peel strength was about one lb/inch. It was observed, 
however, that separation occurred between the facing and the core, rather 
than between the facing and the copper. This indicated a potentially 
higher copper peel strength was available, and that the problem had 
shifted to one of compatibility between the facing layer and the core 
sheets. This was resolved by development of a bonding sheet as described 
in the examples that follow. 
EXAMPLES 2-6 
In the following examples, the laminated core was produced from synthetic 
mice papers of A composition which were pretreated by soaking in 
guanidinium hydrochloride. Each test piece, but the control, had a facing 
layer of a synthetic mica paper containing 70% chopped glass fibers, the 
mica having a composition shown above as B. The paper was impregnated with 
resin and partially cured. In each case, a bonding sheet was assembled 
intermediate the facing and the core. This bonding sheet was resin 
impregnated and partially cured also. The partial curing (B-staging) 
schedules were all at 150.degree. C., but time was varied to provide 
different degrees of resin advancement. 
TABLE I shows, for each example, the mica composition used in producing the 
bonding sheet, and the time of partial curing at 150.degree. C. for the 
bonding and for the facing layer. In examples 2, 3 and 4, both the facing 
layer and the bonding sheet were pretreated, before impregnation, by 
soaking in a solution of guanidinium hydrochloride. In example 6, each was 
similarly pretreated in a solution of octylamine hydrochloride. 
The letters "a" and "b" refer to foils attached to opposite surfaces, and 
the facing layers and bonding sheets associated therewith. 
TABLE I 
______________________________________ 
Bonding sheet Facing Layer 
Example Comp. B-time (min) 
B-time (min.) 
______________________________________ 
2a A 4 4 
2b A 4 8 
3a A 8 8 
3b A 8 12 
4a B 8 8 
4b B 8 16 
5 control (no bonding sheet or facing) 
6a A 8 12 
6b A 8 12 
______________________________________ 
Measurements made on the sample test pieces of TABLE I are shown in TABLE 
II wherein 
RC=Resin Content of the bonding sheet. 
IR=Insulation Resistance in ohms after 300 hours 
Peel=Copper peel in Lb/ /in 
BWA=Boiling Water Absorption (100.degree. C./2 hrs) 
TABLE II 
______________________________________ 
Sample RC (%) Peel IR BWA % 
______________________________________ 
2a 48.2 0.5 3 .times. 10.sup.8 
.41 
2b 0.5 4 .times. 10.sup.8 
3a 52.4 7.4 7 .times. 10.sup.8 
.56 
3b 7.4 7 .times. 10.sup.9 
4a 52 8.0 5 .times. 10.sup.8 
.69 
4b 7.8 8 .times. 10.sup.8 
5a -- 0.4 1 .times. 10.sup.9 
.40 
5b 0.5 9 .times. 10.sup.8 
6a 67.9 7.6 4 .times. 10.sup.11 
.27 
6b 7.0 6 .times. 10.sup.11 
______________________________________ 
Example 2 failed to show any improvement in copper peel strength due to 
using the normal B-staging schedule of 4 minutes at 150.degree. C. for the 
bonding sheet. However, when the partial cure was advanced to eight (8) 
minutes in example 3, the strength increased dramatically to over 7 
lbs/inch. Example 4 confirmed the improvement was not mica composition 
dependent. However, certain other properties were not as good. Example 5, 
a control sample with no bonding sheet or facing layer, confirms the 
normal poor copper peel strength. Example 6 shows the optimum results 
attained when the present invention is combined with a pretreatment with 
an octylamine solution. 
The mixed resin used in these examples provided essentially the 
characteristics of the BT resins, that is a glass transition temperature 
of about 200.degree. C., a dielectric constant not over 4.0 and a 
Z-expansion not over 50 ppm/.degree.C. 
In order to determine the general applicability of the foregoing findings, 
the tests were essentially duplicated using the DER-566 epoxy resin alone 
for impregnation purposes. The tests reported above were run and showed 
copper peel values in excess of 7 lbs./in. and IR values of over 10.sup.10 
ohms after 300 hours. These results were obtained with the octylamine 
treated synthetic micas, Advanced B-staging for a bonding sheet containing 
the composition B mica and 70% chopped fibers was 160.degree.-200.degree. 
C. for four minutes. That for a composition A bonding sheet containing 20% 
chopped glass fibers was 170.degree.-200.degree. C. for four minutes. The 
facing layer, composed of composition B mica with 70% chopped glass 
fibers, was advance B-staged at 220.degree. C. for four minutes. 
The following examples illustrate the effect on insulation resistance (IR) 
values that the attainable under circumstances where otherwise low values 
are encountered. 
A series of one oz. copper foils was prepared for sealing by brushing over 
the underside of each foil, a 50% varnish of the mixed resin using the 
same catalyst level as above. After B-staging under varying conditions, 
the copper foils were assembled with laminated pre-pregs for hot pressing. 
The resin pick-up was in the range of 9-11% and thickness about 1.1 mils. 
The following examples illustrate application of the inventive method under 
varying conditions and the comparative results achieved. 
EXAMPLE 7 
A pre-preg prepared with the mica of composition A was treated with a 
solution of guanidine to open the surface pores. A varnished copper foil 
was B-staged for 12 minutes at 150.degree. C. and then assembled over one 
face of the stacked prepreg while the opposite face was assembled with an 
unvarnished foil. The assembly was then hot pressed at 180.degree. C. for 
a period of 120 minutes at 1000 psi pressure. This was followed with a 
post-cure of four hours at 200.degree. C. under the same pressure. The 
resulting circuit board blank was operated for 112 hours and insulation 
resistance measured at that time. The resistance of the varnished face was 
about 10.sup.6 ohms whereas the resistance of the unvarnished face was 
10.sup.5 ohms. This indicated B-staging at 150.degree. C. for 12 minutes 
was insufficient to substantially alter insulation resistance in this 
example. 
EXAMPLE 8 
The procedure of Example 7 was repeated, except that the varnished copper 
foil was advance B-staged for 15.5 minutes. Insulation resistance of 
6.times.10.sup.8 ohms was measured after 306 hours and one of 
3.times.10.sup.8 ohms was measured after 596 hours. For comparison, the 
unvarnished surface measured 2.7.times.10.sup.5 after 306 hours. These 
data indicate that a partial cure of 15 minutes duration at 150.degree. C. 
is sufficient to improve insulation resistance substantially. 
EXAMPLE 9 
The procedure of Example 8 was repeated on four separate pre-pregs. In each 
case, the procedure was varied in two respects. The surface of the mica 
paper was pretreated with diammonium hexane, rather than guanidine. Also, 
the varnished copper was advance B-staged for 15 minutes at 150.degree. C. 
Insulation resistances measured on the varnished surface varied from 
8.times.10.sup.9 to 2.times.10.sup.10 ohms, while that of the unvarnished 
surface varied from 5.6 to 7.times.10.sup.6 ohms. 
EXAMPLE 10 
The procedure of Example 7 was repeated except that the mica of composition 
B was used in preparing the laminate. Both copper foils were varnished and 
partially cured at 150.degree. C., one for 16 minutes; the other for 14 
minutes. After 306 hours use, the foil staged for 16 minutes had an IR of 
3.times.10.sup.7 ohms while the 14 minute one was 2.times.10.sup.7 ohms. 
For reference, an unvarnished comparison sample showed 2.5.times.10.sup.5 
ohms after 112 hours. 
EXAMPLE 11 
Example 10 was repeated except that the mica paper was pre-treated with 
diammonium hexane before application of the varnished and advance B-staged 
copper foils. Both the top foil (16 minutes) and the bottom foil (14 
minutes) showed 1-2.times.10.sup.8 ohms IR on duplicate samples measured 
after 596 hours. A comparison unvarnished sample showed 1.times.10.sup.6 
ohms after 112 hours. 
The data obtained in examples 9-11 indicate the efficacy of advanced 
B-staging in improving insulation resistance where conditions of treatment 
and materials are varied. 
EXAMPLE 12 
The prior examples 7-11 were made without a facing inserted between the 
copper foil and the laminate. Customarily such a facing, e.g., a layer of 
glass fiber fabric, is provided. Accordingly, a test piece was prepared 
with a core in accordance with Example 7, and a varnished foil on top and 
bottom. The top foil was partially cured at 150.degree. C. for 15.5 
minutes, and the bottom foil 10 minutes. A compatible facing layer 
containing 70% chopped glass fibers was assembled between the foils and 
the pre-preg. After 306 hours, the IR of the top foil (15.5 minutes) was 
about 1.times.10.sup.9 ohms, while the bottom (10 minutes) was 
9.times.10.sup.5 ohms. This indicated that ten minutes was too short a 
time with this particular test piece to obtain advanced B-staging.