MANUAL OF

METAL-TO- CERAMIC
SEALINO TECHNIQUES

ELECTRONIC TUBE DIVISION

SPERRY GYROSCOPE COUPANV
IN DIVISION OF SPERRY RAND CORPORATION
OREAT NECK, NEW YORK

Prepared by
ENGINEERING DEPARTMENT
C.W. Johnson
PUBLICATIONS DEPARTMENT
E.W. Cheatham
1.

Prepared under the sponsorship of

ROME AIR DEVELOPMENT CENTER
AIR FORCE SYISTEIS COMMAND
UNITED STATES AIR PORCE
GRIFFISS AIR FORCE BASE, NEW YORK

Contract No. AF301602)-2371

Project No. 5573 Task No. 557303

I Copy No.

IC
Sperry Pub. No. NA-27-90\ ~ ' May 196*3

.0 N
PATENT NOMICE
When Government drawings, specifications, or other data
are used for any purpose other than in connectiontwith a
definitely related Government procurement operation, the
United States Government thereby incurs no responsibility
nor any obligation whatsoever and the fact that the Gov-
ernment may have formulated, furnished, or in any way
supplied the said drawings, specifications or other data
is not to be regarded by implication or otherwise as in
any manner licensing the holder or any other personor
corporation, or conveying any rights or permission to
manufacture, use, or sell any patented invention that may
in any way be related thereto.

ASTIA NOTICE
Qualified requestors mayobtain copies of this document
from the Defense Documentation Center forScientific and
Technical Information, ArlingtonHall Station, Arlington
12, Virginia. DDCservicebfor the Department of Defense
contractors are available through the "Field of Interest
Register"ona "need-to-know" certified by the cognizant
military agency of their project or contract.
 CONTENTS

I Section Page

I INTRODUCTION 1
II SELECTION OF RAW
MATERIALS 5
2-1. Ceramics 5
2-2. Metals 6
III SEAL DESIGN 9
3-1. Basic Thin-Wall
OD Compression
Seals 9
3-2. ID or Pin Seals 11
"3-3. Butt Seals 13
3-4. Nonstandard
Seals 17
IV PARTS PREPARATION
"AND METALLIZING 19
4-1. Cleaning of
Ceramics 19
4-2. Cleaning and
Plating of Metals 21
4-3. Metallizing 21
"4-4. Evaluation of
Metallizing 28
4-5. Electroplating 30
V FIXTURES AND
ASSEMBLY 33
jVI YPRAZING 37

iii
 CONTENTS (Cont)

Section Page

VII CONTROL TESTING 39

VIII FAILURE ANALYSIS 4

*Appendix

A RECOMMENDED
TOLERANCES AND
METALLIZING
ALLOWANCES 43
B METALLIZING MIXTURES 45
C COMBINATIONS OF
CERAMIC BODY,
METALLIZING MIXTURE,
AND SINTERING CYCLE 47
D BRAZING MATERIALS 49
E TYPICAL PLATING
BATHS AND
PROCEDURES 51

iv
I

I ILLUSTRATIONS

Figure Page
1 Expansion Coefficients
of Typical Ceramic
I Bodies and Metals 7
2 Thin-Wall OD
I Compression Seal 10
3 Transitions to Thick-
Wall Metal Members 12
I4 Self-Jigging Seals in
Which Metal Member
I Performs Jigging 14
5 Hollow-Pin Seals
Strengthened with
Solid Pin Insert 14
6 Typical Butt-Seal
I Assemblies 15
7 Back-up of Ductile
Metal Seal with
I
Second or Blank
Ceramic 16
I 8 Thick-Wall Seal 18

9 Rectangular Seals 18
I 10 Hand-Coating Equipment 23
11 Roller-Coating Equipment 24

12 Spray-Coating Equipment 24
13 Dewpoint Cup 27
I
V

I
I
SECTION I
I NITRODUCT ION

"Metal-to-ceramic seals have been in

existence for thousands of years, since early
man first made decorative enamel-copper trin-
kets. Little progress was made until the nine-
teenth century with the advent of the internal
combustion engine and the need for spark plugs.
Since then, there have been considerable tech-
nological advances, particularly with regard to
the electronic tube industry. To further knowl-
edge in this area, the Electronic Tube Division
of Sperry Gyroscope Company conducted a seal
technology study for the Rome Air Development
Center, Air Force Systems Command, under
Contract No. AF30(602)-Z371. ýThis manual,
discussing the procedures to fabricate metal-
to-ceramic seals, is based on the study program.
During the study program, the effects
known variables had upon the ultimate strength
of metal-to-ceramic seals were investigated.
Five statistically designed experiments were
performed to examine the significance of the
variables and their interactions on over-all
seal strength. The results of the program were
masked to a great extent by a high residual
error due to uncontrolled and/or unknown vari-
ables; only the most marked effects could be
observed and assigned statistical significance.
I
11
 Because of the high coefficients of varia-
tion (or dispersion) of the experiments, many
of the recomn ended processes are of an advi-
sory nature, .ather than of specific scientific
preciseness.lThe main contribution of this
manual is in cataloging the variety of processes
required to fabricate metal-to-ceramic seals,
with recommendations as to the relative degrees
of importance in controlling various phases of
the operations. Some of the techniques em-
ployed in fabricating seals and controlling the
known variables are described; the ceramic
engineer can select those suitable for his par-
ticular needs .It must be remembered, how-
- vever, that fabrication of seals depends to a
large extent upon the skills of the operators;
and reproducibility and reliability can be
gained only through proper training and practice.
K--• The organization of this manual is
patterned after the normal work flow, from raw
materials to finished product:
"* Selection of raw materials
"* Seal design
"* Parts preparation and metallizing
"* Fixtures and assembly
"* Brazing
"* Control testing
"* Failure analysis.
I
It must be emphasized that this manual
provides recommendations and not absolute
instructions. With the large variety of ceramics
and metallizing mixtures available, an almost
infinite number of combinations is possible,
each possessing individual characteristics. It
is true that many combinations with similar
characteristics can be grouped together, but
proficiency in the use of a particular ceramic-
metallizing combination is obtained through
* careful observation of all phases of the operation.
Many techniques which can be employed
in the processing steps have not been mentioned
for the sake of brevity. Substitutions can be
made because of available equipment and facili-
ties. For instance, if electroplating facilities
are not available, the application of nickelous
"oxide or cuprous oxide, with subsequent reduc-
ing atmosphere sintering, can replace the elec-
troplated layers. Similarly, some electroless
"platings can be used if phosphorous contamina-
tion is permissible. The ceramic engineer
must select several combinations of ceramic,
metallizing, metal members, and brazing
materials and then work with these to establish
basic parameters of operation. A skilled
engineer soon develops a basic understanding
of his selected combinations, and is better able
to employ these materials for his purposes.
Considerable work still remains to
determine the basic mechanisms involved in
these processes. Several investigators have
hypothesized the mechanisms, but none has
succeeded in accurately describing the observed

3
phenomena. Manufacturing of metal -to -ceramic
seals currently straddles the line between
science and skilled craft; a few years ago it
was completely craft. The immediate future
should benefit as mo re scientific data are
collected, as the mechanisms are more com-
pletely understood.

4
I
I
SECTION I I
SELECTION OF RAW MATERIALS

I
2-1. CERAHICS
j The choice of the ceramic body is basic
to the process of sealing. Selection is primarily
determined by the application of the device in
which the seal is incorporated, because of the
mechanical and electrical properties of different
ceramics. Some of the properties to consider
are thermal shock resistance, r-f losses,
softening temperature, thermal expansion,
porosity or vacuum-tightness, degassing, and
physical strength.
The percentage of alumina in the body is
proportional to the ease of sealing. Many
ceramics from a variety of suppliers are avail-
able covering a range from 85- to 100-percent
alumina. Dense vacuum-tight aluminas are
made by mixing a small percentage of the
vitreous or glassy material referred to as a
flux with the refractory polycrystalline material,
and heating the composition beyond the melting
point of the flux. Two alumina bodies with the
same percent alumina content will not neces-
sarily have the same physical and electrical
properties, because the composition of the flux
can vary. In general, ceramics increase in
price as the purity of alumina increases. The

5
I
lowest-purity alumina which will be satisfactory
provides the greatest cost savings in raw
material and yield in processing.

2. 2. METALS
The selection of the metal member of
the seal is somewhat governed by the physical
and electrical requirements, but it is more
dependent upon the ceramic body chosen.
Characteristics such as magnetic properties,
r-f and d-c conductivity, and structural
strength determine the selection of a group of
metals. A particular metal member is then
selected on the basis of its coefficient of
expansion. Ideally, the ceramic and the metal
should be identically matched, so that they
expand and contract at the same rate throughout
the brazing and operating cycles; stresses
caused by dissimilar expansion characteristics
are thereby eliminated. It should be noted that
the stringent thermal cycling tests performed
to evaluate the reliability of finished seals are
based on the stresses developed between the
dissimilarly expanded ceramic and metal
members.
Expansion coefficients of some typical
ceramic bodies and some of the metals more
commonly employed in sealing to the ceramics
are shown in figure 1. No ideal combinations
are presently available, but the metals and
ceramics currently used allow construction of
highly reliable seals. Some of the metals

6
!

100
i I I
GLASS-SEALING ALLOY
(GRAY) KOVAR A AND FERNICO
,-. 90 - -

7"C -
70 - - N-1413.3) -- J

" 0 -
/NO.4
SYLVANIA

ALLOY T
Z
2 Cu(017.7)

c ALUMINA
50O -L -
-JJ C SAl- 00)
g ,(WESGO AL-300O
/-0 (WESGOAL-IO09)

40 - - -

CARIBON STEEL

0W (4.6)

d" No 15.3)

10 d

0
0 l00 200 300 400 500 600 700 600 900
TEMPERATURE (*C)

i FIGURE I. EXPANSION COEFFICIENTS OF TYPICAL

CERAMIC BODIES AND METALS
7
7
I
normally encountered in fabricating the
variety of seals described in Section III are
Kovar, No. 42 alloy, Sylvania No. 4, nickel,
molybdenum, and copper.

8
I
SECTION III
SEAL DESIGN

3-1. BASIC THIN-WALL O COMPRESSION 'SEAL'S

The thin-wall OD compression seal

(figure 2) employs a thin surrounding metal
member which relies on its compliance to com-
pensate for the mechanical stresses caused by
the thermal expansion mismatch. Wall thick-
ness usually ranges from 0. 005 inch to 0. 025
inch. The metal member should be as thin as
possible while still offering the mechanical
strength required for handling and processing.
In designing this type of seal, the
engineer must rely heavily on judgment and pre-
vious experience. Small seals-those about
0. 25 inch and less in diameter--are more diffi-
cult to seal successfully than are seals having,
for example, a diameter of 1 or 2 inches.
Wherever possible, small seals should use
thinner metals. Seal thickness or length is an
important factor. Extremely thin ceramic
members, which provide short leak paths, are
to be avoided wherever possible. Seals having
thicknesses or leak distances of 0. 125 inch or
greater offer fewer problems than those in the
0. 050- to 0. 030-inch range. In designing
seals, it is necessary to allow proper clear-
ance between the ceramic and metal member

I 9
 ROOM
TEMPERATURE

Ii II

MISMATCH
STRESS

HEATED

METAL CERAMIC

FIGURE 2. THIN-WALL OD COMPRESSION SEAL

10
I
for the application of the metallizing and
plating. Basic rules for allowances are given
in Appendix A.
When sealing a ceramic assembly to a
massive metal member, a transition metal of
thinner cross section or a thinned-down metal
similar to the massive metal member can be
employed (figure 3). The transition metal is
then selected to match with the alumina body;
the massive metal member can be any material
to which the transition metal can be joined.
Thin-wall OD compression seals should
be designed to be self-jigging to facilitate
manufacturing. This is accomplished by in-
corporating a shoulder to position the ceramic
and metal with respect to each other (figure 4).
The step or shoulder should not be part of the
ceramic because of the extra expenses involved.

3-2. ID OR PIN 'SEAL'S

The pin material dictates the seal design

to a large extent. If the pin is made of lower-
expansion molybdenum, it is usually solid.
With nickel, copper, or other higher-expansion
metals, the pins are usually hollow to allow more
compliance so that the metal can yield to the
mismatch.
Wall thickness usually varies from 0. 005
inch to about 0. 015 inch, but can be made larger
for very large ID holes. In general, the total
wall thickness should not exceed 20 percent of
I 11
I
 SPACE~

SCERAMIC
HEAVY METAL
C TRANSITION METAL

FIGURE 3. TRANSITIONS TO THICK-WALL

METAL MEMBERS

12
I
the diameter of the ceramic hole, following
the rule that the metal member should be as
thin as possible. When tubular pin material
is employed, plugs can be welded or brazed
at both ends of the tubing to produce a vacuum-
tight seal. For high current-carrying capa-
bilities, a center conductor with proper clear-
ance can be employed; brazing should be at
* both ends of the compliant metal tubing
(figure 5).

T 3--3. BUTT 'SEAL'S

Butt seals are desirable because of the

ease of assembly. The mating surfaces can
be easily and inexpensively ground flat;
metallizing procedures include silk screening,
roller coating, and spraying with simple fix-
tures and jigging. Typical butt-seal assemblies
are illustrated in figure 6.
The transverse loading stress, normally
encountered in horizontally supported tubular
devices, will peel simple butt seals more
readily than the more physically rugged com-
pression seal. Back-up ceramic rings have
been used to reduce the thermal expansion
mismatch sheer stresses, and consequently
increase the reliability of the seals (figure 7).
Nickel is frequently used, although Kovar
"is employed where more strength is required.
As ductile and thin a metal as possible should
jbe used. Seal leak paths should be at least
0. 125 inch for a safe low working limit. The

I
13
I
 METAL CERAMIC

FIGURE I4. SELF-JIGGING SEALS IN WHICH METAL

MEMBER PERFORMS JIGGING

M-BRAZE

'WELD.m

METAL CERAMIC

FIGURE 5. HOLLOW-PIN SEALS STRENGTHENED WITH

SOLID PIN INSERT

14
I
Ii

] CERAMIC

= METAL

FIGURE 6. TYPICAL BUTT-SEAL ASSEMBLIES

|I
115
 WON,

CERAMIC

E : IMETAL

FIGURE 7. BACK-UP OF DUCTILE METAL SEAL

WITH SECOND OR BLANK CERAMIC

16
I
thickness of the metal should be the same order
of magnitude as with the OD compression seal.
In the special case in which two metallized
ceramics are fabricated into a vacuum-tight
joint, a spacing material of 0. 010- or 0. 020-
inch nickel is normally employed between the
ceramics to minimize stresses.

3-4. NONSTANDARD 'SEAL'S

Some typical nonstandard seals are
illustrated in figures 8 and 9. Figure 8 shows
a thick-wall seal employed for windows and
similar types of dielectrics. Rectangular
seals, which can be sealed into thin- or thick-
wall metal members,,are shown in figure 9.
Rectangular ceramic pieces should have sub-
stantial radii of no less than 0. 030 to 0. 040
inch because the metallizing of square corners
is extremely difficult and leads to stress con-
centrations in the finished seals.

T
I 1

17
!
 METAL CERAMIC

FIGURE 8. THICK-WALL SEAL

METAL CERAMIC SLAB WAVESUIDE

WINDOW SEAL WAVEGUIDE SEAL

FIGURE 9. RECTANGULAR SEALS

18
I
I
SECTION IV
PARTS PREPARATION AND METALLIZING

I
"4-1. CLEANING OF CERAHICS

After the ceramic and metal members

have been selected and the seal has been
designed, the parts must be prepared for met-
allizing and electroplating prior to assembly.
Ceramics received from the suppliers contain
organic and (occasionally) metallic contaminants
on the surface of the body. These must be com-
pletely removed so as not to interfere with the
metallizing operations. The ceramic parts
are placed in a clean Pyrex container in which
all chemical cleaning is performed. The parts
must not be handled with metallic utensils.

The ceramic parts are boiled in clean

concentrated nitric acid (CP grade) for 30
minutes. The nitric acid solution is then de-
cantered, and the parts flushed with clean dis-
tilled or deionized water. * The parts are then
boiled for 10 minutes in fresh deionized water,
followed by a flush with clean, cold deionized
water. Ultrasonic rinsing in cold deionized
water as a final rinse is recommended, but not

*Distilled water can be substituted for deionized

water, whenever specified.

19
I
absolutely necessary. After all water rinses,
the ceramic parts are rinsed in methanol (CP
grade) in the same Pyrex container.
After decantering the excess methanol,
the Pyrex beaker containing the ceramics is
dried in an air oven at 1200C. The parts are
then placed on a clean ceramic plate, pre-
viously air-fired, and the plate and parts are
air-fired at 1000 * 250C for 10 minutes; care
must be taken to raise and lower the tempera-
ture at a rate which will avoid thermal shock
and cracking of the ceramic pieces. After
cooling, the parts are placed in clean poly-
ethylene bags and stored until ready for met-
allizing. A maximum of 3 days can elapse
between cleaning and metallizing of the ceramics;
if this time is exceeded, air-firing is recom-
mended to remove accumulated organic con-
tamination.
The clean ceramic parts must be handled
with clean rubber finger cots and/or plastic-
coated nylon gloves to prevent finger oils and
other contaminants from being transferred to
the ceramic surfaces. Organic materials,
such as finger oils, which are allowed to remain
on the ceramics during subsequent operations
are reduced to carbon, and metallic salts are
reduced to metals and metal oxides; these stain
the ceramics and cause sparking, electrical
leakage, and r-f losses in many applications.
Tweezers, spatulas, and other tools used in
handling the ceramics must be ceramic or
plastic coated.

20
I
4-2. CLEANING AND PLATING OF HETALS•

Metal parts require cleaning similar to

that of ceramics to produce satisfactory seals.
Nickel, monel, Kovar, and stainless steel
receive the following treatment. The metal
parts are ultrasonically cleaned in trichloro-
ethylene, followed by a hot tap-water rinse and
a concentrated hydrochloric acid pickle long
enough to eliminate scale and oxides. This is
followed by a cold tap-water rinse, and then a
cold deionized water rinse. A methanol dip,
similar to the ceramic drying procedure,
followed by oven drying at 120 0 C prepares the
part for further processing.
All metal parts generally require metal
plating. A copper flash is also required in the
regions to be brazed; this prewets the joint
areas, allowing the brazing material to flow
more readily. The plating serves different
purposes for the various materials (nickel is
not plated). Molybdenum is plated to assist in
the wetting by the solder. Stainless steel is
afforded protection against oxidation during
brazing in the wet reducing atmosphere. Kovar
is plated to prevent intergranular penetration
during brazing with silver-containing alloys,
and also to protect against oxidation during
storage.

4-3.. HETALLIZING

The cleaned ceramics are now ready for

the application of the metallizing mixture.
Typical mixtures are listed in Appendix B.

21
1
There are several techniques of applying the
metallizing (figures 10, ll,and 12); the one
chosen depends on the specific requirements.
Metallizing thickness does not appear to have a
large significance in seal strength, but does
cause some variations. Therefore, efforts to
standardize the procedure will result in a more
controlled and reproducible process, in addition
to enabling tighter dimensional tolerances to
be maintained for easier fits during the assem-
bly stage s.
Handpainting with a sable brush is the
mo st commonly employed method for small
lots and oddly shaped pieces. Wherever
possible, the part to be coated should be
mechanically rotated so that the total seal area
can be coated uniformly before the mixture
dries. This prevents nonuniformity of thick-
ness in the seal area and guarantees freedom
from voids. Spray coating and roller coating
require masking and more fixtures; thus, they
are suitable for large production runs. The
viscosity of the mixture determines, to some
extent, the ease of application and the resultant
thickness and uniformity; it should be main-
tained at a constant value. As the mixture is
used, evaporation rei-uces the amount of solvent.
With time, the mixture becomes more viscous,
requiring periodic replacement from the stock
bottle. In no case should any unused portion of
metallizing mixture be returned to the stock
mixture, as this will cause variations in con-
tent and viscosity.

22
I
I
!
!
1

I
 FIUR CAR MOTICO•
OLAGE AI

ROLT IN CERAMIC SPPCIMNE

DJUSTABLE AUXILM
ROLLER IWIPER)

MAGETAIL sIZRNG

FIGURE 12. SRAY-COATING EQUIPMENT

METALIZNG ONTINE

ON AIN AIROBRUS
A •uSABNET $ OLERSEE

COTO M CONPTROLPCIEN

FIGURE 12. SROLLE-COATING EQUIPMENT

4OOL
TUNT.
I
After the ceramics have been coated, it
is essential that the solvents be completely
evaporated before sintering; otherwise,there
will be explosive expulsion of the materials
during sintering, with subsequent damage to
the coated surface. The ceramics are then
placed in clean molybdenum boats, or for
large assemblies on corrugated molybdenum
sheets or mesh screening lining the bottom of
j the boat. (The coated ceramics can be placed
temporarily on lint-free tissue before being
placed in the boats. ) The corrugated metal
and screening reduce the thermal path between
the ceramic and the bottom of the boat resting
in the hot zone of the furnace, thereby mini-
mizing temperature shock conditions which
can damage or destroy the ceramic. The parts
are positioned to prevent the relatively soft,
unsintered metallized coating from rubbing onto
unmetallized surfaces.
The ceramics are then placed in the
furnace for sintering. The type of furnace
employed governs the particular procedure.
For periodic batch furnaces, the ceramics are
normally protected against thermal shock by
the thermal inertia of the furnace, which acts
as the limiting heat rate to which the parts are
subjected. A pusher, or horizontal, furnace
through which the boat is passed must be
handled with extreme precaution to minimize
shocking the ceramics; several progressive
steps moving the boat closer to the hot zone,
finally into the hot zone, followed by graded
movements into the cooling chamber are

25

I
required to protect the pieces. The furnace
atmosphere can be forming gas, dissociated
ammonia, or pure hydrogen; but in all cases
the dewpoint of the gas must be maintained at
a level no less than 30 0 C. As long as the
minimum dewpoint is maintained, the seal
strength is not drastically affected.

Measurement of dewpoint in the region

0
of 30 C warrants description. The dewpoint
cup employed (figure 13) consists of a closed
chamber (with a glass viewing window) through
which the sample gas is passed, and a project-
ing polished chrome-plated cup upon which the
gas impinges. Hot water at approximately
55 0 C (after the chamber is purged for at least
15 minutes at a flow rate of approximately 2
cfh) is placed in the metal cup. Observing the
polished surface of the cup, which is immersed
in the sample gas of the chamber, a thermom-
eter of the proper dewpoint range is used to
stir the hot water while small amounts of cold
water are added. This procedure is followed
until a blush forms on the metal cup, at which
time the thermometer is read. This tem-
perature is the dewpoint of the sampled gas.
It is necessary to keep the sampling tubes and
lines above the temperature of the dewpoint
(300C). Otherwise, the cooler walls of the
sampling tube will condense out the water
vapcr, effectively reducing the sample gas
dewpoint and giving an incorrect indication of
the actual dewpoint in the furnace.

26
 I

j FIGURE 13. DEWPOINT CUP

I
 The temperature of sintering and the
soak time at the sintering temperature vary
for the combination of ceramic body and met-
allizing mixture. Some recommended com-
binations of ceramic body, metallizing mixture,
and sintering cycle are listed in Appendix C.
In most cases, a L50 0 C temperature control
is adequate, provided the minimum temperature
is not exceeded. The soak time is somewhat
more sensitive for mixes containing glass and
for high-purity alumina bodies (above 99-percent
alumina), although the molybdenum-manganese
mixes are insensitive for several hours. In
general, the sensitivity to sintering conditions
increases with the purity of the alumina body,
requiring more precise controls as the alumina
content approaches 100 percent. No firm rules
have been established, because the variety of
mixes employed reacts differently under various
conditions. After cooling below 50 0 C in the
furnace cooler, the metallized ceramics are
removed from the boat, using plastic- or
ceramic-coated tools and/or finger cots or
gloves.

4-4. EVALUATION OF HETALLIZING

The metallized area can be evaluated by

the following simple scratch test. A new,
single-edge razor blade is held at approxi-
mately 45 degrees to the surface of the sin-
tered metallizing. The blade is then pressed
against the surface and forced into the met-
allizing, while observing the procedure through
a 10-power microscope.

28
I
Poor Coating - The metallizing is too soft
(usually resulting from under-sintering
in time and/or temperature) if the razor
blade cuts into the metallizing and peels
it, exposing ceramic beneath the surface.

Good Coating - Adequate sintering and

metallizing is indicated if the razor blade
on subsequent passes over the same
place causes a metallic polished spot to
appear without removal of metallizing.
A note of caution-be certain with met-
allizing mixtures containing glass that the
polished surface is not the razor blade being
rubbed off on excess glass in the metallizing
surface, which can give a false indication.
Also, this excess glass covers the metallic
surface of the metallizing and will permit
deposition of electroplating at later stages of
the process, with subsequent failure during
brazing, because the plating and brazing will
not wet the glass-rich surface.
If the metallizing is soft, it can be
resintered at a slightly higher temperature
and/or a longer period of time, but it is
generally advisable to resinter under the same
conditions for a second cycle equal to the
first. For hand application, it is desirable to
apply two or three thin coats of metallizing,
with sintering between each coat so that varia-
tions in the painting operation can be leveled
out on subsequent layers. Application by
spraying and roller coating are uniform and can
be limited to one layer.

29
!
 The thickness of the final metallized
ceramic should be between 0. 0004 and 0. 0015
inch. Thin areas, as low as 0. 0002 inch, are
acceptable, providing bond and texture are
good and the thin areas do not exceed 20 per-
cent of the length of the metallized seal (leak
path direction). Local areas as thick as 0. 002
inch are acceptable providing the aggregate
length of these spots does not exceed 20 percent
of the length of the metallized area.

4-5. ELECTROPLATING

After the final sintering, the ceramics

are electroplated. The plating serves two
functions:
" It protects the metallizing from excess
penetration by the solder. If the solder
is allowed to be molten for extended
periods of time (exceeding one-half hour),
it will penetrate the metallizing coating
and lift it away from the ceramic, causing
leaks.
"* It prewets the surface of the metallized
layer so that the solder employed during
the braze will flow properly and make the
joint.

A nickel plate followed by a thin copper

plate is generally employed. The nickel forms
a barrier against penetration. The copper
visually indicates the presence of electroplated
conductive surfaces and aids in the flow of most

30
I
brazing materials, such as the more commonly
employed OFHC copper, copper-gold alloys,
or copper-silver alloys. Appendix D lists the
I melting data for these brazing materials.
Typical plating procedures and the recommended
plating bath compositions are given in Appendix
E. Accurate thickness control of the plating is
essential for dimensional considerations, as
well as for maintaining a barrier against solder
I penetration.
The electroplated metallized ceramics
and the electroplated metal members are now
prepared for the assembly operation. The
parts should be stored in clean polyethylene
bags in a heated cabinet to minimize oxidation
from humidity and airborne sulphides. Con-
tamination control in the use of plastic-coated
nylon gloves and/or finger cots is essential to
prevent stains on the surfaces to which the seal
will be made. Otherwise, there will be sub-
sequent reduction of seal length and an over-all
loss of reliability in the assembly, if not initial
failure due to a vacuum leak. An oxide imprint,
resulting from the acids of finger oils, will
act as a stopoff barrier preventing the flow of
the molten solder.

3
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!
I
31

1
I
I
SECTION V
FIXTURES AND ASSEMBLY

Cleanliness and contamination control

have been emphasized throughout this manual.
The practice is equally important regarding
fixtures and assembly. All fixtures and jigs
used to assemble the seal must be thoroughly
cleaned to avoid contaminating the parts with
materials which would impede the flow of
solder during the brazing operation. A vapor
degreasing and deionized water rinse are
adequate to remove most organic materials.

Fixtures can be constructed of greened

or oxidized stainless steel, electronic-grade
graphite, or ceramics such as the machinable
lavas and boron nitride. If possible, within the
design requirements of the seal assembly, self-
jigging features should be employed.

In the manufacture of r-f window assem-

blies, it is desirable (because of r-f conduc-
tivity) to construct the outer metal member of
copper. Restraining bands must then be used
to prevent the sleeve from expanding and allow-
ing an excessive gap between the metal and
ceramic member, resulting in improper brazing
and poor fillets. These can be made of several
thin bands of greened stainless steel, restrained
with 0.020- to 0. 030-inch molybdenum wire
twisted together in the form of rings holding the

I
33
!
stainless steel bands in place. Alternatively,
a single-layer winding of 0. 005-inch molybdenum
wire provides a more uniform restraint and
minimizes puckering at the twist points. With
all restrained assemblies, it is desirable to
electroplate 0. 0005 inch of copper on the
brazing site (on either the ceramic or metal
member) to ensure having solder penetrate
along the seal length. Overrestraint will
prevent an adequate flow of solder into the
joint, resulting in a weak bond.
Magnesium oxide is recommended as a
stopoff when the flow of solder along the metal
member must be inhibited. It is easily applied
by spraying or brushing, and being extremely
refractory, effectively prevents wetting by the
brazing metal. The oxide is removed from the
assembly by dissolving it in a 5-percent acetic
acid solution, followed by several deionized
water rinses. Other commonly used stopoffs-
such as chrome oxide, aquadag, and alumina
dispersions-are to be avoided as they are
difficult to remove and may contaminate the
device if not completely eliminated.
The fixtures should be designed so that
the ceramic member is not exposed to the
sudden heat of the furnace. It should be pro-
tected by a cover, thus allowing uniform heating
of the whole assembly. Ceramic windows and
other structures can shatter, even though pre-
heated, when exposed to the hot zone of the
furnace because of the intense radiant heat
produced in this zone. The thermal inertia of
the fixture should be such that thermal shock is

34
I
not a problem. However, caution should be

used in overcompensating for thermal shock,

because an excessively high thermal mass will
allow the brazed joint to remain at the melting
temperature for a period of time much greater
than is scheduled for the hot zone. Extended
periods of time at the brazing temperature may
damage the seal by dissolving or penetrating
the metallizing excessively and weakening the
bond at the interface.

[
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I
[
I
I 35

I
I
I SECTION VI
I BRAZING

Brazing can be accomplished with a push-

through horizontal furnace, belt furnace, or
bell-jar brazing. The brazing schedule must
account for the heating rate of the assembly,
so that the ceramic members are not thermally
1 shocked. It is desirable to preheat the assem-
bly to approximately 50 0 C below the melting
point of the solder. This allows the metal and
ceramic members to come to thermal equili-
brium and attain the proper gap relationship as
planned in the original design. After the ther-
jmal soaking below melting temperature, a
short excursion into the melting range and back
down again, in as brief a time as possible,
allows a good flow with a minimum of penetra-
tion of the metallizing and alloying with the
metal members.
The sensitivity to the brazing time and
temperature depends upon the brazing material
and the metal member. With copper-gold
eutectic alloys being brazed to copper members,
an extended brazing time and temperature will
allow erosion of the copper member and
possible freezing of the solder joint. Nickel
plating of metal members which are compo-
nents of eutectic brazing materials will mini-
mize this alloying effect. Although the alloying
effect is damaging on initial brazes, it can be

3
37
I
successfully employed in attempting reworks
of assemblies which leak upon first brazing.
Adding a small piece of brazing alloy to the
area where the leak is detected will enable a
seal to be made at the brazing temperature
without disturbing the surrounding brazed
areas; the solution of some of the metal mem-
ber sufficiently raises the melting point of the
first brazed joint to prevent it from reliquefying
and disturbing the alignment of the original seal.
Through careful temperature monitoring, two
successive brazes on the same assembly with
the same brazing material can be effected with
this technique. Subsequent step brazes can then
be employed with brazing materials having
prqgressively lower melting points.

After brazing, a helium mass-spectrometer

leak detector is used to check the hermetic seal
for vacuum tightness. At the maximum sensitiv-
ity of most commercial leak detectors (10-10
atmosphere cc per second),minute leaks are not
readily detectable; devices being stored could
then fail because of a gradual rise of the pres-
sure above the minimum operating level.
Equipment to measure extremely small leak
rates due to a pressure rise can be employed.
A penetrating leak check dye has been success-
fully used in many large-scale operations in
which hundreds of seals have been simultaneously
checked. The sensitivity to submicroscopic
leaks is somewhat limited, but the leaks in the
helium mass-spectrometer range can be detected
with careful techniques.

38
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I
I
SECTION VII
CONTROL TESTING

!
A moderately severe test of the completed
hermetic seal is thermal cycling to some tem-
perature slightly above the maximum tempera-
ture at which the device will be used. This can
be repeated several times with helium mass-
spectrometer leak checks after each cycle. The
first or second cycle will usually eliminate the
majority of substandard seals, but there is no
T clear correlation between additional cycles and
the degree of reliability of the seals in the popu-
lation being tested. All testing, if possible,
j should be performed on representative samples
of units being manufactured and should simulate
conditions similar to the end use. For parts of
electronic tubes which are to be subsequently
brazed to metal members by induction or resis-
tance heating one end of the assembly, testing
should duplicate these uneven stresses applied
to the seal. Thermal shock, below the level
which would cause the ceramic to fail, can be
employed to test small, bead coaxial antenna
seals. Pressurization of thin ceramic windows
has been used to stress the seal area.
ASTM samples are not recommended as a
precise indication of the reliability and quality
of seals belonging to a population of which the
ASTM members were a part. The present ASTM
test has been found to have a large coefficient of

I 39

!
variation due to many uncontrollable variables;
differences in seal strength which may occur
from lot to lot in the metallizing and brazing
operations tend to be masked. Coefficients of
variation from 25 to 35 percent were observed
in the study program on which this manual is
based. A modification employing a metal mem-
ber brazed to one-half of an ASTM set is more
desirable and more representative of a typical
metal-ceramic seal.
Statistically sampling quantities of
manufactured seals which are subjected to
destructive testing is also indicative of varia-
tions from batch to batch. The destructive
testing can be accomplished by tearing the
ceramic and metal member apart in a repro-
ducible fashion. For example, with coaxial
antenna assemblies, extremely sensitive moni-
toring is possible by pulling the center coaxial
lead while restraining the outer conductor until
the ceramic bead fractures. Amore sensitive
indication of seal quality is obtained by simul-
taneously monitoring the hermetic tightness of
the seal during the application of stress.

40
!

SECTION VIII1
FAILURE ANALYSIS

It is desirable to evaluate seals which have

failed to determine the cause of failure. A
direct approach is the use of a penetrant dye
"which carbonizes after air-firing. This leaves
a carbon track which can be traced through sub-
sequent sectioning and polishing operations.
"An even more direct and simpler approach is
peeling the metal member from the ceramic
member, and observing the condition of the
peeled interface. It is difficult to ascertain the
quality of a seal from the condition of the peeled
interface on a new ceramic metallizing com-
bination. However, considerable experience
with various combinations of ceramics and met-
allizing mixtures will enable a trained operator
to detect differences in the degree of sintering,
the thickness of the metallizing, and the site of
the failure-whether at the metal-ceramic inter-
face, metallizing striation, or metallizing-
electroplating interface.
The adage that pulled ceramic indicates a
good seal is not true for all cases of ceramic-
metallizing combinations. The high alumina
ceramics (above 99-percent alumina) with met-
allizing mixes containing glass-forming ingre-
dients (such as silica and calcia) generally peel
clean from the ceramic when they fail, giving the
appearance of a poor bond; but many seals of this
I appearance have withstood the most severe en-
vironmental conditions imposed upon them.

41
I
I APPENDIX A
RECOMMENDED TOLERANCES AND
METALLIZING ALLOWANCES

I A-1. OD 'SEALS MORE THAN 0.150 INCH OD TO

THIN-WALL 'SLEEVES
Metal Tolerance: k 0.001 inch
Ceramic Tolerance: ± 0. 001 inch

Metallizing Allowance: 0. 006 inch on a diameter

figured on the mean from ceramic and metal
sizes plus the amount of plate in thousands
if metal is plated prior to sealing.
Example: A ceramic with OD of 0. 200 ± 0. 001
inch is to be sealed into a metal which is
to receive 0. 001 inch thickness of plate
prior to sealing. The machined metal
will be 0. 200 + 0. 006 + 0. 002 inch for a
final size of 0. 208 k 0. 001 inch.

j A-2. OD 'SEALS, LESS THAN 0.150 INCH OD TO

THIN- WALL 'SLEEVES
Same as Case A-l, except metallizing
j clearance to be 0. 004 inch.

A-3. ID'SEALS MORE THAN 0.150 INCH ID TO

THIN-WALL 'SLEEVES
Same as Case A-1.

4
43
A-4. ID SEALS LESS THAN 0.150 INCH 10 TO
THIN-WALL SLEEVES
Metal Tolerance: k 0. 0005 inch
Ceramic Tolerance: + 0. 002 inch

Metallizing Allowance: 0. 005 inch and 0. 007

inch figured on the mean from ceramic
and metal sizes.
Example: A ceramic with ID of 0. 045 * 0. 002
inch will require metal sleeves to be
specified at 0. 038 ± 0. 0005 inch and
0. 040 + 0. 0005 inch. The closest fitting
sleeve is used in assembly.

A-5. ID 'SEAL'S TO 'SOLID PINS

Metal Tolerance: ± 0. 0005 inch
Ceramic Tolerance: * 0.002 inch
Metallizing Allowance: 0. 004 inch, 0. 005 inch,
0. 006 inch, 0. 007 inch, 0.-008 inch
figured on the mean from ceramic and
metal sizes.
Example: A ceramic with ID of 0. 045 * 0.002
inch will require pins specified at 0. 037
inch, 0. 038 inch, 0. 039 inch, 0. 040 inch,
0. 041 inch (all *0. 0005 inch). The
closest fitting pin is used.

A-6. HETALLIZED CERAMICS

Design as described above, but metallizing
thickness is 0. 0015 inch minimum.

44
I
I APPENDIX B
g METALLIZING MIXTURES

I Mixture Composition

I Standard Mo-Mn Mix 240 grams Mo

60 grams Mn
Active Metal Mo-Mn 294 grams Mo
Mix (65C)* 6 grams Ti
Lithium Molybdate 270 grams Mo
I Mix (91B) 30 grams LiMoO 3

Calcia Mix (72C) 295. 2 grams Mo

18.4 grams CaO 2

Glassy Mix (50B) 290 grams Mo

24 grams SiO 2
1 11 grams Mn

*Parenthetical references designate Sperry mix

numbers. Refer to Final Technical Report, Metal-
Ceramic Seal Technology Study and Final Technical.
Report, Ceramic-Metal Seals for High-Power Tubes,
Sfurnished by Sperry Gyroscope Co. to U. S. Air
Force under Contracts AF30(602)-2047 and
AF30(602)-2371, respectively.

4
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45

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I
I APPENDIX C
COMBINATIONS OF CERAMIC BODY,
METALLIZING MIXTURE, AND

i SINTERING CYCLE

Recommended
Sintering Cycle
I Time at
Mix (see Tempera- Heat
* Alumina Body Appendix B) ture (°C) (hours)

"94% AD-94 Standard 1500 4-8

* 94% AD-94 91B 1500 6
9416 AD-94 65C 1500 4
I 9616 AL-300 65C 1500 4
96%o AL-300 72C 1575 4-8
J 99. 5%
AD-995 50B 1575 4-8

Atmosphere: Dissociated ammonia with a dew-

point greater than 30 0 C.

1I 4

I
I
I
APPENDIX D
BRAZING MATERIALS
I
I
Brazing
[Material Temperature

Silver-Copper Eutectic* 790 0 C

! 37. 5%
Gold, 62. 5%
Copper Alloy 1030 0 C
I Copper (OFHC) 1100 0 C
Silver 980 0 C

I Stopoff: Milk of magnesia; dry thoroughly

I before firing.

*Kovar must be nickel-plated to prevent

intergranular penetration.

!
1 49
I
APPENDIX E
TYPICAL PLATING BATHS
AND PROCEDURES

E-1. QUALITY OF PLATE

Platings should be smooth, continuous,

uniform in appearance, and not coarsely crystal-
line. Pin holes, blisters, modules, pits,
indications of "burning", and other harmful
defects should not be present. All details of
workmanship should conform to the best plating
practices.
For a simple quality test, fire the plated
item for 10 minutes at 900 0 C in hydrogen or
dissociated ammonia. The plate should show no
blisters or separation from the basis metal at
the interface when examined at a magnification
of approximately 4X.

E-2. COPPER PLATING BATH (ROCHELLE)

a. Equipment

1. Tank - Steel, or sulfur-free rubber-

lined steel, provided with stainless-steel heating
coils, proper temperature controls, and an
exhaust hood.

j 2. Anodes - Pure cast copper, with the

exception of 5 percent of the anode area which

51

!
should be made of steel. The total anode area
should be at least twice the cathode or work
area.

3. Power Supply - Adjustable 0-6 volts

dc, with an accurate ammeter of proper range
for work size (depends upon work area plus
bare rack area).
b. Plating Bath
I.. Composition

Sodium Carbonate 4. 0 oz/gal

Sodium Cyanide 4. 5 oz/gal
Copper Cyanide 3. 5 oz/gal
Rochelle Salt 6.0 oz/gal
Free Sodium Cyanide 0. 75 oz/gal
Temperature 65 0 C :k 50 C
pH 12.0-12.8

2. Preparation

(a) Dissolve sodium carbonate and sodium

cyanide in warm distilled or deionized
water.
(b) Add copper cyanide slowly, while
stirring the solution.
(c) When the copper cyanide is completely
dissolved, add the rochelle salt.

52
!
(d) Purify the bath at low current densi-
"tiesfor 10 to 12 hours before use.
Use a large stainless-steel plate run
at 2 amperes per square foot.

3. Maintenance

(a) Weekly chemical analyses for the

purpose of adjusting the composition
to correct concentration.

(b) Daily addition of deionized or dis-

tilled water to account for losses
(solution evaporated or carried out of
tank on work and rack).
(c) Frequent or continuous filtering
through activated charcoal filters
to remove organic and particulate
contamination.

c. Plating Procedure

1.
S Immerse metallized ceramic parts
in plating bath for 30 seconds with
no current flowing. Apply current
and plate at 20 amperes per square
foot for 30 seconds, or until a uni-
form copper film covers the part.
2. Cleaned metal parts are given a 2-
percent sodium cyanide dip prior to
plating at a current density of 15-40
amperes per square foot. The plating
rate at 15 amperes per square foot
is 0. 0012 inch per hour, and at 40
amperes per square foot is 0. 002 inch
per hour.

53
I
E-3. NICKEL PLATING BATH ('SULFAMATE)
a. Equ••ment

I. Tank - Rubber-lined steel tank or

equivalent equipped with a Karbate or tantalum
steam coil. Stainless-steel (300 series) or
inconel steam coils can be used when the bare
metal cannot become anodic. Means for agitat-
ing the work and continuous filtrations of the
solution are recommended.

2. Anodes - 99-pprcent rolled, depolarized

nickel. The anodes should be properly bagged
and at least twice the work area.

b. Plating Bath
1. Composition

Nickel Sulphamate 60 oz/gal

Boric acid 4 oz/gal
Anti-pit agent SNAC
(Barrett Chemical
Products Co. ) 0. 05 oz/gal
Temperature 500C

pH 3.5-4.5
Density (Baume) 29-31

2. Preparation

Dissolve all salts in warm water with

stirring.

54
I
3. Maintenance
(a) Weekly chemical analyses for the
purpose of adjusting the composi-
tion to correct concentration.

(b) Weekly (or more frequent) filtering

j through activated charcoal.
(c) Daily check of pH.

I c. Plating Procedure

1. After plating metallized ceramics

with copper as in paragraph E-2,
immediately rinse in deionized
water.
2. Electroplate in nickel plating tank
at 50 amperes per square foot.
Plating rate is approximately 0. 003
inch per hour.

3. For metal parts, "strike" first in

following bath at 65 amperes per
square foot for I minute:
I Nickel Chloride 32 oz/gal
Hydrochloric Acid 4. 8 av
20OBe (tech grade) oz/gal

Room temperature
99-percent nickel
anodes
Follow immediately with step 2
above.

1 55
p