the j o i n i n g welding process used to engineering

1. The j o i n i n g of c e r a m i c s : m i c r o j o i n i n g
The properties of ceramics
- Metals are characteristically ductile, have high thermal and electrical
conductivity, and relatively high coefficients of thermal expansion;
-Ceramics are brittle, have low thermal and electrical conductivity, and
their thermal expansion coefficients tend to be somewhat lower than
those of metals;
-Superconducting ceramics have been developed with a critical
temperature of up to — 1500C;
- Dispersions of SiC whiskers in various ceramic matrices (Al2O3 for
example) have given fracture toughness values of up to 15MNm-3/2,
which is to be compared with a value of about 20MNm-3/2 for flake
graphite cast iron ;
-Thermal expansion characteristics are not necessarily unmatchable;
-The chief obstacle to direct metal-ceramic bonding is the combination
of brittleness and thermal expansion mismatch;

2. Cont’d
- In consequence, fusion welding is generally regarded as
inapplicable to ceramic metal joints;
-Much metal-ceramic joining is mechanical in character;
- Press-fitting or shrink fitting is extensively used in electrical
equipment;
-The attachment of fireproofing and other insulating material to
steel is by means of studs or frame work welded to the metal
surface, and is primarily mechanical;
-The main technical interest lies in those applications where a bond
is formed at a metal-non-metal interface;

3. G l a s s - m e t a l s e a l s
General
-The sealing of glass to metal has been in use for many years in the
production of incandescent lamps and electron tubes and more
recently for vapour lamps and housings for semiconductors.;
- Most early incandescent lamps were sealed with lead zinc borate types
of glass, which soften and flow at relatively low temperatures;
- Glass is available ranging from soft glass (sodalime silica or lead
oxide/mixed alkali silica) with working temperatures of 800-10000C
and thermal expansion coefficients above 5 x 10-6K-1 to hard glasses
(borosilicate, alumino silicate and vitreous silica) with working
temperatures of 1000-13000C and thermal expansion coefficients
below 5 x 10-6K-1.
Joint design
-Glass-metal joints may be divided into two types: matched and
unmatched seals;
-In matched seals the thermal expansion coefficient of the glass is
matched as closely as possible to that of the metal, and there is
chemical bonding at the glass metal interface;

4. Cont’d
-Unmatched seals fall into two categories. The first is the compression
seal, in which the glass surrounds the metal conductor or duct, and
is itself surrounded by a metal ring which, on cooling to room
temperature, contains residual tensile stress, so putting the glass in a
state of compression;
-A chemical bond between metal and glass is not necessary but is
desirable ;
-A glass ring is sealed on to a copper tube, the end of which has been
tapered to a point;
- Differential expansion is accommodated by flexure of the tube
without placing any significant stress on the glass;
- A glass tube or other shape is then fused to the ring;
- There is good chemical bonding between copper and glass;
Housekeeper seals may be made with other metals but are particularly
useful for copper, whose thermal expansion is higher than that of any
commercial glass.

5. Residual strain in a matching seal
-On cooling from the liquid state, glass, being an amorphous substance,
does not crystallize and solidify at a fixed point but becomes
progressively more viscous;
There is a working temperature at which the viscosity is l0kgm-1 s-1and
below which the glass can no longer be manipulated;
-At the softening temperature the viscosity is 10 4.5 kg m-1 s-1 and
below this the glass behaves generally as an elastic solid;
- At the annealing temperature it takes longer than 15 min to relieve
residual stress and the viscosity is 1010 kg m-1 s-1 and finally at the strain
temperature it takes longer than 4h to relieve stress and the viscosity
is 10 11. 6 kg m-1 s-1;
-Between the strain point and the annealing point is the set point, and in
glass-metal systems this point is taken as the temperature below which
elastic strain can be generated in the glass if there is a mismatch in the
contraction;
-Higher differential strains require smaller and thinner assemblies.

6. Cont’d
Bonding mechanisms
-In principle, the chemistry of a glass-metal bond is very simple;
- The oxide layer on the metal surface is dissolved by the liquid glass,
which wets and is directly bonded to the metal surface;
-So far as oxide formation is concerned, metals may be divided into three
categories: those of low reactivity, a medium-activity group and a
high-activity group;
-The first category includes gold, silver and platinum;
-Oxidation in air does not produce bulk oxide layers but oxides may be
adsorbed; however, these dissociate on heating;
- In the case of silver, Ag2O dissolves in the glass and a silver silicate forms
at the interface, resulting in a chemical bond;
-Gold and platinum do not react in this way;
-Iron, nickel and cobalt fall into the medium-activity group;

7. Cont’d
-Iron is not much used for electrical or electronic
applications, but in porcelain and vitreous enamel coatings
a powdered glass is applied to an iron or steel substrate and
subsequently fired to form a glass over layer which is
bonded to the metal;
The oxide layer formed before and during the firing operation
dissolves in the glass, and provided that the glass is saturated with
FeO at the interface the bond strength is high;
-When the oxide concentration falls owing to diffusion into the
glass, the bond becomes progressively weaker. However, if cobalt
is added to the glass, this is reduced by iron at the interface to
form FeO: Fe + CoOglass = FeOglass + Co;
-The cobalt alloys with iron to form dendrites that grow into the
glass and improve bonding;

8. Cont’d
-Kovar (29 Ni-17 Cr-54 Fe) is a low-expansion alloy much used for glass-metal
seals. This falls into the medium-activity group but the bond strength is
largely governed by the thickness of a preformed oxide layer;
-The high-activity group includes chromium, titanium and zirconium;
-Chromium in particular is present in many ferrous alloys;
- Chromium may reduce silicates in glass to form CrSi x crystals which grow
into glass from the interface;
- In alloys reactions may be complex owing to the formation of multilayer
oxides on the metal surface.
Forming a glass-metal seal
-The metals used for glass-metal seals include platinum, tungsten,
molybdenum, copper and various alloys of iron, nickel, cobalt and
chromium;
-Tungsten and Kovar both have low expansion coefficients, and a number of
proprietary matching glass compositions are available for these metals;
-The first step in forming a seal is to clean the metal surface by heating in a wet
hydrogen atmosphere at 11000C; this removes hydrocarbons and other
contaminants and results in some etching and roughening;

9. Cont’d
- Secondly an oxide layer is formed by heating in air;
-The glass is applied as a powder coating on the metal surface, and the
assembly heated to the sealing temperature, typically 10000C;
-The bulk glass is then sealed to the coating;
- Alternatively metal pins and powdered glass may be put together in a metal
frame and then heated to the sealing temperature;
- The assembly is then cooled at a controlled rate to room temperature;
- The critical temperature is around the set point, and the glass must be
cooled slowly through this temperature range, or annealed, in order to avoid
damaging thermal stresses.
Glass-ceramics
General
-Glass has the useful property that, when molten, it will wet and bond to an
oxidized metal surface; However, being an amorphous substance, it softens
at elevated temperature and lacks the rigidity and strength which most
ceramics. exhibit under such conditions;
- With glass-ceramics it is possible to combine these desirable characteristics by
forming and bonding the substance when it is in a glassy state, and then
causing it to crystallize and transform into an elastic, non viscous solid.

10. Cont’d
-Crystallization (devitrification) of a common sodium silicate glass is well
known as a form of deterioration; the process starts at the surface and
results in an opaque, coarse-grained substance that has little coherence;
-The crystallization of glass-ceramics, however, is nucleated internally so as to
produce a uniform, fine grained polycrystalline material;
-The majority of the substances that form glass-ceramics are silicates;
-They are made by melting the raw materials together in the temperature range
1000-17000C to form a glass;
-Suitable nucleating agents such as P2O, TiO2 and ZrO2 are incorporated;
- The molten glass is then worked to the required form by normal glass-
making techniques and cooled to room temperature;
- Conversion to the polycrystalline form is accomplished by first heating to a
temperature at which the nuclei precipitate, and then to a higher
temperature at which the crystallization takes place;
-In some cases both nucleation and crystallization occur at the same
temperature;
-So far as metal-ceramic joints are concerned one advantage of glass-ceramics
is the wide range of thermal expansion properties that are available,
making it possible;

11. Cont’d
Making glass-ceramic seals
-The metal parts are cleaned, roughened and pre-oxidized and then
placed in the mould;
- Molten glass is then introduced and formed by means of a plunger;
- When set, the assembly is transferred to a furnace where the
nucleation and precipitation heat treatments are carried out.;
- In a second method, the metal parts are placed together with
preformed glass in a graphite mould;
- The whole assembly is heated in an inert atmosphere to a temperature
at which the glass flows and bonds to the metal;
- In this technique the glass-ceramic transformation usually takes place
on cooling in the mould;
-Glass-ceramic seals are used for high-voltage and high-vacuum devices,
and for thermocouple terminations;

12. Cont’d
Coatings
-Glass-ceramics may be applied as a coating to metals in much the
same way as vitreous enamelling;
-The metal is prepared and glass is applied by spraying a powder
suspension either as a uniform coat or through a wire mesh
screen where it is required to develop a pattern;
- Alternatively dry powder is deposited electrostatically;
-The coatings are dried when necessary and then the assembly is
fired so as to flow the glass and make a bond;
- Crystallization normally takes place during the firing cycle;
-Glass-ceramic coatings have the advantage over vitreous enamel
that they are refractory. Thus they can withstand the firing
temperature of 850-900 0C which is required for the application
of thick-film circuitry.

13. Brazing
General
-Many of the technically important ceramics that have been developed in
recent years, including alumina, zirconia, beryllia, silicon carbide,
titanium carbide, silicon nitride, boron nitride and sialons, are not
wetted by the commonly used brazing metals. An exception to this rule
is tungsten carbide, which is used for machining, and is attached to a
steel shank by silver brazing.;
-There are two ways of overcoming the non-wetting problem;
-The first is to metallize the surface of the ceramic and then make a
brazed joint between the metallized layer and the bulk metal;
-The second method is to employ a brazing alloy incorporating an
ingredient that interacts chemically with the ceramic surface. This
procedure is known as active metal brazing.
Metallization
-The metallization technique most commonly used, particularly for
alumina, is the manganese-molybdenum process;
-The primary metallizing layer is applied initially as a paint consisting of
a mixture of molybdenum, manganese oxide, a glass frit, a carrier
vehicle and a solvent. This coating is fired at 15000C in an atmosphere
of hydrogen or cracked ammonia containing water vapour;

14. Cont’d
-During this operation the glass bonds with the ceramic (which itself must
contain a glassy intergranular phase) and the metallic particles sinter
together;
-The second step is to nickel-plate the surface either by electroplating or using
an electroless process, after which the component may be once again
sintered in a hydrogen atmosphere or go directly to brazing;
-The brazing filler metal is usually the 72 Ag-28 Cu eutectic, which has a low
melting point (780 0C) and good ductility to accommodate any thermal
contraction mismatch;
-For higher-temperature applications a nickel-based brazing metal may be used;
-The manganese-molybdenum process requires the presence of a glassy phase
in the ceramic. Where this is not acceptable or not practicable the metal
coating must be applied directly, for example by vapour deposition or by ion
sputtering;
Active metal brazing
-The active element in all commercially available ceramic-metal brazing
alloys is titanium;
-TiO is metallic in character and is wetted by brazing metal. Similar reactions
would appear to occur with carbides and nitrides; at any rate these types of
ceramics are wetted by Ti-bearing alloys;

15. Cont’d
-Prior to brazing, both ceramic and metal must be clean and free from
organic contaminants;
- Rough ceramic surfaces are less easily wetted, and for strong joints it
is desirable to finish the surface by lapping or by refiring at high
temperature;
-Brazing is carried out in a vacuum furnace at a pressure of less than 0.1
Nm-2,preferably 10-2-10-3 N m-2;
-Alternatively an atmosphere of purified argon or helium, with an
oxygen content of less than 1 ppm, or a hydrogen atmosphere with a
dew point lower than —510C may be used;
- Assemblies are heated at a rate of 10-200C min-1 and the temperature
is held for at least 15min below the solidus temperature of the solder
to ensure uniformity;
Then it is raised slowly to500C above the liquidus and held for 30min or
longer;
-The wetting reaction is sluggish and the fluidity of the brazing metal is
relatively low;

16. Other techniques
Diffusion bonding
-As compared with the diffusion bonding of metal-metal joint, the ceramic-
metal joint presents several additional problems. Firstly, only one
component, the metal, is capable of significant plastic deformation.
Secondly, there is no inter diffusion.
Thirdly, there must be some means of accommodating thermal mismatch;
- The last condition usually requires the use of interlayers, which are thin
wafers of metal having a thermal expansion coefficient intermediate between
those of the metal and the ceramic; Thus alumina has been bonded to copper
for electronic applications using a gold interlayer;
- One of the largest applications to date is the accelerator tube for the 20 MV
van de Graaff generator .
-The tube consists of a sandwich structure of insulating rings which form an
evacuated envelope for the ion beam .

17. Cont’d
-The thermal match between titanium and alumina is relatively good, but it was
decided nevertheless to use an aluminium interlayer;
- Early bonding trials were made in high-purity argon but it proved impossible
to maintain the oxygen level required to avoid damage to the titanium
electrodes, and eventually a press was designed which operated in a vacuum
below 10-3 N m-2 at 6500C, equivalent to an inert gas purity of 0.01 ppm by
volume;
- The individual subassemblies were subject to a final test capable of detecting
leaks of 10-10 mbar 1 s-1 for helium, and the reject rate was maintained below
10%.; This tube has operated successfully and no failures of the metal-
ceramic joints have been experienced either in commissioning or in service;
- The thermal match between titanium and alumina is relatively good, but it
was decided nevertheless to use an aluminium interlayer;
- Early bonding trials were made in high-purity argon but it proved
impossible to maintain the oxygen level required to avoid damage to the
titanium electrodes, and eventually a press was designed which operated in a
vacuum below 10-3 N m-2 at 6500C, equivalent to an inert gas purity of 0.01
ppm by volume.

18. Cont’d
-The individual subassemblies were subject to a final test capable of detecting
leaks of 10-10 mbar 1 s-1 for helium, and the reject rate was maintained
below 10%. This tube has operated successfully and no failures of the
metal-ceramic joints have been experienced either in commissioning or in
service;
Electrostatic bonding
-This process is also known as field-assisted bonding;
-If a smooth metal plate and a smooth ceramic surface are heated in contact in
a vacuum, and if a voltage is applied to the joint, metal ions will migrate
from the ceramic material into the metal, leaving an ion-depleted zone
close to the interface. This, in turn, gives rise to an electrostatic field
which generates a bonding force.
-Applied voltage is typically in the range 100-500 V, and the depth of the ion-
depleted zone is of the order of micrometres.
-There is significant interdiffusion, and silver appears to be particularly
mobile, having replaced sodium in the ion-depleted zone;

19. Cont’d
-In the reaction bonding of Al-2.5 Mg to zirconia, the interdiffusion is similar
to that in the aluminium-glass joint; there is a gradient of both aluminium
and magnesium contents on the zirconia side of the joint;
-The current that flows during this operation is very small, of the order of
microamperes in the case of an aluminium/glass bond;
-For such a joint, a minimum quantity of electricity must pass before bonding
will occur;
-The amount of electricity required is reduced if the axial pressure during the
bonding operation is increased;
-Field-assisted bonding has been applied to the joining of metals to glass, β
alumina and other electrical insulators;
Friction welding
-The technique used is similar to that for metal-metal joints; usually the
ceramic is held stationary and the metal is rotated. Rotation continues for a
predetermined period, then it stops and the joint is upset, giving the
characteristic flash;
-The nature of the bond is uncertain and may in part be mechanical.

20. Cont’d
Various metal ceramic joints have been made experimentally and this technique
may find a use where aluminium heat sinks need to be attached to AlN
substrates;
Ceramic-ceramic bonds
-Glasses and glass ceramics may be used as bonding media for oxide ceramics and
for those ceramics that have an intergranular glassy phase. They cannot
accommodate a thermal mismatch and are therefore most suitable for joining a
material to itself;
-Alumina has been joined using glasses of Al2O3-MnO-SiO2and Al2O3-CaO-MgO-
SiO2 compositions;
- Silicon nitride has been joined using an Al2O3-MgO-SiO2 glass, which simulates
the intergranular phase;
- Bonding was carried out in a nitrogen atmosphere at a temperature between 1550
and 16500C;
Alternatively, bonds can be made using sintering additives (MgO-Al2O3 or Y2O3-
Al2O3), again in a nitrogen atmosphere and at temperatures above 15000C, with
or without pressure.
-Ceramics may also be joined by metallizing and brazing or diffusion bonding:
however, such joints are not refractory and may not be suitable for elevated
temperature applications.

21. Microjoining
-Microjoining is employed in the assembly and fabrication of electronic devices;
-The soldering of components to printed circuit boards ;
-There are many other joining requirements, however;
- In most instances active electronic devices must be protected by means of a
package, which is a sealed metal or ceramic box;
- The silicon chips must be bonded to a substrate, and interconnecting wires
bonded to the chips and to other circuitry;
-Joints must be made between devices and heat sinks;
-A high-power silicon chip is soldered to a BeO heat spreader;
- BeO has high thermal conductivity combined with electrical insulating properties:
hence its use for this purpose;
-The heat spreader has been metallized on the underside by the manganese-
molybdenum process .This is then diffusion bonded to the copper heat sink
at 30O0C with a pressure of less than 20 N mm-2 and with a bonding time of5-10
min;
-An intermediary foil of silver may be desirable to reduce contraction strains;
-The lid of metal packages may be bonded to the base by various methods;
-The nickel plating acts as a brazing metal;
Gold plating is used for the same purpose.

22. Cont’d
-Adhesive bonding is made with an epoxy or polyamide adhesive loaded with
metal powder where electrical or thermal conductivity is required;
- In such cases the chip may be metallized with an Al-Cr-Ni-Au alloy;
- The same type of metallization is used for soldering.;
- A 95Pb5Sn solder preform is used and may be fused manually with flux or
furnace (reflow) soldered in a hydrogen atmosphere;
-The other important type of joint is that between wire and, on the one hand, the
aluminium pad on the surface of the chip and, on the other hand, the circuit
track on the substrate;
- Aluminium or gold wires are used, and they are attached by ultrasonic
welding;
The joint is made either with a flattened length of wire (wedge bonding) or
with a ball formed at the wire tip by melting (a ball bond);
- High-power semiconductor devices are used for switching and regulating
current in welding power sources, in heavy-duty motor drives and other
large electrical systems.

23. Cont’d
-The devices are capable of switching over 1000 A in microseconds, and groups
of them may be used to handle a power of several megawatts. They consist
essentially of a circular disc of silicon containing the required diffused-in
junctions, normally 0.5 mm thick and up to 125 mm in diameter. During
operation heat is generated at a rate of 1 kW in a 50 mm diameter device,
and this is dissipated through a copper heat sink;
-The thermal mismatch between copper and silicon is too great to permit direct
bonding, however, so normal practice is to braze the silicon to an
intermediate plate of molybdenum using an aluminium-silicon eutectic
brazing alloy;
-A mechanical pressure joint is then made between the copper and the
molybdenum, to produce a copper-molybdenum-silicon-molybdenum-
copper sandwich ;
-This technique is satisfactory for diameters up to about 75 mm but for larger
sizes and for higher efficiency it has disadvantages;
- In order to wet the molybdenum it is necessary to use a brazing temperature of
680-750 0C and at this level some of the silicon is dissolved and the wafer
may be partly eroded;
-Therefore new joining methods are being developed.

24. Cont’d
-One technique is to use vapour deposition to apply a layer of the
aluminium-silicon eutectic to the silicon wafer, and to coat the
molybdenum with a layer of MoSi2. This makes it possible to
braze at a lower temperature. Alternatively the two elements
are joined by diffusion soldering. This method employs a
silver or silver-indium solder which is initially liquid and wets
both surfaces. The joint is then held at about 2000C to diffuse
out the solute element, leaving an intermediate layer which is
mainly silver and has a high remelt temperature, as required for
this type of joint;
-Micro-joining is characterized by a great diversity of techniques
and of metal-metal and metal-non-metal combinations.