JOINING

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JOINING
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SHIELDED METAL ARC WELDING
50 of all welds are fabricated in this manner.
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MIG WELDING
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MICROSTRUCTURAL EVOLUTION IN THE WELD REGION
BASED ON THERMAL HISTORY
In the heat affected zone (HAZ) far from the weld
bead, the material experiences lower temperature,
and recrystallized grains are small. The metal
there is hard. Very close to the weld bead, the
material experienced temperatures high up in the
austenite phase field. The grains are larger,
and the material is soft. Under load, the soft
region deforms much more than the surrounding
regions. Consequently failure tends to occur
there. This problem can be reduced through
appropriate heat treatment (stress relieving).
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MORPHOLOGY OF DENDRITES IN THE WELD REGION
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Fe-C PHASE DIAGRAM, ILLUSTRATING DIFFERENT
TEMPERATURES IN THE WELD AND HAZ.
The temperatures in the HAZ determine the weld
microstructure.
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DEFECTS IN WELDS
Porosity is generally due to trapped hydrogen and
moisture.
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CRACKING OF WELDS
Largely caused by shrinkage of the weld on
solidification. Often occurs in the zone of last
solidifying metal. However, there are transverse
cracks as well as cracks in the HAZ (toe cracks)
as well.
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ROLL BONDING AS A FORM OF JOINING
Traditionally used for cladding. Examples pure
Al on Al-Zn (7xxx) alloys, or stainless steel on
mild steel. More recently, the method offers a
scope for making super strong materials with very
fine grain size.
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JOINING SOLDERING OF ELECTRONIC COMPONENTS
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SURFACE MOUNTING OF ELECTRONIC COMPONENTS
Use of screen printing in depositing solder paste.
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FLIP CHIP TECHNOLOGY IN SURFACE MOUNTING OF IC
CHIPS ON PRINTED CIRCUIT BOARDS
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DETAILS OF MOUNTING OF CHIP
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PHOTOLITHOGRAPHY, MICROMACHINING AND MEMS
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PHOTO-LITHOGRAPHY IN FABRICATION OF ELECTRONIC
COMPONENTS
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ANISOTROPIC VERSUS PARTIALLY ANISOTROPIC ETCH
In Si, different chemicals have different rates
of dissolution of different planes e.g., (001),
(111) planes etc.). The chemicals are designed
to be anisotropic, so as to obtain sharp edges
without undercut. Figure (a) shows such a
chemical cut. SiO2 is formed by oxidizing the Si
single crystal at 900 - 1200 C. Silica dissolves
much faster than Si.
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CHEMICAL MICROMACHINING THAT TAKES ADVANTAGE OF
THE HIGHLY ANISOTROPIC ETCH CHARACTER OF Si WITH
SPECIFIC CHEMICALS
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MICROMACHINING OF A MEMBRANE
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MEMS
Micro Electro Mechanical Systems (MEMS) are
mostly Si-based microscopic devices whose
processing is compatible with a chip
manufacturing process. As a result, the
mechanical and electronic components of the
assembly are highly integrated. At the same
time, the designer of the component must select
materials and designs that are compatible with
the process.
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MICROMACHINING OF A CANTILEVER IN MEMS
APPLICATIONS
Silica is formed by oxidizing the Si substrate.
Polysilicon is deposited by chemical vapor
deposition (CVD). The silica is etched away
(mostly by plasma etching process), since silica
dissolves much faster than Si. Often a silicon
nitride layer is introduced before step 3 to get
stronger joints between the polysilicon and the
Si substrate.
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MEMS CANTILEVER SYSTEM
Opposite charge attracts, like charge repels.
A charge difference between the bottom substrate
and the top cantilever provides a Coulomb force.
An oscillating signal can provide rapid vibration
of the system (kHz to MHz range).
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A MEMS MIRROR SYSTEM
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INTRICATE DESIGN OF A HINGE IN A MEMS SLIDER
SYSTEM
Fractures
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LIGA PROCESS IN MEMS MANUFACTURING
Requires a conductive substrate to deposit Ni by
electroplating. The process offers larger
structures and greater flexibility with materials.
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LIGA PRODUCT
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PRINTED CIRCUIT BOARDS
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PC BOARD
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PRINTED CIRCUIT BOARDS
Uses photolithography and etching of Cu to
produce the wiring.
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MULTI-LAYER PC BOARD
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SURFACE MOUNTING ON PC BOARDS