Transcript EIN 3390 Chap 19 Non..

Chapter 19
Electronic Electrochemical
Chemical and Thermal
Machining Processes
(Review)
EIN 3390
Manufacturing Processes
Fall, 2010
19.1 Introduction
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Non-traditional machining (NTM) processes
have several advantages
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Complex geometries are possible
Extreme surface finish
Tight tolerances
Delicate components
Little or no burring or residual stresses
Brittle materials with high hardness can be
machined
◦ Microelectronic or integrated circuits (IC) are
possible to mass produce
NTM Processes
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Four basic groups of material removal using NTM
processes
◦ Chemical:
 Chemical reaction between a liquid reagent and
workpiece results in etching
◦ Electrochemical
 An electrolytic reaction at workpiece surface for
removal of material
◦ Thermal
 High temperature in very localized regions
evaporate materials, for example, EDM
◦ Mechanical
 High-velocity abrasives or liquids remove
materials
Limitations of Conventional
Machining Processes
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Machining processes that involve chip
formation have a number of limitations
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Large amounts of energy
Unwanted distortion
Residual stresses
Burrs
Delicate or complex geometries may be
difficult or impossible
Conventional End Milling vs. NTM
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Typical machining parameters
◦ Feed rate (5 – 200 in./min.)
◦ Surface finish (60 – 150 min) AA – Arithmetic
Average
◦ Dimensional accuracy (0.001 – 0.002 in.)
◦ Workpiece/feature size (25 x 24 in.); 1 in. deep
NTM processes typically have lower feed
rates and require more power
consumption
 The feed rate in NTM is independent of
the material being processed
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19.2 Chemical Machining
Processes
Typically involves metals, but ceramics
and glasses may be etched
 Material is removed from a workpiece by
selectively exposing it to a chemical
reagent or etchant
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◦ Gel milling- gel is applied to the workpiece in
gel form.
◦ Maskant- selected areas are covered and the
remaining surfaces are exposed to the etchant.
This is the most common method of CHM.
Table 19-1 Summary of NTM Processes
Masking
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Several different
methods
◦ Cut-and-peel
◦ Scribe-and-peel
◦ Screen printing
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Etch rates are slow
in comparison to
other NTM processes
Figure 19-1 Steps required to produce a stepped contour
by chemical machining.
Defects in Etching
Figure 19-2 Typical chemical milling defects: (a) overhang: deep cuts with improper
agitation; (b) islands: isolated high spots from dirt, residual maskant, or work material
inhomogeneity; (c) dishing: thinning in center due to improper agitation or stacking of parts in
tank.
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If baths are not agitated properly, defects
result
Advantages and Disadvantages
of Chemical Machining
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Advantages
◦ Process is relatively
simple
◦ Does not require
highly skilled labor
◦ Induces no stress or
cold working in the
metal
◦ Can be applied to
almost any metal
◦ Large areas
◦ Virtually unlimited
shape
◦ Thin sections
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Disadvantages
◦ Requires the handling
of dangerous
chemicals
◦ Disposal of
potentially harmful
byproducts
◦ Metal removal rate
is slow
Photochemical Machining
Figure 19-4 Basic steps in photochemical machining (PCM).
Design Factors in Chemical
Machining
If artwork is used, dimensional variations can
occur through size changes in the artwork of
phototool film due to temperature and
humidity changes
 Etch factor (E)- describes the undercutting of
the maskant
◦ Areas that are exposed longer will have more
metal removed from them
◦ E=U/d, where d- depth, U- undercutting
 Anisotropy (A)- directionality of the cut,
A=d/U, and Wf = Wm + (E d), or
Wm = Wf - (E d)
 where Wf is final desired width of cut
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d/3
Chemical-Mechanical Polishing
(CMP)
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Uses the synergy
of chemistry and
mechanical
grinding to obtain
flatness on the
order of 50 nm.
CMP is used to
fabricate
integrated
circuits (ICs)
Figure 19-6 Schematic of chemical-mechanical
polishing (CMP).
Photochemical Machining for
Electronics
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Most common method for creating maskants
Involves the use of UV (Ultra-Violet) lightsensitive emulsions, called photoresists
Photoresists are applied to the surface of the
workpiece and selectively exposed to an intense
ray of UV light
ICs use semiconductor materials that can be
made to be either electrically conducting or
insulating
◦ Doping modifies these electrical properties by
introducing impurity atoms into
semiconductors.
Silicon is the most widely used semiconductor
material
How ICs are Made
Ability to selectively modify the electrical
properties of semiconductors is the backbone of
microelectronic manufacturing
On Fig 19-8, the sequence of processes or steps
required to manufacture a simple metal-oxidesemiconductor (MOS) is show up.
Photolithography shown on Fig 19 - 9 is used to
produce a polymeric mask over the oxide layer,
which allows only selected areas of the oxide layer
to be etched.
IC Manufacturing and Economics
Small circuits are inexpensive, but the
cost of packaging, testing, and
assembling the completed circuits into an
electronic system must be taken into
account
 Ways to improve the economics
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◦ Increase wafer size
 Increases the usable area
◦ Increase the number of chips per wafer by
decreasing chip dimensions
◦ Improve die yield
IC Packaging
Serves to distribute electronic signals
and power
 Provides mechanical interfacing to test
equipment and printed circuit boards
(PCBs)
 Protect the delicate circuitry from
mechanical stresses and electrostatic
discharge during handling and in corrosive
environments
 Dissipate heat generated in the circuits
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Steps in IC Packaging
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Two main methods in which components are
connected to the circuit on the PCB
DIP is an example of through-hole (TH)
technology, or pin-in-hole (PIH)
◦ IC packages and discrete components are inserted
into metal-plated holes in the PCB and soldered
from the underside
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Surface mount (SM) technology places
electronic components onto solder paste pads
that have been dispensed onto the surface of
the PCB
IC Packaging
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SM technology
◦ Packages are more cost-effective than TH
◦ Designed for automated production
◦ TH components have only one lead geometry
and SM have many different designes
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Lead geometries
◦ Butt lead or J-lead
◦ Gull wing leads
◦ Solder balls
PCB Fabrication Process
Figure 19-13 Typical base materials used may
be epoxy-impregnated fiberglass, polyimide, or
ceramic. Epoxy-impregnated fiberglass is the
cheapest substitute for interconnecting leaded
packages. Fiberglass is used to increase the
mechanical stiffness of the device for
handling, while epoxy resin imparts better
ductility. The fiberglass is impregnated on a
continuous line where resin infiltrates the
fiberglass mat in a dip basin, and the soaked
fabric passes through a set of rollers to
control thickness and an oven where the
resin is partially cured. The resulting glass resin
sheet is called prepreg. Multiple prepregs are
then pressed together between
electroformed copper foil under precise heat
and pressure conditions to form a copperclad laminate or PCB.
19.3 Electrochemical Machining
Process
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Electrochemical
machining (ECM)
removes material
by anodic
dissolution with
a rapidly flowing
electrolyte
The tool is the
cathode and the
workpiece is the
electrolyte
Figure 19-17 Schematic diagram of
electrochemical machining process
(ECM).
Table 19-3 Material Removal Rates for ECM Alloys
Assuming 100% Current Efficiency
Electrochemical Processing
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Pulsed-current ECM (PECM)
◦ Pulsed on and off for durations of
approximately 1ms
Pulsed currents are also used in
electrochemical machining (EMM)
 Electrochemical polishing is a modification
of the ECM process
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◦ Much slower penetration rate
Table 19-4 Metal Removal Rates for ECG for Various
Metals (Electrochemical Grinding – ECG)
Advantages and Disadvantages
of Electrochemical Machining
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Advantages
◦ ECM is well suited for
the machining of
complex twodimensional shapes
◦ Delicate parts may be
made
◦ Difficult-to machine
geometries
◦ Poorly machinable
materials may be
processed
◦ Little or no tool wear
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Disadvantages
◦ Initial tooling can
be timely and
costly
◦ Environmentally
harmful by-products
19.4 Electrical Discharge
Machining
Electrical discharge machining (EDM)
removes metal by discharging electric
current from a pulsating DC power
supply across a thin interelectrode gap
 The gap is filled by a dielectric fluid, which
becomes locally ionized
 Two different types of EDM exist based on
the shape of the tool electrode
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◦ Ram EDM/ sinker EDM
◦ Wire EDM
Figure 19-21 EDM or spark erosion machining of metal, using high-frequency spark discharges in
a dielectric, between the shaped tool (cathode) and the work (anode). The table can make X-Y
movements.
EDM Processes
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Slow compared to
conventional
machining
Produce a matte
surface
Complex
geometries are
possible
Often used in tool
and die making
Figure 19-22 Schematic diagram of equipment
for wire EDM using a moving wire electrode.
Effect of Current on-time and
Discharge Current on Crater Size
MRR = (C I)/(Tm1.23),
Where MRR – material removal rate in in.3/min.; C –
constant of proportionality equal to 5.08 in US customary
units; I – discharge current in amps; Tm – melting
temperature of workpiece material, 0F.
Example:
A certain alloy whose melting point = 2,000 0F is to be
machined in EDM. If a discharge current = 25A, what is the
expected metal removal rate?
MRR = (C I)/(Tm1.23) = (5.08 x 25)/(2,0001.23)
= 0.011 in.3/min.
Figure 19-25 The principles of
metal removal for EDM.
Effect of Current on-time and
Discharge Current on Crater Size
From Fig 19 – 25: we have the conclusions:
◦ Generally higher duty cycles with higher
currents and lower frequencies are used to
maximize MRR.
◦ Higher frequencies and lower discharge
currents are used to improve surface finish
while reducing MRR.
◦ Higher frequencies generally cause increased
tool wear.
Considerations for EDM
Graphite is the most widely used tool
electrode
 The choice of electrode material depends
on its machinability and coast as well
as the desired MRR, surface finish,
and tool wear
 The dielectric fluid has four main functions
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Electrical insulation
Spark conductor
Flushing medium
Coolant
Table 19-5 Melting Temperatures for Selected EDM
Workpiece Materials
Advantages and Disadvantages
of EDM
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Advantages
Applicable to all
materials that are
fairly good
electrical
conductors
Hardness,
toughness, or
brittleness of the
material imposes
no limitations
Fragile and delicate
parts
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Disadvantages
Produces a hard
recast surface
Surface may
contain fine cracks
caused by
thermal stress
Fumes can be toxic
HW for Chapter 19 (due date 11/30/2010)
Review Questions:
7, 17, 18 (page 521, 5 points for each question )