Reading Assignment
- 19.2 Chemical Machining Processes
Recommended Reading
- Chemical Machining (chapter preview)
See Also
Outline
Chemical Machining (or milling) Processes
Typically involves metals, but ceramics and glasses (usually with hydrofluoric acid) may be etched
Material is removed from a workpiece by selectively exposing it to a chemical reagant or etchant
- Gel milling- gel is applied to the workpiece
- Maskant- selected areas are covered and the remaining surfaces are exposed to the etchant
Masking
Several different methods
- Cut-and-peel
- Screen printing
Chemical milling: single-sided blind etching
Chemical blanking: through etching of a workpiece.
Etch rates are slow in comparison to other NTM processes
Defects in Etching
If baths are not agitated properly, defects result
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.
Advantages and Disadvantages of Chemical Machining
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
Disadvantages
- Requires the handling of dangerous chemicals
- Disposal of potentially harmful byproducts
- Metal removal rate is slow
Photochemical Machining
Uses light to create a mask. Etched as described above.
Chemical-Mechanical Polishing (CMP)
- Uses the synergy of chemistry and mechanical grinding to obtain flatness
- CMP is used to fabricate integrated circuits (ICs)
Photochemical Machining for Electronics
- Most common method for creating maskants
- Involves the use of UV light-sensitive 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
- Silicon is the most widely used semiconductor material
How ICs are Made (NEC Electronics: How Semiconductors are Made)
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
- 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
Steps in IC Packaging
Attach the clip to the die
- Wire bonding (video,video)
- Tape-automated bonding (TAB) (link)
- Flip-chip (similar to BGA, but smaller scale) (link)
- Dual-in-line packaging (DIP) is the most well-known IC chip package
- DIP refers to the two sets of in-line pins that go into holes in the PCB
- IC packages are grouped mainly on the arrangement, shape, and quantity of leads
- Two main methods in which components are connected to the circuit on the PCG
- 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
- Surface mount (SM) technology places electronic components onto solder paste pads that have been dispensed onto the surface of the PCB
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
- Lead geometries
- Butt lead or J-lead
- Gull wing leads
- Solder balls (BGA, ball grid array)
Electrochemical Machining Process
(See: http://www.electrochemicalmachining.com/ecm)
Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyte
The tool is the cathode and the workpiece is the electrolyte
- 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
- Much slower penetration rate
Other Electrochemical Processing
- Electrochemical hole machining
- Used to drill small holes with high aspect ratios
- Electrostream drilling
- High velocity stream of charged acidic, electrolyte
- Shaped-tube elecrolytic machining (STEM)
- Capable of drilling small holes in difficult to machine materials
- Electrochemical grinding (ECG)
- Low voltage, high-current variant of ECM
- Electrochemical deburring
- Electrolysis is accelerated in areas with small interelectrode gaps and prevented in areas with insulation between electrodes
Design factors in electrochemical machining
- Current densities tend to concentrate at sharp edges or features
- Control of electrolyte flow can be difficult
- Parts may have lower fatigue resistance
Advantages and Disadvantages of Electrochemical Machining
Advantages
- ECM is well suited for the machining of complex two-dimensional shapes
- Delicate parts may be made
- Difficult-to machine geometries
- Poorly machinable materials may be processed
- Little or no tool wear
Disadvantages
- Initial tooling can be timely and costly
- Environmentally harmful by-products
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