Control usually is achieved by means of effective high velocity and low volume local exhaust ventilation at the solder tip. Devices that return filtered air to the workplace may, if the filtration efficiency is inadequate, cause secondary pollution which can affect people in the workroom other than those soldering. Filtered air should not be returned to the workroom unless the amount of soldering is small and the room has good general dilution ventilation.
After wafer fabrication is completed, each intrinsically finished wafer undergoes a wafer sort process where integrated circuitry on each specific die is electrically tested with computer-controlled probes. An individual wafer may contain from one hundred to many hundreds of separate dies or chips which must be tested. After the test results are finished, the dies are physically marked with an automatically dispensed one-component epoxy resin. Red and blue are used to identify and sort dies which do not meet the desired electrical specifications.
With the devices or circuits on the wafer tested, marked and sorted, the individual dies on the wafer must be physically separated. The imperfection in the crystal structure caused by scribing allows the wafer to be bent and fractured along this line. Laser scribing is a relatively recent die separation technique.
A laser beam is generated by a pulsed, high-powered neodymium-yttrium laser.
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The beam generates a groove in the silicon wafer along the scribe lines. The groove serves as the line along which the wafer breaks. Sawing can either partially cut scribe or completely cut dice through the silicon substrate. A wet slurry of material removed from the street is generated by sawing. The individual die or chip must be attached to a carrier package and metal lead-frame. Carriers are typically made of an insulating material, either ceramic or plastic. Plastic carrier materials are either of the thermoplastic or thermosetting resin type.
The attachment of the individual die is generally accomplished by one of three distinct types of attachment: eutectic, preform and epoxy. Eutectic die attachment involves using an eutectic brazing alloy, such as gold-silicon. In this method, a layer of gold metal is predeposited on the backside of the die. Preform bonding involves the use of a small piece of special composition material that will adhere to both the die and the package. A preform is placed on the die-attach area of a package and allowed to melt.
The die is then scrubbed across the region until the die is attached, and then the package is cooled. Epoxy bonding involves the use of an epoxy glue to attach the die to the package.
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A drop of epoxy is dispensed on the package and the die placed on top of it. The package may need to be baked at an elevated temperature to cure the epoxy properly. Once the die is physically attached to the package, electrical connections must be provided between the integrated circuit and package leads.
This is accomplished by using either thermocompression, ultrasonic or thermosonic bonding techniques to attach gold or aluminium wires between the contact areas on the silicon chip and the package leads.
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Ball bonding, which is used only with gold wire, feeds the wire through a capillary tube, compresses it, and then a hydrogen flame melts the wire. In addition, this forms a new ball on the end of the wire for the next bonding cycle. Wedge bonding involves a wedge-shaped bonding tool and a microscope used for positioning the silicon chip and package accurately over the bonding pad. The process is performed in an inert atmosphere.
Ultrasonic bonding uses a pulse of ultrasonic, high-frequency energy to provide a scrubbing action that forms a bond between the wire and the bonding pad. Ultrasonic bonding is primarily used with aluminium wire and is often preferred to thermocompression bonding, since it does not require the circuit chip to be heated during the bonding operation.
Thermosonic bonding is a recent technological change in gold wire bonding.
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It involves the use of a combination of ultrasonic and heat energies and requires less heat than thermocompression bonding. The primary purpose of encapsulation is to put an integrated circuit into a package which meets the electrical, thermal, chemical and physical requirements associated with the application of the integrated circuit. The most widely used package types are the radial-lead type, the flat pack and the dual-in-line DIP package.
The radial-lead type of packages are mostly made of Kovar, an alloy of iron, nickel and cobalt, with hard glass seals and Kovar leads. Flat packs use metal-lead frames, usually made of an aluminium alloy combined with ceramic, glass and metal components. Dual-in-line packages are generally the most common and often use ceramic or moulded plastics. Transfer moulding is the predominant plastic encapsulation method. In this method, the chips are mounted on untrimmed lead frames and then batch loaded into moulds.
Powdered or pellet forms of thermosetting plastic moulding compounds are melted in a heated pot and then forced transferred under pressure into the loaded moulds. The system usually consists of a mixture of:. Injection moulding uses either a thermoplastic or thermosetting moulding compound which is heated to its melting point in a cylinder at a controlled temperature and forced under pressure through a nozzle into the mould.
The resin solidifies rapidly, the mould is opened and the encapsulation package ejected. A wide variety of plastic compounds are used in injection moulding, with epoxy and polyphenylene sulphide PPS resins being the newest entries in semiconductor encapsulating.
The final packaging of the silicon semiconductor device is classified according to its resistance to leakage or ability to isolate the integrated circuit from its environment. These are differentiated as being hermetically airtight or non-hermetically sealed. Leak testing is a procedure developed to test the actual sealing ability or hermetism of the packaged device.
Two common forms of leak testing are in use: helium leak detection and radioactive tracer leak detection. In helium leak detection, the completed packages are placed in an atmosphere of helium pressure for a period of time. Helium is able to penetrate through imperfections into the package.
After removal from the helium pressurization chamber, the package is transferred to a mass-spectrometer chamber and tested for helium leaking out of imperfections in the package. Radioactive tracer gas, usually krypton Kr , is substituted for helium in the second method, and the radioactive gas leaking out of the package is measured. Under normal conditions, personnel exposure from this process is less than 5 millisieverts millirems per year Baldwin and Stewart Controls for these systems usually include:.
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Also, materials that come in contact with Kr e. Leach-Marshal provides detailed information on exposures and controls from Kr fine-leak detection systems. Burn in is a temperature and electrical stressing operation to determine the reliability of the final packaged device. Devices are placed in a temperature-controlled oven for an extended period of time using either ambient atmosphere or an inert atmosphere of nitrogen.
Because of the large number and the complexity of the tests required, a computer performs and evaluates the testing of numerous parameters important to the eventual functioning of the device. Physical identification of the final packaged device is accomplished by the use of a variety of marking systems. The two major categories of component marking are contact and non-contact printing.
Contact printing typically incorporates a rotary offset technique using solvent-based inks.
Non-contact printing, which transfers markings without physical contact, involves ink-jet head or toner printing using solvent-based inks or laser marking. The solvents used as a carrier for the printing inks and as a pre-cleaner are typically composed of a mixture of alcohols ethanol and esters ethyl acetate.
Most of the component marking systems, other than laser marking, use inks which require an additional step for setting, or curing. These curing methods are air curing, heat curing thermal or infrared and ultraviolet curing. Ultraviolet-curing inks contain no solvents. Laser marking systems utilize either a high-powered carbon dioxide CO 2 laser, or a high-powered neodymium:yttrium laser.
These lasers are typically embedded in the equipment and have interlocked cabinets that enclose the beam path and the point where the beam contacts the target. This eliminates the laser beam hazard during normal operations, but there is a concern when the safety interlocks are defeated.itlauto.com/wp-includes/trace/4271-offrir-un-telephone.php
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The most common operation where it is necessary to remove the beam enclosures and defeat the interlocks is alignment of the laser beam. During these maintenance operations, ideally the room containing the laser should be evacuated, except for necessary maintenance technicians, with the doors to the room locked and posted with appropriate laser safety signs. However, high-powered lasers used in semiconductor manufacturing are often located in large, open manufacturing areas, making it impractical to relocate non-maintenance personnel during maintenance.
For these situations, a temporary control area is typically established. Normally these control areas consist of laser curtains or welding screens capable of withstanding direct contact with the laser beam. Entrance to the temporary control area is usually through a maze entry that is posted with a warning sign whenever the interlocks for the laser are defeated.
Other safety precautions during beam alignment are similar to those required for the operation of an open-beamed high-powered laser e. Even after power is off, a significant shock potential exists within the tool and must be dissipated prior to working inside the cabinet. Along with the beam hazard and electrical hazard, care should also be taken in performing maintenance on laser marking systems because of the potential for chemical contamination from the fire retardant antimony trioxide and beryllium ceramic packages containing this compound will be labelled.