Bond head cooling apparatus

A cooling apparatus is provided for a bond head which has a collet to hold a semiconductor die and a heater to heat the semiconductor die held by the collet. The cooling apparatus includes a dielectric liquid supply for supplying a dielectric liquid and a gas supply for supplying a gas. A spray nozzle is located next to a surface of the heater, and is connected to both the dielectric liquid supply and the gas supply. In order to cool the heater, the spray nozzle sprays a liquid-gas mixture of the dielectric liquid and the gas towards the surface of the heater.

FIELD OF THE INVENTION

The invention relates to bonding machines for electronic devices, and in particular to bond heads comprised in bonding machines which require heating and cooling of the bond heads during operation.

BACKGROUND AND PRIOR ART

In a bonding machine for electronic devices, such as a die bonding machine, an important module is the bond head. During a bonding process, the bond head is operative to hold a semiconductor die while a heater incorporated in the bond head heats the die. The heater then presses the die against a bonding site using a predetermined force and temperature profile. To heat the semiconductor die rapidly and thus reduce the bonding time, pulse heating is sometimes applied, followed by cooling (such as air cooling) to reduce the temperature of the die.

Conventional methods of actively cooling the bond head may involve the injection of compressed gas into the bond head. While introducing compressed gas on its own to cool the bond head is adequate for most purposes, the cooling rate is limited. It would be beneficial if one could achieve a better cooling rate and reduce gas consumption by employing improved cooling means.

SUMMARY OF THE INVENTION

It is thus an object of the invention to seek to provide a cooling apparatus for a bond head that achieves a faster cooling rate as compared to conventional air or gas-cooled systems.

According to a first aspect of the invention, there is provided a cooling apparatus for a bond head having a collet adapted to hold a semiconductor die and a heater which is adapted to heat the semiconductor die held by the collet, the cooling apparatus comprising: a dielectric liquid supply for supplying a dielectric liquid and a gas supply for supplying a gas; and a spray nozzle located next to a surface of the heater, the spray nozzle being operatively connected to both the dielectric liquid supply and the gas supply; wherein the spray nozzle is operative to spray a liquid-gas mixture comprising the dielectric liquid and the gas towards the surface of the heater for cooling the heater.

According to a second aspect of the invention, there is provided a bonding apparatus comprising: a collet adapted to hold a semiconductor die, and a heater which is adapted to heat the semiconductor die held by the collet; a dielectric liquid supply for supplying a dielectric liquid and a gas supply for supplying a gas; and a spray nozzle located next to a surface of the heater, the spray nozzle being operatively connected to both the dielectric liquid supply and the gas supply; wherein the spray nozzle is operative to spray a liquid-gas mixture comprising the dielectric liquid and the gas towards the surface of the heater for cooling the heater.

According to a third aspect of the invention, there is provided a method for cooling a bond head having a collet adapted to hold a semiconductor die and a heater which is adapted to heat the semiconductor die held by the collet, the method comprising; supplying a dielectric liquid from a dielectric liquid supply and a gas from a gas supply to a spray nozzle located next to the surface of the heater, the spray nozzle being operatively connected to both the dielectric liquid supply and the gas supply; and spraying from the spray nozzle a liquid-gas mixture comprising the dielectric liquid and the gas towards the surface of the heater for cooling the heater.

It would be convenient hereinafter to describe the invention in greater detail by reference to the accompanying drawings which illustrate one preferred embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.

FIG. 1is an isometric view of a bond head10comprising a cooling apparatus according to the preferred embodiment of the invention. The bond head10has been inverted to illustrate a collet14of the bond head which is adapted hold a semiconductor die (not shown) during die bonding.

During a die bonding process, the bond head10may be heated to about 350° C. for melting solder balls attached to the semiconductor die so as to bond the semiconductor die to a substrate. Thereafter, it is beneficial to cause the solder balls to solidify as quickly as possible to harden the solder ball joints, with the objective of increasing productivity and quality.

At one end of the bond head10, there is a base12which is adapted for attaching the bond head10to a bond head column of a die bonding machine (not shown) in use. A collet14is situated at an opposite end of the bond head10, on which a semiconductor die may be held during die bonding. The collet14is in turn supported on a block comprising a heater16. The heater16is operative to heat the collet14and a semiconductor die which is held on the collet14during a die bonding process.

An insulation block18is located behind the heater16to reduce the transmission of heat from the heater16to the base12. For the control of dielectric liquid and gas ejected towards the heater16, at least one pair of liquid solenoid valves20and at least one pair of gas solenoid valves22are mounted next to the base12. Preferably, the pairs of liquid and gas solenoid valves20,22are mounted on opposite sides of the bond head10to promote even distribution of the dielectric liquid and gas that are introduced. Gas and liquid inlets24introduce gas and dielectric liquid into the cooling apparatus for the purpose of cooling the bond head10. Thereafter, gas and dielectric liquid used for cooling the bond head10are extracted from the bond head10via one or more exhaust outlets26.

FIG. 2is an isometric view of the bond head10ofFIG. 1with part of its collet14and heater16removed to illustrate its cooling apparatus. Underneath the heater16, gas connectors28are mounted for transmitting gas from a gas and liquid chamber34incorporated within the bond head10to a plurality of spray nozzles30located next to a surface of the heater16for directing cooling sprays comprising a mixture of dielectric liquid and gas towards the surface of the heater16. In addition to the insulation block18, insulative O-rings32are installed between the heater16and the insulation block18for sealing purposes.

FIG. 3is an isometric view of the cooling apparatus that is incorporated into the bond head10ofFIG. 1. The cooling apparatus generally includes gas and liquid inlets24which receive dielectric liquid from a dielectric liquid supply21and gas from a gas supply23, and respective liquid and gas solenoid valves20,22which control the introduction of dielectric liquid and gas through the gas and liquid inlets24into the gas and liquid chamber34.

The gas and liquid chamber34comprises a gas chamber connected to a gas inlet and a liquid chamber connected to a liquid inlet of the gas and liquid inlets24. The gas and liquid chamber34is connected to the spray nozzles30so that gas and dielectric liquid respectively which are received in the gas and liquid chamber34may be ejected in the form of sprays comprising both gas and dielectric liquid from the spray nozzles30when cooling of the heater16is required.

FIG. 4is a sectional view of a spray nozzle30comprised in the cooling apparatus. Compressed gas received in the gas and liquid chamber34is introduced to the spray nozzle30by way of the gas connectors28, whereas dielectric liquid received in the gas and liquid chamber34is introduced to the spray nozzle30by way of separate liquid connectors36. The gas connectors28and liquid connectors36operatively connect the spray nozzles30to the gas supply23and the dielectric liquid supply21. A liquid-gas mixture that is mixed or combined in the spray nozzle30is ejected from the spray nozzle30onto the heater16, preferably in the form of a spray of mist, from a spray outlet40of the spray nozzle30.

FIG. 5is a recirculation system which may be used with the cooling apparatus illustrated inFIG. 1. The recirculation system generally comprises a liquid pump42, a liquid filter44, a gas supply23in the form of a compressed gas supply50, exhaust tubes54and a dielectric liquid supply21in the form of a radiator and liquid tank56.

A liquid pump42is used to pump a cooling dielectric liquid through a liquid filter44, the liquid filter44being operative to remove debris from the dielectric liquid. A pressure gauge46and a flowmeter48determine the pressure and amount of dielectric liquid that is being transmitted to the spray nozzles30. At the spray nozzles30, compressed gas from the compressed gas supply50is added to the dielectric liquid and the mixture is sprayed onto the heater16in order to cool the heater16.

Thereafter, the used gas and dielectric liquid are exhausted from the one or more exhaust outlets26and through exhaust tubes54to the radiator and liquid tank56, where the used dielectric liquid is cooled by a radiator before it is collected in a liquid tank for recycling. Recycled dielectric liquid is introduced to the pump42again for re-use, with a flow control valve58controlling the feeding of dielectric liquid from the radiator and liquid tank58to the pump42. The pump42will again pump dielectric liquid to cool the heater16.

FIG. 6is a liquid spray control system which may be used with the cooling apparatus illustrated inFIG. 1. A heater controller60receives temperature feedback62from the heater16, and relying on such temperature feedback controls a power supply63for powering the heater16. When the heater controller60senses that the heater16needs to be cooled, the heater controller60activates a microcontroller board64to send signals to driver boards66for initiating the generation of a cooling spray. The heater controller60is further configured to control respective proportions of the dielectric liquid and the gas in the liquid-gas mixture being ejected from the spray nozzles30, by way of its control of the liquid solenoid valves20and the gas solenoid valves22via the microcontroller board64.

The driver boards66will then activate the liquid solenoid valves20to inject dielectric liquid into the gas and liquid chamber34and the gas solenoid valves22will inject compressed gas into the gas and liquid chamber34. From the gas and liquid chamber34, gas and dielectric liquid are separately introduced to the spray nozzles30, where the mixture of gas and dielectric liquid is sprayed onto the heater16in the form of a mist to cool the heater16.

Hence, dielectric liquid is ejected and mixed with compressed gas to form a spray or mist which is propelled onto a hot surface of the heater16for rapid cooling. In order to enhance the cooling performance, the surface of the heater16may include fins and may further be coated with an evaluation layer to reduce the film boiling effect. The cooling rate of the heater16is controllable by adjusting the operating parameters of the solenoid valves20,22. The microcontroller board64will signal the solenoid valves20,22to be activated to deliver a desired gas and dielectric liquid mixture onto the heater16. An evaporation temperature of the dielectric liquid selected is lower than an operating temperature of the surface of the heater16to ensure that no dielectric liquid droplets are left in the cooling chamber next to the surface of the heater16during cooling. Furthermore, the heated vapor will be cooled down by means of the radiator to be condensed, and then fed into the liquid tank for recycling.

The cooling rate during the whole cooling process may be adjusted by applying different dielectric liquid/gas flow rates in the supply of the dielectric liquid and the gas. This control allows flexibility to adjust the cooling rate in real time. Furthermore, the cooling rate can be manipulated so that the heater16is more durable and has a longer lifespan. For instance, a constant cooling rate may be used all the time, or the cooling rate may be slower at first, followed by a faster cooling rate.

It should be appreciated that thermal cooling rate of the heater16may be significantly improved by using the cooling apparatus according to the described embodiment of the invention. Sprays comprising compressed gas and dielectric liquid are propelled directly onto the surface of the heater16, and the sprays are more easily evaporated as compared to conventional liquid-cooled approaches to cool down the heater16. It has been found that the use of a dielectric liquid, such as distilled water, in the aforementioned liquid-gas spray cooling approach allows the cooling apparatus to achieve a cooling rate of 100° C. per second or higher.

It has also been found that by using latent heat (or evaporative) cooling, the greatly reduced water flow rate and compact structures led to reduced water flow rate and supply pressure. This makes the entire cooling apparatus more compact. Further, since the solenoid valves20,22are located on the sides of bond head, and are connected to the spray nozzles30by short connectors28,36, closed loop control over the spraying process to achieve a rapid cooling rate is enabled.

In terms of application flexibility, the two sets of solenoid valves20,22with individual cycle control enables the creation of different compositions of gas and dielectric liquid to form the cooling sprays or jets. As such, the cooling rate during the cooling process is adjustable by applying different gas/liquid flow rates. The temperature insulation measures used ensure that the bond head10as a whole is not sensitive to temperature changes at the heater16, and more precise temperature control is possible.