Weighing scales – Weigher responsive material control
Reexamination Certificate
2000-08-01
2002-06-04
Gibson, Randy W. (Department: 2841)
Weighing scales
Weigher responsive material control
C177S116000, C702S173000, C427S096400, C222S055000, C029S592100, C438S106000
Reexamination Certificate
active
06399902
ABSTRACT:
The invention relates in general to the field of integrated circuit packaging, more particularly, the present invention provides for an apparatus and method for an inline measuring system controlling the volume of flux dispensed onto a semiconductor flip chip or a packaging substrate.
BACKGROUND OF THE INVENTION
In a semiconductor device assembly, a semiconductor IC chip or die may be bonded directly to a packaging substrate without the need for a separate leadframe or for separate I/O connectors, i.e., wire or tape. Such chips are formed with ball-shaped beads or bumps of solder affixed to I/O bonding pads on the chip. During packaging, the chip or die is flipped onto its active circuit surface so that the solder balls form electrical connections directly between the die and conductive traces on a packaging substrate. Semiconductor chips of this type are commonly called flip chips.
Flip-chip technology is well known in the art. A die having solder bumps formed on the active or front side of the die is inverted and bonded to the traces on a packaging substrate through the solder bumps. The bonding is accomplished by heating the flip-chip assembly to a temperature that causes the solder forming the bumps to reflow. A solder joint is formed between the die and the packaging substrate and a narrow gap formed between the die and the substrate. The packaging substrate usually a ceramic or polymer composite.
However, prior to bonding the die to a substrate, a solder flux such as a rosin or a no-clean flux is applied to either the active surface of the die or the packaging substrate surface. The flux serves primarily to remove any surface oxidation or natural oxide films on the surface of the solder, so that during reflow, the solder balls make good contact with traces on the packaging substrate. The solder is typically silver or lead combined with a more reactive metal such as indium or tin. During reflow of the solder, the flux is burned off. The flux must be removed because any remaining residue from the flux may alter the adhesion properties of the underfill material and may also pose a corrosion problem to the semiconductor chip. It is preferred that the flux be removed by heating during the reflow step. However, it is conventional in the industry to also use an organic solvent or aqueous solution to remove the flux.
Following the reflow of the solder and removal of the flux, the mechanical integrity of solder joints of the die to the packaging substrate and the reliability of the integrated package is enhanced by underfilling the gap between the die and the substrate with an underfill material. The underfill is a polymer, in liquid form, such as an epoxy or a plastic polymer. Typically, the underfill is dispensed around at least one adjacent side of the die and the underfill material flows by capillary action to fill the gap between the die and the substrate. The underfill is normally filled with ceramic particles to control its rheology in the uncured state and to improve its thermal and mechanical properties in the cured state. The diameter of the filler particles in the underfill are sized smaller than the height of the gap so as not to restrict flow. Typical formulations have a viscosity of about 10 Pa·s at the dispense temperature. After the underfill material is flowed into the gap, it is cured in an oven at an elevated temperature.
The conventional process dispensing flux onto an IC component involves an offline manual sampling technique to determine the amount of flux to be dispensed. The offline method involves the following steps. The assembly line is interrupted. A representative number of IC components from the assembly line are removed before the flux is dispensed. Each component is weighed and then placed back on the assembly line. Flux, as controlled by the controller, is dispensed onto the IC component. After the flux is dispensed, the component is removed from the assembly line and reweighed. A visual observation is made to determine whether too much or too little flux has been applied. After a representative number of samples have been taken, the controller is manually adjusted to control the amount of flux to be dispensed. Once the offline sampling is completed and the controller is adjusted, the assembly process is put back on line. After a period of time, the offline sampling is repeated again to determine if the controller needs adjustment. This process must be repeated frequently. There is a need to automate the dispensing means to eliminate the need for the frequent sampling and offline weighing of the IC components to adjust the controller.
SUMMARY OF THE INVENTION
The object of this invention is to provide an apparatus for dispensing a flux onto an IC component to eliminate the need for offline manual weighing of the component before and after the flux is dispensed.
A further object of the invention is to provide an apparatus for dispensing a flux onto an IC component to control the amount of flux dispensed onto the IC component to prevent an excess of flux from being dispensed.
Another object of the invention is to provide a method for dispensing a flux onto an IC component to eliminate the need for offline manual weighing of the component before and after the flux is dispensed.
And still a further object of the invention is to provide a method for dispensing a flux onto an IC component to control the amount of flux dispensed onto the IC component to prevent an excess of flux from being dispensed.
The other objects and characteristics of the present invention will become apparent from the further disclosure of the invention which is given hereinafter with reference to the accompanying drawings.
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Accujet Dispenser, brochure by Nordson Corporation, Feb. 1995.
Brownfield Terri J.
Halderman Jonathan D.
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