Apparatus for cleaning items using gas plasma

Details Apparatus for cleaning items using gas plasma Apparatus for cleaning items using gas plasma

1999-07-09

2001-07-31

Mills, Gregory (Department: 1763)

Coating apparatus

Gas or vapor deposition

With treating means

C156S345420, C118S72300R, C118S500000, C118S719000, C118S728000, C422S022000, C204S298370

Reexamination Certificate

active

06267075

ABSTRACT:

TECHNICAL FIELD
This invention relates to an apparatus and method for cleaning a substrate using a gas plasma, and more specifically to an apparatus and method in which at least two electrodes are positioned on two sides of a magazine that holds the substrate to be cleaned.
BACKGROUND ART
Many different methods and apparatus exist for coupling integrated circuits to lead frames. Typically the integrated circuit is coupled centrally within the lead frame to form an integrated circuit/lead frame assembly using an adhesive, such as epoxy. The integrated circuit is then electrically coupled to the lead frame assembly. The electrical connection between the integrated circuit and the lead frame are usually thin gold wire soldered at one end to the integrated circuit and at the other end to the lead frame. The central portion of the lead frame assembly is then encapsulated in a plastic or ceramic material so that only the lead pins or terminals of the lead frame extend from the encapsulating material.
It is very important that the adhesive bond between the integrated circuit and the lead frame be sufficiently strong to prevent any shifting of the components during operations performed on the lead frame assembly. It is also important that the gold wires coupled between the integrated circuit and the lead frame be both strong and electrically conductive. Therefore, it is important that the surface of the lead frame to which the integrated circuit and electrical connections attach be very clean prior to the application of adhesive or soldering of gold wire, in order to assure the proper connections. It may also be desirable to clean the lead frame assembly prior to encapsulation of the integrated circuit, because contamination on the lead frame assembly prior to encapsulation may result in inferior or non-functioning products.
Lead frames are typically stamped metal strips and are typically handled by automated machinery in magazines that hold stacks of lead frames, often 25 or 40 in number.
Various prior art apparatus and methods exist for use in cleaning the surfaces of the lead frames, including the use of gas plasmas such as argon plasma. It is known in the art to use removable magazines having perforated sides to hold the lead frames during cleaning. The perforations in the sides of the magazines allow the plasma to enter the magazine to remove unwanted material, such as oxides, from the lead frame surfaces. Typically, the lead frames are loaded into the magazine outside of the cleaning apparatus. Then, at a later stage, the magazine is moved into the cleaning apparatus before the plasma cleaning process is initiated.
A number of problems have been encountered in such prior art apparatus and methods using plasma for cleaning lead frames. For example, the perforated sides of the lead frame magazines allow contaminants to enter the magazine and deposit on the lead frames or lead frame assemblies during handling. Additionally, during both automated and manual handling of the magazines unwanted materials are deposited on the exterior of the magazine itself. Unwanted material on the exterior of the magazine may cause several potential problems when the magazine is placed in the plasma chamber for cleaning. First, some of the material removed from the magazines may redeposit onto the lead frames. And second, some of the material removed during cleaning operations deposits on the walls and other surfaces of the plasma chamber. Materials deposited on the interior surfaces of the plasma chamber require periodic cleaning. Such materials build up more quickly, and thus require more frequent cleaning, when deposited materials originate from both the exterior surface of the magazine and from the lead frames, rather than just form the lead frames alone.
Prior art plasma cleaning apparatus pass the plasma across the surface of the lead frames in a single direction. Plasma is very short lived. Therefore, the edge of the lead frame nearest the source of the plasma is cleaned significantly better than the edge of the surface furthest from the plasma source. The wider the lead frame, the more significant this problem becomes.
Accordingly, what is needed is an improved apparatus and method for cleaning the surface of lead frames that overcomes the disadvantages of the prior art.
SUMMARY OF THE PRESENT INVENTION
Accordingly, the present invention is an apparatus and method for generating a gas plasma within a chamber and using the plasma to clean a selected surface or surfaces placed within the chamber. More specifically, the invention is an apparatus and method for cleaning lead frames and integrated-circuit/lead-frame assemblies (hereafter referred to as “lead frame assemblies”) using gas plasma. Although the apparatus and method of the invention described below will focus on the use of argon plasma to clean lead frames, in alternate embodiments, any desirable ionizable gas may be used, such as SF
6
, CF
4
, NH
3
, O
2
, and H
2
. Additionally, other items or substrates may be cleaned, including items loaded in boats (magazines) and substrate/PCB products.
The device of the invention is an argon plasma cleaning system including a chamber, preferably a vacuum chamber, with a captured or stationary cassette or magazine positioned therein for holding a stack of lead frames or integrated circuit/lead frame assemblies. The stationary magazine includes perforated sides and is preferably open to the front and back.
At least one, and preferably two, electrodes are positioned on each side of the stationary magazine. In a preferred embodiment using four electrodes, two electrodes are positioned on each side. A first inner electrode is positioned a selected distance from a first side of the magazine, and a second inner electrode is positioned a selected distance from the other side of the magazine. Similarly, a first outer electrode is positioned a selected distance from the first inner electrode, on the side of the inner electrode opposite the stationary magazine, and the second outer electrode is similarly positioned a selected distance form the second inner electrode. In a preferred embodiment, the electrodes are perforated sheets or grids lying in a plane parallel to the nearest side of the stationary magazine. Thus, gaps are formed between each pair of inner and outer electrodes and between each inner electrode and the closest side of the stationary magazine. In a preferred embodiment, the device of the invention includes a means for controlling, and changing as desired, the charge and voltage of each electrode and of the stationary magazine.
The apparatus of the invention includes a means for creating a vacuum in the vacuum chamber, such as a vacuum pump assembly that includes a vacuum pump, valve, and pressure gauge, coupled to an exhaust conduit. The apparatus of the invention also includes a means for allowing a chosen ionizable gas into the vacuum chamber, such as a gas inlet assembly that includes preferably, a pressurized gas cylinder, pressure gauge and valve, coupled to an inlet conduit connected to the vacuum chamber. The ionizable gas used is preferably argon, although other ionizable gasses or mixtures of gasses may be used. In a preferred embodiment, two vacuum outlets and two gas inlets are used, one on each side of the vacuum chamber.
During operation, the plasma gas moves from one side of the vacuum chamber to the other side due to the vacuum generated on the side of the vacuum chamber opposite the side from which the ionizable gas flows into the chamber. A control means is used to give each electrode and the stationary magazine a selected charge and voltage. In the preferred embodiment wherein two electrodes are used on each side of the stationary magazine, when the argon passes between the first outer and first inner electrodes, a voltage potential between the electrodes induces the ionizable gas to ionize, creating a plasma. The plasma gas diffuses through perforations in the stationary magazine and across the lead frames removing unwanted material and carrying the material towards the

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