Conveyors: power-driven – Conveyor section – Endless conveyor
Reexamination Certificate
2000-02-25
2001-05-08
Bidwell, James R. (Department: 3651)
Conveyors: power-driven
Conveyor section
Endless conveyor
C198S836100, C198S608000
Reexamination Certificate
active
06227353
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a die attach material curing apparatus and method and, more specifically, to a radiant energy die attach material curing apparatus and method.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, the cure process for die attach materials is generally achieved via a batch process in a large nitrogen purged convection oven. Die attach involves curing of the die attach material, generally an epoxy, which secures the semiconductor die to the leadframe. Other solutions to the stand alone “snap cure” of die attach material use heater block contact and/or forced convection technology to transfer energy to the target.
In the batch process, the leadframes and semiconductor die (or dies if plural leadframes are used) are secured together with a die on each leadframe by placing the die over the uncured die attach paste material and then placing the leadframe strips in carriers. These carriers are fed into a chamber, which is a forced air and convection oven, for a predetermined period of time, usually several hours, the temperature in the chamber being determined by the type of die attach used and other well known factors. The heat in the chamber heats up the die and the die attach material and causes crosslinking of the die attach polymer adhesive material with the die thereby being secured to the die attach material and to the leadframe.
The prior art die attach material cure processes as described above require a relatively long curing time and also are relatively dirty due to the outgassing caused as a result of the curing process. The materials resulting from the outgassing remain at least in part in the chamber and can deposit on the semiconductor die, thereby causing a possible degrading of the electrical properties of the final semiconductor device. It is therefore apparent that better die attach material curing techniques are highly desirable.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described impediments of the prior art are materially reduced and there is provided a die attach material curing apparatus and method whereby the curing time is substantially diminished relative to the prior art. This is accomplished wherein the gases resulting from the curing of the die attach material are substantially eliminated from the curing chamber during the curing procedure, thereby minimizing contamination of the semiconductor die with these gases.
Briefly, the above is accomplished by providing a system which includes an elevator assembly for receiving carriers, each carrier having stacked leadframe strips therein spaced from each other, such as with shelves in the carrier, with a die held to each leadframe by uncured die attach paste material. The elevator moves in a vertical direction to position a selected one of the leadframe strips therein relative to a gooser assembly whereby the gooser assembly is aligned with a particular leadframe strip in each carrier to move that leadframe strip out of the carrier in a direction normal to the direction of elevator movement. The leadframe strip, which has now been moved out of the carrier, is received by a pinch roller which moves the leadframe strip into a curing chamber. A front flag, a mechanism in the curing chamber, locates the leadframe strip in the center of the curing chamber. After curing of the die attach material in the chamber for a predetermined time, a rear flag of the mechanism in the curing chamber moves the leadframe strip back against the pinch roller which, together with the front flag, move the leadframe strip back into its original position in the carrier. The elevator then lines up another of the leadframe strips in the carrier with the gooser assembly and the procedure is repeated until the die attach material on all of the leadframe strips in the carrier has been cured. The carriers with leadframe strips and die attached thereto with cured die attach material are then removed.
The elevator assembly is capable of receiving carriers of different widths which can accommodate leadframe strips having different widths, yet will center the carriers in the elevator assembly, regardless of carrier width. This is accomplished by providing a pair of rotatable polygonal solids on one side of the carrier, the polygonal solids having an axis disposed such that the distance from the axis to each side of the polygon is different. Air cylinders are provided on the other side of the carrier to push the carrier against the eccentric blocks. In this way, by adjusting the position of each of the polygonal solids about its axis such that a selected like pair of the sides faces the carrier, the dimension between these surfaces and the rotational axis of the rotatable polygonal solids is selected to accommodate the carrier being used.
The leadframe strip handling system is designed to accommodate leadframe strips of different widths. It does so by providing multi-level tracks. Each leadframe strip width has its own distinct leadframe strip track. The tracks are nested within each other. The largest width track is at the top and the smallest width track is at the bottom with the intermediate width being located therebetween. In this way, the larger width leadframe strips are closer to the radiation source than are the smaller width leadframe strips. Different process parameters can compensate for the increased distance for the smaller width lead frames strips.
The designs of the multiple level tracks and the pinch roller allow for quick adjustment among different leadframe strip widths. For each leadframe strip track, there is a freely rotating idler bearing. The idler bearings are sized so that the difference in radii is equivalent to the spacing between the multi-level tracks. All bearings share a common shaft so aligning any of the bearings to its corresponding track automatically aligns the remaining (two being shown in the preferred embodiment) bearings to their corresponding track. A single drive wheel is used to engage any one of the free rotating bearings at a given time. The shaft of the drive wheel is designed with detent positions which correspond to the horizontal distance between each pair of the free rotating bearings. In this way, the drive wheel can be quickly aligned to the free bearing of choice by simple pulling or pushing the shaft so that the correct detent feature is engaged in the spring loaded ball plunger of the drive pulley.
As stated above, the drive wheel shaft can be adjusted axially within the drive pulley. The shaft is located when the spring loaded ball plunger of the drive pulley engages one of the detent positions machined into the drive shaft. In this way, the drive wheel can be quickly aligned horizontally to any of the, for example, in the preferred embodiment, three free rotating bearings.
The drive pulley is mounted on an arm which is in turn mounted to a linear bearing. The linear bearing has enough travel so that the drive wheel can engage the smallest of the (three in the preferred embodiment) rotating bearings as well as be driven to a position higher than the largest of the bearings to allow unobstructed passage of a leadframe strip.
The drive pulley is driven by a flexible belt. The tension of the belt is used to generate a force between the drive belt and the driven bearing sufficient to translate the captured leadframe strip along the leadframe strip track. The belt is stretched over the pulleys and must be sufficiently flexible to allow travel of the drive pulley to a position higher than the largest of the free rotating bearings.
The rapid cure process (RCP) modules or curing chambers include, in addition to a lamp of the type described in Ser. No. 08/255,197, filed Jun. 7, 1994, the contents of which are incorporated herein by reference, a blower which blows air over the leadframe strip and an exhaust system which removes gases out of the module to remove all volatiles released during the die attach operation. Such a module is set forth in Ser. No. 08/9
Amador Gonzalo
Buendia Jessie
Go Chill
Heinen Katherine Gail
Stark Leslie E.
Bidwell James R.
Brady III Wade James
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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