1. Field of the Invention.
The present invention generally relates to a method and apparatus for transporting lead frames, and to an in-line system using the same.
2. Description of the Related Art.
Usually, a semiconductor chip-assembly process begins with a die-bonding process in which an individual chip ("die") separated from a silicon wafer, and having a plurality of integrated circuit elements, is attached to a lead frame. A lead frame is a plate having leads for electrically connecting the chip to external electrical elements, and is made of copper alloys or iron-nickel alloys. It also supports the chip during subsequent steps in the assembly process. The lead frame onto which the chip is attached is electrically coupled to the chip through wires or by directly bonding parts ("inner leads") thereof to metal pads of the chip. Then, the chip semi-assembly is encapsulated to provide protection from environmental effects, such as moisture, dust, and physical and electrical shocks. A thus-obtained package is then subjected to a cutting/forming process to cut and form the leads to adapt the package which includes the chip to being mounted on a circuit board. Each package thus manufactured is then subjected to various electrical and reliability tests, and the packages which pass those tests are supplied to consumers.
As described above, the semiconductor package-assembly process includes many steps, and the capacity of each step is different. Therefore, carrying out continuously the steps as a succession of steps is very difficult. For example, four chips can be attached to die pads of four lead frames in the die-bonding process, while only one chip can be electrically connected to the lead frame in the wire-bonding process during the same period of time. Moreover, for the LOC (Lead On Chip) package assembly process, it approximately 0.8-2.0 seconds are required to bond the chip to the die pad with a polyimide tape by applying heat and pressure thereon, which is about half the time required for the subsequent wire-bonding process. To solve the problems associated with such differences between the required time in each of the steps, and to improve the productivity of the package-assembly process, an integration system called an in-line system has been proposed, in which one die-bonding apparatus is coupled with more than two wire bonding apparatuses. Such in-line system is particularly advantageous for large-scale assembly and production factories, such as memory element production factories.
FIG. 1 is a schematic diagram of an in-line system. The system (40) comprises one die-bonding apparatus (10), four wire-bonding apparatuses (20a, 20b, 20c, 20d) and a transport apparatus (30) for transporting the lead frames (16).
Each of a plurality of lead frames in a stocker (not shown) is fed into a working rail (9) by a lead frame separator (1). Lead frames (2) on the rail (9) have lead patterns which are to be electrically connected to the electrical pads of respective chips, these patterns being connected to each other through a side rail, forming a strip. The wafer cassette (5) contains wafers having integrated circuit elements and which have already been subjected to back-lapping, scribing and back-tape-attaching. When a wafer (not shown) is mounted on an xy-table (6), chip separator (7) separates a plurality of individual chips from the wafer. The separated chips are then transported to the die-bonding head (4) by the chip transport module (8). The die pad (not shown) of the lead frame (2) at the rail (9) beneath the bonding head (4) is supplied with dots of an adhesive such as Ag-epoxy. For the case where the chip is directly attached to the lead of the lead frame without a die pad, for example in LOC packages, the dotting of the adhesive is not required.
The bonding head (4) aligns the the lead frame and the chip at an appropriate position, where they will be attached together, and presses them under heat and pressure to bond them together.
The lead frame onto which a chip is attached by die-bonding apparatus (10) is then moved into a prebake chamber (12) where the adhesive is cured at a predetermined temperature and time.
Buffer (14) is composed of a plurality of inlet magazines (18), into which the lead frames are transported after the completion of die-bonding, and stacked, before they are transported to the wire-bonding stage. The reasons that a plurality of magazines are used to feed lead frames into the wire-bonding stage are that, first, even if the die-bonding apparatus (10) does not work, the lead frames can be continuously fed into the wire-bonding stage. Second, when the die-bonding apparatus (10) cannot output sufficient die-bonded lead frames, lead frames that have been die-bonded by a separate, ex-line die-bonding apparatus are fed into buffer (14) in order continuously to carry out wire-bonding. A lead frame (16a) having a chip attached thereto, which is transported to the lead frame-transport module (30) from one of the inlet magazines (18), undergoes wire-bonding by one of four wire-bonding apparatuses (20a through 20d).
Each wire-bonding apparatus (20) is composed of a loader (24), an unloader (26) and a wire-bonding head (22). The wire-bonded lead frame (16) is transported by the transport module (30) to one of the outlet magazines (28), from which the lead frame will be transferred to a subsequent assembly step, such as molding of a capsule of plastic material in protective relation to the chip and its connections to the frame. The outlet magazines (28) have the same structure as that of the inlet magazines (18) of buffer (14).
The control part (15) controls the transport of the lead frames and the overall operation of the system, and comprises a microprocessor. Its initial operational values are determined by the size and shape of the lead frames as well by the structure of the chips.
The lead frame transport module (30) in such an in-line system employs conveyor belts for transporting the lead frames, as shown in FIG. 2A. The lead frame transport module (30) is composed of two rail bodies, including first (50a) and second (50b) rail bodies, which have the same structure as one another. The rail body (50) has a large number of rollers (52), onto which belts (54) are wound. The first rail body (50a) is interconnected to the second rail body (50b) by a connector board (56). The width (W) of the board (56) is adjusted, depending on the width of the lead frame (16) which is to be transported. The power for moving the conveyor belt is supplied by a motor, which is located between the first rail body (50a) and the wire-bonding head (22), although not shown in FIG. 2A. The rotational power of the motor is transferred to the power transfer shaft (57), one end of which is coupled to the power transfer roller (58) of the first rail body, while the other end thereof is coupled to the power transfer roller (58) of the second rail body, thereby making it possible to control the moving speed of the conveyor belts (54) of the first rail body so that it is the same as that of the second rail body. The side rail of the lead frame is disposed in contact with the conveyor belt (54), and moves upon the moving of the conveyor belt (54) powered by the motor.
When the lead frame (16) reaches a certain predetermined position along the rail (50), sensor a (70) detects it and transfers the detected signal to a stop (72). The ""-shaped stop (72) is hidden within a split (74) formed in rail body (50) when the lead frame passes, and is projected outside the split when a signal is received from the sensor (70) to stop the lead frame. The moving lead frame stops when it collides with the stop (72). Since the moving and stopping of the lead frame is controlled by the control part of the in-line system (for example, the controller (15) in FIG. 1), the motor can be controlled to stop the conveyor belt (54) and consequently the lead frame. Nevertheless, in such case, since the lead frame leans toward its traveling direction by the action of inertia, the stop (72) should be employed in order accurately to control the stopping of the lead frame at a proper position.
Detection of the position of the lead frame is required when a lead frame arrives at the loader (24) and the unloader (26) of a certain wire-bonding apparatus among a plurality of wire-bonding apparatus. FIG. 2A shows a state in which the die-bonded lead frame (16a) is moved to the wire-bonding apparatus from the rail (50) by the loader (24) of wire-bonding apparatus, and the wire-bonded lead frame (16c), which is placed on unloader (26), stands ready for return to rail (50).
Since the loader the (24) and unloader (26) have the same structure and operation, only the loader (24) in FIG. 2A will be described. The loader (24) employs a belt for transporting lead frames. Thus, the loader and unloader are each composed of a belt (82), a roller (84) onto which belt is wound, and a motor (80) supplying power to the belt. The belt wound onto the roller, which is on the rail (50), is hidden in the split (74) formed in the rail (50) during the transport of lead frame. When the sensor (70) detects the position of the lead frame and the lead frame stops due to the stop (72), the belt (82), hidden in the split, and the roller (84) are moved upward, above the shaft (83), and the lead frame (16a) is transported to the wire-bonding apparatus, according to movement of the belt (82) of the loader.
The bonding head (22) of the wire-bonding apparatus is composed of a capillary (60), into which gold or aluminum wire (62) is fed, and a trimmer (64) for cutting the wire after the completion of wire-bonding between the leads of the lead frame and the metal pads of the chip. After completion of wire-bonding of each individual chip (66) attached to a lead frame (16b), the lead frame is moved to the unloader (26), where the lead frame is moved to the rail by movement of the belt of the unloader (26).
The above-described lead frame transport module (30) of a conveyor belt-type has problems associated with use of many rollers (52) (for example, 400 rollers) and belts (for example, 16 belts). For example, abrasions of parts thereof due to mechanical friction therebetween. That is to say, as shown in FIG. 2B, an individual roller (52) is composed of a pivotal shaft (53), a bushing (55) and a bearing (51), which will be abraded during rotation of the roller, and, therefore, reach a point where it needs to be replaced. Moreover, dust generated during the abrasions are critical contaminants, which cause significant semiconductor failures. Further, even if only one part is worn out, the whole system should be stopped in order to replace the worn part, resulting in a reduction in productivity, and in an increase in the production cost.
Such problems are not limited to in-line system, but are common to all apparatus employing conveyor belts for transport.
Besides, the conveyor belt-type transport systems also have other problems associated with instability of the transport rails, which will be described with reference to FIGS. 3 and 4.
FIGS. 3A and 3B are sectional views showing the transport of the lead frames using a conventional transport having a conveyor belt. In order to simplify the drawings, the sensor, motor and power transferring parts are omitted. As the belt (54) moves in the direction indicated by the arrow, a lead frame (16), which is disposed in contact with the belt through its side rail, moves accordingly. Although FIG. 3B shows wire-bonding of a chip (66) to a lead frame (16) by means of wires (62), if it shows lead frame fed into loader (24 in FIG. 2) of wire-bonding apparatus, the lead frame is just die-bonded.
As described above, since the surface the of belt (54) is not peripheral to the moving direction of the lead frame, and the belt (54) passes through split (74) of the lead frame stop (72) or the loader/unloader of the wire-bonding apparatus, many parts of the belt (54) cannot be contacted with the side rail of lead frame. Further, since each roller (52) is at a different height, and the belt (54) droops due to tension, the lead frame (16) moves on uneven surfaces of belt (54).
In particular, the lead frame (16), passing the vicinity of the split (74) is apt to be curled into the space between the belt (54) and a roller (52), causing a failure, as can be seen from FIG. 4A.
The difference between the moving speeds or heights of the rails (50a, 50b) may cause a reversion of the lead frames (see FIG. 4B), overlapping of two lead frames (see FIG. 4C), or, in the worst case, cause a swerving or derailment of the lead frame from the rail (see FIG. 4D).
Moreover, since the lead frame moves with waves, in the case of a very thin lead frame, for example, a TOSP (Thin Small Outline Package), the sagging of the bonded is so large that they may contact the, chip be or broken during the transport of lead frame (see FIG. 4E). According to the observation of the inventors, a thin lead frame with less than a 6 mil thickness cannot be transported using a conveyor belt-type system.
If LOC (Lead On Chip) packages each having a chip attached to the lower surface of a lead frame are applied to a conveyor belt-type transport system, they may be damaged, due to contact of the chip with the belt, and consequently they cannot be applied to a conveyor belt-type transport system.