A SYSTEM FOR IDENTIFYING DEFECTIVE ELECTRONIC DEVICES

Abstract: A system for identifying defective electronic devices includes an inspection device (2, 3) located at a first location during manufacture of electronic devices. The inspection device (2, 3) determines whether a device is defective. A memory device (4) receives information relating to defective devices from the inspection device (2, 3) and stores the information. The information received by the memory device is sufficient to identify the defective device at a second location in a subsequent stage of manufacture. A machine (6) at the second location retrieves the stored information (10) from the memory device (4) to identify a defective device at the second location.

Description: A SYSTEM FOR IDENTIFYING DEFECTIVE ELECTRONIC DEVICES The invention relates to a system for identifying defective electronic devices, and in particular, a system to enable the identification of defective electronic devices during manufacture . During the manufacture of electronic devices, a silicon wafer having a number of electronic circuits thereon is attached to a plastic adhesive tape and the silicon wafer is cut into individual electronic circuits, each of which is known as a "die" . The dies are held in position with respect to each other by the adhesive tape which is not cut during the silicon wafer cutting process. The dies attached to the adhesive tape then pass to a die attach machine which transfers each die from the adhesive tape to a mounting surface (or stage) on a lead frame. The dies are mounted on the mounting surfaces usually by some form of adhesive, such as an epoxy adhesive. Prior to placing each die on the respective mounting surface, the die attach machine carries out an inspection (known as a pre-bond inspection) which checks that the epoxy has been dispensed correctly onto the stage and also ensures that the lead fingers associated with each stage are not damaged. After each die has been attached, the die attach machine carries out a further inspection (known as a post-bond inspection) to ensure that each die is correctly positioned on the respective stage and that, for example, the die is not too close to an edge of the stage and that the die is mounted squarely on the stage. This pre-bond and post-bond inspection process is referred to as "Integrated Quality Control" (IQC) . After the die attach process, the lead frame with the dies attached is positioned in a magazine with a number of other lead frames and passed to an oven in which the epoxy adhesive is cured by heating. The magazine then passes to a wire bond machine which bonds wire from the circuit contacts on each die to respective lead finger contacts on the lead frame. It is possible that defects may be produced by the wire bond machine itself and this may be checked by Visual Bond Inspection (VBI) . After wire bonding, the magazine passes to a moulding stage where black resin is moulded around each stage and die on the lead frame . Originally, movement of the lead frames between each phase in the manufacturing process was manual and each stage in the process was a stand alone operation. However, the movement of the magazine may now be performed by a robotic handling machine and this requires each magazine to be bar coded to enable machine identification of each magazine. This enables individual magazines to move to the next stage in the production line without requiring all the magazines in the batch to finish that particular stage. However, a disadvantage of this mechanised handling system is that it is difficult to trace defects, especially in the wire bond process, as the magazines may move around independently of each other to different stages in the assembly process. In the conventional system, if a defect is identified during the die attach process the defect is indicated manually by a marking on the lead frame for visual identification by an operator. The wire bond machine is not aware of the defect and is unable to determine a defective device from the marking on the lead frame. Accordingly, the wire bond machine will try to bond wire to the die and the lead fingers until it determines that it is unable to do so correctly. Only then does the wire bond machine become aware of a defective device and then signals to an operator that there is a problem with a particular die or wire bond process and ask the operator for further instructions. The operator then has to instruct the wire bond machine to skip or reject that particular device. This double detection of defective devices reduces the efficiency of the assembly process and wastes machine time. In addition, after the moulding process, defects in a device are no longer visible. Hence, defective devices have to be manually marked, for example by scoring the resin moulding to indicate that they are defective, prior to singulation of the devices from the lead frames, so that defective devices may be identified and removed after singulation. This manual marking of the moulding to indicate defective devices further reduces the efficiency of the assembly operation. In accordance with one aspect of the present invention, a method of indicating defective electronic semiconductor devices comprises identifying a defective device and electrically connecting a wire between two contacts on the device to short circuit the two contacts. In accordance with another aspect of the present invention, a system for identifying defective electronic devices comprises an inspection device located at a first location during manufacture of electronic devices, the inspection device determining whether a device is defective, a memory device receiving information relating to a defective device from the inspection device and storing the information, the information being sufficient to identify the defective device at a second location in a subsequent stage of manufacture, and a machine at the second location retrieving the stored information from the memory device to identify a defective device at the second location. An advantage of the invention is that by storing information relating to the defective device and passing the information to a later stage in the assembly process, duplication of identification and documentation of defective devices is reduced and the efficiency of the assembly process is increased. Another advantage of the invention is that it is not necessary to visually mark defective devices, as defective devices can be detected by testing the devices for a short circuit which is a normal part of the testing process . Preferably, the later stage in the process to which the information is sent is a wire bond process and in response to the information, the wire bond machine electrically connects two respective contacts on the device to short circuit the contacts . Preferably, the contacts which are electrically connected are two contacts on the lead frame and typically, two lead fingers. Preferably the information stored which identifies the defective device, includes information relating to the stage number on the lead frame strip on which the defective device is located, information relating to the position of the lead frame in a magazine and information relating to the identity of the magazine in which the lead frame is located. Typically, the defective devices are then rejected during the testing stage as any devices which have two contacts short circuited will fail the test stage and be rejected. Preferably, the contacts short circuited by the wire bond machine are predetermined. An example of a method and a system for identifying defective electronic semiconductor devices will now be described with reference to the accompanying drawings in which :- Figure 1 is a schematic diagram of an assembly process for electronic semiconductor devices in which the invention is incorporated. Figure 1 shows an assembly process for electronic semiconductor devices from a die attach stage through to moulding. During die attach, a die attach machine 1 removes individual dies from an adhesive tape on which the silicon wafer is mounted and positions each die on a respective stage on a lead frame . Each lead frame has a number of stages so that a number of dies are mounted on each lead frame. During the die attach process, the die attach machine 1 carries out a pre-bonding inspection 2 in which the machine 1 ensures that the adhesive, which is normally an epoxy adhesive, has been correctly applied to the central portion of the respective stage. The machine 1 also ensures that the lead fingers associated with each stage are not broken or otherwise damaged. If lead fingers are damaged or the epoxy has not been dispensed correctly, and the die attach machine 1 cannot correct the problem, a defective device signal is sent to a memory device 4 which stores information on defective devices as a "reject map" 10. If the die attach machine 1 detects that the epoxy adhesive has been dispensed properly and that the lead fingers are not damaged, the die attach machine 1 removes a die from the adhesive tape and places the die on the stage on the epoxy adhesive . The die attach machine 1 then carries out a post-bond inspection 3 in which the machine 1 checks that the attached die is not too close to the edge of the stage and that the die is not twisted or skewed on the stage. If the die is not positioned properly, a defective device signal is passed to the memory 4 and the reject map 10 is updated. After a die is attached to each stage on the lead frame, the lead frame is inserted into a magazine which has a unique identification code. The identification code of that magazine is stored in the memory 4 and associated in the reject map 10 with each of the defective devices which are located on lead frames that are in that magazine. A number of magazines containing lead frames with dies attached are then passed to a curing oven 5 in which the lead frames and the dies are heated to cure the epoxy adhesive and to properly bond each die to the respective mounting stage. After curing, the magazines are passed to wire bond machines 6. Typically one magazine is passed to each wire bond machine 6. Each wire bond machine 6 identifies the magazine it has received and retrieves the reject map 10 containing the defective device information for that magazine from the memory 4. As the wire bond machine 6 comes to each device to perform the wire bond operation, it determines from the reject map 10 whether the device is a defective device. If the device is defective, the wire bond machine bonds the wire between two predetermined lead fingers to short circuit the lead fingers and no further wire bonding is performed on that device. If a problem is detected with a device during the wire bond operation, or if the wire bond machine detects that a wire has become broken during the wire bond operation, and the problem cannot be corrected, the wire bond machine 6 sends a defective device signal to the memory device 4 to report that the device is defective and the reject map 10 for that magazine is updated. The information that passes to the memory 4 to update the reject map 10 is the location of the defective device on the lead frame, the position of the relevant lead frame in the magazine, and the magazine identification code. In addition, the wire bond machine 6 also bonds a wire between the two predetermined lead fingers to short circuit the lead fingers. Typically, the lead fingers between which the wire is bonded are always the same two so that all defective devices have a wire bonded between the same two lead fingers. After the wire bonding process, all the defective device information and the relevant lot or batch identification may be stored in a memory 7. After wire bonding, the magazines are passed to a moulding machine which moulds black resin around each device. The devices are then singulated and tested for short circuits at an short open circuit testing machine 9. Hence, the short circuit testing machine 9 rejects all devices which contain short circuits and therefore rejects all the defective devices as short circuits in all the defective devices were generated by the wire bond machine 6 bonding wires between the two predetermined lead fingers. The invention has the advantage that by creating a reject map 10 of each magazine containing the relevant positions of each defective device on each lead frame in the magazine, defective chips may be identified by the wire bond machine 6 and indicated appropriately by bonding wires between two lead fingers . In addition, by short circuiting two lead fingers by bonding a wire between two lead fingers on defective chips enables removal of the defective chips during testing and does not require manual marking of the chips which is labour intensive and slows down the assembly operation. In addition, it may be possible to incorporate an open circuit test in the singulation process to further reduce time spent on identification of defective devices.

Claims: CLAIMS 1. A method of indicating defective electronic semi-conductor devices comprises identifying a defective device and electrically connecting a wire between two contacts on the device to short circuit the two contacts. 2. A method according to Claim 1, wherein the defective device is one of a number of similar devices on a strip of material and the two contacts are two contacts on the strip of material associated with the defective device. 3. A method according to Claim 2, wherein the strip of material is a lead frame and the two contacts are lead fingers. 4. A method according to Claim 3, wherein the two contacts are predetermined. 5. A method according to any of the preceding claims, and further comprising retrieving information identifying the defective device and subsequently connecting the wire between the two contacts on the identified defective device. 6. A method according to any of the preceding claims, the method further comprising connecting the wire to the two contacts using a wire bonding machine. 7. A system for identifying defective electronic semiconductor devices comprising an inspection device located at a first location during manufacture of electronic devices, the inspection device determining whether a device is defective; a memory device receiving information relating to a defective device from the inspection device and storing the information, the information being sufficient to identify the defective device at a second location in a subsequent stage of manufacture; and a machine at the second location retrieving the stored information from the memory device to identify a defective device at the second location. 8. A system according to Claim 7, wherein the machine at the second location is a wire bond machine, and in response to the information, the wire bond machine connects a wire between two contacts on the device to short circuit the two contacts.

Inventors: VADIVAZHAM KALEAMBUR RAJARATHI, VADIVAZHAM, KALEAMBUR, RAJARATHINAM
Applicants: SIEMENS AG, VADIVAZHAM KALEAMBUR RAJARATHI, SIEMENS AKTIENGESELLSCHAFT, VADIVAZHAM, KALEAMBUR, RAJARATHINAM
Application year: 1997
Main classification: H01L21
IPC codes: G01R31, H01L21, H01L23