Fluid level sensor

[0002] The invention relates to a fluid level sensor positioned in a container that dispenses fluid used in forming electronic devices.

[0003] In the exemplary manufacturing process of electronic devices, different types of materials are used. For example, adhesives are commonly used to attach one component of a device to another component. To make an integrated circuit device, a die is mounted to a support structure by means of adhesives, which can be one of several types, including metal alloys (e.g., solder) and organic or inorganic adhesives (e.g., epoxies and polyimides, which can be filled with metal).

[0004] In the integrated circuit device 10 shown in FIG. 1, a die 14 can be mounted to a leadframe 12 by using an adhesive layer 16 to attach the underside 34 of the die 14 to the top surface 30 of leadframe 12. If an epoxy or polyimide type adhesive is used, the adhesive is typically dispensed in fluid form onto portions of the top surface 30 of the leadframe 12. After formation of the adhesive layer 16, the die 14 is contacted to the adhesive layer 16 to bond the die 14 to the leadframe 12. Next, bond pads 24 on the die 14 are wirebonded (using wires 26) to corresponding bond pads 22 on leadfingers 20 of the leadframe 12. The leadframe/die assembly is then encapsulated using an encapsulant 18 (e.g., plastic).

[0005] An adhesive originally in fluid form is dispensed onto a target support structure, such as the leadframe 12 in FIG. 1, using a dispensing container in a die attach machine. Various automated methods have been used to detect the level of the fluid adhesive inside the dispensing container. One method that has been used is magnetic sensing. The dispensing container includes a ferromagnetic coated piston that sits on top of the fluid adhesive. As the adhesive is being dispensed, the level of the fluid decreases, and the ferromagnetic piston falls inside the cylindrical container with the fluid level. When the ferromagnetic coated piston reaches a predetermined position in the container, it activates a magnetic sensor to stop the die attach machine. However, certain types of adhesives contain ferromagnetic fillers that may interfere with the magnetic sensing mechanism.

[0006] Various fluid level sensors and detectors have been proposed to detect different types of fluid (e.g., gasoline, water, oil) stored in different types of containers. Such sensors include optical sensors and electrical sensors.

[0007] Generally, the invention is directed to a sensor used in a fluid dispensing container, the sensor including a pair of electrical conductors positioned in the dispensing container to which a voltage to detect the amount of fluid in the container.

[0008] The invention has one or more of the following advantages. Accurate sensing of the fluid level in a fluid dispensing container can be performed with a large variety of adhesives, including adhesives filled with a ferromagnetic filler. The fluid level sensing can be accomplished without the use of moving parts inside the container, which improves reliability and facilitates handling.

[0009] In general, in one aspect, the invention features a system for dispensing an adhesive onto an electronic device. The system includes a container storing the adhesive and a port through which the adhesive can flow. Electronic conductors are positioned in the container. A voltage is applied across the conductors so that a current flow is induced through the adhesive in the container. The current is measured to determine a level of the adhesive in the container.

[0010] In general, in another aspect, the invention features a method of applying an adhesive onto an electronic device. The adhesive is stored in a container having electrical conductors positioned in the container. A voltage is applied across the conductors. The current flow through the adhesive is measured to determine the adhesive level in the container. The adhesive is supplied to the electronic device until the adhesive level drops below a predetermined amount.

[0011] Other features and advantages will become apparent from the following description and from the claims.

[0012] FIG. 1 is an enlarged, cross-sectional view of a prior art integrated circuit device.

[0013] FIG. 2 is a diagram of an adhesive dispensing container having a fluid level sensor according to the invention.

[0014] FIG. 3 is a graph showing resistance of the adhesive in the adhesive dispensing container with respect to the level of the adhesive in the container.

[0015] FIG. 4 is a flow diagram of the process for dispensing adhesive onto electronic devices.

[0016] Referring to FIG. 2, a fluid dispensing container 208 (e.g., a syringe) contains an adhesive 224 in fluid form. The adhesive 224 can be made of various materials, including organic or inorganic adhesives such as thermoset or thermoplastic compounds (e.g., epoxies and polyimides) or some mixture of such compounds. The adhesive can be filled with such electrically conductive fillers as gold (Au), silver (Ag), nickel (Ni), or nickel-plated polystyrene. Nickel and nickel-plated polystyrene (which are ferromagnetic fillers) are examples of fillers used in epoxies or polyimides that may interfere with a fluid level sensing system that is based on magnetic sensing. The ferromagnetic fillers may inadvertently activate the magnetic sensor to cause a false empty indication.

[0017] In the electrical sensing system used in the adhesive dispensing container 208 shown in FIG. 2, however, numerous types of electrically conductive adhesives can be used, including adhesives that are filled with a ferromagnetic filler. The fluid dispensing container 208 includes two strips of electrical conductors placed along the inner wall 226 in the longitudinal or vertical direction Y of the dispensing container 208. The conductors 210A and 210B can be formed of any electrically conductive material, such as copper, gold, silver, or a tungsten coated material.

[0018] The conductors 210A and 210B can be coated to the inner wall 226 of the container 208, or the conductors can be removable probes inserted into the container 208.

[0019] The conductors 210A and 210B extend from the top 230 of the container 208 and run to a predetermined depth into the container 208. The electrically-conductive, fluid adhesive 224 is dispensed through a nozzle 206 under control of control signals provided by a controller 222 in a control block 225. The controller 222 can selectively activate or deactivate the assembly line 200 and notify operators of the container empty condition. Adhesive layers 204 can be formed on target electronic structures 202 (e.g., wafers, leadframes, printed wiring boards, and other integrated circuit or packaged devices). The adhesive layers 204 can later be subjected to a high temperature curing process to bond a die to a support structure, for example.

[0020] To detect current flow through the fluid adhesive 224, a voltage V is applied across electrodes 212A and 212B connected to the conductors 210A and 210B, respectively, in the container 208. The electrodes 212A and 212B are connected by lines 214A and 214B, respectively, to a voltage source 220 that is programmable to control the voltage V across electrodes 212A and 212B. A current meter 218 is connected to detect the current flow through the circuit formed by the voltage source 220, electrodes 212A, 212B, conductors 210A, 210B and the fluid adhesive 224.

[0021] From the voltage V and current I, the resistance R of the fluid adhesive 224 can be determined. For a predefined type of adhesive material, a threshold resistance value is stored in a memory 240 in the controller 222. The measured resistance is compared to the threshold resistance to determine whether or not to shut off the line 200.

[0022] Certain types of adhesives are more conductive then others. For the less conductive adhesives, a higher voltage V can be applied across the electrodes 212A and 212B to increase current flow through the adhesive 224. For highly conductive adhesives, the current V is decreased to decrease the current flow I.

[0023] Referring to FIG. 3, a graph shows the resistance R through the adhesive 224 as a function of the level of the adhesive in the container 208, as measured by the height h of the adhesive 224 from an initial position Y₀along the Y direction. The resistance R increases generally linearly with a decrease in the height h of the adhesive 224. When the adhesive level 232 drops below the position Y₀, (i.e., the fluid level drops below the bottom edge of the conductors 210A and 210B), the resistance R rises sharply to infinity.

[0024] Referring to FIG. 4, the flow of a portion of the controller 222 is illustrated. During the die attach operation, the controller 222 causes the nozzle 206 of the container 208 to dispense adhesive onto target structures 202 on the assembly line 200 (step 400). Next, based upon the applied voltage V and the detected current I, the resistance R representing the amount of adhesive 224 in the container 208 is determined (step 402). The controller 222 then accesses the controller memory 240 to obtain a threshold resistance R_thresholdthat is based on the type of adhesive material used (step 404). The detected resistance R is then compared to the threshold resistance R_threshold(step 406). If the measured resistance R is less than R_threshold, then the die attaching system remains on and the dispenser 208 continues to dispense adhesive (step 400). However, if R is not less than R_threshold, then the container 208 is considered empty and the nozzle 206 is shut off and the line is temporarily stopped (step 408) by setting control signals from the controller 222 to appropriate states. The operator is then notified of the empty container condition (step 410).

[0025] Other embodiments are within the scope of the following claims. For example, although the fluid level sensor has been described in conjunction with sensing the level of an adhesive in a container, the levels of other fluids can also be similarly detected. Other configurations of the electrical conductors in the container can be used. Instead of long strips of conductors, smaller electrodes can be positioned at a predetermined depth inside the container. Using such electrodes, current flow will occur until the fluid level drops below the electrodes, at which point the current will sharply decrease.