GB2238876A

GB2238876A - Solder paste dispensing sensor

Info

Publication number: GB2238876A
Application number: GB9025118A
Authority: GB
Inventors: Jerrold S Pine; Stephan Peana; Sunil Lakhani
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1989-11-20
Filing date: 1990-11-19
Publication date: 1991-06-12
Also published as: GB9025118D0; JPH03167831A

Abstract

A change in the inductance of a coil 16 on the passage therethrough of dispensed conductive material (solder paste 32) affects the frequency of an oscillator circuit 20 and thereby controls the operation of a dispenser 12 to ensure that a desired amount is dispensed. <IMAGE>

Description

SOLDER PASTE DISPENSING SENSOR Field of the Invention This invention relates in general to the field of sensors and more specifically to a sensor for monitoring and controlling the amount of solder paste being dispensed in response to a change in inductance.

Background of the Invention Microelectronic component assembly relies heavily on automated surface mount assembly techniques due to the repetitive nature of the assembly process and the size of the components. The assembly process generally involves the placement of a number of microelectronic components on printed circuit boards. Solder paste is the most conventional material used in the electronics industry to couple the components to printed circuit boards.

The primary concern in dispensing solder paste is the amount of mass in the resulting solder joint. Solder paste is a mixture of solid particles in a liquid flux binder solution. This flux evaporates during the reflow process.

To obtain the required mass after reflow, the same mass has to be contained in the area in which the solder dot is dispensed. Current technology relies on volumetric control of the solder paste and the assumption that the density of the solder paste is constant, which is not reliable since the mass before and after the reflow process is not constant. In practice, the flux has a tendency to separate and move away from the solder particles. This problem is increased at high pressure and can result in flux depletion as the dispenser is emptied, therefore, any shift in the material properties (shifts which may occur during manufacturing, mixing, handling, or dispensing), will result in major changes in specific gravity, thus effecting the solder mass dispensed.

Currently, the electronics industry uses several types of application methods to dispense solder paste on a printed circuit board. These methods include: mesh screen printing, metal mask stencil printing, pin transfer, pneumatic dot and line dispensing, and positive displacement dispensing.

Mesh screen printing and metal mask stencil printing are viable printing methods when solder paste is to be dispensed on a flat, component-free printed circuit board. If components have previously been placed on the printed circuit board, mesh screen printing and metal mask stencil printing are not feasible application methods due to the potential interference of the components with the printing process.

Pin transfer involves dipping the leads of a leaded component, which results in a small dot of solder paste adhering to the end of the leads. The problem with this application method is that the design of the component's pin pattern must exactly correspond to the solder pad pattern on the printed circuit board. Another problem with this application is in assuring that the pin's tip receives a uniform wetting of the solder paste. Therefore, the pin transfer application method is not desirable for use in microelectronic assembly using automated machinery.

One of the more versatile methods to dispense solder paste on a variety of printed circuit board configurations is by using a pneumatic dispenser (e.g., needle and syringe). The needle size may vary to dispense a variety of dot sizes or lines of solder paste. The problem with pneumatic dot and line dispensing is that the needle may become clogged, resulting in little or no solder paste being dispensed. Furthermore, the amount of solder paste being dispensed will vary depending on the type and age of the solder paste used in the application. Solder paste that is not uniformly dispensed results in components that will either short to other components (excess solder) or that are not soldered securely to the board (insufficient solder). Both cases result in a defect that may be detected in the factory or at a later time by the product's end user.

Pneumatically dispensing solder paste may be accomplished manually or programmed to be automatically dispensed. The manual application is extremely time consuming and not considered a viable option.

Programmable pneumatic dispensing is a process that allows the solder paste to be dispensed in a controlled and repeatable pattern, but is typically done with no feedback. System's that utilize no feedback from the dispensing apparatus to the computer program are called open-loop systems.

These system 5 are conventionally used within the industry.

Vision systems have been employed to detect excess or insufficient solder paste conditions, providing feedback from the vision system to the computer program; however, the vision system cannot tell if a void is present below the surface of the solder paste. Current pneumatic dispensing systems use a time-pressure dispensing valve or a positive volumetric dispensing valve located at the end of the automated machinery. Both types of dispensing valves operate using open-loop computer program control, without providing feedback information on the solder paste being dispensed. Without this feedback, the system does not know if the correct amount of solder paste was dispensed.

All of the above mentioned application methods are limited by their inability to detect insufficient solder and the occurrence of solder shorts during the solder dispensing process. Other variations within the dispensing apparatus (e.g., solder paste consistency, flux segregation, dogs, temperature) or outside variations (e.g., board warp age, plating) also impact the dispensing quality. Conventional automated machinery used to dispense solder paste is unable to detect defects in the dispensing process due to the system's open-loop control.

Thus, a need exists for a dispensing apparatus that controls the amount of solder paste being dispensed in response to a change in inductance.

Summary of the Invention Accordingly, it is an object of the present invention to provide an improved sensor.

In carrying out the above and other objects of the invention in one form, there is provided a method comprising the step of controlling the amount of a substance being dispensed in response to a change in inductance caused thereby.

Brief Description of the Drawings FIG. 1 is a schematic diagram of the preferred embodiment of the present invention.

FIG. 2 illustrates the magnetic field resulting from an electric current passing through a wound coil.

Detailed Description of the Invention Referring to FIGS. 1 and 2, a coil 16 is wound around a ferrite core 15 which is contiguous to a channel (e.g., needle) 14. A conductive (metallic) substance (e.g., solder paste 32) is stored in a dispenser 12 (e.g., syringe) and extruded through the needle 14 as required. When a magnetic field exists around the coil 16, an oscillator circuit 20 monitors (i.e. measures or detects) the amount of solder paste 32 deposited on the solder pad 34 from the needle 14. As the coil 16 approaches a desired solder pad 34 on a printed circuit board 36, the inductance of the coil 16 is changed. This variation is due to the change in permeability of the material intersecting the coil's magnetic field. This field is the most sensitive where its flux lines are densest (regions 28 and 30 of the coil 16).Since the solder dispenser 12 is within the region 30, the permeability is constant within that region 30.

Therefore, the region 28 provides the variable inductance. The closer the needle 14 moves to the solder pad 34, the higher the resulting inductance.

This change in inductance varies the frequency of the oscillator circuit 20. A frequency counter 22 monitors the varying frequencies and transmits the data to a microprocessor 24. The microprocessor 24 may calibrate the change in frequency to the distance in which the coil 16 is above the solder pad 34.

This information may be transmitted to the controller 26 to accurately set the height of the needle 14 for dispensing.

The frequency baseline is recorded once the needle 14 height is located above the solder pad 34. As the solder paste 32 is extruded out of the needle 14 and onto the solder pad 34, the inductance of the coil 16 will change, increasing the frequency of the oscillator 20. This change is caused by the permeability change of the region 28 present between the needle 14 and the solder pad 34. By monitoring the frequency of the oscillator 20, the microprocessor 24 measures the amount (mass) of the solder paste 32 being dispensed. This information may then be transmitted to the controller 26, which would incrementally or continuously dispense solder paste until the desired amount had been obtained. This system, without controller feedback (i.e. open-loop), may simply monitor the solder paste dispensing process and provide statistical process control. Additional information provided by the coil 16 may include data of the solder paste 32 quality and the dispenser functionality. The added information may also include a solder flow status warning if a clog is encountered, indicating that the solder paste 32 has separated from the needle, as well as other desired information.

This information may also be used by the controller 26 to modify the amount of solder paste 32 dispensed, resulting in reliable solder joints.

Controlling both the dispensing height and solder paste 32 mass are critical in achieving a consistent solder mass deposition and ultimately a reliable solder joint between an electronic component and the solder pad 34.

Claims

1. An apparatus for monitoring the amount of a conductive substance dispensed by a dispensing means to provide a dispensed amount thereof, the improvement which comprises: dispensing means for dispensing a conductive substance to provide a dispensed amount thereof; and monitor means coupled to said dispensing means for monitoring the dispensed amount of said conductive substance in response to a change in inductance caused thereby.

2. The apparatus according to claim 1 wherein said monitor means further comprises an information means coupled to said oscillator circuit for providing an indication of the amount of said conductive substance dispensed.

3. The apparatus according to claim 1 further comprising a control means coupled to said dispensing means and said monitor means for controlling the dispensed amount of said conductive substance in response to the change in inductance.

4. The apparatus according to claim 3 wherein said control means further comprises an information means coupled to said oscillator circuit for providing an indication of the amount of said conductive substance dispensed.

5. An apparatus for dispensing a conductive substance upon a pad, comprising an inductive coil having a magnetic field, dispensing means operating within said magnetic field of said inductive coil for dispensing said conductive substance upon said pad, the improvement comprising: measuring means for measuring the amount of said conductive substance dispensed upon said pad.

6. The apparatus according to claim 5 further comprising: computer means coupled to said dispensing means for controlling the dispensed amount of said conductive substance.