Method and Apparatus for Dispensing Spherical-Shaped Quantities of Liquid Solder

This invention relates generally to dispensing of liquid solder. More particularly, but not by way of limitation, this invention relates to methods and apparatus for producing spherical-shaped quantities of melted solder of accurately known size and applying the spherical-shaped quantities with great accuracy to very small soldering areas to be wetted.

Although this invention is applicable to the dispensing of various liquids, it has been found particularly useful in the environment of dispensing very small solder balls to very small soldering areas. Therefore, without limiting the applicability of the invention to "dispensing very small solder balls to very small soldering areas", the invention will be described in such environment.

In a high density electronic manufacturing process, semiconductor integrated circuit chips are bonded to a substrate by a solder reflow process. In the prior art and with reference to FIG. 1, the semiconductor integrated circuit chip or chip 10 is shown operatively joined to substrate 12 after the solder reflow process has been completed.

Also in the prior art and with reference to FIG. 2, the interconnect material between the chip 10 and the substrate 12 is in the form of a solder bump 14 prior to the solder reflow process. The solder bumps 14 are placed on the pads 16 of the integrated circuit chip or chip 10 by one of various prior art processes which include 1) deposition through a mask plus solder reflow, 2) electroplating plus solder reflow, 3) pick-and-place of $ solder bumps, etc. The actual bonding process utilizes solder bumps which are deposited on the wettable metal pads 16 on the chip 10. Chip 10 (the upside-down chip or flip chip) is then flipped over (see FIG. 2) and the solder bumps 14 are aligned with the correct matching o footprint of solder wettable terminals or connect pads or bond pads 18 on the substrate 12. Heat is then applied and all joints or interconnects between the pads 16 on the chip 10 and the solder wettable terminals or connect pads or bond pads 18 on the substrate 12 are made simultaneously by reflowing the solder in the solder bumps 14. Typically, the interconnects are made with solder bumps 14 which are 100 to 150 .mu.m (microns) in diameter with several hundred to several thousand solder bumps per chip. The electronic manufacturing process is moving toward more interconnects per chip which requires solder bumps of a smaller size since the interconnects are closer together.

With reference to FIGS. 3 and 4, the prior art method of deposition of solder through a mask plus reflow for creating solder bumps 14 is disclosed. Solder is evaporated through apertures or holes 20 in a metal mask 22 which has been aligned over the metal pads 16 on chip 10 with the metal pads 16 being surrounded by insulation 24. In this way an array of solder deposits 26 comprising tin 28 and lead 30 is created on the surface of the chip 10. This method is a batch process. The composition of the evaporated solder deposits 26 depends upon the starting composition of the solder and the vapor pressures of the respective metals comprising the solder. Reflow in a furnace homogenizes the solder deposits 26 and brings the solder deposits 26 to a spherical shape or solder bumps 14 as shown in FIG. 4. The metal pads 16 are wettable by the solder bumps 14 while the insulation 24 is not. Some disadvantages of this process include the high overall cost of the process. The quality and repeatability of the solder alloy is not good and the masks get coated with the solder materials and must be continuously cleaned or thrown away.

Another process for creating solder bumps 14 is the electroplating process plus reflow and consists of application of photoresist material on the chip, exposure to a masked ultraviolet beam to define a pattern in the photoresist material, development of the pattern in the photoresist material and then electroplating with the solder composition. Reflow in a furnace brings the solder deposits to a spherical shape similar to that in FIG. 4. Some disadvantages of this process include the high overall cost of the process because of the photolithograph and the solder plating step presents environmental waste treatment problems. Furthermore the solder alloy is not consistent and usually contains impurities.

Another process for creating solder bumps 14 is the pick-and-place method wherein solder balls are created off line using processes developed for the creation of lead shot. The solder balls are then inspected and those which do not meet specifications are discarded. The solder balls are arranged in grooves in a tray and a robot arm generates the pick-and-place action and picks up solder balls, one at a time and, with the application of heat and pressure, positions each solder bump on an individual pad 16 on chip 10. Some disadvantages of this process include the high cost of the equipment needed for the process, the fact that the process is not suitable for volume production and the danger to individuals in handling and controlling the small solder balls.

This invention provides a method and apparatus for producing solidified solder balls and for ejecting generally spherical-shaped drops of liquid solder through a controlled atmosphere to impact on a surface to be wetted. The apparatus comprises a preload reservoir to hold and maintain solder in a liquid state, an ejection chamber to hold and maintain the solder in a liquid state, a connection between the preload reservoir and the ejection chamber to allow liquid solder in the preload reservoir to be transferred to the ejection chamber while maintaining the oxides in the preload reservoir, means to pressurize the ejection chamber with an inert gas and an ejection device operatively connected to receive liquid solder from the ejection chamber and to eject generally spherical-shaped drops of liquid solder to a specific location on a surface to be wetted. Means are included to provide an atmosphere of inert gas between the ejection device and the specific location on a surface to be wetted.

The method includes the steps of maintaining solder in a liquid state in a reservoir, transferring solder in the liquid state from the reservoir to an ejection chamber, maintaining the transferred solder in a liquid state in the ejection chamber, pressurizing the ejection chamber with an inert gas, transferring solder in the liquid state from the ejection chamber to an ejection device, and ejecting generally spherical-shaped crops or liquid solder from said ejection device to a specific location on a surface to be wetted. An atmosphere of inert gas is provided between the ejection device and the specific location on a surface to be wetted.

Other advantages and features of the invention will become more apparent with reference to the following detailed description of a presently preferred embodiment thereof in connection with the accompanying drawings, wherein like reference numerals have been applied to like elements, in which:

FIG. 1 is a simplified pictorial illustrating the prior art integrated circuit chip bonded to a substrate;

FIG. 2 is a simplified pictorial illustrating one step in the prior art bonding of an integrated circuit chip to a substrate;

FIG. 3 is a simplified pictorial illustrating one step in the prior art deposition method of providing solder bumps on an integrated circuit chip;

FIG. 4 is a simplified pictorial illustrating a solder bump after reflow in the prior art deposition method;

FIG. 5 is a simplified pictorial illustrating apparatus constructed in accordance with the present invention for dispensing spherical-shaped quantities of liquid solder;

FIG. 6 is a simplified pictorial illustrating apparatus constructed in accordance with the present invention dispensing spherical-shaped quantities of liquid solder to soldering areas to be wetted;

FIG. 7 is a simplified pictorial illustrating the stacking of solder balls on an integrated circuit chip;

FIG. 8 is a simplified pictorial illustrating the application of flux and solder on a pad;

FIG. 9 is a graphic illustration of break-off length vs voltage of solder balls formed under various conditions;

FIG. 10 is a simplified pictorial illustrating the formation of solder balls by the present invention;

FIG. 11 is a graphic illustration of droplet temperature vs distance from the ejection device of the present invention; and

FIGS. 12-14 depict various arrays of solder balls produced by the present invention.

Referring to the drawing and FIG. 5 in particular, shown therein and generally designated by the reference character 40 is solder ejecting apparatus constructed in accordance with the present invention and which is capable of not only producing and ejecting spherical-shaped quantities of liquid solder which can be caught in a container but is capable of producing and ejecting spherical-shaped quantities of liquid solder and applying the spherical-shaped quantities of liquid solder with great accuracy to very small soldering areas or surfaces to be wetted which includes wettable metal pads on an integrated circuit chip and connect or bond pads on a packaging substrate.

As illustrated, the solder ejecting apparatus 40 comprises a preload reservoir 42 for initially receiving and holding solder 44 in a liquid state. Heating element 46 is operatively positioned in preload reservoir 42 to maintain the solder 44 in a liquid state. Thermocouple 48 is connected to power source 50 so the solder 44 will be maintained in a liquid state. An ejection chamber 52 is connected to the outlet of preload reservoir 42 through conduit 54 and valve 56 to allow liquid solder 44 to flow from the preload reservoir 42 into ejection chamber 52. The oxides and impurities tend to stay at the top of the liquid solder 44 in the preload reservoir 42 and do not enter the ejection chamber 52. Heating element 58 is operatively positioned in ejection chamber 52 to maintain the solder 44 in a liquid state. Thermocouple 60 is connected to power source 62 so the solder 44 will be maintained in a liquid state. The ejection chamber 52 is pressurized through inlet port 64 by an inert gas from gas source 66 which forces liquid solder 44 through filter 68 into the ejection device 70 to purge the system in preparation for operation. Although the ejection chamber 52 is not pressurized during operation, the space above the liquid solder 44 is filled with an inert gas to eliminate any oxygen in the atmosphere above the liquid solder 44. Heater 72 surrounds ejection device 70 and controls the temperature of the liquid solder 44 within the ejection device 70. Heater 72 is connected to power source 74. Programmable controller 76 provides activating signals to drive electronics 78 whose output causes ejection device 70 to eject a drop or drops 80 of liquid solder 44 which is controlled by the particular program in programmable controller 76.

The atmosphere surrounding the drops 80 of liquid solder 44 leaving ejection device 70 is filled with an inert gas to eliminate oxygen in the space traveled by the drops 80. A gas source 82 provides a flow of inert gas into the space traveled by the drops.

It is critical that the atmosphere in the ejection chamber 52 and in the space traveled by the drops 80 of liquid solder 44 be devoid of oxygen. The inert atmosphere greatly enhances forming spherical-shaped drops of liquid solder which are solidified into spherical-shaped balls of solder.

As illustrated in FIG. 5, ejection device 70 comprises a drop-on-demand ejection device employed to produce drops 80 of solder 44 in the form of solder balls 86. The balls 86 are solidified drops 80 of solder 44 and which are very uniform in shape size caught in container 84 for use where needed. The solder balls 86 are spherical in shape. In the preferred embodiment, solder balls 86 can be provided which range in diameter from 25 .mu.m to 200 .mu.m with the size of the solder balls 86 varying with the size of the nozzle of the ejection device 70 and the magnitude and frequency of the pulses from drive electronics 78.

As illustrated in FIG. 6, ejection device 70 comprises a charge-and-deflection ejection device employed to produce drops 80 of solder 44 in the for of solder balls 86 which are solidified drops 80 of solder 44 after being ejected to metal pads 16 on chip 10 and connect or bond pads 18 on substrate 12. The solder balls 86 are spherical in shape and are very uniform in shape and size and are accurately positioned on the metal pads 16 on chip 10. With the charge-and-deflection type of ejection device, ejection chamber 52 is pressurized by an inert gas from gas source 66 and is maintained in a pressurized state during operation to produce the ejection of the spherical-shaped quantities of liquid solder.

In this embodiment, the ejected drops 80 of liquid solder 44 are charged to a predetermined polarity by applying a voltage of the proper polarity and magnitude to charge plates 88. The charged drops of liquid solder travel through an electric field created by the application of a high voltage to a pair of deflection plates 90. Depending upon the charge applied to the drops 80 of liquid solder 44, the drops 80 will be deflected to desired metal pads 16 on chip 10 or will be deflected to catcher 92. The metal pads 16 are maintained at a cooler temperature than the drops 80 which form solder balls 86 on the pads 16.

It will be appreciated that rather than deflecting or moving the drops 80 of liquid solder 44, chip 10 could be moved by mounting chip 10 on an X-Y table 94 which is equipped with an X-axis positioning servo 96 and a Y-axis positioning servo 98. Programmable controller 76 is operatively connected to the X-axis positioning servo 96 and the Y-axis positioning servo 98 and is structured to provide programmed control signals thereto to move the X-Y table 94 to a particular desired location and/or a predetermined sequence of locations with respect to ejection device 70.

It will be appreciated that the drop-on-demand of ejection device FIG. 5 could be used in lieu of the charge-and-deflection ejection device of FIG. 6 with the X-Y table 94 being used to position the chip 10.

With reference to FIG. 7, the height of the stack 100 of solder balls 86 can be easily changed by ejecting more than one drop 80 of liquid solder 44 onto a pad 16 of chip 10. Higher stacks of solder balls 86 provide better reliability during temperature cycling (less shear stress in the solder joint).

With reference to FIG. 8, an additional ejection device 70 is operatively connected to a different ejection chamber 52 which contains liquid flux 102. It will be appreciated that spherical-shaped drops of liquid flux 102 may be dispensed just before or at the same time the drop or drops 80 of liquid solder 44 is dispensed so that the drop or drops of liquid flux 102 arrives at the wettable metal pads 16 just before or at the same time as the arrival of the drop or drops 80 of liquid solder 44.

In the preferred embodiment, the ejection device 70 comprises a piezoelectric transducer which operates in the normal manner. The basic difference from an ink jet transducer is that the materials are selected which will withstand the temperature of liquid solder.

FIG. 9 depicts the results of operation of the drop-on-demand ejection device 70 under various conditions of pressurization of the ejection chamber 52, frequency of excitation of the ejection device 70 and the voltage to the piezoelectric transducer of the ejection device 70. The generally linear curves demonstrate that the ejection device 70 operates in a very controlled manner with good repeatability. Break-off length describes the distance from the orifice at which the droplets detach from the jet.

FIG. 10 depicts the ejection of generally spherical-shaped quantities of liquid solder 44 from ejection device 70 of the charge-and-deflection type with an orifice diameter of 58 .mu.m and operating at a frequency of 15 kilohertz and a velocity of 3.5 m/sec. The diameter of the solder drops 80 is 100 .mu.m. The wavelength .lambda. of the stream of solder drops 80 is 230 .mu.m.

FIG. 11 depicts the comparison of results of the operation of ejection device 70 with comparable curves derived by a computer model for droplet temperature vs distance from the ejection device 70 and shows that the operation of the inventive system is predictable and that the temperature of the solder drops or droplets 80 which is desired at the point of deposit on a pad, etc., can be chosen by merely placing the pad at the correct distance from the ejection device 70.

FIGS. 12-14 illustrate magnified views of solder balls 86 with a diameter of 110 microns produced by the present invention.

With reference to FIG. 5, the operation of present invention begins with the solid solder being placed in preload reservoir 42. The solder comprises an eutectic alloy of at least two of the elements lead, tin, bismuth and indium. Power is applied to heating element 46 to bring the temperature of the particular solder to the melting point. After the solder is melted in the preload reservoir 42, a portion thereof is transferred through conduit 54 and valve 56 to the ejection chamber 52 where the solder 44 is maintained in a liquid state by heating element 58. Ejection chamber 52 is pressurized with an inert gas, e.g. nitrogen, from 10 to 30 psi. The pressure forces liquid solder 44 through filter 68 into ejection device 70. Ejection device 70 is structured to have a nozzle opening of 25 to 100 microns in diameter and to be excited from drive electronics 78 with a signal having a frequency between 2000 to 100,000 hertz. Upon excitation of the ejection device 70 drops 80 of liquid solder 44 are ejected to become individual solder balls 86, as shown in FIG. 5, or solder balls 86 deposited on pads 16 of chip 10, as shown in FIG. 6.

The advantages of the present invention are many and include the ability to easily adjust the diameter size of the solder ball 86. In the drop-on-demand ejection device 70, small changes in the diameter of the solder ball 86 require only a change in the excitation voltage. Large changes require a change in the size of the orifice 104 of the ejection device 70. In the charge-and-deflection ejection device 70, changes are only required in frequency of the excitation voltage. The present invention can produce several thousand solder balls 86 per second. The equipment is less expensive than the prior art equipment. Personnel need not handle the individual solder balls which can be dangerous because of their small size. Flux may also be dispensed. Different layouts of pads on the chip only require a program change to the programmable controller.

From the foregoing detailed description it can be appreciated that solder ejecting apparatus constructed and operated in accordance with this invention provides an improved apparatus and method of making solder balls and also of applying solder balls on pads of a chip which is faster and more accurate resulting in a less costly operation.

Although the present invention has been described with reference to a presently preferred embodiment, it will be appreciated by those skilled in the art that various modifications, alternatives, variations, etc., may be made without departing from the spirit and scope of the invention as defined in the appended claims.