HK1009036B - Apparatus and method for enhancing stability of ejecting droplets of a molten solder - Google Patents

Description

Apparatus and method for improving stability of molten solder drop during jetting

The present invention relates generally to an apparatus and method for improving the stability of a solder drop spray when dispensing a droplet of molten metal. More particularly, the present invention relates to improvements in the size, shape and composition of any pump-ejected droplet of molten solder.

With the repeated design, digital electronic systems, particularly personal computers, have been greatly reduced in size and increased in functionality, with the attendant reduction in the size of the integrated circuit components and their input/output terminals, which directly affects the pattern of printed circuit board contacts.

Conventional techniques for soldering components of integrated circuit devices to printed circuit boards are either increasingly approaching their technological limits or are becoming more expensive due to the shrinking dimensions of typical advanced designs. The current widely used technique of stencil printing solder paste onto printed circuit boards has reached its technical limits, while another widely used technique is the deposition of solder onto printed circuit boards by electroplating, which is expensive.

A similar situation exists when forming solder-infused arrays on integrated circuit dies (e.g., in the case of flip-chip devices) or on ceramic integrated circuit assemblies. Attempts have been made to reflow solder beads or columns to an integrated circuit die or ceramic package, which has also been a complicated and costly endeavor.

Thus, a hot plume of solder is raised which is directed or sprayed into the integrated circuit die (discrete or in wafer form), ceramic integrated circuit assembly and printed circuit board. While the quality of the molten solder droplets can be controlled in terms of composition, volume and location of injection, there are many benefits to using a solder droplet dispensing or jetting method. It has been found that it is entirely possible to accurately project the molten solder droplets into position during deposition onto the substrate. However, to date, none of the molten solder dispensing systems has been reliable enough to be applied to production to the extent that the molten solder droplets are consistent in composition, volume, and structure.

Us patent 5,377,961 describes an electric pump for dispensing molten solder that is relatively controlled in volume. The electric pump described therein represents a significant advance over solder dispensing machines that employ electrostrictive transducers such as piezoelectric devices, and thus forms the basis of a dispensing system in which molten solder can be relatively controlled. While early tests confirmed the effectiveness of such electric pumps for dispensing molten solder droplets, evaluation of manufacturing specifications critical to the repeatability of droplet dispensing, consistency of droplet composition, and stability of droplet size indicated that further improvements are needed. In this regard, testing has determined that molten solder in the vicinity of the nozzle orifice does not always form a homogeneous, pure solder mixture over time. Metallographic analysis of the solder nozzle residue and photographic analysis of the droplet dispensing process confirmed that the droplets contained a mixture of pure solder, tin oxide and lead oxide, thereby discordating the physical properties of the formed droplets such as surface tension, melting point and affinity (wettability) with other materials. Enclosing the nozzle portion in a nitrogen (oxygen-free) environment does not solve the above-mentioned problems.

From tests carried out on an electric pump, it can be seen that the solder droplets differ significantly over a period of several hours, regardless of how the input parameters determining the pressure pulse that generates a droplet of solder that is ejected once are constant. Further investigation results also confirmed that the instability was caused by the unevenness of the solder in the vicinity of the nozzle hole, which uneven solder consisted of the above-mentioned pure solder and a mixture of tin oxide and lead oxide.

The problem of studying the uniformity of solder droplets during long periods of operation of an electric solder pump has been a consideration in determining under what conditions, other than a nitrogen atmosphere, the surrounding nozzle can deliver solder droplets uniformly and stably over a long period of time in molten solder containing tin and lead.

The present invention employs a stability enhancing apparatus in a dispensing or dispensing system for stabilizing the size and composition of a droplet of molten solder, said stability enhancing apparatus comprising: a nozzle connected to the dispensing system and having an aperture for delivering droplets of molten solder; an injection device for injecting flux into the nozzle opening; and a flux infiltration apparatus for infiltrating the flux injected into the nozzle opening and the molten solder exposed through the opening by capillary action. In accordance with another aspect of the invention, the apparatus for infiltration of flux injected at the nozzle orifice is provided with a nozzle having a temperature sufficient to activate the flux and a nozzle orifice of a size and composition sufficient to allow the liquid flux to infiltrate via capillary action. In addition, improvements have been made to achieve periodic flux injection using a nozzle conveyor by moving the nozzle into contact with a source of flux.

In a specific embodiment of the invention, an electric pump is employed to dispense droplets of molten solder in accordance with an electric pulse, and a gantry-type delivery device is employed to position the nozzle of the solder pump relative to the substrate on which the solder droplets are to be injected, and to move the nozzle to a source of flux. The flux may be added to the nozzle by the following method: the nozzle of the solder dispensing head is immersed in the flux reservoir, contacted with the flux carrying medium or passed through a thin film of flux extracted from the reservoir. In addition, the improvement comprises injecting flux into the nozzle bore by transporting the flux into the molten solder column or out of the nozzle. In various embodiments, flux is applied to the nozzle orifices and penetrates into the nozzle orifices by capillary action, chemically decomposing the solder oxides, removing oxides from the walls of the orifices, and facilitating the venting of gases trapped in the molten solder near the nozzle orifices.

These and other features of the present invention will be more clearly understood and appreciated from a reading of the following detailed description of the embodiments.

FIG. 1 schematically illustrates a droplet manufacturing apparatus employing an electric pump to inject molten solder droplets onto a substrate and stabilized in operation by a flux application device;

FIG. 2 schematically illustrates a cross-sectional view of a nozzle area of a motorized pump head;

FIG. 3 schematically illustrates a cross-sectional view of a nozzle region along with non-uniform solder and infiltrated flux;

FIG. 4 schematically illustrates the insertion of a nozzle into a flux reservoir;

FIG. 5 schematically illustrates the transfer of flux from the intermediate medium into the nozzle;

FIG. 6 schematically illustrates the transfer of flux from a film onto a nozzle;

FIG. 7 schematically illustrates the transfer of flux through a tube in molten solder onto a nozzle;

fig. 8 schematically illustrates the transfer of flux into the nozzle through the spray tube outside the solder reservoir.

The molten solder droplets ejected from the nozzle of the droplet pump are stabilized by directing flux at the nozzle while operating the droplet pump. While flux is widely used in printed circuit board assemblies to chemically alter the metal surfaces of mechanically safe, metallurgically pure soldered joints, the present invention uses flux on the solder dispensing apparatus itself.

The addition of flux to the nozzle of a solder droplet pump destroys oxides in the solder composition and renders the molten solder at the nozzle orifice of the solder pump pure and uniform. Initial studies of the effect of flux during the dispensing of solder drops have shown that flux has three effects. First, the flux decomposes and reduces the tin and lead oxides in the solder column at or near the interface between the molten solder and the nozzle surface, thereby creating a homogeneous mixture of fusible pure solder. Second, the flux breaks down and facilitates the removal of oxides from the inner surface of the nozzle bore itself, making the bore smooth and primarily acting to lubricate the bore surface with a layer of flux material. Finally, the flux reduces the surface tension of the solder material in the nozzle hole itself, thereby improving the wetting of the nozzle surface around the hole by the solder material to eliminate trapped gas at that location. Such trapped gas is generally believed to reduce the uniformity of droplet dispensing.

The flux facilitates the solder droplet dispensing process by simply applying the flux to the nozzle of the solder droplet pump with a swab. The heat of the solder drop pump accelerates the capillary movement of the flux through the orifice, activating the flux, causing it to break up oxides, reducing surface tension, and accelerating the movement of the solder drop through the nozzle orifice. Flux residue appears to be unproblematic and, even if residue is present, is not difficult to remove with simple mechanical scraping methods or cleaning devices.

FIG. 1 schematically illustrates one embodiment of the present invention in a solder dispensing system. As can be seen from the figure, the solder dispensing head 1 is fixed at the XYZ gantry 2, and is positioned with reference to not only the substrate 3 on which the solder droplets are to be injected but also the flux application apparatus 4 and the residue removal apparatus 6. The solder dispensing head 1 causes solder drops to be ejected through the nozzle 7 onto the substrate 3 in a size typically in the range of 4 to 20 mils in diameter. The substrate 3 may be a printed circuit board with a copper contact pattern, a wafer of a plurality of integrated circuit dies (chips) with a discrete pad pattern, or a subassembly holding a plurality of discrete chips or ceramic components. In any event, gantry 2 positions nozzles 7 at designated locations that enable one or more droplets of molten solder to be selectively injected onto substrate 3.

Experience has shown that the solder in the dispensing head 1 is preferably composed of 63/37 pure tin/lead and is thus a solder variety available from many stores. However, if the solder is to be poured onto a wafer or chip, it is likely that a different ratio (typically 90/10 of lead/tin) of high temperature solder will be used.

The flux application apparatus 4 is only schematically shown in the drawings, and specific embodiments will be described later. The device 4 comprises as simple a component as the flux-coated swab described above. The main feature of the general schematic of fig. 1 is that the nozzle 7 is periodically supplied with flux and possibly cleaned of flux, which is performed while droplets of molten solder are deposited in a pattern onto the substrate 3.

Although there are many ways of applying flux to the nozzle, including dipping, spraying, contact transfer or direct injection into the molten solder, preliminary studies have shown that the application of a constant period of time is sufficient to achieve the stability required by current droplet spraying equipment. The electromagnetic devices associated with the above-mentioned recommended solder drop spray heads are complex and large, and appear to be better than the complex design of flux injection into solder through a conduit using a periodic external addition. The basic principle is the same, i.e. the uniform ejection of solder droplets is facilitated by the solder flux.

Fig. 2 shows a schematic view of the nozzle assembly 7 and the relevant lower part of the solder dispensing head assembly 1. It can be seen that the nozzle assembly 7 is connected to the lower part of the spray head assembly and includes an outer sleeve 8 and nozzles 9. The molten solder column 11 is in the nozzle housing 8 and remains in a liquid state due to heating of the jet head assembly. Periodic electromagnetically induced pressure pulses from the jetting head assembly (as described in the above-mentioned U.S. patent 5,377,961) eject individual solder drops through the nozzle orifices 12. In accordance with the present invention, the stability of the molten solder droplets is enhanced by applying flux to the tip region 13 of the nozzle assembly 7.

Fig. 3 shows a schematic enlargement of the nozzle 9, the nozzle eye 12, the molten solder 11 and the injected flux 16. The nozzle 9 of this embodiment is made of a sapphire disk. The outer surface of the molten solder 11 generally has an oxide/dross region 14. The flux applied to the nozzle tip 13 (fig. 2) forms a flux layer 16 by capillary action on the inner wall of the aperture 12 and the exposed surface of the molten solder 11. The flux is activated by the temperature of the nozzle assembly, and the activated flux acts to remove oxides/dross 14 from the solder in the vicinity of the clearance hole 12. Further, the flux 16 also functions to stabilize the surface characteristics of the molten solder 11 and lubricate the inner wall of the hole 12.

The composition of the flux is of considerable importance. In evaluating a series of fluxes, it was found that the flux was suitable for repairing solder parts (commonly referred to as repair flux), while hot air braze leveling (HASL) flux had the greatest effect on enhancing molten solder droplet stability. Repair solders and HASL solders have high thermal stability at high temperatures (above 150 ℃) and high reactivity (i.e., the ability to break thick oxide layers). Since the study has focused on a narrow temperature range of the nozzle and spray head, i.e., the range of 220 ℃ - > 250 ℃, there may be other solder fluxes dispensed that are suitable for improving performance at measurable temperatures below or above this evaluation range.

For evaluation, 63/37 tin/lead solder dispensed through a nozzle at about 230 ℃ was used to determine conditions under which the flux did not coke during use and did not produce a residue that could clog the perforations. Preliminary tests conducted under these conditions of use and through the use of a wiping flux showed that 10,000-20,000 well-formed, dimensionally stable droplets could be dispensed. There are two commercially available fluxes with characteristics suitable for the chosen conditions, one Kester 450B repair flux and one Kester 2438 HASL flux. The activation temperatures of these two fluxes are in the range of 100 to 130 ℃ and are stable up to the above-mentioned 230 ℃ temperature of the nozzle.

The present invention contemplates a series of different methods of applying flux to the nozzle tip 13 (fig. 2) to initiate capillary action and form flux 16, as shown in fig. 3. Fig. 4 illustrates an example in which the tip 13 of the nozzle assembly 7 is inserted into the liquid flux 17 in the flux reservoir 18. The flux reservoir 18 thus represents one version of the flux application apparatus 4 shown in fig. 1.

Fig. 5 shows another version of the flux application apparatus, in which liquid flux 17 in a flux reservoir 18 is delivered to the nozzle assembly 7 by a sponge 19. The transfer of flux by contact with the sponge body 19 or the same medium of action limits the volume of flux actually applied to the nozzle tip 13.

Another method of depositing a flux film on the tip 13 of the nozzle assembly 7 is shown in fig. 6. As shown in step (r), the ring 21 is inserted into the liquid flux 17 stored in the flux reservoir 18. When the ring 21 is lifted from the liquid flux 17, a thin film of flux is left behind through the annulus. The nozzle assembly 7 is placed in a flux film to deposit a thin layer of flux on the nozzle tip 13, as shown in step two.

Another way of injecting flux into the bore 12 (fig. 2) of the nozzle 9 is shown in fig. 7. As can be seen, the thin tube 22 extends down into the molten solder 11 of the nozzle assembly 7, with the mouth of the tube 22 being located adjacent the aperture 12 of the nozzle 9. Flux 23 is sprayed into the nozzle adjacent the aperture 12. Excess flux and flux residue is discharged through the apertures 12 of the nozzle 9 in preparation for dispensing of droplets of molten solder.

The case where the flux is directly sprayed to the nozzle tip 13 is shown in fig. 8. Also, the primary objective is to inject flux into the region of the hole in an amount sufficient to cause the flux to penetrate into the hole by capillary action and onto the surface of the molten solder 11 at the hole 12. In the embodiment of fig. 8, a flux pump 24 is used in conjunction with a flux sprayer 26 to dispense flux in a desired dosage and position.

The above series of examples illustrate the primary object of the invention to deliver flux to the molten solder in the orifice region of the nozzle hole and the orifice itself in an amount sufficient to remove solder oxides and dross and to lubricate the orifice at the operating temperature of the molten solder. The flux thus provided ensures that droplets of molten solder can be ejected evenly from the dispensing head thousands of times, thereby providing a molten solder dispensing machine that is adaptable to the manufacturing environment.

It will be understood by those skilled in the art that the above-described embodiments are merely examples of various aspects of the invention, and that equivalent embodiments may be substituted without departing from the scope of the invention as defined in the appended claims.

Claims (16)

1. An apparatus for improving stability of a molten solder dispensing system upon ejection of a solder drop, comprising:

a nozzle, coupled to the dispensing system, having an aperture for delivering droplets of molten solder;

a flux injection device for injecting flux at the nozzle opening; and

a flux infiltration apparatus for allowing flux injected at the nozzle opening to infiltrate the opening and the molten solder exposed through the opening by capillary action.

2. The apparatus of claim 1 wherein the means for causing the flux at the nozzle opening to infiltrate via capillary action has a nozzle temperature suitable for activating the flux and the nozzle opening is sized and configured to cause the liquid flux to infiltrate via capillary action.

3. The apparatus of claim 1 wherein the flux injection means is a conveyor which functions to periodically bring the nozzle into contact with a source of flux.

4. The apparatus of claim 2 wherein the flux injection means is a conveyor which functions to periodically bring the nozzle into contact with a source of flux.

5. The apparatus of claim 1 wherein said flux injection means is a tube positioned to inject flux at the nozzle orifice.

6. The apparatus of claim 2 wherein said flux injection means is a tube positioned to inject flux at the nozzle orifice.

7. The apparatus of claim 1 wherein said flux injection means is a nozzle conveyor operative to periodically move a nozzle past a film of liquid flux extracted from a reservoir.

8. The apparatus of claim 2 wherein said flux injection means is a nozzle conveyor operative to periodically move the nozzle past a film of liquid flux extracted from a reservoir.

9. A method of improving the stability of a molten solder dispensing system during the ejection of a droplet of solder, said system having a nozzle connected to deliver a droplet of molten solder through an orifice of the nozzle, said method comprising the steps of:

injecting flux into the nozzle hole; and

flux injected at the nozzle orifice is caused to penetrate into the orifice and onto the molten solder exposed through the orifice by capillary action.

10. The method of claim 9, wherein the step of causing the flux injected at the nozzle orifices to infiltrate via capillary action comprises heating the nozzle to a suitable temperature sufficient to activate the flux and causing the liquid flux to infiltrate via capillary action using an orifice sized and configured appropriately.

11. The method of claim 9, wherein the step of periodically injecting flux comprises moving the nozzle into contact with a source of flux with a conveyor.

12. The method of claim 10, wherein the step of periodically injecting flux comprises moving the nozzle into contact with a source of flux with a conveyor.

13. The method of claim 9, wherein the step of periodically injecting flux comprises injecting flux onto the nozzle bore with an injection needle.

14. The method of claim 10, wherein the step of periodically injecting flux comprises injecting flux onto the nozzle bore with an injection needle.

15. The method of claim 9, wherein the step of injecting flux comprises periodically moving a nozzle with a conveyor through a film of liquid flux extracted from a reservoir.

16. The method of claim 10, wherein the step of injecting flux comprises periodically moving a nozzle with a conveyor through a film of liquid flux extracted from a reservoir.