SPLIT PISTON

Technical field

The invention disclosed herein relates to jetting of viscous medium onto a substrate. More precisely, it relates to an ejector and a method utilizing an impacting device that comprises an actuating part and an impacting part and which can be arranged in a first state in which the actuating part and the impacting part are separated from each other and in a second state in which the actuating part exerts a force on the impacting part to cause the viscous medium to be jetted onto the substrate.

Background

Ejectors and methods are known in the art for jetting droplets of viscous medium or fluid, such as e.g. solder paste or adhesive, onto a substrate such as a printed wiring board (PWB), thus forming deposits on the substrate prior mounting components thereon. Such an ejector generally comprises a chamber for accommodating a volume of the viscous medium prior to the jetting thereof, a jetting nozzle communicating with the chamber, and an impacting device for impacting and jetting the viscous medium from the chamber through the nozzle in the form of droplets. Further, a feeding mechanism may be utilised to feed the medium to the chamber.

High production accuracy and reliability are factors of interest when manufacturing e.g. printed circuit board (PCB) assemblies. In particular, the reliability, such as e.g. the accuracy and repeatability of the jetting process is of interest due to its effects on the performance and the quality of the final product, such as e.g. the PCB assembly. Too small volumes of deposited medium may e.g. lead to dry joints or loosening components, whereas too large volumes of deposited medium may result in short-circuiting caused by e.g. solder balls, or defective contacts due to contamination of adhesive or underfill. To increase process reliability and performance, an improved control of the application of the deposited medium is desirable so as to reduce the risk for unintentional shortcuts, contamination or erroneous volumes. Summary

An object of the present inventive concept is accordingly to provide an improved and more reliable application of jetted droplets onto a substrate. Additional and alternative objectives may be understood from the following.

According to an aspect of the present inventive concept there is provided an ejector for jetting a viscous medium onto a substrate. The ejector comprises a jetting chamber adapted to accommodate the viscous medium, a nozzle communicatively connected to the chamber, and an impacting device adapted to impact a volume of the viscous medium in the chamber such that viscous medium is jetted through the nozzle towards the substrate. The impacting device comprises an actuating part and an impacting part, and is possible to arrange in a first state in which the actuating part and the impacting part are separated from each other, and in a second state in which the actuating part engages the impacting part and exerts a force on the impacting part, wherein the force causes the impacting part to impact the volume of the viscous medium in the chamber.

By separating the impacting device into an actuating part and an impacting part, two different lengths of stroke can be achieved. Considering the length of stroke as the total distance the actuating part and the impacting part, respectively, moves during a jetting cycle, the separation in the first state allows for the length of stroke of the actuating part to be greater than the length of stroke of the impacting part. Put differently, when comparing the motion of the actuating part and the impacting part, the actuating part may retract further away from the nozzle.

The length of stroke of the actuating part is a parameter of interest in the jetting process, as it may affect the maximum velocity at which the impacting device impacts the viscous medium to be jetted. Increasing the length of stroke gives the actuating part an increased distance over which it may accelerate during the stroke. The velocity at the impact may, in turn, affect the jetting in terms of e.g. exit speed of the jetted droplet, its shape, spreading on the substrate, breakoff etcetera. These parameters will be described in further detail below.

The length of stroke of the impacting part can be considered to correspond to the displacement or stroke volume in the jetting chamber, and should, at least according to some examples, be matched to the volume of the viscous medium to be jetted. A too long stroke, or too large displacement, may lead to air being sucked into the jetting chamber, via the nozzle, as the impacting device is retracted from the nozzle. This may in particular be the case when the displacement volume is larger than the feeding rate of viscous medium to the chamber. The entrapped air in the jetting chamber may impair the quality of the jetting process and make the process unreliable. A too short length of stroke may, on the other hand, result in a too low volume to be jetted as the displacement is too low to expel the desired amount through the nozzle.

The present inventive concept is hence particularly advantageous in connection with jetting of smaller droplet volumes, such as droplets having a volume of e.g. 5 nanolitres or less, as such volumes in general require a relatively small displacement of viscous medium in the jetting chamber. In general, smaller volumes also tend to require a relatively high impact (and thus velocity) in order to break off and be able to leave the ejector at a sufficiently high speed. The splitting of the impacting device into two parts that are allowed to at least partly move independently from each other thus allows for relatively small droplets to be ejected at a relatively high velocity and with a relatively low displacement within the jetting chamber.

In the context of the present application, it is to be noted that the term "viscous medium" should be understood as a medium comprising e.g. solder paste, solder flux, adhesive, conductive adhesive, or any other kind of medium or fluid used for fastening components on a substrate, conductive ink, resistive paste, or the like.

For at least some solder paste applications, the solder paste may include between about 40% and about 60%, inclusive, by volume of solder balls and the rest of the volume may be solder flux. The solder balls are typically about 20 microns in diameter, or between about 10 and about 30 microns, inclusive, in diameter.

In at least some solder paste applications, the volume percent of solder balls of average size may be in the range of between about 5% and about 40%, inclusive, of the entire volume of solid phase material within the solder paste. In other applications, the average diameter of the first fraction of solder balls may be within the range of between about 2 and about 5 microns, inclusive, while the average diameter of a second fraction of solder balls may be between about 10 and about 30 microns, inclusive.

The term "jetted droplet", or "shot" should be understood as the volume of the viscous medium that is forced through the jetting nozzle and moving towards the substrate in response to an impact of the impacting device. It will however be appreciated that a plurality of droplets may be expelled from the nozzle in response to a single stroke of the impacting device.

By the term "jetting" is meant a non-contact deposition process that utilizes a jet to form and shoot droplets of a viscous medium from a jetting nozzle onto a substrate, as compared to a contact dispensing process, such as "fluid wetting". In contrast to a dispenser and dispensing process where a needle in combination with, for contact dispensing, the gravitation force and adhesion force with respect to the surface is used to dispense viscous medium on a surface, an ejector or jetting head assembly for jetting or shooting viscous medium should be interpreted as an apparatus including for example a piezoelectric actuator and a plunger or piston for rapidly building up pressure in the jetting chamber by the rapid movement (e.g., rapid controlled mechanical movement) of the impacting device (e.g., the rapid movement of a plunger) over a period of time that may be longer than about 1 microsecond, but less than about 50 microseconds, thereby providing a deformation of the fluid in the chamber that forces droplets of viscous medium through the jetting nozzle. In one implementation, an ejection control unit applies a drive voltage intermittently to a piezoelectric actuator, thereby causing an intermittent extension thereof, and a reciprocating movement of a plunger with respect to the assembly housing of the ejector or jetting assembly head. The jetting of viscous medium may be performed in a sequence of shots while the jetting nozzle is in motion without stopping at each location on the workpiece or substrate where viscous medium is to be deposited.

A volume of each individual droplet to be jetted onto the workpiece may be between about 0.1 nanolitres and about 30 nanolitres. A dot diameter for each individual droplet on the substrate may be between about 0.1 mm and about 1 .0 mm. The speed of the jetting, i.e. the speed of each individual droplet, may be between about 5 m/s and about 50 m/s. The speed of the jetting mechanism, e.g. the impacting mechanism for impacting the jetting nozzle, may be as high as between about 5 m/s and about 50 m/s but is typically smaller than the speed of the jetting, e.g. between about 1 m/s and about 30 m/s, and depends on the transfer of momentum through the nozzle.

The term "formation" of a droplet may refer to the break-off of a fluid filament induced by the motion of the fluid element. This may be contrasted to a slower natural break-off akin to dripping where the break-off of a fluid filament is driven for example by gravity or capillary forces.

Without acquiescing to a particular physical model, the volume and/or the shape of the jetted droplet is believed to depend on the actual speed of the droplet and its viscosity at the break-off, such that an increased viscosity may require an increase speed while it reduces the plasticity or elasticity of the filament and promotes an earlier and more well-defined break-off.

Typically, an ejector is software controlled. The software needs instructions for how to apply the viscous medium to a specific substrate or according to a given (or alternatively, desired or predetermined) jetting schedule or jetting process. These instructions are called a "jetting program". Thus, the jetting program supports the process of jetting droplets of viscous medium onto the substrate, which process also may be referred to as "jetting process" or "printing process". The jetting program may be generated by a pre-processing step performed off-line, prior to the jetting process.

As discussed herein, the term "deposit size" refers to the area on the workpiece, such as a substrate, that a deposit will cover. An increase in the droplet volume generally results in an increase in the deposit height as well as the deposit size. A "workpiece" may be a board (e.g., a printed circuit board (PCB) or flexible PCB), a substrate for ball grid arrays (BGA), a flexible substrate (e.g., paper) chip scale packages (CSP), quad flat packages (QFP), wafers, flip- chips, or the like.

According to an embodiment, the ejector may comprise a spring mechanism that is arranged to provide a preload for the actuating part and, in the first state, to allow it to retract from the nozzle. The preload may thus be directed in a length direction of the impacting device.

According to an embodiment, the impacting part may be preloaded, preferably by a spring mechanism, to cause the impacting part to retract away from the nozzle and to in the first state allow the jetting chamber to be refilled with viscous medium to be jetted. Alternatively, or additionally, the impacting part is caused to retract by a pressure exerted on the impacting part by the viscous medium in the jetting chamber.

Irrespective of the actual mechanism causing the impacting part and the actuating part to retract from the nozzle, it is advantageous if, during the retraction into the first state, the velocity of the impacting part is lower than the velocity of the actuating part such that the actuating part is allowed to retract a further distance than the impacting part (and thus separate from the impacting part) before the next stroke is initiated.

According to an embodiment, the actuating part is formed of a material allowing for a length of the actuating part to be varied in a controlled manner, preferably by means of a control unit. The actuating part may e.g. comprise a piezoelectric material that is actuated by an applied voltage provided from e.g. the control unit.

According to an embodiment, the impacting part comprises at least one channel for supplying the viscous medium to the jetting chamber. The channel may e.g. be axial, extending within the impacting part or in an outer surface of the impacting part. Alternatively, or additionally the impacting part may comprise a helical channel or groove extending along the impacting part for feeding the viscous medium to the jetting chamber.

According to an embodiment, the ejector may comprise a feeding mechanism for supplying the viscous medium to the jetting chamber. The feeding mechanism may be variable such that a specific feeding rate can be selected to deliver a desired volume of the viscous medium to the jetting chamber. The delivered volume can be selected such that it complies with the stroke volume of the impacting part and/or a determined volume to be jetted. In one example, the feeding mechanism may be reversed to as to reduce a pressure in the jetting chamber and/or to decrease a volume of the viscous medium present in the jetting chamber.

According to an embodiment, the ejector may comprise a rotating mechanism adapted to rotate the impacting part around a length axis of the impacting device, wherein the impacting part is configured to rotate such that shearing is induced in the viscous medium to be jetted. This is a particular advantage when the ejector is used with a viscous medium that exhibits shear thinning (or shear thickening) properties. By rotating the impacting device and thus subject the viscous medium to shearing, the viscosity of the viscous medium may be affected as the viscous medium passes the rotating impacting device towards the jetting chamber. The ejector hence allows for the viscosity of the viscous medium to be varied or controlled prior to the impact by the impacting device. In case the ejector is used with a shear shinning medium, the viscosity of the viscous medium may be reduced in response to the shearing so as to facilitate feeding and pumping of the viscous medium in the ejector and reduce the risk of blocking or clogging of the nozzle. In case the ejector is used with a shear thickening medium, the viscosity of the viscous medium may be increased in response to the rotation of the impacting device so as to promote a more distinct break-off point for the filament that is formed during droplet formation and allow the droplet to be more accurately positioned on the substrate. The rotating mechanism may also induce shear in the viscous medium already present in the jetting chamber. This is of particular interest at the beginning or start-up of a jetting sequence, as the viscous medium already present in the jetting chamber otherwise tend to have a viscosity that differs from the viscosity of the viscous medium that is being feed to the jetting chamber during the jetting sequence. By subjecting the viscous medium in the jetting chamber to shearing before the jetting sequence is started, that is, before the first droplets are jetted onto the substrate, the viscous medium may be given an initial viscosity that is closer a steady state viscosity of a continuous jetting sequence.

The rotation of the impacting device could further be adjusted to compensate for variations in viscosity between different types or batches of viscous media, thereby allowing for a broader variety of viscous media types to be jetted by the ejector.

According to a second aspect, a method for jetting a viscous medium onto a substrate is provided, wherein the jetting is performed by means of an ejector according to the first aspect. The method comprises providing viscous medium to the jetting chamber, and arranging the impacting device in a first state in which the actuating part and the impacting part are separated from the other, and in a second state in which the actuating part engages the impacting part and exerts a force on the impacting part. The force causes the impacting part to impact the volume of the viscous medium in the chamber, thereby jetting viscous medium through the nozzle towards the substrate.

According to an embodiment, the method further comprises retracting the impacting part from the second state at least partly by means of the force of a spring mechanism.

According to an embodiment, the method comprises retracting the impacting part from the second state without using a spring mechanism. The retraction may be achieved solely by the force induced by the pressure difference caused by the viscous medium in the jetting chamber during and after a portion of the viscous medium has been ejected through the nozzle.

According to an embodiment, the volume of the viscous medium in the jetting chamber is supplied to an inlet communicatively connected to or associated with the impacting part of the impacting device. This may be achieved by means of a feeding mechanism.

According to an embodiment, volume of the viscous medium supplied through an inlet of the housing of the ejector corresponds to the viscous medium jetted through the nozzle. The supplied volume may further be determined by a selected feeding rate of the feeding mechanism.

The inventive concept disclosed may be embodied as computer readable instructions for controlling a programmable computer in such manner that is causes an ejector or system to perform the method outlined above. Such instructions may be distributed in the form of a computer- program product comprising a non-volatile computer-readable medium storing the instructions.

It will be appreciated that any of the features in the embodiments described above for the ejector according to the first inventive concept may be combined with the system and method according to the other aspects of the inventive concept disclosed.

Further objective of, features of, and advantages with the present inventive concept will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art will realise that different features of the present inventive concept can be combined to create embodiments other than those described in the following. Brief description of the drawings

The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present inventive concept, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

Figure 1 is a schematic illustration of an ejector.

Figure 2 shows a spring loaded impacting part and an actuating part of an ejector.

Figures 3a-d are schematic illustrations of the relative positions of the actuating part and the impacting part during a jetting process.

Figure 4 is a schematic illustration of a system for jetting viscous medium onto a substrate.

Figure 5 is a flow chart illustrating a method for jetting viscous medium onto a substrate. Detailed description of preferred embodiments

Detailed embodiments of the present inventive concept will now be described with reference to the drawings. The present inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the inventive concept to those skilled in the art.

With reference to figure 1 , there is shown an ejector 1 comprising an assembly housing 10 and an impacting device 4, which in this implementation may include an actuating part 5 including e.g. a piezoelectric actuator, and an impacting part 6 such as e.g. a plunger or piston 6 operatively connected to the actuating part 5. The piston 6 may be axially moveable while slideably extending through a cylinder bore hole in a bushing 8. Cup springs 9 may be provided to resiliently balance the actuating part 5 against the assembly housing 10, and for providing a preload for the piezoelectric actuator. An eject control unit (not shown) may apply a drive voltage intermittently to the piezoelectric actuator 7, thereby causing an intermittent extension thereof, and hence a reciprocating movement of the piston 6 with respect to the assembly housing 10, in accordance with solder pattern printing data.

Furthermore, the ejector 1 may comprise jetting nozzle 3, which may be operatively directed against a substrate 23 onto which droplets 22 of viscous medium are to be jetted. The nozzle 3 provides an outlet 3 through which the droplets 22 are jetted towards the substrate 23.

A jetting chamber 2 may be defined between an end surface 1 1 of the piston 6 and the nozzle 3. Axial movement of the piston 6 towards the nozzle 3 may cause a rapid decrease in the volume of the chamber 2. Such an impact by the piston 6 may thus cause a rapid pressurisation and jetting of viscous medium through the nozzle 3.

Viscous medium may be supplied to the jetting chamber 2 from a supply container (not shown in figure 1 ), via a feeder 12. The feeder 12 may comprise an electric motor (not shown) having a motor shaft 13 partly provided in a tubular bore that extends through the ejector housing 10 to an outlet port 21 communicating with the chamber 2. At least a portion of the rotatable motor shaft, or feed screw 13 may be surrounded by a tube 14 made of an elastomer or the like arranged coaxially therewith in the tubular bore, wherein the threads of the rotatable feed screw 13 may be in sliding contact with the innermost surface of the tube 13. Viscous medium captured between the threads of the feed screw 13 and the inner surface may then be forced towards the chamber 2 in accordance with the rotational movement of the feed screw 13.

Figure 2 illustrates a portion of an ejector that may be similarly configured as the ejector of figure 1 . In the present embodiment, the impacting part 6, or piston 6, is preloaded by a spring mechanism 20, such as a cup spring 20, for retracting the piston 6 into the first state. The ejector 1 further comprises an inlet 21 for supplying viscous medium to the jetting chamber 2. As indicated by the arrows in figure 2, the viscous medium may pass from the inlet 21 to the jetting chamber 2 and the nozzle 3 via a gap or slit 15 between an outer sidewall of the piston 6 and an inner sidewall of the cylindrical bore in which the piston 6 slides during the stroke. In an

alternative, or additional embodiment the piston 6 may be rotatable around its length axis so as to induce shearing in the viscous medium passing through the gap 15, thereby allowing the viscosity of shear thinning (or shear thickening) viscous media to be varied.

Figures 3a-d show the relative positions, or states, of the actuating part 5 and the impacting part 6 of an ejector during a jetting process. The ejector may be similarly configured as the embodiments disclosed in connection with figures 1 and 2. In this particular example, the actuating part 5 comprises a piezoelectric actuator 5 and the impacting part 6 a spring loaded piston 6. The piston 6 is spring loaded against the housing 10 by means of a cup spring 20, biasing the piston 6 in a direction away from the nozzle 3.

Figure 3a shows the piezoelectric actuator 5 in a non-activated state, i.e., without any voltage applied. In this state the piezoelectric actuator 5 rests against the piston 6 and the axial position (as seen in the length direction of the piezoelectric actuator and the piston) is determined by the counter force generated by the cup spring 20. In figure 3b the piezo 5 is activated to press the piston 6 towards the nozzle 3 until a desired chamber volume 2 is achieved. The chamber volume 2 may be selected to correspond to a desired volume to be jetted, and may then be filled with viscous medium, such as e.g. solder paste, which preferably may correspond to the volume to be jetted. The viscous medium may be supplied from the inlet 21 via a gap 15 defined by the piston 6 and the housing 10. Alternatively, the chamber 2 is already filled with viscous medium when the piezo 5 is activated, such that excess viscous medium is discharged from the chamber 2 as the piston 6 is pushed downwards, i.e., towards the nozzle 3.

Once the desired chamber volume 2 (or volume of viscous medium in the chamber 2) is reached, the piezoelectric actuator 5 may be rapidly retracted to its first position, in which the piezoelectric actuator 5 and the piston 6 are axially separated from each other as shown in figure 3c. This separation may e.g. be achieved by retracting the piezo 5 faster than the piston 6 is able to retract from the nozzle 3. The slower movement of the piston 6 may e.g. be due to the dimensioning of the spring force (or spring constant) of the cup spring 20, which may be selected to be sufficiently weak to allow the piezo 5 to move faster than the piston 6. It will however be appreciated that such spring mechanism 20 is optional, and that the piston 6 instead may be retracted by means of a pressure difference acting on the piston 6. The difference in speed of retraction is illustrated by arrows in figure 3c, wherein the length of the arrows indicates the speed.

In figure 3d, the piezoelectric actuator 5 and the piston 6 are arranged in the second state, in which the piezo 5 has been rapidly extended to force the piston 6 downwards to compress the chamber 2 and thus to jet at least part of the viscous medium in the chamber 2 through the nozzle 3. The speed of the piezo 5 (and thus the piston 6) may in this stroke be higher than the speed of the initial, chamber-defining activation of the piezo 5 as discussed above in connection with figure 3b. In the case of figure 3b, the piston 5 was pushed towards the nozzle 3 to define the volume of the viscous medium to be jetted by the stroke of figure 3d. The speed of the piezo 5 may therefore in the first, volume-defining case be sufficiently low to discharge excess volume in the chamber without causing any actual jetting.

The above-illustrated procedure of figures 3b-d may be repeated for each shot, and in particular when jetting "single" shots, i.e., shots not forming part of any sequence of several shots. In case a plurality of shots are jetted in a sequence, the volume defining step of figure 3b may be performed only for the first shot in that sequence. The subsequent shots may be ejected by means of the steps of figures 3c and 3d, i.e., by letting the piezoelectric actuator 5 and the piston 6 alternate between the first state and the second state.

With reference to figure 4, there is illustrated a jetting machine 41 in which substrates 47 will be provided with droplets of viscous medium, such as e.g. adhesive or solder paste. A software program may be run on a computer 43, which communicates with the machine 41 . The software program has a database, which holds principal manufacturing data about substrates, e.g., PCBs, machine data for the machine in which the substrates are to be processed. Substrate data 45 about the substrate may be imported to the database, preferably in the form of CAD data comprised in a CAD file. The program may be adapted for generating a jetting program controlling the jetting process. The jetting program may e.g. comprise parameters affecting the impacting force of the impacting device, the supply of viscous medium to the jetting chamber, and a rotational speed of the impacting device so as to provide a desired viscosity. The software program may be provided on a computer readable medium which is illustrated by a CD ROM 49 in figure 5.

Figure 5 is a flowchart illustrating an example of a method for jetting viscous medium onto a substrate by means of an ejector or system according to any of the above embodiments. In a first step 501 , viscous medium is provided to the jetting chamber 2. This may e.g. be realised by means of a feeder 12 connected to a supply container 19. Preferably, the viscous medium may be feed through an opening 21 in the housing 10 and enter the gap 15 defined by the outer sidewall of the piston part 6 of the impacting device and the cylinder bore 8 of the housing 10. In a next step, the impacting device 4 is arranged 502 in a first state in which the actuating part 5 and the impacting part 6 are separated from each other. In a further step, the impacting device 4 is arranged 503 in a second state in which the actuating part 5 engages the impacting part 6 and exerts a force on the impacting part 6, which in response impacts a volume of the viscous medium in the chamber 2 such that viscous medium is jetted through the nozzle 3 towards the substrate 23.

In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily

appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.