EP3463685A1

EP3463685A1 - Method, device and compound for changing viscosity of viscous medium

Info

Publication number: EP3463685A1
Application number: EP17727543.5A
Authority: EP
Inventors: Gustaf MÅRTENSON; Lars ESSÉN; Kezhao XING
Original assignee: Mycronic AB
Current assignee: Mycronic AB
Priority date: 2016-05-26
Filing date: 2017-05-24
Publication date: 2019-04-10
Also published as: WO2017202982A1

Abstract

A method in an ejector (1) for jetting droplets (22) of a viscous medium onto a substrate (23) is disclosed. In the method, the viscous medium and a viscosity controlling compound is provided to a chamber (5) of the ejector, and then impacted by an impacting device (6, 7) such that a volume of the viscous medium in the chamber is jetted through a nozzle (4) of the chamber towards the substrate. The viscosity controlling compound is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of the droplet.

Description

METHOD, DEVICE AND COMPOUND FOR CHANGING VISCOSITY OF

VISCOUS MEDIUM

Technical field

The invention disclosed herein relates to jetting of viscous medium onto a substrate. More precisely, it relates to a viscous medium having a viscosity controlling compound, and a method and an ejector for forming jetted droplets of such a viscous medium.

Background

Ejectors and methods are known in the art for jetting droplets of viscous medium or fluid, 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 nozzle space, and an impacting device for impacting and jetting the viscous medium from the chamber through the nozzle in the form of droplets. Further, a feeder may be utilised to feed the medium into the nozzle space. The amount, or volume, of the deposited viscous medium at different locations on the substrate may be varied by applying several droplets on top of each other, thus forming a larger deposit, or by varying the volume of the jetted droplet by e.g. feeding a larger or smaller volume of the viscous medium into the chamber.

High production accuracy and reliability are factors of interest for the manufacturing of e.g. printed circuit board (PCB) assemblies. In particular, the reliability, such as e.g. the accuracy and the repeatability of the jetting process is of interest due to its effects on the performance and the quality of the final product, e.g. the quality of a 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 lead to 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, droplet landing position inaccuracy and erroneous volumes, which are associated with jet printing of low viscosity fluids such as solder paste.

The "viscosity modifying agents", or "thixotropic agents", mentioned in EP 1760123 A2 and other prior art solutions are related to printing, e.g. inkjet printing, of a high viscosity, stress-dependent shear-thinning fluid with thixotropic properties.

Summary

An object of the technology disclosed is to provide an improved and more reliable application of jetted droplets onto a substrate.

This and other objects of the technology disclosed are achieved by means of a method, viscous medium and ejector having the features defined in the independent claims. Different implementations of the technology disclosed are defined in the dependent claims.

Hence, according to a first aspect of the technology disclosed, a method in an ejector for jetting droplets of a viscous medium onto a substrate is provided. The method comprises the steps of providing the viscous medium and a viscosity controlling compound to a chamber of the ejector, and impacting, by means of an impacting device, a volume of the viscous medium in the chamber such that viscous medium is jetted through a nozzle of the chamber towards the substrate. The viscosity controlling compound is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased at least during the formation of the droplet.

According to a second aspect, a viscous medium is provided. The viscous medium is adapted to be supplied to a chamber and jetted, via a nozzle of the chamber, onto a substrate by means of an impacting device. Further, the viscous medium comprises a viscosity controlling compound adapted to change a viscosity of the viscous medium upon impact by an impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased at least during formation of the jetted droplet.

According to a third aspect, an ejector for jetting droplets of viscous medium onto a substrate is provided. The ejector comprisesa chamber, which is adapted to accommodate the viscous medium to be jetted and a viscosity controlling compound. Further, the ejector comprises a nozzle 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 ejector is further comprising or is provided with a viscosity controlling compound which is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased at least during formation of the droplet.

The present invention is based on the realisation that a viscous medium is subject to considerably varying shear rates during the jetting process, and that different rheological characteristics of the medium would be required for different shear rates. In other words, different rheological regimes apply for different stages of the jetting process. From one point of view, it would be advantageous if the viscous medium were shear thinning at relatively low shear rates, such as below 100 s^"1, to facilitate pumping and feeding the viscous medium to the chamber. A reduced viscosity may reduce the hydrodynamic resistance through chamber and the nozzle, and allow for smaller droplet volumes to be pumped in a more repeatable and accurate way. From another point of view, it would be advantageous if the viscous medium were shear thickening at relatively high shear rates, such as e.g. above 10.000 s^"1, to promote a more distinct break-off point for the filament that is formed during droplet formation. Thus the formation of jetted droplets of well-defined shape and/or volume may be promoted, whereas deformation or spraying when leaving the nozzle outlet may be reduced. Alternatively, or additionally, the increase in viscosity during the formation of the droplet may allow the droplet to be more accurately positioned on the substrate and further reduce the tendency of the droplet to deform, spray or spread upon impact on the substrate.

The technical effect of the "viscosity modifying agent", or "thixotropic agent", mentioned in EP 1760123 A2 and other prior art solutions are limited to stress-dependent shear-thinning fluids with thixotropic properties. These prior art solutions do not address, or are concerned with, the correlation to the imposed pressure that will cause the mentioned shear-thinning. The "viscosity controlling compound", or "viscosity modifying agent", according to the method and ejector system disclosed differs from general "thixotropic agents" mentioned in prior art solutions in two specific ways. Primarily, the "viscosity controlling compound" of the technology disclosed could both cause a shear- thinning of the fluid, as well as a shear-thickening, depending on the effect desired for the specific application. Secondly, the "viscosity controlling compound" of the technology disclosed has a specific activation time, or delay, that can be primed for which part of the ejection profile that is to be used as viscosity trigger. This activation trigger may be the pressure increase from the movement caused by the piezo stimulation or the imposed stress from the movement of the viscous medium, e.g. paste, through the nozzle. The "ejection profile" may comprise at least one of the actuation movement of the impacting device and the geometry of the chamber including the nozzle space, but may in certain aspects of the technology disclosed also include the rheological characteristics of the viscous medium.

The design of a viscosity modifying agent, or viscosity controlling compound, according to the method and ejector system disclosed where the fluid changes viscosity as it travels through the nozzle (decreasing and then either decreasing further of increasing) is fundamentally different for the ejection of low viscosity fluids compared to inkjet printing of high viscosity fluids. The specific control of the rheological activation and relaxation times of a fluid through the addition of a modification agent is very different than the addition of a general thixotropic agent that affects the whole behavior over a large range of shear stresses. The break-off process for low viscosity fluids according to the

technology disclosed is controlled by the surface tension of the fluid and the introduction of either a random or controlled point for the initiation of an instability on the cylindrical surface of the fluid as it exits the nozzle. As the viscosity of the fluid increases, the effect of surface tension decreases and the break-off process will increasingly be dominated by viscous forces.

The present invention address the problems with varying shapes and volumes of the droplets as well as the problem with inaccuracy in time for when the break-off occurs (which in turn leads to a position inaccuracy for when the droplets land on the surface of the workpiece, or substrate), by introducing a viscosity controlling compound which is adapted to achieve local viscosity gradients in the viscous medium to thereby promote a more distinct break-off point for the filament that is formed during droplet formation. After an initial time frame of -5-30 microseconds when an initial temporary drop in viscosity caused by the initial movement, or accelerating, of an impacting device towards the nozzle outlet exposing the viscous medium to a rapidly increasing pressure and thus rate of deformation or shear, the viscosity controlling compound according to the method and ejector system disclosed is adapted to increase the local viscosity of a frontal portion of the viscous medium (difference in the viscosity of the viscous medium in the spatial domain, i.e. viscosity gradients) with a further increase of the shear rate due to the temporal development of the velocity of the viscous medium, or fluid, such that the main increase in viscosity occurs for a frontal portion of the viscous medium after the ejection of said frontal portion of the viscous medium through the nozzle.

According to certain aspects of the technology disclosed, the viscosity controlling compound has a certain activation time, or delay, that is primed by which part of the ejection profile that is to be used as viscosity trigger, said viscosity controlling compound being adapted to change the viscosity such that the main increase in viscosity occurs after ejection of the viscous medium through the nozzle, thereby achieving viscosity gradients, e.g. local viscosity gradients, in the viscous medium that promote a more distinct break-off point for the filament that is formed during droplet formation.

According to certain aspects of the technology disclosed, the filament that is formed during droplet formation, or frontal portion of the viscous medium forming a droplet to be ejected through the nozzle, essentially, or to a large extent, corresponds to the frontal portion of the viscous medium which has already reached its highest viscosity, where the rest of the viscous medium, which has not yet reached its highest viscosity, retracts to remain in the chamber after break-off and the droplet is ejected and is moving towards the substrate.

The technical effect that the viscosity modifying agent according to the technology disclosed has a specific activation time, or delay, that can be primed in order to achieve or generate viscosity gradients in the viscous medium, is used to promote a more distinct break-off point for the filament that is formed during droplet formation. Hence, according to the method and ejector of the technology disclosed, a more distinct break-off point for the filament that is formed during droplet formation is achieved by introducing a viscosity modifying agent with a specific activation time that can be primed for which part of the ejection profile that is to be used as viscosity trigger in order to introduce local differences in viscosity, i.e. viscosity gradients, for the viscous medium, thereby promoting the more distinct break-off point for the filament.

The filament that is formed during droplet formation, i.e. the frontal portion of the viscous medium forming a droplet to be ejected through the nozzle, is essentially, or to a large extent, defined by the volume of the frontal portion of the viscous medium which has already reached its highest viscosity, and where the rest of the viscous medium, which has not yet reached its highest viscosity, will retract and remain in the chamber after the droplet has been ejected and is moving towards the substrate.

According to certain aspects of the technology disclosed, the movement, acceleration and impact of the impacting device, as well as the geometry of the chamber and the viscous medium is designed in such a way that the break-off process will increasingly be dominated by viscous forces as the viscosity of the viscous medium, or fluid, increases and the effect of surface tension decreases.

According to certain aspects of the technology disclosed, the

movement, acceleration and impact of the impacting device, as well as the geometry of the chamber and the viscous medium is designed in such a way that the break-off position for the filament essentially corresponds to the position in space where the viscous medium reaches its highest viscosity, where this position preferably is outside the nozzle orifice in order to promote a more distinct break-off.

According to certain aspects of the technology disclosed, a method in an ejector for jetting droplets of a viscous medium onto a substrate is provided. The method comprises the steps of providing the viscous medium and a viscosity controlling compound to a chamber (5) of the ejector and impacting, by means of an impacting device (6, 7), a volume of the viscous medium in the chamber such that viscous medium is jetted through a nozzle (4) of the chamber towards the substrate, wherein the viscosity controlling compound is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet, thereby promoting a more distinct break-off point for the filament that is formed during droplet formation to achieve droplets with a more well-defined shape and/or volume and prevent deformation or spraying caused by the ejected viscous medium breaking up into smaller pieces forming debris.

According to the technology disclosed, the viscosity controlling compound has a certain activation time that, according to the method disclosed, is primed by which part of the ejection profile that is to be used as viscosity trigger, said viscosity controlling compound being adapted to change the viscosity such that the main increase in viscosity occurs after ejection of the portion of the viscous medium through the nozzle, thereby generating a viscosity gradient that promotes a more distinct break-off point for the filament that is formed during droplet formation. The above-mentioned ejection profile comprises at least one of the actuation movement of the impacting device and the geometry of the chamber including the nozzle space, but may in certain aspects of the technology disclosed also include the rheological characteristics of the viscous medium, or fluid. Further, as the viscosity of the viscous medium increases, the effect of surface tension decreases and the break-off process will increasingly be dominated by viscous forces.

According to certain aspects of the technology disclosed, an ejector and a method in an ejector for jetting droplets of a viscous medium onto a substrate is provided. The method comprises the steps of providing the viscous medium and a viscosity controlling compound to a chamber (5) of the ejector and impacting, by means of an impacting device (6, 7), a volume of the viscous medium in the chamber such that viscous medium is jetted through a nozzle (4) of the chamber towards the substrate, wherein the viscosity controlling compound has a specific activation time that can be primed for which part of the ejection profile that is to be used as viscosity trigger in order to introduce a local difference in viscosity of the viscous medium such that viscosity gradients in the viscous medium are created (preferably in and/or close outside the nozzle space) during the formation of said droplet, wherein said viscosity gradients are promoting a more distinct break-off point for the filament that is formed during droplet formation in order to achieve droplets with a more well-defined shape and/or volume.

The method according to the technology disclosed may also be used in order to prevent deformation or spraying caused by the ejected viscous medium breaking up into smaller pieces forming debris. As the viscosity of the viscous medium or fluid increases, the effect of surface tension decreases and the break-off process will increasingly be dominated by viscous forces.

According to certain aspects of the technology disclosed, a method in an ejector for jetting droplets of a viscous medium onto a substrate is provided. The method comprises the steps of providing the viscous medium and a viscosity controlling compound to a chamber (5) of the ejector and impacting, by means of an impacting device (6, 7), a volume of the viscous medium in the chamber such that viscous medium is jetted through a nozzle (4) of the chamber towards the substrate, wherein the viscosity controlling compound is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the main increase in viscosity occurs after ejection of the portion of the viscous medium through the nozzle, thereby promoting a more distinct break-off point for the filament that is formed during droplet formation to achieve droplets with a more well-defined shape and/or volume, and prevent deformation or spraying caused by the ejected viscous medium breaking up into smaller pieces forming debris.

The viscosity is also an important parameter for underfill material, which may be formed by deposits of viscous medium. If the viscosity of the deposit is too low, the deposit may tend to form a flat surface that may trap air bubbles when the component is being attached. If the viscosity is too high, the force needed to have the bump of the component penetrate through the material may be outside the function range of the mounting machine.

A distinct and repeatable break-off point during the formation of the droplet, i.e., as the droplet is separated from the remaining filament of viscous medium, is particularly advantageous in connection with jetting of smaller droplet volumes, such as droplets having a volume of e.g. 5 nanolitres or less, as the relative contribution to the volume variations tend to increase with a decreasing total volume. Thus, it is of particular interest to reduce variations in the break-off point when jetting smaller droplets.

The inventors have realised that by providing a viscosity controlling compound, preferably by adding it to the viscous medium directly in the chamber or in advance, a better control of the viscosity of the viscous medium may be achieved. In particular, the viscosity of the viscous medium may be increased during the formation of the droplets, e.g. as the droplet breaks off from the remaining viscous medium in the ejector. The increase in viscosity may e.g. be induced immediately after the viscous medium has been ejected through the nozzle, and may be triggered e.g. by the impacting device impacting the viscous medium in the chamber and/or a shear rate being above a threshold value.

The viscosity controlling compound should be understood as a material or compound capable of varying the viscosity of the viscous medium during different stages or regimes of the jetting process. The viscosity controlling compound may e.g. be shear thickening, i.e., capable of increasing the viscosity of the viscous medium when subject to increasing relative

deformation or shear strain, or shear rates above a certain level. Further, the viscosity controlling compound may be shear thinning, or, in other words, cause the viscosity of the viscous medium to reduce when subject to increasing shear rates. The viscosity controlling compound may also be thixotropic, which should be understood as having a time-dependent shear thinning or shear thickening property. In one example, the increase in viscosity could take place within a range of 1 microsecond to 1 millisecond from the impact of the impacting means. The actual time required for the increase in viscosity may e.g. be determined by the specific requirements for the jetting process, the shape of the nozzle, the desired exit speed of the droplet, etcetera. In one example, the viscosity controlling compound is adapted to change the viscosity such that the main increase in viscosity of the viscous medium occurs after ejection of at least a portion of the viscous medium through the nozzle. Preferably, the main portion of the increase in viscosity occurs on the way between the nozzle and substrate, i.e. prior to the break-off of the filament.

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. The viscous medium, without addition of viscosity controlling compound, may have a viscosity that is relatively unaffected by deformation. Alternatively, the viscous medium may be shear thinning in response to deformation.

One of the more important findings of the inventors is that it is in particular the shape and geometry of the nozzle space, e.g. in relation to the shape and geometry of the chamber of the ejector, that have a significant impact on the shear rates that can be achieved by adding a certain viscosity controlling compound, or thixotropic agent, to the viscous medium. The shear affecting the fluid as it travels through the ejector is dependent on the change of fluid speed as a function of the distance from the wall, e.g. the wall in the nozzle space. The shear rate, expressed as du/dy, where du is the change in fluid speed as we travel a distance dy in the wall-normal direction away from the wall, e.g. a wall in the nozzle space, will increase for an decrease in dimension or an increase in volume flux.

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.

In the context of the present application, it is noted that the term

"jetting" should be interpreted as a non-contact deposition process that utilizes a fluid 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 an impacting device, such as an impacting device including, for example, a piezoelectric actuator and a plunger, for rapidly building up pressure in a fluid chamber by the rapid movement (e.g., rapid controlled mechanical

movement) of an 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 30 microseconds, thereby providing a deformation of the fluid in the chamber that forces droplets of viscous medium through a 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 while the at least one jetting nozzle is in motion without stopping at each location on the workpiece or substrate where viscous medium is to be deposited. For the movement of the impacting device to be rapid enough to build up a pressure impulse in the fluid chamber to form individual droplets or shots of the relatively highly viscous fluids (with a viscosity of about or above 100 mPa s) that are forced out of the chamber through the jetting nozzle, the break-off may be induced by the impulse of the shot itself and not by gravity or the movement of a needle, e.g. the movement of a needle together with its adhesion assembly to the surface by the deposit. A volume of each individual droplet to be jetted onto the workpiece may be between about 0.1 nanolitres and about 50 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.

In order to distinguish "jetting" of droplets of a viscous medium using a "jetting head assembly" such as an ejector-based non-contact jetting technology from the slower natural dripping break-off driven by gravity or capillary forces, we introduce below non-dimensional numbers that describe a threshold for the dripping-jetting transition for filament break-off for different cases and fluids that are driven by different physical mechanisms.

For elastic fluids, the terms "jetting" and "jetting head assembly" refer to the definition of jetting droplets by reference to the Weissenberg number, where e is the dominant relaxation time of the fluid, Uj_et is the speed of the fluid and R is the radius of the jet, can be used and the threshold for dripping-jetting is approximately 20 < With < 40.

For fluids where break-off is controlled by viscous thinning, the terms "jetting" and "jetting head assembly" refer to the definition of jetting droplets by reference to the Capillary number, described by Ca= goUjet a, where go is the yield viscosity and a is the surface tension, can be used to introduce a threshold for dripping-jetting of Ca_th ^s 10.

For fluids where break-off is dominated by inertial dynamics, the terms "jetting" and "jetting head assembly" refer to the definition of jetting droplets by reference to the Weber number, expressed as hU²j_etR a, where h is the fluid density, can be used to introduce a jetting-dripping threshold of We_th ^s 1 .

The ability to eject a more precise and/or accurate volume of viscous medium from a given distance at a specific position on a workpiece while in motion are hallmarks of viscous jetting. These characteristics allow the application of relatively highly viscous fluids (e.g., above 1 Pa s) while compensating for a considerable height variation on the workpiece (h = about 0.4 to about 4 mm). The volumes are relatively large compared to ink jet technology (between about 100 picolitres and about 50 nanolitres) as are the viscosities (viscosities of about or above 100 mPa s).

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

At least some example implementations of the technology disclosed provide increased speed of application due to the jetting "on the fly" principle of ejector-based jetting technology applying viscous medium without stopping for each location on the workpiece where viscous medium is to be deposited. Hence, the ability of ejector-based jetting technology of jetting droplets of the viscous medium onto a first (horizontal) surface is performed while the at least one jetting nozzle is in motion without stopping at each location provides an advantage in terms of time savings over capillary needle dispensing technology.

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.

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

In at least some solder paste applications, the volume percentage of solder balls of average size may be in the range of between about 5% and about 40% 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, while the average diameter of a second fraction of solder balls may be between about 10 and about 30 microns.

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 viscosity controlling compound may be adapted to maintain the increased viscosity after impact of the jetted droplet onto the substrate. The increased viscosity may be maintained for a certain period of time, such as e.g. several hours, during which the substrate may be subject to subsequent processing.

According to an embodiment, the viscosity controlling compound may be further adapted to reduce the viscosity upon impact of the jetted droplet onto the substrate. Reducing the viscosity upon impact, or after a certain period of time on the substrate, may be advantageous for subsequent processing of the substrate. A reduced viscosity of the deposit may e.g. facilitate attachment of components and improve underfill characteristics, as less force may be required to attach the components or deform the underfill.

According to an embodiment, the viscosity controlling compound may be added to the viscous medium directly in the chamber. The viscosity controlling compound may e.g. be supplied by a supplying means, which may be connected to a reservoir for continuous or temporary supply to the viscous medium. Supplying the viscosity controlling compound to the chamber may allow for an improved mixing with the viscous medium prior to jetting.

According to an embodiment, the viscosity controlling compound may be added to the viscous medium at a position in close proximity or adjacent to the nozzle, i.e. be added directly into the nozzle space at a position which is below a certain distance from the working space of the frontal surface of the impacting device such as a plunger or piston 6. The supply may e.g. be realised by a supply means as mentioned above. By adding the viscosity controlling compound close to the nozzle the added viscosity controlling compound may be limited to only a portion of the viscous medium present in the chamber. Consequently, small amounts of viscosity controlling compound may be supplied to only one or a few consecutively jetted droplets. This allows for some droplets, such as relatively small droplets, to be jetted with the addition of the viscosity controlling compound and other droplets, such as relatively large droplets, to be jetted without (or with close to insignificant) amounts of viscosity controlling compound.

According to certain embodiments of the technology disclosed, the viscosity controlling compound may be added to the viscous medium in a device comprising a small jetting chamber, or nozzle space, which is below a membrane or diaphragm. In a certain method that may be used for some of these embodiments, quantities of a highly viscous fluid are dispensed from an outlet of a dispensing orifice by the use of an expelling mechanism that deforms a flexible, elastomeric membrane or diaphragm into a jetting chamber having sides, an open top covered by the diaphragm and an outlet in the bottom of the jetting chamber that is in fluid communication with the dispensing orifice, the dispensing orifice, outlet and jetting chamber being on a common longitudinal axis, the method comprising: forcing the highly viscous fluid to the jetting chamber through an inlet channel in fluid communication with the jetting chamber, deforming the flexible, elastomeric

diaphragm/membrane into the jetting chamber a distance sufficient so that the diaphragm/membrane blocks the opening in the bottom of the chamber, the deformation occurring with sufficient force to expel a volume of material from the dispensing orifice as a single drop. Further details about this embodiment are given in e.g. U.S. Patent No. 875751 1 .

The technology disclosed may be embodied as computer readable instructions for controlling a programmable computer in such manner that it causes an ejector 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 method according to the first aspect of the present invention may be combined with the viscous media and the ejector according to the other aspects of the present invention.

Further objectives of, features of, and advantages with the present invention 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 invention can be combined to created embodiment other than those described in the following.

Brief description of the drawings

The above, as well as additional object, feature and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of embodiments of the present invention. Reference will be made to the appended drawings, on which:

Figure 1 is a schematic cross section of an ejector according to an embodiment of the present invention, comprising an impacting device, a nozzle and a chamber;

Figures 2a and b are schematic cross sections of a chamber and nozzle similarly configured as in figure 1 , further comprising a supply means for the viscosity controlling compound; Figure 3 is a cross section of a chamber and nozzle of an ejector according to an embodiment of the present invention, schematically illustrating the formation of a droplet during the jetting process; and

Figure 4 is a diagram illustrating the displacement of the impacting device, the pressure increase and the viscosity, as a function of time during the jetting process.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

Detailed description

With reference to figure 1 , there is shown a schematic view of an ejector according to an implementation of the technology described.

The ejector 1 comprises an assembly housing 10 and an impacting device, which in this implementation may include a piezoelectric actuator 7 and a plunger or piston 6 operatively connected to the piezoelectric actuator 7. The plunger 6 may be axially moveable while slideably extending through a bore hole in a bushing 8. Cup springs 9 may be provided to resiliently balance the plunger 6 against the assembly housing 10, and for providing a preload for the piezoelectric actuator 7. 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 plunger 6 with respect to the assembly housing 10, in accordance with solder pattern printing data.

Furthermore, the ejector 1 may comprise jetting nozzle 2, which may be operatively directed against a substrate 23 onto which droplets 22 of viscous medium are to be jetted. The nozzle 2 may according to the present embodiment comprise a nozzle space 3 and a nozzle outlet 4 through which the droplets 22 are jetted towards the substrate 23.The nozzle outlet 4 may be located at one end, such as a lower portion, of the nozzle 2.

A chamber 5 may be defined between an end surface of the plunger 6 and the nozzle 2. Axial movement of the plunger 6 towards the nozzle 2 may cause a rapid decrease in the volume of the chamber 5. Such an impact by the plunger 6 may thus cause a rapid pressurisation and jetting of viscous medium through the nozzle outlet 4.

In other implementations of the technology disclosed using a different type of ejector, the plunger comprising a piston may be replaced by another type of impacting device such as e.g. a membrane or diaphragm.

In a particular embodiment, a jetting device is used in which quantities of a highly viscous fluid are dispensed from an outlet of a dispensing orifice by the use of an expelling mechanism that deforms a flexible, elastomeric diaphragm into a jetting chamber having sides, an open top covered by the diaphragm and an outlet in the bottom of the jetting chamber that is in fluid communication with the dispensing orifice, the dispensing orifice, outlet and jetting chamber being on a common longitudinal axis, the method comprising: forcing the highly viscous fluid to the jetting chamber through an inlet channel in fluid communication with the jetting chamber, deforming the flexible, elastomeric diaphragm into the jetting chamber a distance sufficient so that the diaphragm blocks the opening in the bottom of the chamber, the deformation occurring with sufficient force to expel a volume of material from the dispensing orifice as a single drop. Further details about this embodiment are given in e.g. U.S. Patent No. 875751 1 .

According to the technology disclosed, this particular embodiment and device including a flexible, elastomeric diaphragm provides for a method in an ejector for jetting droplets of a viscous medium onto a substrate (23), the method comprising:

providing the viscous medium and a viscosity controlling compound to a jetting chamber below a flexible, elastomeric diaphragm of the ejector; and deforming the flexible, elastomeric diaphragm into the jetting chamber a distance sufficient so that the diaphragm blocks the opening in the bottom of the chamber, the deformation occurring with sufficient force to expel a volume of material from the dispensing orifice as a single drop;

wherein said deformation is impacting a volume of the viscous medium in the jetting chamber such that viscous medium is jetted through a nozzle of the jetting chamber towards the substrate; and wherein the viscosity controlling compound provided to the jetting chamber is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet.

wherein said deformation is impacting a volume of the viscous medium in the jetting chamber such that viscous medium is jetted through a nozzle of the jetting chamber towards the substrate; and

wherein viscosity controlling compound is adapted to change the viscosity such that the main increase in viscosity occurs after ejection of the portion of the viscous medium through the nozzle, thereby promoting a more distinct break-off point for the filament that is formed during droplet formation.

Certain aspects of the technology disclosed define an ejector and a method in an ejector (1 ) for jetting droplets (22) of a viscous medium onto a substrate (23), the method comprising:

providing the viscous medium and a viscosity controlling compound to a chamber (5) of the ejector; and

impacting, by means of an impacting device (6, 7), a volume of the viscous medium in the chamber such that viscous medium is jetted through a nozzle (4) of the chamber towards the substrate;

wherein the viscosity controlling compound has a certain activation time in order to be adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet;

wherein, during an initial time frame in the range of 5-30 microseconds from when the impacting device (6, 7) starts moving, or accelerating, towards the nozzle outlet 4 and expose portions of the viscous medium to a rapidly increasing pressure and/or rate of deformation or shear depending on the localisation of the fluid element, an increasing shear rate triggers a shear thinning behaviour of the viscous medium leading to a drop in viscosity of the viscous medium; and

wherein the viscosity controlling compound is adapted to, after the initial time frame and drop in viscosity, introduce a viscosity gradient in that the main increase in viscosity for a frontal portion of the viscous medium occurs after the ejection of said frontal portion of the viscous medium through the nozzle, thereby creating a local difference in the viscosity that is promoting a more distinct break-off point for the filament that is formed during droplet formation.

wherein the viscosity controlling compound is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet;

wherein, during an initial time frame in the range of 5-30 microseconds from when the impacting device (6, 7) starts moving, or accelerating, towards the nozzle outlet 4 and expose portions of the viscous medium to a rapidly increasing pressure and/or rate of deformation or shear depending on the localisation of the fluid element, an increasing shear rate triggers a shear thinning behaviour of the viscous medium leading to a temporary drop in viscosity of the viscous medium; and

wherein the viscosity controlling compound is adapted to, after the initial time frame and temporary drop in viscosity, increase the local viscosity of a frontal portion of the viscous medium with a further increase of the shear rate due to the temporal development of the velocity of the fluid such that the main increase in viscosity for the frontal portion occurs after the ejection of the frontal portion of the viscous medium through the nozzle, thereby creating a local difference of the viscosity of the different portions of the viscous medium in the nozzle space which is caused by the temporary drop in viscosity for a portion of the viscous medium behind the frontal portion of the viscous medium and the increase in viscosity for the frontal portion after ejection of the frontal portion through the nozzle, said local difference in space of the viscosity for different portions of the viscous medium (i.e.

achieving viscosity gradients) is promoting a more distinct break-off point for the filament that is formed during droplet formation.

All these impacting devices have in common that they are configured to provide for a non-contact jetting process to form and shoot droplets of a viscous medium from a jetting nozzle onto a substrate by quickly generating a pressure impulse by the reciprocating, or vibrating movement of the impacting device.

The impacting devices may move from a starting position towards an end position during a time period of about 1 to 30 microseconds in order to shoot individual droplets having a deposit volume between about 0.1 nanolitres and 50 nanolitres, such as e.g. 1 to 5 nanolitres, 5 to 15 nanolitres, 10 to 20 nanolitres, or 10 to 50 nanolitres. The speed of the impacting device for impacting the nozzle with a pressure impulse may be between about 5 m/s and about 50 m/s.

Viscous medium may be supplied to the nozzle space 3 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 communicating with the chamber 5. 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. Viscous medium captured between the threads of the feed screw 13 and the inner surface may then be forced towards the chamber 5 in accordance with the rotational movement of the feed screw 13.

Figures 2a and b show examples of supplying means for providing viscosity controlling compound to the viscous medium when it is located in the chamber. The chamber, ejector and viscous medium may be similarly configured as the embodiment described with reference to figure 1 .

Figure 2a discloses a chamber 5, a plunger 6 and a nozzle 4 according to an implementation of the technology wherein the viscosity controlling compound may be added to the viscous medium at a position in close vicinity of the nozzle or nozzle outlet 4. This may be realised by means of a channel 15 having its outlet in a wall defining the nozzle outlet 4. Thus, the viscosity controlling compound may be added locally to the portion of viscous medium that is to be ejected, e.g. during the next impact or stroke of the plunger 6.

Figure 2b discloses an alternative, or additional, configuration of the chamber shown in figure 2a. In figure 2b, the supplying means or channel 15 has its outlet closer to the end surface of the plunger 6, such that viscosity controlling compound may be added to, and mixed with, viscous medium in the chamber 5 before the mixture is forced towards the nozzle outlet 4.

In both of the above examples, the channel 15 may be connected to a reservoir comprising the viscosity controlling compound. The compound may e.g. be supplied to the chamber by means of a pump or any other kind of arrangement suitable for delivering a preferably controlled amount of compound to the chamber. In one example, a rotatable feed screw and elastic tube as described above in connection with figure 1 may be used.

Figure 3 illustrates the ejection of a portion of viscous medium through the nozzle upon impact by the impacting device according to an exemplary implementation of the present invention. The chamber 5 may be defined by the end surface of the impacting device, such as a plunger 6, and the innermost surface of a nozzle plate 2. The outlet of the chamber 5 may be formed by a nozzle outlet 4 connected to the chamber via a nozzle space 3.

In the present figure, the plunger 6 is shown in a retracted state allowing viscous medium in the chamber 5 to be replenished. The position of the plunger 6 during impact, i.e., when the plunger 6 has been forced towards the nozzle outlet 4 so as to cause an ejection of viscous medium through the outlet 4, is indicated by a dashed line. During impact, the plunger 6 will expose the viscous medium in the chamber to an increased pressure that accelerates at least a portion of the viscous medium towards the nozzle outlet 4. During this process, the viscous medium may be exposed to an extreme deformation or shear rate. Shear rates in the range of from 10.000 s^"1 to 100.000 s^"1, or even more, may be reached within a few microseconds.

The shape and geometry of the nozzle space 3 (e.g. relative the dimensions of the chamber 5), in combination with the force imposed on the viscous medium by the impacting device, have a significant impact on the ability to obtain shear rates in the range of from 10.000 s^"1 to 100.000 s^"1 that may be achieved by first adding a certain viscosity controlling compound, or thixotropic agent, to the viscous medium. The shear affecting the fluid as it travels through the ejector is dependent on the change of fluid speed as a function of the distance from the wall in the nozzle space 3. The shear rate, expressed as du/dy, where du is the change in fluid speed as we travel a distance dy in the wall-normal direction away from the wall, e.g. a wall in the nozzle space 3, will increase for a decrease in dimension or an increase in volume flux.

The position of the portion of the viscous medium to be jetted is indicated by A, B, and C, each representing different stages of the ejection or jetting process. In region A, the portion of the viscous medium may be more or less at rest, and hence exposed to a relatively low shear or stress.

In region B, the portion of the viscous medium may be forced, by the impacting plunger, towards the nozzle outlet 4. In this region, the viscosity of the medium may be reduced in response to increasing shear or deformation, thereby facilitating transport and ejection of the medium through the nozzle outlet.

In region C, the portion of the viscous medium has been ejected from the nozzle outlet and is on its way towards the substrate. The present figure indicates the onset of a droplet formation, wherein the break-off point for the filament that is formed as the viscous medium is ejected from the nozzle outlet is indicated by a waist.

Figure 3 may be read together with figure 4, which is a schematic representation of the piston displacement I (upper curve), the pressure p (middle curve) in the chamber 5 (middle graph), and the viscosity η (lower curve) as a function of time t during the ejection of the viscous medium. The momentaneous regimes for positions A, B and C of the portion of the viscous medium to be ejected are represented as vertical, dashed lines indicating the actual piston displacement, pressure increase and viscosity for each position or region A, B, C.

In region A, the piston 6 has not yet started moving towards the nozzle outlet 4 and the portion of the viscous medium is hence not exposed to any substantial pressure increase or deformation.

In region B, the piston 6 is moving, or accelerating, towards the nozzle outlet 4 and exposing the portion of the viscous medium to a rapidly increasing pressure and thus rate of deformation or shear. The time range, during which the major portion of the pressure increase takes place, may in some examples be 5 to 20 microseconds. Initially, the increasing shear rate may trigger a shear thinning behaviour of the viscous fluid, leading to a drop in viscosity to e.g. 0.1 -1 Pas. The shear thinning may e.g. be caused by the viscosity controlling compound, an additional shear thinning compound, and/or an inherent property of the viscous medium. The reduced viscosity may result in a decrease in hydrodynamic resistance through the nozzle, which may facilitate generation of smaller droplet volumes.

As the pressure or shear rate reaches a critical or maximum value, which may be about 10.000 s^"1 to 100.000 s^"1 or more, a shear thickening behaviour may be triggered that causes the viscosity to increase to e.g. to 1 - 10 Pas or above. As shown in figure 4, there might be a time delay for the activation of the shear-thickening behaviour. Due to the time delay, the viscosity may continue to increase at C even if the pressure in the chamber 5 has started to sink. This delay may thus allow the portion of the viscous medium to exit the nozzle before the viscosity of said portion reaches its maximum. Further, the time delay may be short enough to allow the viscosity to reach a level that is high enough to impact the break-off process of the droplet.

The time range between region B and region C, i.e., or, other words, the time it may take for the portion of the viscous medium to be expelled from the nozzle space 3, may correspond to about 1 to 30 microseconds. The critical shear rate, caused by the pressure induced by the plunger 6, may thus be used to activate the viscosity controlling compound that may have been added to the viscous medium in the supply container, in the chamber 5 or in a close vicinity of the nozzle outlet 4. The shear thickening process may according to an example be understood as an avalanche reaction of e.g. a polymeric material, leading to a viscosity in the range of from 1 to10 Pas or more.

The time delay for the activation of the shear-thickening behaviour may be referred to as a thixotropic character that allows the ejected droplet to maintain a relatively high viscosity during a measurable period of time. Such a period of time may e.g. refer to microseconds, such as within the range of 1 to 10 microseconds, or more. The thixotropic character may at least partly be determined by the composition of the viscosity controlling compound, and may allow for the ejected droplet to reach the substrate before the viscosity is reduced again, preferably back to a viscosity within the same range as prior to ejection. However, the shear thickening may also be more or less permanent, or at least have an impact on the viscosity that lasts for a period of time that is sufficient for influencing subsequent processing of the substrate. The viscosity of the jetted droplet may remain at the relatively high level, or even continue to increase for at least some period of time after the ejection.

The increase in viscosity during the break-off process may promote the formation of droplets having a well-defined shape and/or volume, and prevent deformation or spraying caused by the ejected viscous medium breaking up into smaller pieces forming debris. Further, the increase in viscosity and, possibly, the thixotropic behaviour, may help preventing the droplet from spreading on the substrate. A low spreading or footprint of the droplet is of particular interest for e.g. low pitch applications and small pad size

applications in order to obtain sufficient deposit volume. Alternatively, the viscosity may be reduced at the impact onto the substrate, or shortly after the impact, so as to promote area coverage and/or capillary filling on the substrate. This is particularly advantageous in connection with e.g. VIA-filling, underfill, and mono-layered deposits.

In certain aspects of the technology disclosed, the time delay for the activation of the shear-thickening behaviour may be in the range of 3 to 7 microseconds following a design choice in selecting the volume percentage of the viscosity controlling compound of the viscous medium used for a certain ejector configuration. By carefully selecting the composition of the viscosity controlling compound and the volume percentage of the viscosity controlling compound (of the viscous fluid) to obtain a certain time delay (e.g. in the range of 3 to 7 microseconds) for the activation of the shear-thickening behaviour for a certain ejector configuration, a more distinct and repeatable break-off process may be achieved in order to promote the formation of droplets having a well-defined shape and/or volume, thereby also mitigating the negative effects of deformation or spraying caused by the ejected viscous medium breaking up into smaller pieces forming debris. As mentioned above, this thixotropic character of the viscous medium may at least partly be determined by the composition of the viscosity controlling compound, and may, according to certain aspects of the technology disclosed, further allow for the ejected droplet to reach the substrate before the viscosity is reduced again, preferably back to a viscosity within the same range as prior to ejection.

It will be appreciated that any one of the embodiments described above with reference to figures 1 to 4 is combinable and applicable to a viscosity controlling compound as described above. Such a compound should be understood as a material or agent capable of increasing the viscosity of the viscous medium when subject to increasing relative deformation or shear strain. The compound may be provided in the viscous medium during manufacturing of the viscous medium, during a separate preparation step or in connection with the use of the viscous medium in an ejector according to the present invention. Hence, the shear thickening and/or shear thinning capability of the viscous medium may be an inherent capability and/or a capability provided by addition of a separate agent.

Examples of viscosity controlling compounds and thixotropic/anti- thixotropic agents may include castor oil and modified castor oil derivatives, such as hydrogenated castor oils, amine-modified castor oils, and amine- modified hydrogenated castor oils. Further examples include waxes, such as honey wax and carnauba wax; polyamides and fatty acid amides such as stearamide, stearic acid amide, ethylene bis stearic acid amide; polyethylenes such as polybutene; amorphous fumed silica and silylated amorphous fumed silica, such as CAB-O-SIL TS-720 and CAB-O-SIL M-5 (from Cabot

Corporation); and polyalkyl methacrylate. The viscosity controlling compound may e.g. be added to a flux, which in particular may form part of a viscous medium being a solder paste. In flux may in some example comprise 0.5 to 12 wt% of at least one of the compounds or agents listed above.

Claims

1 . A method in an ejector (1 ) for jetting droplets (22) of a viscous medium onto a substrate (23), the method comprising:

wherein the viscosity controlling compound is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet.

2. The method according to claim 1 , wherein viscosity controlling compound is adapted to change the viscosity such that the main increase in viscosity occurs after ejection of the portion of the viscous medium through the nozzle, thereby promoting a more distinct break-off point for the filament that is formed during droplet formation.

3. The method according to any of the preceding claims, wherein the viscosity controlling compound has a certain activation time that is primed by which part of the ejection profile that is to be used as viscosity trigger, said viscosity controlling compound being adapted to change the viscosity such that the main increase in viscosity occurs after ejection of the viscous medium through the nozzle, thereby achieving viscosity gradients in the viscous medium that promote a more distinct break-off point for the filament that is formed during droplet formation.

4. The method according to claim 3, wherein the ejection profile that is used as viscosity trigger comprises at least one of the actuation movement of the impacting device and the geometry of the chamber including the geometry of the nozzle space.

5. The method according to any of the preceding claims, wherein the break- off process will increasingly be dominated by viscous forces as the viscosity of the viscous medium, or fluid, increases and the effect of surface tension decreases.

6. The method according to any of the preceding claims, wherein the main increase in viscosity occurs after ejection of the portion of the viscous medium through the nozzle and droplet and before impact of the droplet onto the substrate.

7. The method according to any of the preceding claims, wherein said filament that is formed during droplet formation corresponds to the frontal portion of the viscous medium which has already reached its highest viscosity, and where the rest of the viscous medium, which has not yet reached its highest viscosity, retracts and remains in the chamber after the droplet is ejected.

8. The method according to any of the preceding claims, wherein the break- off position in space for the filament corresponds to the position in space where the viscous medium reaches its highest viscosity.

9. The method according to any of the preceding claims, wherein the break- off position for the filament, corresponding to the position where viscous medium reaches its highest viscosity, is outside the nozzle orifice.

10. The method according to any of the preceding claims, wherein the viscosity controlling compound is further adapted to maintain the increased viscosity after impact of the jetted droplet onto the substrate.

1 1 . The method according to any of the preceding claims, wherein the viscosity controlling compound is further adapted to reduce the viscosity upon impact of the jetted droplet onto the substrate.

12. The method according to any one of the preceding claims, wherein the viscous medium is provided with the viscosity controlling compound prior to the step of providing the viscous medium and the viscosity controlling compound to the chamber.

13. The method according to any one of claims 1 to 1 1 , wherein the viscosity controlling compound is supplied to the viscous medium when said viscous medium is located in the chamber.

14. The method according to any one of claims 1 to 1 1 , wherein the viscosity controlling compound is supplied to the viscous medium locally at a position in close vicinity of the nozzle.

15. The method according to any of the preceding claims, wherein the viscosity controlling compound is shear thickening for shear rates above 10.000 s^"1.

16. The method according to any one of the preceding claims, wherein the viscosity controlling compound further is a shear thinning compound for shear rates below 100 s^"1.

17. The method according to any one of the preceding claims, wherein the viscosity controlling compound is provided only for jetted droplets having a volume of 5 nanolitres or less.

18. Viscous medium for being supplied to a chamber and jetted, via a nozzle of the chamber, onto a substrate by means of an impacting device, the viscous medium comprising a viscosity controlling compound that has a certain activation time primed for which part of the ejection profile that is to be used as viscosity trigger, and adapted to change a viscosity of the viscous medium upon impact by an impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet, thereby a viscosity gradient is achieved in order to promote a more distinct break-off point for the filament that is formed during droplet formation.

19. Viscous medium for being supplied to a chamber and jetted, via a nozzle of the chamber, onto a substrate by means of an impacting device, the viscous medium being adapted to receive a viscosity controlling compound that has a certain activation time primed for which part of the ejection profile that is to be used as viscosity trigger, and adapted to change a viscosity of the viscous medium upon impact by an impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet, thereby a viscosity gradient is achieved in order to promote a more distinct break-off point for the filament that is formed during droplet formation.

20. The viscous medium according to claim 18 or 19, wherein the ejection profile that the activation time is primed for comprises at least one of the actuation movement of the impacting device and the geometry of the chamber including the geometry of the nozzle space.

21 . The viscous medium according to claim 18 or 19, wherein the viscosity controlling compound is further adapted to maintain the increased viscosity after impact of the jetted droplet onto the substrate.

22. The viscous medium according to claim 18 or 19, wherein the viscosity controlling compound is further adapted to reduce the viscosity upon impact of the jetted droplet onto the substrate.

23. The viscous medium according to claim 18 or 19, wherein the viscosity controlling compound is shear thickening for shear rates above 10.000 s^~1.

24. The viscous medium according to claim 18 or 19, wherein the viscosity controlling compound further is shear thinning for shear rates below 100 s^~1.

25. The method according to any one of the preceding claims, wherein the viscosity controlling compound is a polymeric compound.

26. An ejector for jetting droplets of viscous medium onto a substrate, the ejector comprising:

a chamber adapted to accommodate the viscous medium to be jetted and a viscosity controlling compound

a nozzle connected to the chamber;

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;

wherein the viscosity controlling compound is adapted to change a viscosity of the viscous medium upon impact by the impacting device such that the viscosity of at least a portion of the viscous medium forming a droplet to be ejected through the nozzle is increased during the formation of said droplet; and

wherein the viscosity controlling compound is configured to have a certain activation time that is primed for which part of the ejection profile, e.g. the movement of the impacting device or the geometry of the chamber, is to be used as viscosity trigger, said viscosity controlling compound is further adapted to change the viscosity such that the main increase in viscosity occurs outside the nozzle orifice to achieve viscosity gradients in the viscous medium that is promoting a more distinct break-off point for the filament outside the nozzle orifice.

27. The ejector according to claim 26, further comprising a supplying means adapted to supply the viscosity controlling compound to the viscous medium.

28. The ejector according to claim 27, wherein he supplying means (15) is adapted to supply the viscosity controlling compound to the viscous medium when said viscous medium is located in the chamber.

29. The ejector according to claim 27, wherein the supplying means is adapted to supply the viscosity controlling compound locally at a position in close vicinity of the nozzle.

30. The ejector according to claim 27, adapted to supply the viscosity controlling compound to the viscous medium only when jetting droplets having a volume of 5 nanolitres or less.