Abstract:
This invention relates to a device for jetting droplets of a particle filled viscous medium. The device has a nozzle, an eject mechanism connected to the nozzle, a medium feeding mechanism connected to the eject mechanism, a particle filled viscous medium container connected to the medium feeding mechanism, and a filter arranged between an outlet of the particle filled viscous medium container and the medium feeding mechanism.

Description:
TECHNICAL FIELD 
   This invention relates to a device for jetting droplets of a particle filled viscous medium, such as solder paste. 
   TECHNICAL BACKGROUND  
   In the non-contact viscous medium dispensing technology called jetting, which has been developed in the course of the recent years, a central issue is how to generate droplets of a predetermined size at a predetermined pace. In many respects this technology is superior to other available dispensing technologies. However, at an early stage, when jetting viscous medium containing a substantial amount of particles, such as solder paste, a problem encountered was an unexpected interference and unintentional interruption of the series of consecutive solder paste droplets jetted from the device. 
   Since the solder paste is a most special type of medium consisting of a solder powder, i.e. small metal grains, solved in a viscous flux, and since the jetting technology is rather an extreme variety of dispensing, it was initially presumed that the problem could be overcome by improving the mechanical parts of the jetting device. Indeed some mechanical causes were found, such as smearing of the solder powder at narrow passages. These deficiencies have been corrected, and yet occasionally similar problems occur. The reasons therefore are still to be found. 
   SUMMARY OF THE INVENTION  
   The object of this invention is to provide a solution to the problem of interference and interruption described above. 
   The object is achieved by a device according to the appended claims. 
   In accordance with one aspect of the present invention there is provided a device for jetting droplets of a particle filled viscous medium, comprising a nozzle, an eject mechanism connected to the nozzle, a medium feeding mechanism connected to the eject mechanism, a particle filled viscous medium container connected to the medium feeding mechanism, and a filter arranged between an outlet of the particle filled viscous medium container and the medium feeding mechanism. 
   Surprisingly it turns out that the filter eliminates, or at least substantially reduces, the problems of interference, and temporary and lasting interrupts of the series of droplets jetted from the apparatus. This indicates a presence of impurities or some kind of large particles in the solder paste. This could not be expected since so far, within this technical field, there had been no indications of this kind of problems with the solder paste. It has then been discovered that the major reason for the problems of different kinds of sudden interferences and temporary or remaining interrupts described above has been contaminations in the solder paste, which have led to clogging of narrow passages of the device. Further, it should be noted that, in addition to solder paste, the device is useful also with other particle filled viscous media having similar viscous properties. Such other particle filled viscous media are, for example, electrically conductive adhesive, and resistive paste. However, for reasons of explanatory simplicity, below the particle filled viscous medium will be exemplified with solder paste. 
   Further, in accordance with this aspect of the invention, the filter is to be arranged between the particle filled viscous medium container and the mechanism for feeding the particle filled viscous medium into the eject mechanism. This has proven to be the most effective position for the filter, due to existing narrow passages of the medium feeding mechanism which otherwise are likely to be clogged. 
   According to an advantageous embodiment of the device the filter comprises a sieve, which preferably is a metal sieve. The sieve can be provided with appropriately sized meshes such as to prevent too large particles from passing through the sieve, while not causing an undesired increase in the flow resistance. 
   According to another embodiment of the device the filter is arranged in a duct, extending between the outlet of the container and the medium feeding mechanism, and the filter further comprises consecutive first and second O-rings. The O-rings abut the inner wall of the duct, and the sieve is positioned between the first O-ring and the second O-ring. The O-rings are preferred because they provide a good seal against the wall of the duct while keeping the sieve in place without any risk of deforming it. If the filter additionally is positioned such that the end of the outlet of the container abuts the first O-ring, the O-ring will also provide a seal between the outlet and the duct preventing the solder paste from passing between the outlet and the internal wall of the duct. 
   According to yet another embodiment of the inventive device it is arranged to be releasably mountable in a particle filled viscous medium jetting machine. In order to obtain this, the device further comprises a device holder mating with a stand of a jetting machine, and an interface adapted to a complementary interface of the stand, wherein said interface comprises electric, and pneumatic elements for controlling the operation of the device. 
   Further objects and advantages of the present invention will be discussed below by means of exemplifying embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     Exemplifying embodiments of the invention will be described below with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic cross-sectional view of a part of an embodiment of a jetting device according to the present invention; 
       FIG. 2  is an enlargement of a portion of  FIG. 1 , and illustrates an embodiment of a filter according to the present invention; 
       FIG. 3  in a schematic cross-sectional view illustrates another embodiment of a filter mounted in another embodiment of a jetting device, according to this invention; and 
       FIG. 4  is a schematic cross-sectional view of yet another embodiment of the filter. 
   

   DESCRIPTION OF EMBODIMENTS  
   In  FIG. 1  the principal structure of an embodiment of a device according the present invention is disclosed. The device  1  comprises a nozzle  3 , through which solder paste is ejected, an ejection mechanism  5 , a feeding mechanism  7 , connected to the ejection mechanism for feeding solder paste into the ejection mechanism, and a solder paste container  9 . All those parts are mounted in a body  11  having necessary recesses, borings etc., for mounting the different parts and for guiding the solder paste from the container  9  to the nozzle  3 . The path which the solder paste follows on its way to the nozzle comprises an inlet duct  12  extending from a container holder  13 , where an outlet portion  15  of the container  9  is received, to the feeding mechanism  7 , and more specifically to a feeding tube  17 , which extends to the ejection mechanism  5 , and more specifically to an eject chamber  19 . 
   The container holder  13  has a cylindrical end portion  14 , which has an internal thread. The outlet portion  15  of the container  9  has a corresponding cylindrical outlet end portion  16 , which has an external thread, that is matched with said internal thread. 
   The feeding mechanism comprises a rotatable feeding shaft  21 , which in this embodiment is shaped as a feed screw, and which extends along the interior of the feeding tube  17 . The ejection mechanism comprises a reciprocatable ejection shaft  23 , the end of which defines a wall of the eject chamber  19 , connected to an actuator  25 , which causes the movement of the ejection shaft  23 . 
   A filter  27  is arranged in the inlet duct  12 . In this preferred embodiment, the filter  27  is mounted at a filter holding portion  29  of the inlet duct. The filter holding portion is widened so as to reduce the undesired effect of the flow resistance inevitably caused by the filter  27 . The filter comprises a sieve  31  and an O-ring  33 , as best shown in  FIG. 2 . A circumferencial edge portion  35  of the filter  27  is supported by a seat, or shelf,  37  formed in the body  11 . An O-ring is arranged on a side of the edge portion opposite of the side resting on the seat  37 . In this embodiment, the container holder  13  is an individual part, which is mounted in the body  11 , wherein the container holder  13  is engaged with the O-ring  33  and keeps the filter  27  in place by forcing, via the O-ring  33 , the sieve  31  against the seat  37 . The O-ring  33  provides for a seal of the narrow passage between the periferal edge of the sieve  31  and the inner wall of the inlet duct  12 , and also protects the sieve from being deformed in conjunction with the mounting of the container holder  13 . 
   When the jetting device  1  is operated the solder paste is fed, by means of the feeding mechanism  7  to the eject chamber  19 , and then the actuator  25  is energised, and thereby the ejection shaft  23  is moved rapidly towards the nozzle  3 . A droplet is thus jetted from the nozzle towards a surface arranged in a close vicinity of the nozzle. The ejection shaft  23  is then returned to a resting position, and the operation is repeated. The feeding mechanism is supplied with the solder paste through either the influence of gravity or the use of a pressure applied in the container forcing the solder paste out of the container  9 . In either case the solder paste is guided through the inlet duct to the feeding mechanism, while passing through the filter  27 . The sieve is provided with meshes, i.e. a large number of apertures through which the solder paste passes. The size of the apertures is chosen to be larger than the metal grains of the solder paste but small enough to prevent the troublesome impurities, which are particles significantly larger than the metal grains, of the solder paste from passing the sieve. In the long run the filter  27  is clogged to such a degree that the throughput is decreased to such an extent that the jetting operation is disturbed. Then the filter has to be exchanged. However, this is a simple operation, which is typically performed in conjunction with an exchange of the container  9 . Thus, the filter exchange does not significantly affect the effective production time of the jetting device  1 . 
   However, in an alternative embodiment of the device, it is constructed as an exchangeable cassette, or assembly, containing, in excess to the above described parts, an assembly holder mating with a stand of a solder paste jetting machine, and an interface adapted to a complementary interface of the stand. The interface comprises electric, and, possibly, pneumatic elements for controlling the operation of the assembly. In this embodiment, simply, the whole assembly is exchanged when the container is empty or another type of solder paste is to be used. In other words, the jetting device according to this embodiment is releasably mountable in a jetting machine. 
   In yet another embodiment of the jetting device the fastening of the container in the body is different from that of the embodiment described above. Rather than the threaded portion and the outlet portion the container is provided with a single cylindrical outlet portion  41 , which is introduced into a complementary cylindrical portion  43  of the body. The container is retained by means of a resilient locking shackle  44 . Further, the filter  45  is arranged at the very end of the outlet portion  41 , i.e. at the very beginning of the inlet duct  47 . The diameter of the filter  45  is larger than that of the inlet duct  47  in order to reduce the increase of the flow resistance caused by the filter  43 . The filter comprises a first, or upper, O-ring  49 , a second, or lower, O-ring  51 , and a sieve  53  arranged intermediate of the O-rings  49 ,  51 . Thus, in this embodiment of the filter  43 , the upper O-ring forms a seal between the end of the outlet portion  41  and the inner wall of the inlet duct  47 . 
   An advantageous alternative embodiment of the filter is a double sieve construction, as shown in  FIG. 4 . This double sieve construction comprises a first, or upper, sieve  55 , a second, or lower, sieve  57 , an first, or upper, O-ring  59 , and a second, or lower, O-ring  61 . This filter is mountable in the embodiment of the jetting device as shown in  FIG. 1 . Complementing with a third, lowermost, O-ring would make it useful also in the other embodiment of the jetting device as shown in  FIG. 3 . The first sieve  55 , which is the first one as seen in the direction of the flow of solder paste, has a mesh which is coarser than that of the second sieve  57 . This embodiment of the filter may stop more contaminations than may the single sieve filter, which has the same mesh size as the second sieve  57  before clogging. This is obtained without any significant increase of the flow resistance. 
   Above embodiments of the device according to the present invention have been described. These should be seen as non-limiting examples. Many modifications will be possible within the scope of the invention as defined by the claims.