Abstract:
Surface mount technology may be utilized to join two surfaces together that may include relative surface irregularities. By varying the volume of surface mount material applied to electrically and physically join the two surfaces, surface-to-surface irregularities may be compensated for. Various techniques may be utilized to vary the volume of the interconnection material in a high speed fashion.

Description:
BACKGROUND 
     This invention relates generally to surface mounting technology utilized to electrically and physically interconnect electronic components. 
     In surface mount technology (SMT), two surfaces may be electrically interconnected using solder bumps or balls that are deposited upon one of the surfaces and then reflowed or heated to solder the two surfaces together. SMT may be an advantageous process because it is not necessary to work in the region between the two surfaces to be joined and because a large number of components may be connected in the same reflow step. 
     A problem may arise, however, when one of the surfaces to be bonded is irregular. For example, one surface to be surface mounted to another surface may be warped. When the surface mount bumps of uniform size are deposited, an open or poor connection may result. Conversely, if the warp of one surface extends toward the other, solder balls in the intervening area may be excessively deformed causing them to squeeze out and short other electrical components. 
     Referring to FIG. 6, a first relatively flat substrate  10  may be surface mounted to an irregular substrate  12 . In this case, the substrate  12  is warped for illustration purposes. The solder bumps or balls  14  may be applied to the flat surface  12 . However, when the two surfaces  10  and  12  are brought together, some of the solder balls may not make effective contact with the surface  12  because of its curvature, while other balls may be excessively deformed raising the possibility of shorts to adjacent components. 
     Thus, there is a need for a way to deal with the possibility that the spacing between two surfaces to be surface mounted may not be uniform and regular throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of one embodiment of the present invention; 
     FIG. 2 is a side elevational view of an embodiment using a glue dispensing apparatus; 
     FIG. 3 is a side elevational view of another embodiment in accordance with the present invention; 
     FIG. 4 is a side elevational view of still another embodiment of the present invention; 
     FIG. 5 is a flow in accordance with one embodiment of the present invention; and 
     FIG. 6 illustrates the problem that may arise with the prior art. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the surface mounted substrates  10  and  12  are well joined despite the fact that the surface  12  is irregular. For example, substrate  12  is illustrated as having a warp which causes it to warp away from the substrate  10  near the right side in FIG.  1 . By making the solder bumps  14  of substantially equal volume on the left side because of the substantially uniform spacing therein and progressively increasing the volume of the solder bumps  14   a ,  14   b  and  14   c , the irregularity between the two substrates  10  and  12  can be compensated for by the surface mounting technique. 
     A variety of different techniques may be utilized to apply different volumes of material for forming the solder bumps  14  of programmably variable volume. For example, conventional printing and needle dispensing techniques utilized for applying solder bumps may be utilized. However, with conventional technology, great pains are taken to ensure that equal amounts of solder are applied to form each solder bump. In connection with embodiments of the present invention, the same technology may be utilized to dispense variable amounts of solder bumps deliberately to account for bonded surface irregularities. 
     Referring to FIG. 2, a glue dispensing machine  30  may dispense the bumps  14  at a very high rate of speed. A reservoir  22  contains the material to be deposited which may be solder or other conductive adhesive. Examples of materials utilized for making bumps may include solder, solder paste, conductive epoxy, and isotropic conductive materials. Material in the reservoir  22  is pumped by the pump  16  through a valve  18  and ultimately through a dispensing needle or nozzle  20  to be dispensed on the substrate  10 , forming the solder bumps  14 . By programmably controlling the aperture of the needle  20 , in one embodiment, the volume of material that is deposited to form the bumps  14  may be programmably adjusted. 
     In one embodiment, a variety of measuring techniques may be used to determine the surface profile of the substrates  10  and  12 . Flatness of a substrate  10  or  12  may be measured using various mechanical and optical profilometers. Measurement of nonflatness of a substrate  10  or  12  may also be accomplished by scanning a mechanical feeler gauge over the substrate  10  or  12  surface or by using a linear or area array of gauges. Alternatively, optical techniques may be utilized, such as looking at the off-axis image of a grating that can be used to determine the flatness profile. Ultrasonic profile measurement devices may be used as well, as still another example. 
     Once the substrate  10  and/or  12  profile is known, the thickness of any particular bump  14  may be calculated. The bump  14  volume may then be programmed into the bump  14  forming apparatus. 
     Referring to FIG. 3, the variable bump  14  volume may be achieved using a multiple deposition apparatus  32  in another embodiment. In one embodiment, the two dispensing needles  20  and  20   a  may work in tandem. For example, a perfect pair of substrates may utilize only the needle  20  in one embodiment. When variable bump volumes are needed, both needles  20  and  20   a  may be utilized. Similarly, when less volume is needed, the needle  20   a  may be utilized. As still another alternative, both needles  20  and  20   a  may work in parallel to apply the desired amount of material at the highest possible rate. Any number of additional nozzles  20  may be utilized as well. 
     As before, each nozzle  20  or  20   a  may be connected to a valve  18  or  18   a , a pump  16  or  16   a  and ultimately to a reservoir  22  of dispensed material. The composite formed by depositing dots from two different sources may result in a solder ball that is not materially different than other solder balls except for the amount of the volume applied, in one example. 
     Referring next to FIG. 4, an ink jet system may be utilized to deposit the solder bumps, in another embodiment. Use of ink jet equipment to deposit solder is well known. Some ink jet or microjet printers are capable of printing various solder compositions. In general, a microjet printer  34  includes a reservoir  22  storing the liquid material, a pump  16 , and a transducer  18  that breaks up the liquid stream into droplets. A charging electrode  24  applies a charge to each droplet and an electrostatic field is applied by electrodes  26  to deflect the droplet to its precise location on the substrate  10 . 
     The ink jet technique can produce liquid droplets on the order of 20 to 150 microns in diameter, at a frequency of 1,000 per second, in an on-demand stop and go mode, and at a frequency of up to 20,000 droplets per second in continuous mode. The capability has been demonstrated for 70 micrometer droplets of molten Sn63 solder at a deposition rate of 600 droplets per second. Higher rates may be achieved with multiple ink jet devices. One such ink jet printer suitable for this application is a Microjet by Microtab Technologies, Inc., Plano, Tex. 75074, for the Jetlab Printing Platform. 
     The required volume of solder deposited at a given location can be achieved by simply programming the printer  34  (or multiple printers on the same manufacturing line to increase throughput). The technique to handle random variations in substrate camber is well suited for depositing liquid or low viscosity paste materials on substrates that require low to high density of contacts. 
     Referring FIG. 5, initially the surface irregularity is measured as indicated at block  40 . Once the surface irregularity has been determined, the necessary contact size may be calculated as indicated in block  42 . This may be implemented using automated processes in some embodiments. The variable contacts may then be deposited using a variety of different techniques as indicated in block  44 . 
     Thereafter, the substrate  10  and  12  surfaces may be aligned as indicated in block  46 . Pressure and/or heat may then be applied to the surfaces of the substrates  10  and  12  to implement surface mounting as indicated in block  48 . 
     Following deposition of the solder material, it may be desirable to do additional processing before forming contacts. For example, it may be desirable to do a reflow process to form or reform solder bumps  14 . Alternatively, it may be desirable to add a flux or cleaning agent. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.