Abstract:
An apparatus and method of forming fluxless solder balls includes forming solder balls from a supply of solder. A coating is formed on the solder balls for limiting naturally occurring oxide growth on the solder balls before significant natural oxide growth on the solder balls has occurred. The coating allows the solder balls to be soldered without using flux.

Description:
BACKGROUND  
         [0001]    The use of small solder balls positioned in ball grid arrays for making electrical interconnections in electronic chip packages is becoming increasingly popular. A typical ball grid array may contain over 1000 solder balls between 12-30 mils in diameter and 50 mils apart. Such a ball grid array allows a large number of electrical connections to be made in a small area. During manufacturing of an electronic chip package which employs solder balls for electrical interconnections, the solder balls are placed in the desired ball grid array configuration upon the chip package at the appropriate location, and then later bonded thereto as well as to any mating surfaces by reflowing the solder balls in a reflow oven. Prior to reflow, flux is applied for chemically removing surface oxides from the solder balls and appropriate surfaces so that the solder balls can be properly bonded thereto. The flux also maintains a protective layer over the cleaned surfaces during soldering and removes reaction products. After soldering, highly corrosive flux residues remain behind which are later removed with solvents in a cleaning process.  
         SUMMARY  
         [0002]    The present invention provides fluxless solder forms which may be placed in ball grid arrays on chip package substrates. The solder forms are formed from a supply of solder. A layer or coating is formed on the solder forms for limiting naturally occurring oxide growth on the surface of the solder forms. The layer or coating allows the solder forms to be soldered without using flux.  
           [0003]    In preferred embodiments, the solder forms are solder balls or spheres which are formed from molten solder at a solder ball forming station by a droplet spray process. In the droplet spray process, the molten solder is caused to fall as droplets which solidify while falling to form the solder balls. The layer or coating is formed on the solder balls at a coating station while the solder balls are falling and before significant natural oxide growth on the solder balls has occurred. The layer or coating is formed by treating the solder balls with plasma products including fluorine which forms an oxyfluoride layer on the solder balls. The coating station includes a chamber containing the plasma products therein. The solder ball forming station is positioned at the upper end of the chamber. The chamber is supplied with the plasma products from a plasma generator which generates plasma from a gas containing fluorine.  
           [0004]    The fluxless solder forms or solder balls provided by the present invention may eliminate the step of applying flux before soldering and the step of removing flux residues after soldering. This not only reduces manufacturing time but also reduces the inventory of materials and equipment required to be on hand since the flux and the cleaning solvents associated with the eliminated steps as well as corresponding equipment are no longer needed. The elimination of such steps, materials and equipment reduces the manufacturing costs of soldering or reflowing ball grid arrays.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0006]    [0006]FIG. 1 is a schematic drawing of an embodiment of the present invention solder ball apparatus.  
         [0007]    [0007]FIG. 2 is a schematic drawing depicting the conversion of an oxide layer to an oxyfluoride layer on a newly formed solder ball when treated with fluorine radicals F + .  
         [0008]    [0008]FIGS. 3A and 3B are schematic drawings depicting the reaction of fluorine (F) atoms with tin (Sn) atoms to form tin oxyfluorides.  
         [0009]    [0009]FIG. 4 is a graph depicting the atomic concentration of oxygen at particular depths for both newly produced solder balls and fluxless solder balls having an oxyfluoride layer.  
         [0010]    [0010]FIG. 5 is a graph depicting the atomic concentration of lead (Pb), tin (Sn), fluorine (F), oxygen (O) and carbon (C) relative to depth within 63 Sn/37 Pb fluxless solder balls having an oxyfluoride layer.  
         [0011]    [0011]FIG. 6 is a schematic drawing of the solder droplet generator depicted in FIG. 1.  
         [0012]    [0012]FIG. 7 is a schematic drawing of the plasma generator depicted in FIG. 1.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    Referring to FIG. 1, solder ball apparatus  10  is an apparatus which produces treated solder balls  16  that do not require flux when soldering or reflowing. Apparatus  10  includes a solder ball forming station having a solder droplet generator  12  that produces uniform droplets  47  of molten solder which solidify while falling into solder balls  15 . The solder droplet generator  12  is positioned within the interior  18   a  of a gas tight chamber  18  at the upper portion of the chamber  18 . Chamber  18  serves as a coating or treating station in which the solder balls  15  are coated or treated. A plasma generator  14  for generating plasma  13  is coupled to the interior  18   a  of chamber  18  by a conduit  28  and supplies chamber  18  with plasma products  13   a  for coating or treating the falling solder balls  15 . The plasma products  13   a  contain atomic fluorine radicals F +  that react with the solder balls  15  to form treated or coated solder balls  16  which have an oxyfluoride layer or coating  19  (FIG. 2) thereon. A container  20  is positioned at the bottom of the chamber  18  for collecting the coated or treated solder balls  16 . A fitting  24  at the bottom of chamber  18  is connected to a vacuum pump for evacuating the system.  
         [0014]    In use, the interior  18   a  of chamber  18  and the interior  30   a  of vessel  30  of plasma generator  14  are evacuated to remove gases from the system through fitting  24 . After evacuation, nitrogen gas (N 2 ) is introduced therein and then removed to facilitate the removal of oxygen. The system may be evacuated to a pressure of 0.15 Torr and then filled with N 2  gas to a pressure of 13 kPa before being removed. The introduction and subsequent evacuation of N 2  gas may be repeated to ensure that most of the oxygen in the chamber  18  and plasma generator  14  is removed. Once the chamber  18  and plasma generator  14  are sufficiently evacuated, the plasma generator  14  is turned on and supplied with sulfur hexafluoride gas (SF 6 ) from a gas source via inlet  26 . The SF 6  gas enters plasma generator  14  at a controlled rate for obtaining a desired pressure (for example, 1.5 Torr). The plasma generator  14  dissociates the SF 6  gas to produce a plasma  13  containing highly reactive atomic fluorine radicals F + . The following equation describes electron-impact induced molecular dissociation of SF 6  gas:  
           e +SF 6 →SF 6-x +XF+ e x≦ 6.   (Eq. 1)  
         [0015]    Preferably, pure SF 6  gas is supplied to plasma generator  14  to obtain the maximum concentration of atomic fluorine radicals F +  within the generated plasma  13  for treating solder balls  15 . N 2  gas may be mixed with the SF 6  gas to accelerate the decomposition of the SF 6  gas, however, this reduces the concentration of the atomic fluorine F +  within the plasma  13 . Plasma products  13   a  including fluorine radicals F +  flow from the inner vessel  30  of plasma generator  14  into the interior  18   a  of chamber  18  via conduit  28  which is coupled therebetween.  
         [0016]    The heaters  42  (FIG. 6) of solder droplet generator  12  are turned on to melt solder  38  contained within the crucible  40  of solder droplet generator  12 . The solder droplet generator  12  is then operated to form uniform droplets of molten solder  47  which fall downwardly within the interior  18   a  of chamber  18  and solidify into solder balls  15  while falling. As the solder balls  15  fall within chamber  18 , the solder balls  15  fall or pass through the plasma products  13   a  contained therein. The fluorine radicals F +  surround and contact the surfaces of each falling solder ball  15 . Oxides formed on the surface of the solder balls  15  in a layer  17  (FIG. 2) are treated by the fluorine radicals F + . The fluorine radicals F +  react with the oxides in layer  17  causing layer  17  to undergo an oxide conversion process which transforms the layer  17  into an oxyfluoride layer or coating  19 . In this manner, solder balls  15  become treated or coated solder balls  16 . The treated solder balls  16  are collected in container  20  which may contain a quantity of oil  22  such as silicone oil for cooling the solder balls  16 . The treatment time of solder balls  15  may be as little as 225 milliseconds. However, the height of chamber  18  may be sized to provide longer treatment times.  
         [0017]    In solder balls  15  having a typical eutectic 63 Sn/37 Pb tin/lead solder composition, the oxide conversion process may be generally described as a conversion from Sn/Pb oxide to Sn/Pb oxyfluoride as follows:  
         SnPbO x +yF + →SnPbO x F y .   (Eq. 2)  
         [0018]    More specifically, tin oxides (SnO and SnO 2 ) are usually the main surface oxide components on untreated 63 Sn/37 Pb solder balls  15 . FIGS. 3A and 3B depict the conversion of tin oxides on the surface of solder balls  15  into oxyfluorides by the bonding of fluorine atoms (F) with the tin atoms (Sn). The conversion of the tin oxides may be described as follows:  
         SnO x +yF→SnO x F y    (Eq. 3)  
         [0019]    In addition to the existence of oxyfluorides in the oxyfluoride layer  19  of treated solder balls  16 , there may also be some tin-fluoride compounds (for example, SnF 2  and Sn 2 F 6 ).  
         [0020]    As seen in the graph of FIG. 4, the plasma product treatment significantly reduces the atomic concentration of oxygen on the surface of a treated solder ball  16  as well as the penetration of the oxygen into the treated solder ball  16  in comparison to solder balls that are not treated. For example, the atomic concentration of oxygen on the surface of a treated solder ball  16  is about  34 % while the atomic concentration of oxygen on the surface of a newly produced untreated solder ball  15  is about 41%. In addition, the penetration of oxygen in a treated solder ball  16  is about 60 Å deep while the penetration of oxygen in a newly produced untreated solder ball  15  is about 90-100 Å deep. The graph of FIG. 5 depicts the relative atomic concentrations of lead (Pb), tin (Sn), fluorine (F), oxygen (O), and carbon (C) relative to solder ball depth for plasma product treated 63 Sn/37 Pb solder balls  16 .  
         [0021]    Once treated, the oxyfluoride layer  19  (FIG. 2) on the treated solder balls  16  allows the treated solder balls  16  to be reflowed or soldered in a ball grid array without the use of flux for cleaning the solder balls  16 . For purposes of description, soldering includes reflowing. The thin oxyfluoride layer  19  formed on the treated solder balls  16  has a structure that is sufficiently brittle to fracture or break up into small pieces when the treated solder balls  16  melt, allowing clean solder to flow, thereby permitting reflow as well as proper joining. Consequently, the use of the treated solder balls  16  can eliminate the steps of applying flux prior to soldering and then cleaning flux residues afterwards. The oxyfluoride layer  19  also limits subsequent oxide growth  17  on the treated solder balls  16  so that the treated solder balls  16  can be stored for a period of time in air (for example, about six days) before use. After about seven days, sufficient oxide growth may form on the treated solder balls  16  to prevent fluxless soldering. When performing a reflow process with treated solder balls  16 , a rapid reflow which occurs in about five seconds or less provides the best results because there is little time for sufficient oxides to grow to hamper the reflow process. If desired, the treated solder balls  16  can also be used with various fluxes such as water soluble flux, no-clean flux, and rosin-based flux.  
         [0022]    A more detailed description of solder ball apparatus  10  now follows. Referring to FIG. 6, solder droplet generator  12  includes a housing  35  in which the lower portion forms a crucible  40  where solder  38  is melted and contained. Heater  42  extends around crucible  40  for heating and melting the solder  38  contained within crucible  40 . A thermocouple  33  monitors the temperature of the molten solder  38  for maintaining the proper temperature. For a 63 Sn/37 Pb solder composition, the molten solder  38  may be maintained at about 235° C. A piezoelectric actuator  32  is clamped against the upper disk  36   a  of a vibration transmitting member  36  at the upper portion of housing  35  by a clamping plate  37  and bolts  39 . The upper disk  36   a  is positioned over an opening  33  at the upper portion of housing  35  with the piezoelectric actuator  32  clamped against the top surface of the upper disk  36   a . Vibration transmitting member  36  transmits vibrations produced by piezoelectric actuator  32  to the molten solder  38 . Vibration transmitting member  36  includes a shaft  36   b  extending downwardly from upper disk  36   a  which is connected to a lower disk  36   c . The lower disk  36   c  is extended into the lower portion of crucible  40  within the molten solder  38 . Vibrations produced by piezoelectric actuator  32  are transferred downwardly through upper disk  36   a  and shaft  36   b  to the lower disk  36   c  of vibration transmitting member  36  for perturbing the molten solder  38 .  
         [0023]    Pressurized gas, for example, an inert gas such as nitrogen, argon or helium, is employed to pressurize the interior of housing  35  via inlet  34 . The pressurized gas is employed for forcing molten solder  38  from crucible  40  through the orifice  44  located at the bottom of crucible  40 . A pressure differential of only about 35 kPa (5 lb./in. 2 ) is required to force a falling jet of molten solder  45  from crucible  40  through orifice  44 , however, a pressure differential of between about 135-700 kPa (20-100 lb./in. 2 ) is preferred. By vibrating piezoelectric actuator  32  at a periodic oscillation having a wave length greater than the circumference of the jet diameter, the falling jet  45  of molten solder  38  breaks into a train of solder droplets  47  while falling. The droplets  47  pass through an opening  46   a  in a charging plate  46  located below the crucible  40 . The charging plate is provided with a voltage which charges the falling droplets  47  by electrostatic induction to prevent merging of the droplets  47  during flight.  
         [0024]    The diameter of orifice  44 , the pressure differential within crucible  40  and the vibration frequency of piezoelectric actuator  32 , varies depending upon the size of the solder balls  16  to be made. For example, an orifice  44  diameter of 406 μm, a pressure differential of 34.4 kPa and a vibration frequency of 1430 Hz may be used to produce solder balls  15  that are 760 μm in diameter; an orifice  44  diameter of 254 μm, a pressure differential of 44.8 kPa, and a vibration frequency of 2582 Hz may be used to produce solder balls  15  that are 500 μm in diameter; and an orifice  44  diameter of 178 μm, a pressure differential of 68.9 kPa, and a vibration frequency of 4370 Hz may be used to produce solder balls  15  that are 300 μm in diameter. In order to obtain a particular solder ball  15  diameter, shrinkage of the solder while cooling is also taken into account. The different orifice  44  diameters and pressure differentials within crucible  40  provides different initial velocities of the jet  45  of molten solder. For example, an orifice  44  diameter of 406 μm and a pressure differential of 34.4 kPa provides an initial jet velocity of 2.8 m/s; an orifice  44  diameter of 254 μm and a pressure differential of 44.8 kPa provides an initial jet velocity of 3.2 m/s; and an orifice  44  diameter of 178 μm and a pressure differential of 68.9 kPa provides an initial jet velocity of 4.1 m/s. For producing solder balls that are small in diameter, the higher initial jet velocities in combination with the small ball diameters allows over a million solder balls to be produced in just a five-minute period of time.  
         [0025]    During the operation of solder droplet generator  12 , variations in the target diameter of the solder balls  15  may be controlled by a closed loop control system where the size of the falling solder droplets  47  is measured by a CCD camera and the vibration frequency of the piezoelectric actuator  32  adjusted in response to the measurements. Typically, the solder droplets  47  are measured by digital image analysis where the images from the CCD camera are transformed into a pixel array and then the pixel values are transformed into length units. The CCD camera is calibrated to account for any optical distortion and image amplification. Such a control system can produce solder spheres with a size variation smaller than ±2.5% of the target size.  
         [0026]    Although solder droplet generator  12  has been shown and described for use in solder ball apparatus  10 , other suitable molten droplet generators may be employed, for example, the devices described in U.S. Pat. Nos. 5,266,098 and 5,431,315, the contents of which are incorporated herein by reference in their entirety.  
         [0027]    Referring to FIG. 7, plasma generator  14  is a microwave plasma generator which dissociates the SF 6  gas with microwaves. Plasma generator  14  may be formed from a microwave oven  14   a  within which a Pyrex® inner vessel  30  is mounted. The power of plasma generator  14  is controlled by controls  54 . An iron oxide polymer and aluminum mesh is used to prevent microwaves from radiating to the outside environment from plasma generator  14 . Two flow meters  48   a  and  48   b  with respective inlets  26   a  and  26   b  are coupled in communication with inlet  26 . The flow of N 2  gas and SF 6  gas into inner vessel  30  is controlled by respective flow meters  48   a  and  48   b . A pirani type pressure gauge  52  measures the pressure within inner vessel  30 . Inlet  26  and conduit  28  include elongate tubes  31  extending within the interior  30   a  of inner vessel  30  for delivering the gases to and removing the plasma products  13   a  containing fluorine radicals F +  from the center of the microwave oven  14   a . Pressure gauge  52  also includes an elongated tube  31  for measuring the pressure at this central region. The length of conduit  28  is kept to a minimum and is coupled to chamber  18  at a position for quickly delivering plasma products  13   a  to the falling solder balls  15  in order to minimize the recombination of the fluorine radicals F +  with neutral species before contacting the solder balls  15 .  
         [0028]    A stable plasma  13  may be generated by plasma generator  14  at a power of 1000 watts, a frequency of 2.45 GHz and an SF 6  gas pressure of 0.15 to 5 Torr. Typically, microwave power above 600 watts provides maximum dissociation of SF 6  gas. The higher SF 6  gas pressures are preferred to provide a higher concentration of atomic fluorine F + . A high concentration of atomic fluorine is desirable to ensure sufficient treatment of solder balls  15  because the treatment time of the falling solder balls  15  is very short. The pressure of the plasma  13  within the system may be controlled by the SF 6  gas flow rate. For example, a plasma  13  pressure of 0.8 Torr may be obtained by a SF 6  gas flow rate of 100 SCCM (standard cubic centimeters per minute), a plasma  13  pressure of 1 Torr may be obtained by a SF 6  gas flow rate of 476 SCCM, a plasma  13  pressure of 1.5 Torr may be obtained by a SF 6  gas flow rate of 985 SCCM, and a plasma  13  pressure of 3 Torr may be obtained by a SF 6  gas flow rate of 1302 SCCM.  
         [0029]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.  
         [0030]    For example, although solder ball apparatus  10  is depicted to position solder ball generator  12  in a chamber  18  that receives plasma products  13   a  containing fluorine radicals F +  from plasma generator  14  via conduit  28 , alternatively, plasma generator  14  may be modified to house solder ball generator  12  within vessel  30  so that both the plasma  13  and the solder balls are produced in vessel  30 . In such a case, solder ball generator  12  must be shielded from the microwaves. In addition, although plasma generator  14  is described as a microwave plasma generator, alternatively, plasma generator  14  may generate plasma by other suitable methods, such as by radio frequency. Furthermore, although conduit  28  preferably is mounted to the top of chamber  18  for delivering plasma products  13   a  into chamber  18 , alternatively, conduit  28  may be mounted at other suitable locations, with the vacuum fitting  24  also being positioned in an appropriate position relative to conduit  28  to maintain an adequate plasma concentration in the chamber  18 . Although the present invention has been described for treating solder balls having a composition of 63 Sn/37Pb, it is understood that solder balls of other compositions may be treated, such as 10 Sn/90 Pb solder balls. Also, chamber  18  may be employed for treating solder balls that have been previously formed. The solder balls may be loaded into a feed device which drops the solder balls through the plasma product  13   a  filled chamber  18 . The solder balls may be stored in an inert environment before and/or after treatment. The container  20  for collecting treated solder balls may be replaced by a conveyance system. Finally, solder forms not necessarily spherical in shape may also be formed and treated with plasma products  13   a.