Solder paste mixture

Abstract: A solder paste is composed of a vehicle (or flux) system and a mixture of at least two solder powders. One component of this mixture is a eutectic or near-eutectic Sn/Pb alloy powder, while the other component comprises powders selected from at least one elemental metal powder or at least one solder alloy powder or at least one elemental metal powder and at least one solder alloy powder. Said other component is a powder or combination of powders each of which has a liquidus temperature which is lower by at least 5 degrees Centigrade (°C) than the solidus temperature of said eutectic or near-eutectic Sn/Pb alloy powder or a solidus temperature which is higher by at least 5°C than the liquidus temperature of said eutectic or near-eutectic Sn/Pb alloy powder. The eutectic or near-eutectic Sn/Pb powder makes up from 5 to 95 weight percent of the total powder mixture. Alternatively, not all powders which comprise the second component need to obey this rule so long as at least 30% by wt. of the entire powder mixture has a solidus temperature which is at least 5°C higher than the highest liquidus temperature of the eutectic or near-eutectic Sn/Pb alloy powder. This solder paste results in an uncompromised consolidation and wetting process in which all of the solder joints or balls are formed on and wetted to an appropriate contact by the solder reflow process.

Description: Field of the Invention This invention concerns an improved solder paste. Background of the Invention Solder paste is widely used in the electronic industry. For example, solder paste reflow is used for surface mount assembly of packaged electronic components onto circuit boards, and to form solder balls (bumps) on ball grid array (BGA) electronic packages. For surface mounting applications which use solder paste/reflow technology, interconnections to an electronic component such as a packaged IC unit (integrated circuit chip or multi-chip module) or a discrete component (a resistor, a capacitor, a transistor or a diode) take the form of leaded or unleaded arrays of solderable I/O (input-output) connections. Examples include surface mountable IC packages with perimeter arrays of solder wettable butt, J or gull-wing shaped metal leads, surface mountable discrete components with solder wettable contact pads and surface mountable BGA packages with area or grid arrays of solder wettable I/O pads, each with its own solder ball. The "surface mount" assembly process involves placing individual deposits of solder paste over each of the solder wettable contact pads of the main circuit board and aligning the array of I/O contacts (IC leads, discrete component contact pads or BGA solder balls) on the component to the pattern of solder paste deposits that was placed on the contact pads of the main circuit board. The solder wettable I/O contacts of the surface mountable package unit are then placed into the solder paste deposits. The process of alignment and placement is repeated until the main circuit board is fully populated. This assembly is then heated so as to melt the solder in the paste such that, upon subsequent cooling and solidification, the solder forms joints. As a result of forming these solder joints, all of the I/O contacts on every component are bonded electrically and mechanically to the appropriate contact pads on the main circuit board. This process is know as a solder paste "reflow" assembly. Another application of solder paste is to form solder balls on an area array of solder wettable metal pads that comprise the I/O contacts on a surface mountable ball grid array (BGA) package. Such an application is described by Y. Degani, T. D. Dudderar and W. L. Woods, Jr. in U.S. Application Serial No. 08/128492 filed September 23, 1993. This patent application discloses an expedient process for producing solder balls (or bumps) on a grid array of solder wettable metal I/O pads. In this process solder paste is stencil printed onto the solder wettable metal I/O pads on the bottom side of the BGA and then reflowed to form solder balls (or bumps) on each pad. This process is known as printed solder "bumping", and the resulting solder bumps or balls comprise the I/O contacts for applications such as surface mount reflow assembly of the BGA packages onto printed circuit boards as described above. In both of these applications the solder paste is comprised of a flux system plus a solder alloy in powder form. In order to assure that the resulting solder joints be consistently uniform and reliable, it is important that the paste be designed so that most of the solder powder will consolidate into a single mass during the reflow process. Here "most of the solder" means at least 97%, preferably 99%, most preferably 99.5% (by weight) of the total solder powder. Furthermore, for surface mount assembly it is also necessary that this single, well consolidated mass of reflowed solder be formed on and wetted to the solder wettable metal pad on which the paste was initially printed and robustly wetted to the packaged IC lead, discrete component contact pad or BGA solder ball. Solder pastes which incorporate solder powders of eutectic or near-eutectic alloys, such as 63/37 Sn/Pb or 60/40 Sn/Pb, respectively, will occasionally fail to meet these criteria. For example, (1) in the surface mount assembly of components to a circuit board, occasionally a stray non-consolidated (eutectic or near-eutectic alloy) solder ball will be found under the component after reflow, or (2) in bumping a BGA with a eutectic or near eutectic alloy solder paste as described above, occasionally a well-consolidated solder ball will be found in a location well off of the solder wettable metal I/O pad after reflow. Since such a misplaced solder ball is not in contact with the circuitry, the entire BGA package is rendered unusable. On the other hand, in the case of example (1), the stray solder ball is a potential reliability risk in terms of either an insufficient solder joint volume or an electrical short. (See H. H. Manko, Soldering Handbook for Printed Circuits and Surface Mounting, pages 290 to 294, Van Nostrand Reinhold (New York) 1986. U.S.-A-4,680,141 discloses a solder paste comprising a liquid vehicle, finely divided solder metal particles of tin and lead, and additional infusible metal particles selected from brass, bronze and copper. EP-A-0 164 906 discloses a solder composition comprising a solder paste and disposed in the solder paste is a filler comprising discrete particles which are solid at a temperature at which the solder is molten. Patent Abstracts of Japan Vol. 13 No. 577 (M-910) 20 th December 1989 and JP-A-01 241 395 (Matsuo Handa KK) 26 th December 1989 discloses a solder paste comprising two or more solder alloy powders of different melting points and a flux. US-A-5 328 521 discloses a solder paste containing finely divided particles of eutectic or pseudo-eutectic metal alloy or mixtures; finely divided particles of a metallic additive; and an organic medium. Clearly, a need exists for solder paste compositions which, when applied to surface mount assembly or BGA solder ball formation, result in an uncompromised consolidation and wetting process in which all of the solder joints or balls are uniform, and every uniform solder joint or ball is formed on and wetted to an appropriate contact by the solder reflow process. According to the present invention, there is provided a solder paste as defined in claim 1 or a process as defined in claim 2. Detailed Description of the Invention A solder paste embodying this invention is composed of a vehicle (or flux) system and a mixture of three or more solder powders. One component of the mixture is an Sn/Pb alloy powder having a eutectic or near-eutectic alloy composition such as 63/37 Sn/Pb alloy or 60/40 Sn/Pb alloy, respectively. The 63/37 or 60/40 means a ratio of Sn to Pb in wt.%. The other component of the mixture is a combination of powders, each of which has a liquidus temperature which is lower by at least 5°C than the solidus temperature of the eutectic or near-eutectic Sn/Pb alloy powder, or a solidus temperature which is higher by at least 5°C than the liquidus temperature of the eutectic or near eutectic alloy powder, or at least one of which has a solidus temperature which is higher by at least 5°C than a liquidus temperature of said eutectic or near-eutectic Sn/Pb powder. The solder paste includes a vehicle (or flux) and a solder powder mixture. The vehicle is present in the paste in an amount sufficient to allow proper dispensing or application of solder paste in methods such as screen or stencil printing, typically in a range of from 7 to 12 wt.% of the solder paste with the remainder of the solder paste being the solder powder. The vehicle can be selected from the vehicles described in the above-identified U.S. application 08/128492 or in U.S. Patent 5,211,764 or from any other vehicle (or flux) that a person experienced in the art will find suitable. The vehicle described in the above U.S. application includes 1-10 wt.% of an organic acid, 1-5 wt.% of a rheological additive comprising a polymer having a molecular weight greater than 50,000, 60-98 wt.% of solvent system, and 0-25 wt.% of further additive whose residue maintains high surface insulation. This invention will be described with reference to an Sn/Pb alloy having a 63/37 eutectic composition; however the use of near-eutectic alloys such as 60/40 Sn/Pb is also contemplated. The eutectic or near eutectic Sn/Pb powder is present in the solder mixture in an amount ranging from 5 wt.% to 70 wt.%, the remainder being the other component of the mixture. This other component is a powder comprised of a combination of at least one single elemental metal powder and at least one solder alloy powder. Each powder in this other component is, preferably, selected so that either its liquidus temperature is lower by at least 5 degrees Centigrade (°C) than the solidus temperature of the eutectic or near eutectic Sn/Pb alloy or its solidus temperature is higher by at least 5°C than the liquidus temperature of the eutectic or near eutectic Sn/Pb alloy. In the case of the other component being comprised of two or more different powders, the liquidus or solidus temperature requirements of all of these powders should satisfy the above conditions. This means that either the liquidus temperatures of all such other powders should be lower than the solidus temperature of the eutectic or near eutectic Sn/Pb alloy powder or the solidus temperatures of all such other powders should be higher than the liquidus temperature of the eutectic or near eutectic Sn/Pb alloy powder. This insures that particles of the composition with the lower liquidus temperature melt before the rest of the solder powders begins to melt, so as to wet the surfaces of the contact pads of the wettable leads (if present) and of the remaining unmelted solder particles having the higher melting temperature. Alternatively, not all powders which comprise the second component need to obey this rule so long as at least 30% by wt. of the entire powder mixture has a solidus temperature which is at least 5°C higher than the highest liquidus temperature of the eutectic or near-eutectic Sn/Pb alloy powder . The eutectic or near eutectic Sn/Pb alloy powder in some mixtures plays the role of a low melting temperature alloy and in other mixtures the role of a high melting temperature alloy. This means that, when the solder paste is heated, the solder in the paste does not melt homogeneously during the heating process but the low melting alloy powders, those whose liquidus temperature is lower than the lowest solidus temperature of other powders, will start to liquefy well before the higher melting alloy powders start to liquefy. As a result, the metal pads and the unmelted solder powder are wetted by the lower melting solder before all the solder mass is in a liquid form. This consolidates the solder paste into a single mass. When all the solder finally melts it is already in contact with the metal pad thus allowing surface tension to pull the rest of the molten solder to the pad. In those instances where the eutectic or near eutectic Sn/Pb powder is used as the low melting component, this powder is present in a range of from 5 wt.% to 70 wt.% of all the solder powder. In those instances where the eutectic or near eutectic Sn/Pb powder is present as a high melting component, this powder is present in a range of from 30 wt.% to 70 wt.% of all the solder powder. In a preferred exemplary embodiment, said one of the powders is the eutectic 63/37 Sn/Pb alloy powder with a melting point (both the liquidus and the solidus temperature) of about 183°C. The solder composition may be exemplified by the following examples. Example: 1 (comparative) Experiments were conducted in a BGA bumping process on a sample of 1,000 solder pads with a solder paste containing a 63/37 Sn/Pb powder and another alloy powder. The paste composition was 8 wt.% vehicle and 20 wt.% of 43/43/14 Sn/Pb/Bi solder powder (with a solidus temperature of 143°C and a liquidus temperature of 163°C), and 72 wt.% 63/37 Sn/Pb solder powder (with a eutectic melting point of 183°C) based on the total weight of solder paste. The printing process to make a BGA was used as described in the above-identified application 08/128492, namely on an array of (30 mils) 0.762 mm. in diameter pads spaced with (100 mils) 2.54 mm. center to center distance. The solder paste was stencil printed as paste deposits (70 mils) 1.778 mm. in diameter and (21 mils) 0.5334 mm. in height. The solder paste was deliberately misprinted so that each printed paste deposit was only in partial, e.g., about 50%, contact with each solder wettable pad instead of being centered on it Such a large misalignment of the printed solder paste deposits with the solder wettable pads was done to amplify the occurrence of the off-the-pad consolidated solder balls. The printed samples were reflowed under a certain temperature-time profile. With this paste composition all of the solder balls formed from the mixed alloy paste were on the pads. Example: 2 (comparative) In this experiment arrays of paste deposits were stencil printed similarly to Example 1 but with solder paste containing only a single eutectic 63/37 Sn/Pb alloy powder. The paste deposit arrays were treated under the same temperature-time profile as in Example 1. The result was that as many as 30% of the solder balls formed from this paste were placed off the pads rendering the arrays unsuitable for further use. Example: 3 The effect of using mixed alloy powders was further evaluated by using a system of two eutectic solder alloy powders. One alloy was 63/37 Sn/Pb alloy with melting temperature of 183°C. The other alloy was 96.5/3.5 Sn/Ag alloy the melting temperature of which at 221°C was higher than the melting temperature of the 63/37 Sn/Pb alloy. In this experiment a series of different weight percent ratios of these two solder alloy powders were mixed with a flux vehicle to form a solder paste. Each resulting paste with a different ratio of solder alloy powders was tested on a separate sample of 1000 solder pads. Each paste was printed on a board that had 10 arrays of 10 by 10 solder wettable pads each 30 mils in diameter and spaced 100 mils center-to-center distance. Each solder paste print was 70 mils in diameter and 21 mils in height and was misaligned from the wettable pad by about 50 percent. As in Example 1, such a large misalignment was done to amplify the occurrence of the off-the-pad consolidated balls. The boards were reflowed and then checked for defects, that is for solder balls that consolidated off the pad instead of on the pad. The results of this experiment were as follows: Sn/Ag % 100 98 95 80 50 30 15 --- Sn/Pb % --- 2 5 20 50 70 85 100 # of defects 950 13 0 0 0 0 3 400 In Table 1 below is shown a number of further exemplary composition mixtures of the 63/37 Sn/Pb powder and of the other component. When more than one powder is present in the other component, an exemplary amount of the presence of each powder in the other component is shown in wt.% based on the total amount of powder in the composition. Examples of Alloy Mixtures that Suppress off the Pad Solder Balling One Component Other Component Weight % of 63/37 Sn/Pb alloy 2nd Alloy or Elemental Metal 3rd Alloy Composition wt.% Solidus °C Liquidus °C Comp. wt.% Solidus °C Liquidus °C 70 100 Sn 18.3 232 232 5/95 Sn/Pb 11.7 308 312 70 100 Sn 17.7 232 232 10/90 Sn/Pb 12.3 268 302 70 100 Sn 30 232 232 N/A N/A N/A N/A 70 96.5/3.5 Sn/Ag 30 221 221 " " " " 70 95/5 Sn/Sb 30 235 240 " " " " 70 86/9/5 Sn/Zn/In 30 188 188 " " " " 95 43/43/14 Sn/Pb/Bi 5 144 163 " " " " 95 42/58 Sn/Bi 5 138 138 " " " " 95 52/48 Sn/In 5 118 131 " " " " Furthermore, the other component of the powder mixture includes x/(100-x) Sn/Pb alloy wherein x is the percentage of Sn based on the total weight of the x/(100-x) Sn/Pb alloy and 1<x<18. This alloy is one of at least two solder powders of the other component, for example, as a combination of an elemental Sn and the x/(100-x) Sn/Pb alloy wherein x is the percentage of Sn in the x/(100-x) Sn/Pb alloy and 1<x<18. The combination of Sn and x/(100-x) Sn/Pb alloy would be such that after melting the components of the powder mixture, the molten liquid will form an alloy with a eutectic or near-eutectic composition. The above examples have been described using a 63/37 Sn/Pb eutectic alloy powder, with a melting point (both liquidus and solidus) of 183°C, as the first component. However other alloys, such as a near-eutectic 60/40 Sn/Pb alloy powder, may be used as the first component so long as the liquidus and solidus temperature requirements of the other component, described above with reference to the eutectic 63/37 Sn/Pb alloy, are satisfied. This means, for example, that for the near-eutectic 60/40 Sn/Pb alloy powder the liquidus temperature of the other component of the solder powder mixture should be at least 5°C lower than the solidus temperature (183°C) of the 60/40 Sn/Pb alloy powder or the solidus temperature of the other component should be at least 5°C higher than the liquidus temperature (190°C) of the 60/40 Sn/Pb alloy powder. Of course the selection will depend on whether the 60/40 Sn/Pb alloy powder is being used as a lower or a higher melting component, respectively.

Claims: A solder paste comprising a vehicle and a mixture of at least two solder powders, one component of said mixture being a eutectic or near eutectic Sn/Pb alloy powder and another component of said mixture being selected from at least one elemental metal, at least one solder alloy and a mixture of at least one elemental metal and at least one alloy, wherein powders of said other component are selected from powders 1) each of which has a liquidus temperature which is lower by at least 5°C than solidus temperature of said eutectic or near-eutectic Sn/Pb powder, 2) each of which has a solidus temperature which is higher by at least 5°C than a liquidus temperature of said eutectic or near-eutectic Sn/Pb powder, and 3) at least one of which has a solidus temperature which is higher by at least 5°C than a liquidus temperature of said eutectic or near-eutectic Sn/Pb powder, said at least one of the powders with said higher solidus temperature making up at least 30 wt. % of the powders in the total powder mixture, wherein said eutectic or near-eutectic Sn/Pb alloy powder has a lower liquidus temperature than the lowest solidus temperature of at least one of the powders of said other component of the powder mixture, said eutectic or near-eutectic Sn/Pb powder comprising from 5 to 70 wt.% of the powder mixture, characterized in that said other component comprises an elemental Sn and a Sn/Pb alloy with the composition x /(100-x) wherein x is the percentage of Sn based on the total weight of the x /(100-x) Sn/Pb alloy, 1<x<18, and the Sn powder and the x /(100-x) Sn/Pb alloy powder are proportioned such that after melting the two components of the mixture, the molten liquid will form an alloy with a eutectic or near-eutectic Sn/Pb composition. A process for forming a solder paste mixture, comprising the steps of providing a vehicle for solder paste; providing a plurality of particles of one component of the solder mixture, said one component being a eutectic or near-eutectic Sn/Pb alloy; providing a plurality of particles of another component of the solder mixture, said other component being selected from at least one elemental metal, at least one solder alloy and a mixture of at least one elemental metal and at least one solder alloy, wherein powders of said other component are selected from powders 1) each of which has a liquidus temperature which is lower by at least 5°C than the solidus temperature of said eutectic or near-eutectic Sn/Pb powder, 2) each of which has a solidus temperature which is higher by at least 5°C than a liquidus temperature of said eutectic or near-eutectic Sn/Pb powder, and 3) at least one of which has a solidus temperature which is higher by at least 5°C than a liquidus temperature of said eutectic or near-eutectic Sn/Pb powder, said at least one of the powders with said higher solidus temperature making up at least 30 wt.% of the powders in the total powder mixture, wherein said eutectic or near-eutectic Sn/Pb alloy powder has a lower liquidus temperature than the lowest solidus temperature of at least one of the powders of said other component of the powder mixture, said eutectic or near-eutectic Sn/Pb powder comprising from 5 to 70 wt.% of the powder mixture, and mixing said vehicle, said one component and said other component to form a paste mixture, characterized in that said other component comprises an elemental Sn and a Sn/Pb alloy with the composition x /(100-x) wherein x is the percentage of Sn based on the total weight of the x /(100-x) Sn/Pb alloy, 1<x<18, and the Sn powder and the x /(100-x) Sn/Pb alloy powder are proportioned such that after melting the two components of the mixture, the molten liquid will form an alloy with a eutectic or near-eutectic Sn/Pb composition.

Inventors: BLONDER GREG E, DEGANI YINON, DUDDERAR THOMAS DIXON, BLONDER, GREG E., DEGANI, YINON, DUDDERAR, THOMAS DIXON
Applicants: AT & T CORP, AT&T CORP.
Application year: 1995
Main classification: B23K35
IPC codes: B23K35, H05K3, B23K101