Patent Publication Number: US-2002012609-A1

Title: Solder composition for high temperature semiconductor device

Description:
[0001] This application claims the benefit of U.S. Provisional Application Ser. No. 60/184,516, filed Feb. 24, 2000. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] This invention relates to solder materials and, more specifically, relates to a solder for semiconductor devices with a temperature expansion matched to the temperature expansion of the parts being soldered.  
       [0004] 2. Discussion of the Related Art  
       [0005] Solders are commonly used to solder a silicon semiconductor die or chip to a conductive support, such as a copper heat sink. When such die is operated at high temperatures, for example, as high as 200° C., the thermal expansion of the die, the solder and the heat sink are diverse. After temperature cycling, the soldered connection may fail. For example, silicon has a thermal expansion, in parts per million, of 4.5×10 −6 /° C., and copper has a thermal expansion of 17×10 −6 /° C. When a soldered device is temperature cycled between about −55° C. and +150° C., considerable stress is developed, resulting in failure of the soldered joint. This problem is even further aggravated in higher temperature systems, such as those which operate at up to 200° C.  
       [0006] Attempts have been made to avoid this problem by using a solder which has a temperature coefficient which better approximates that of the parts to be joined. For example, a solder containing small ceramic beads has been used to adjust the overall solder temperature coefficient to one which is between that of silicon and copper. These ceramic-containing solders have not worked satisfactorily and have a reduced thermal conductivity.  
       SUMMARY OF THE INVENTION  
       [0007] The present invention is a non-ceramic solder composition for connecting a semiconductor device to a conductive support. The solder composition is composed of a solder component and a filler component which are proportioned so that the solder composition has an overall coefficient of thermal expansion that is intermediate of the coefficients of thermal expansion of the semiconductor device and the conductive support. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0008] The sole drawing is an exploded, schematic, cross-sectional view of a silicon die, solder layer and copper heat sink in which the solder is compounded in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
     [0009] The drawing shows a portion of a semiconductor silicon die  10  which is to be soldered to a copper heat sink support  11 . Die  10  may be any semiconductor device such as a diode, MOSFET, IGBT or the like and will have suitable PN junctions (not shown) for that purpose, and will have a bottom solderable coating  12 . Coating  12  may be formed of multiple layers, the outermost of which is preferably silver. Other metals can be used.  
     [0010] Heat sink  11  may be a portion of a conventional lead frame which is to receive die  10 , or can be any other kind of conductive support. While the support is shown as copper, other metals may be used.  
     [0011] In accordance with the invention, a novel solder layer  14  is provided which has a coefficient of thermal expansion intermediate to the coefficient of expansion of die  10  and heat sink  11 . Thus, a solder is formulated in which a lead-tin alloy solder  20  is filled with tungsten particles  21  of varying size and filling the lead-tin matrix with about 80% by weight. Tungsten is a preferred material since it has an extremely low temperature expansion coefficient of 4.5×10 −6 /° C. The filler percentage can vary from about 10% to about 90%, depending on the desired temperature coefficient of expansion.  
     [0012] For a solder layer  14  having a thickness of about 0.004 inches, the tungsten particles have a diameter, or a maximum dimension in any direction, of 0.001 to 0.003 inch, and thus can be readily mixed into a conventional lead-tin alloy solder and comprise up to about 80% by weight of the total solder. The solder layer is suitably fabricated and then cut into small wafers, which can be used with a conventional pick-and-place apparatus during the assembly of die  10  and heat sink  11 .  
     [0013] In a preferred embodiment of solder layer  14 , the expansion of the lead-tin component is about 25×10 −6 /° C. while the temperature coefficient of the tungsten is about 4.5×10 −6 /° C. An appropriate combination of the lead-tin component and the tungsten component produces an estimated overall temperature coefficient of 10×10 −6 /° C., which is approximately intermediate of the temperature coefficient of silicon die  10  (4.5×10 −6 /° C.) and copper heat sink  11  (17×10 −6 /° C.). Thus, the solder will better resist failure during temperature cycling, warpage up to 200° C.  
     [0014] While tungsten particles have been described as the solder filler, molybdenum particles and carbon fiber particles also can be used.  
     [0015] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.