Patent Publication Number: US-2007102489-A1

Title: Ultrasonic welding head

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to an ultrasonic welding head for ultrasonic welding connection of a chip to a substrate.  
      2. Description of the Related Art  
      Known ultrasonic welding heads have a welding die with a welding end face. The welding end face can be embodied as being ground smooth. Another option is for the welding end face to be embodied as being indented in pyramidal form. In welding dies with a welding end face ground smooth, stress marks on the metallization of a given chip once the ultrasonic welding has been done cannot reliably be avoided. Welding dies with a pyramidally indented welding end face represent only a positive engagement with the chip edges, so that the chip edges are correspondingly mechanically stressed. Unwanted damage to a given chip because of the non-two-dimensional load is likewise often unavoidable.  
      With such known ultrasonic welding heads and welding dies, variations in the process parameters that must be monitored in the ultrasonic welding connection of a chip to a substrate cannot be avoided.  
     SUMMARY OF THE INVENTION  
      Given these known conditions, it is an object of the invention, by simple means, to prevent such variations in the process parameters in ultrasonic welding of chips to substrates.  
      According to the invention, a static friction element is used to increase the static friction relative to the chip to be welded. The static friction element is secured to the welding die of the ultrasonic welding head and has a flat frictional end face. It has proved advantageous if the static friction element is formed by a ceramic chip. This ceramic chip may for instance be an aluminum oxide chip, a silicon carbide chip, or the like.  
      According to the invention, the flat frictional end face of the ceramic chip is produced by means of lapping and ensuing roughening. The roughening can be produced by etching; it can have a peak-to-valley height of from 5 to 15 μm.  
      The ultrasonic welding head according to the invention has the advantage that the ultrasonic energy of the ultrasonic welding head is transmitted in a process-safe way to the chip that is to be welded. This transmission of the ultrasonic energy from the flat frictional end face of the welding die of the ultrasonic welding head of the invention to the chip is accomplished by means of the static friction between the frictional end face of the welding die and the corresponding chip surface. The static friction force is the product of the normal force and the coefficient of adhesion. Since the normal force is limited by the chip construction and/or the counterpart face, the coefficient of adhesion is the sole variable that is brought about and adjusted as needed by the roughening of the flat frictional end face of the ceramic chip in order to increase the normal force.  
      The ultrasonic energy is introduced with the aid of the static friction element, preferably formed by a ceramic chip, in a defined way into the chip to be welded, and the welding parameters advantageously have only negligibly slight variation or in other words are quasi-constant.  
      Further details, characteristics, and advantages will become apparent from the ensuing description of an exemplary embodiment, shown in the drawing, of the ultrasonic welding head of the invention for ultrasonic welding of chips to substrates.  
      Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In the drawings:  
       FIG. 1  is a side view of the ultrasonic welding head with a chip that is to be welded; and  
       FIG. 2  is an elevation view of the static friction element, formed by a ceramic chip, of the ultrasonic welding head, looking in the direction of the arrows II-II in  FIG. 1 .  
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS  
       FIG. 1  shows an ultrasonic welding head  10 , which in a manner known per se is set into ultrasonic vibrations. These vibrations are indicated schematically by the double arrow  12 . The ultrasonic welding head  10  has a welding die  14 , to which a static friction element  16  is secured. This securing is done by means of a suitable adhesive  18 . Before the static friction element  16  is firmly adhesively bonded to the welding die  14 , the static friction element  16  is first lapped and then roughened on its frictional end face  20  remote from the welding die  14 . This roughening is preferably done by etching. The peak-to-valley height of the roughening of the frictional end face  20  of the static friction element  16  amounts for instance to from 5 to 15 μm.  
      For picking up a chip  12 , to be firmly welded to a circuit, by suction at the ultrasonic welding head  10  or at the frictional end face  20  of the static friction element  16 , a through hole  24  extends through the welding die  14  and is axially aligned with a through hole  25  that is embodied in the static friction element  16 .  
      The ultrasonic welding head  10  can be connected or is connected to a vacuum source  26 , which is shown schematically as a block in  FIG. 1 . The connection of the vacuum source  26  to the ultrasonic welding head  10  is represented by the angled arrow  28 .  
      The static friction element  16  is formed by a ceramic chip  30 , which by way of example has a rectangular outline at the base, as can be seen from  FIG. 2 . Reference numeral  25  also designates the through hole embodied in the ceramic chip  30  in  FIG. 2 .  
      The ceramic chip  30  comprises a wear-resistant ceramic, and the frictional end face  20  of the ceramic chip  30  has a defined surface roughness or in other words roughening, which is preferably produced by etching.  
      The ultrasonic energy of the ultrasonic welding head  10  is introduced into the chip  22  to be welded via the flat frictional end face  20  of the ceramic chip  30  of the static friction element  16 . The welding parameters are advantageously quasi-constant.  
      Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.