Abstract:
A substrate holding apparatus for semiconductor processing like wire bonding process is disclosed. Two or more holding points at both side of center locating point are provided to created a distributed holding force at clamp tool holding shoulder to generate a moment about substrate moving direction to act on clamp tool so that clamp tool can clamp on substrate more evenly at all sides for better process quality.

Description:
BACKGROUND OF INVENTION  
       [0001]     During semiconductor wire bonding process, substrates like leadframe or stripe with die attached are hold between support block and clamp tool with window opening to expose die and welding points on substrate for processing. The function of substrate holding apparatus is to hold substrate steady against support block so that wire can be welded to die and leads in good quality. Due to high dynamic of wire bonding process and shape defects of substrate, sometime poor quality welding happened because of clamping not effective.  
         [0002]     Most substrate clamp tool link at two side shoulders in the middle of its width to wire bond machine. This is because substrate will be processed while traveling in a guide rail by a transport mechanism in front of rail and wire welding mechanism are located at the other side of rail. That leaves the two sides across the moving direction as holding points. Clamp tool link to wire bond machine through a unit called clamp holder, which normally consists of clamp holder and some force mechanism. The holding of clamp tool by clamp holder besides provide clamp force and location for clamp tool, it also need to accommodate the need for quick and easy change over of clamp tool. One way to do this is through a ball or pin driven by some spring mechanism on clamp holder to mate with a hole or slot in clamp tool to locate and hold clamp tool for quick change over. Ball or pin diameter is bigger than hole or slots on clamp tool and hold clamp tool by ball or pin seating on top edge of hole or slot.  
         [0003]     Clamp holder, holding clamp tool, which positioned on top of substrate, is driven by a linear drive unit to move up and down. A support block is normally placed underneath substrate and driven by another linear drive unit to move up and down also. Substrate is hold at a designed position by clamping in between clamp tool and support block through programmed move sequence. One design of clamp force mechanism is clamp tool, besides move up and down with clamp holder, also have a small relative moving stroke with clamp holder within hard stops on clamp holder and pressed against lower stop by spring loaded ball.  
         [0004]     Clamp tool with window opening in center and hold at two side shoulders are a conventional clamp tool design used by most wire bond machines.  
         [0005]     There are open position and close position for substrate holding apparatus. At open position, clamp tool, substrate and support block are apart from each other and substrate can move freely. At close position, substrate is clamped by clamp tool and support block and positioned at working/holding position. At this position, leadframe will be processed.  
         [0006]     U.S. Pat. No. 5,307,978 of Rickertson et al. discloses a leadframe clamping apparatus having an upper clamp plate hold to clamp holder in centerline at two sides.  
         [0007]     U.S. Pat. No. 5,035,034 of Cotney disclose a leadframe clamp design, which have spring fingers attached to clamp tool to hold on leadframe. This clamp tool link to wire bond machine through hole on clamp tool at two sides with pins on clamp holder in wire bond machine.  
         [0008]     To improve quality of bonding, mechanisms have better clamp effect on substrate were exploited. Some special clamp tool designs were disclosed and within them;  
         [0009]     U.S. Pat. No. 6,634,538 of Evers, et al. disclose a leadframe clamp tool design with a membrane attached to clamp tool and the membrane will clamp on leadframe. This design uses the flexibility of membrane to compensate leadframe unevenness to have better clamp effect on leadframe.  
         [0010]     U.S. Pat. No. 6,651,868 of Kawamura disclose a leadframe clamp tool design with slits around clamp window to produce some flexibility around window opening to compensate substrate unevenness.  
         [0011]     These designs go in the direction to modify clamp tool to add in some flexibility to accommodate substrate deformation. But look at the system, clamp tool link to wire bond machine through clamp holder by hold/locate ball or pin in two shoulder of clamp. These hold/locating points will pass clamping force generated from mechanism in wire bond machine or clamp holder down to clamp point in clamp tool to hold on substrate. In view of moment in clamp surface plane, the moment about axis perpendicular to substrate moving direction is much bigger than moment along moving direction because the one-ball linkage design. As clamp window is normally a square shape cut out in the center of clamp tool and contact area are a narrow shoulder around window, moment to hold down substrate in two along substrate moving direction side is much smaller than moment hold down the other two side. This huge difference in moment will cause clamp to have a big difference in hold down force around window opening. This is a main shortage of existing clamping mechanism in hold down substrate and a source for process instability. Even though flexibility is build into clamp tool with the new inventions, which help to compensate some degree of unevenness on substrate, but because clamp tool is hold too much weak in one direction than other by its driving mechanism, poor clamping also happens.  
       SUMMARY OF INVENTION  
       [0012]     The primary object of this invention is to provide an apparatus design with improved connection between clamp tool and clamp holder to have better clamp effect on substrate. In this invention, multiple holding points are provided besides traditional single locating/holding point at clamp tool connection to clamp holder to produce a much bigger holding moment in about substrate moving direction to improve currently used clamp holding method which only have a strong moment in perpendicular to substrate moving direction.  
         [0013]     It is further the objective of this invention that the added on holding points will provide moment acting on clamp tool while clamping on device but does not affect the locating function of center holding point currently using while clamp tool in opening position or during clamp tool change over.  
         [0014]     To fulfill the objectives of this invention, a set of holding points are provided besides center-locating ball at both sides. These holding points will have spring-loaded balls face a flat surface on clamp tool side shoulder top at both sides of center/holding ball. These balls are guided in holes with lower limit to set ball lower position and pressed on top by flat springs. At open position, these holding balls will not touch clamp tool. At clamp closing position, these holding balls will press on clamp tool. These holding points and locating point form an integral part of a substrate holding apparatus. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  Substrate holding apparatus  
         [0016]      FIG. 2  Exploded view of substrate holding apparatus  
         [0017]      FIG. 3  Clamp tool linkage to clamp holder at open position  
         [0018]      FIG. 4  Clamp tool linkage to clamp holder at close position  
     
    
     DETAILED DESCRIPTION  
       [0019]     As shown in  FIG. 1 . Substrate holding apparatus  10 , consists of clamp holder unit  12  and support unit  14 , clamp substrate  15  with certain holding force at desired height for processing. Clamp holder unit  12  have clamp holder  20  link with clamp tool  40  through linkage mechanism, which will be the center of this invention and stated in detail by following description. A linear drive unit  13 , detail not shown here, drive clamp holder  20  move up and down in vertical direction. Clamp tool  40  is placed above substrate  15  while support unit  14  is placed underneath substrate  15 . Support unit  14  consists of support block  71  and linear drive unit  73 , detail not shown here, to move up and down in vertical direction.  
         [0020]      FIG. 2  is an exploded view of substrate holding apparatus  10 . Clamp tool  40  further consists of left hold shoulder  41  and right hold shoulder  46 , hole  44  and slot  45  on shoulder  41  and  46 , window  48  at the center of clamp and clamp shoulder  49  around window  48  in facing substrate side, which is a narrow shoulder further protrude below clamp bottom surface  50 . Surface  42  and  43  are top and bottom surface of shoulder  41 .  
         [0021]     Clamp holder  20  further consists of left holder  21  and right holder  51  with symmetrical design. On left holder  21 , there is an opening  22  with top limit  23  and bottom limit  24 . Hole  25  is located at center on top portion of opening  22  and hole  26  and  27  on both side of hole  25 .  
         [0022]     Ball  34  is placed in and guided by hole  25  and can move up and down while pressed on top by spring  37  which mount to surface  32  of clamp holder  20  with means not shown here. A shoulder  28  at bottom of hole  25  serve as lower limit for ball  34 . Similar arrangement are made for ball  35  and  36  by placed in and guided by holes  26  and  27  with shoulder  29  and  30  and pressed on top by spring  38  and  39  respectively and able to move in vertical direction.  
         [0023]     Right holder  51  of clamp holder  20  is a symmetrical design with left holder  21 . There is an opening  52  and ball  64 ˜ 66  pressed by spring  67 ˜ 69 , which mount to surface  62  with means not shown here, in hole  55 ˜ 57 .  
         [0024]     The substrate holding apparatus is a symmetric design except locating hole on clamp tool normally is one side round hole and the other side a slot with similar function (slot is used for dimensional compensation). Following description will be centered on the clamp apparatus side with hole on clamp tool and same design apply to slot side.  
         [0025]      FIG. 3  shows clamp tool linkage to clamp holder at open position. Clamp tool  40  shoulder  41  will go into clamp holder  20  left open  22  and seating on bottom limit  24  under force from ball  34 , which further pressed by Spring  37 . Ball  34  seating on edge  46  of hole  44  on clamp tool  40 . Ball  35  and  36  pressed by spring  38  and  39 , seating on shoulder  29  and  30  of hole  26  and  27  respectively. There is a gap  16  between ball  35 / 36  and clamp tool  40  shoulder  41  top surface  42 . This design arrangement is to make sure the added new force points did not affect the locating function of center ball and easiness of clamp tool handling at open position.  
         [0026]      FIG. 4  shows a clamp tool linkage to clamp holder in close position. From open position as shown in  FIG. 3 , support block  71 , driven by linear drive unit  73 , move up and raise substrate  15  up to a desired height while clamp holder unit  12  holding clamp tool  40  and driven by linear drive unit  13  move downward. At close position, shoulder  49  of clamp tool  40  clamp on substrate  15  with top surface  72  of support block  71  and hold it for processing. Clamp tool  40 , under pressure from combined travel by support block  71  and clamp holder  20 , raise up its position as refer to clamp holder  20  so that its bottom surface  43  left clamp holder  20  bottom limit  24  and press ball  34  upward and then further compress spring  37 . At close position, there is still a gap  17  between clamp tool  40  top surface  42  and clamp holder upper limit  23 . Ball  35  and  36  also come into contact with clamp tool  40  top surface  42  and raised up from shoulder  29  and  30  in hole  26  and  27  with spring  38  and  39  being further compressed. Gap  18  exists between clamp tool  40  shoulder  41  bottom surface  43  and lower limit  24  on clamp holder  20 . As a result, there are three forces from ball  34 , 35  and  36  pushing on top surface  42  of clamp tool  40  left shoulder  41 . Similarly, on right shoulder  46  of clamp tool  40 , three forces from ball  55 ,  56  and  57  will be pushing on shoulder  46  top surface  47 . These forces will be balanced by the force acting on shoulder  49  around window  48 .  
         [0027]     The distributed forces along holding shoulder by three balls will generate a moment in about substrate moving direction to have a better holding on substrate  15  by shoulder  49  of clamp tool  40 . This substrate holding apparatus, with the moment from distributed force, will strengthen the clamping on two along substrate moving direction side hold on substrate  15  to improve on the weak point of existing design so as to have a better holding effect and then more stable wire bonding process.  
         [0028]     It will be understood that to those skilled in the art that various changes, additions, deletions, modifications may be added in the improved substrate holding apparatus as disclosed herein without departing from the spirit and scope of the invention as defined in the following claims. For example, more holding points can be added and spring can also be changed to other type.