Abstract:
A flip chip test structure is disclosed. The flip chip test structure utilizes a substrate used in flip chip package to replace the conventional transformer of a flip chip wafer probe card. The substrate-transformer replacement reduces the cost and simplifies the flip chip wafer probe card manufacturing process since the substrate is already available and matches the chip being tested while the transformer needs additional design and custom fabrication which are expensive and time-wasting for corresponding chip being tested.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a flip chip test structure, and more particularly to a low cost flip chip test structure.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIGS. 1 and 2 respectively show two conventional flip chip wafer probe cards. As shown in FIG. 1, the flip chip wafer probe card comprises a probe card printed circuit board (PCB)  102  and a transformer  104 . The printed circuit board  102  has circuits for transmitting signals from the transformer  104  to a test equipment (not shown). The transformer  104  is used as a signal distribution interface between the printed circuit board  102  and the tested chips. The transformer  104  has circuits therein and contacts on one side to transmit test signals to the printed circuit board  102 . The transformer  104  has probes  106  which are used to contact input/output pads of the tested chips. The transformer  104  is replaceable and must be redesigned and manufactured for testing various kinds of chips. FIG. 2 shows another configuration of a conventional flip chip wafer probe card. The flip chip wafer probe card comprises a probe card printed circuit board (PCB)  202  and a transformer  206 . The transformer  206  transmits test signals to the probe card printed circuit board  202  through contacts  204 . The transformer  206  has a probe frame  208  used to contact input/output pads of the tested chips.  
           [0005]    When the conventional flip chip wafer probe cards are used to test flip chip package dies, the probe  106  or the probe frame  208  contact with the pads of the flip chip package dies. The probe  106  or the probe frame  208  transmit test signals to the transformers  104  or  206  and the transformers  104  or  206  distribute signals to a bigger signal pitches which is suitable to requirements of PCB layout. The transformers  104  or  206  then transmit the bigger-pitched signals to PCB of the probe card.  
           [0006]    However, the conventional transformer has several disadvantages. The conventional transformers are made of ceramic material and the manufacturing tooling costs are expensive. Moreover, since one type of transformer is usually designed and manufactured only for a certain chip and few number of the transformer are need, the cost of chip test is very high because the expensive tooling charge is divided by only few transformers.  
           [0007]    Similarly, a probe card PCB needs to match a transformer very well so that either an interposer  103  or solder balls  204  can connect probe card PCB and transformer with good electrical performance. The design and tooling cost of probe card PCB is also expensive due to the same reason that only few probe card PCBs are made. The design, tooling cost and actual manufacturing few transformers and probe card PCB can be 5 to 10 times more expensive compared to conventional non-flip chip probe card. This adds burden to flip chip manufacturing in terms of break-even point.  
           [0008]    In view of the drawbacks mentioned with the prior art flip chip test probe card, there is a continued need to develop new and improved flip chip test structures that overcome the disadvantages associated with prior art.  
         SUMMARY OF THE INVENTION  
         [0009]    It is therefore an object of the invention to provide a flip chip test structure which can reduce chip testing cost.  
           [0010]    It is another object of this invention to provide a flip chip test structure which can simplify the chip test process and save the chip test preparation cycle time.  
           [0011]    It is another object of this invention to provide a flip chip test structure which can provide a most suitable testing condition.  
           [0012]    To achieve these objects, and in accordance with the purpose of the invention, the invention provides a flip chip test structure, the test structure comprises a substrate used in flip chip package and means for holding substrate and communicating signal transmitting the distributed test signals from the substrate to a test apparatus, wherein the substrate communicates between a chip being tested and the means for holding substrate and communicating signal, and distributes test signals from the chip being tested. The test apparatus further includes a well-matched PCB that provides good electrical connections between flip chip substrate and testing equipment.  
           [0013]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated it becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 shows a conventional flip chip wafer probe card;  
         [0016]    [0016]FIG. 2 shows another conventional flip chip wafer probe card;  
         [0017]    [0017]FIG. 3 shows a cross-sectional view of a flip chip package structure;  
         [0018]    [0018]FIG. 4 shows a chip and a substrate;  
         [0019]    [0019]FIG. 5 shows a flip chip test structure; and  
         [0020]    [0020]FIG. 6 shows another flip chip test structure.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    It is to be understood and appreciated that the process steps and structures described below do not cover a complete process flow and structures. The present invention can be practiced in conjunction with various fabrication techniques that are used in the art, and only so many of the commonly practiced process steps and structures are included herein as are necessary to provide an understanding of the present invention.  
         [0022]    The present invention will be described in detail with reference to the accompanying drawings. It should be noted that the drawings are in greatly simplified form and they are not drawn to scale. Moreover, dimensions have been exaggerated in order to provide a clear illustration and understanding of the present invention.  
         [0023]    [0023]FIG. 3 shows a cross-sectional view of a flip chip ball grid array package structure. The flip chip package structure comprises a “flipped” chip  304  and a substrate  302 . The substrate  302  is used as a chip carrier and to distribute signals from the chip  304  to solder balls  308  which have a larger pitch fitting the chip  304  for the requirement of printed circuit board typically used in general electronics and products. The chip  304  connects to the substrate  302  by conductors  306 . FIG. 4 shows the chip  304  and the substrate  302  before bonding and underfill processes. The chip  304  is bonded to the substrate  302  via the soldering between the conductors  306  and circuit traces  310 . Accordingly, the conductors  306  of the chips  304  and the circuit trace pads  310  on the substrate  302  are designed and formed to align with each other. The conductors  306  are formed on bonding pads  312  and thus the bonding pads  312  are also designed and formed to align with the circuit trace pads  310 . The conductors  306  comprise solder bumps. The substrate of flip chip package has one function the same as the transformers  104  and  206 , that is, to communicate signal between the chip  304  and PCB. For example, the substrate  302  is also used to distribute signals from the chip  304  to the solder balls  308  which have a larger pitch in order to match and to be mounted on a printed circuit board. Thus the side of the substrate  302  used to bond with the conductors  306  can be utilized to contact with the printed circuit board of the wafer probe card through proper contactor devices. Moreover, since the circuit trace pads  310  of the substrate  302  match the bonding pads  312  of the chip  304 , the side of the substrate  302  having the circuit traces  310  can be used to contact with the chip  304  through proper contactor devices such as springs, pins and probes. The substrate  302  has a multi-level interconnect structure therein to distribute signals from the chip  304  to a printed circuit board similar to the circuit of the transformers  104  and  206 . In general, the substrate  302  is designed to optimize electrical performance and cost for mass production. Therefore, a conventional transformer of a flip chip wafer test probe card can be replaced by a flip chip substrate.  
         [0024]    Conventionally, when the chip  304  is tested, the probes  106  and the probe frame  208  align with and contact the bonding pads  312  of the chip  304 . The substrate  302  used to carrier the chip  304  in flip chip package can also be used to replace the transformers  104  and  206  since the substrate  302  has circuit trace pads  310  which are designed and formed to align with the bonding pads  312  of the chips. Probes or probe frame as the probes  106  and the probe frame  206  can be utilized on the substrate  302  to contact the bonding pads  312  of the chip  304 . Solders or mechanical devices such as springs and pins can also be formed on the circuit trace pads  310  of the substrate  302  to contact the bonding pads  312 . The substrate  302  is produced in a huge quantity to be used in flip chip package to carrier the chip  304  so that it is quite convenient and with ample supply to replace the conventional transformer which is designed and built specifically with a very limited quantity and a long design cycle time and is much expensive compared to a substrate used in flip chip package. Furthermore, the substrate  302  can be made of organic materials which are low cost while the conventional transformers are made of ceramics which are expensive and the manufacturing tooling of the ceramic transformer is also expensive.  
         [0025]    [0025]FIG. 5 shows a flip chip test structure  500  having a substrate  504  which is used in flip chip package. As shown in FIG. 5, the flip chip test structure  500  also has a means for holding substrate, communicating signal  502  and contactor devices  506  and  508 . The means for holding substrate and communicating signal  502  comprises PCB  510  therein which communicate signals between the substrate  504  and a testing equipment through the contactor device  506 . The means for holding substrate and communicating signal  502  holds the substrate  504  to test chips. The substrate  504  having an interconnect structure which communicates signals between a tested chip and the substrate  504  through the contactor device  508  and distribute the signals from the contactor device  508  with a small pitch to the contactor device  506  with a large pitch. The pitch of the contactor device  508  matches the pitch of bonding pads of a chip being tested while the pitch of the contactor device  506  matches the pitch of the solder balls of the substrate  504  which are used to contact and bond with a printed circuit board or a main board. The contactor device  506  comprises solder balls or mechanical devices such as springs. The contactor device  508  comprises probes, springs or pins. The contactor devices  508  contact bonding pads of a tested chip and communicate the chip and the substrate  504 . The means for holding substrate and communicating signal comprises a printed circuit board used in chip testing. This printed circuit board has similar function to communicate signals between chip and PCB. The printed circuit board also aligns signal pads to transformer. On the other side of the PCB, the signal contact mechanism is designed to general test equipment. This printed circuit board can be replaced by load board which is typically used in IC final testing. The load board in general is also designed to optimize for electrical performance. FIG. 6 shows another flip chip test structure  600  having a substrate  604  which is used in flip chip package. The flip chip test structure  600  also has a PCB  602  and contactor devices  606  and  608 .  
         [0026]    The invention provides a flip chip test structure which utilizes a substrate of flip chip package to replace the conventional transformer. It further utilizes load board to replace PCB. The substrate is available, ready-to-use and low cost while the transformer must be particularly designed and built for each kind of chip being tested and with a very few quantity and a long design cycle time. Moreover, the chip-testing cost is further reduced since the transformer is made of ceramics which is expensive and the manufacturing tooling of the ceramic transformer is also expensive. The load board is also available for IC final test while separate PCB has similar cycle time and cost penalty like transformer.  
         [0027]    Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.