Abstract:
A testing head having cantilever probes is presented. The testing head, comprises a backing ring and a resin holder attached to the backing ring, as well as a plurality of contact probes held by the resin holder and formed with respective contact tips arranged to mechanically and electrically contact a plurality of contact pads of at least one device to be tested. The holder is formed with at least one suitably shaped outline to allow different probe rows to emerge in a cantilever manner.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a testing head having cantilever probes, and more particularly to a testing head for use on semiconductor integrated devices. 
     2. Description of the Related Art 
     As is well known, a testing head is basically a device suitable to electrically interconnect a plurality of contact pads of a microstructure and the corresponding channels of a testing machine that is to perform the tests. 
     Integrated circuits are factory tested in order to spot and reject any circuits which are already defective during the production phase. The testing heads are normally employed to electrically test the integrated circuits “on wafer”, prior to cutting and mounting them in a chip package. 
     As schematically shown in FIGS. 1 and 2, a testing head  1  having cantilever probes usually comprises a backing ring  2 , made of aluminum or ceramics, to which a resin holder  3  is attached, and that is suitable to hold a plurality of movable contact elements or probes  4 , being normally wires made of special alloys having good electrical and mechanical properties, the probes being mounted to jut out of the resin holder  3  at plural points  5  and at a suitable angle from a plane β. Such emerging probes are commonly known as “cantilever probes”. 
     In particular, each probe  4  has an end portion or contact tip  6 , which is bent with an angle γ from the rest of the probe so that a plurality of contact pads  7  of a device to be tested is contacted. The bent contact tips  6  are commonly referred to as the “crooks”. 
     The good connection of the probes  4  of the testing head  1  to the contact pads  7  of a device to be tested is ensured by the testing head  1  exerting a pressure on the device, whereby the probes  4  are vertically flexed in the opposite direction from the device movement towards the testing head  1 . 
     As schematically shown in FIG. 3 for a single probe  4 , as the device to be tested vertically moves against the contact tip  6 , the probe  4  flexes, and its elbow point X, situated at the transition from the contact tip  6  to a probe section  8  emerging from the resin holder  3 , describes a circular arc. 
     Thus, the jutting probe section  8  forms a working arm of the probe  4  adapted to flex vertically, and is commonly referred to as the “free length” of the probe. 
     The crooked shape of the probes  4  is designed to allow the contact tips  6  of the probes  4  to skid, upon coming in touch with the contact pads  7  of the device to be tested and during the pad overtravel beyond a pre-set point of contact, across the contact pads  7  along a direction dictated by the arrangement geometry. 
     It should be noted that the force exerted on the contact pads  7  by each probe  4  depends on many factors, among which are especially the type of material forming the probe  4 , the probe shape, the angle α made by the probe contact tip  6 , the length of the probe jutting section or free length  8 , and the amount of overtravel of the pads to be measured. These factors also determine the extent of the contact tips  6  skidding on the contact pads  7 , this being commonly known as the “scrub”. 
     It should be noted that, with a dense distribution of the contact pads  7 , the probes  4  must be arranged in plural rows, and the lengths L 1 , . . . Ln of the crooked ends vary accordingly, as schematically shown in FIG.  4 . 
     Also known is to use backing rings  2 , generally made of aluminum or ceramics, having different shapes depending on the set of contact pads  7  to be tested, so that the free lengths of the probes  4 , and hence the forces exerted by the latter to the contact pads  7 , can be equalized in the interest of even wear and performance of the testing head  1 . 
     In particular, when the probes  4  are arranged in a plurality of rows or levels, as schematically shown in FIGS. 5A,  5 B and  5 C, the emerging points  5  of the probes  4  from the resin  3 , when viewed frontally, make either a diagonal (FIG.  5 A), or straight (FIG.  5 B), or combination pattern (FIG. 5C) that is dependent on constructional requirements. 
     The portions of the probes outside the backing ring  2  are usually soldered on a PC board  9 , as shown in FIG. 1, to establish an electrical connection between the testing head  1  having cantilever probes and the testing machine. 
     It is therefore necessary that the outer portion of any probe  4  can be recognized unfailingly in the probe bunch, so that it can be soldered on the PC board  9  in the correct manner. 
     In addition, the probes  4  extend with their sections outside the backing ring  2  parallel to one another, as shown in FIG. 6A (side A), and the probes  4  for soldering on the PC board  9  are not easily singled out. It is also known the use of probes  4  with a radial spreading in their portion outside the backing ring  2 , as schematically shown in FIG. 6A (side B). 
     The probes  4  can be arranged in a plurality of rows or layers such that they have a diagonal or a straight configuration, in either the case of parallel or radial probes, as shown in FIG.  6 A. 
     FIG. 6B shows, by way of example, an arrangement of the probes  4  in three rows with a radial diagonal configuration, and FIG. 6C shows an arrangement of the probes  4  in three rows with a radial straight configuration. 
     It is, moreover, a known fact that certain electronic devices, e.g. memories, have contact pads disposed along two sides only. Accordingly, a number of such devices can be tested in parallel if they are set in a single row. 
     A row of devices can be tested by suitably calibrating the inside dimensions of the backing ring  2 . 
     When several rows of devices are to be tested in parallel—usually two rows of eight devices or four rows of eight devices—multi-bridge backing rings, schematically shown in FIGS. 7A and 7B, are used. 
     In particular, a multi-bridge backing ring  2   b  includes a plurality of bridges  2   c  having a width dimension P inside the ring  2   b  perimeter, which bridges are suitable to carry probes for several devices to be tested in parallel. There are various techniques that can be used in order to obtain the desired pressure uniformity on the probes  4  against the corresponding contact pads  7 . 
     A first known technique uses a multi-bridge backing ring  2   b  having plural bridges  2   c  inside its perimeter to define plural device rows FILA 1 , FILA 2 , . . . as schematically shown in FIGS. 7A and 7B. 
     The shape and dimensions of the multi-bridge backing ring  2   b  and the inner bridges  2   c  are selected such that the jutting sections or free lengths FL 1 , FL 2 , FL 3 , FL 4 , . . . of all the probes will be equalized. In this way, the probes  4  are all caused to abut on the contact pads  7  with the same force. 
     A limitation comes to this prior technique from that the minimum width Lmin of a device to be tested cannot be less than the sum of the minimum length FLmin of the jutting section or minimum free length FL 1 , FL 2 , . . . of the probes  4  and the minimum theoretical width Pmin of each inner bridge  2   c , i.e.: 
     
       
         Lmin≧FLmin+Pmin,  (1) 
       
     
     as schematically shown in FIG.  7 A. 
     A second prior technique uses probes of different types bound to the same backing ring  2 , as schematically shown in FIGS. 8A and 8B. 
     In particular, probes  4  of a larger diameter are used for the innermost contact pads  7  within the backing ring perimeter, to have equal forces exerted on the contact pads  7  even though the jutting sections or free lengths FL 1 , FL 2 , . . . may be different. 
     Using this technique, however, the dimensions and free lengths of the probes  4  are difficult to calibrate for an even pressure on all the contact pads  7 . In addition, the probes  4  which are to reach devices located farther inwards than the probe emergence points on the backing ring  2  will be those having the largest dimensions, as having the greatest jutting sections or free lengths  8 , thus enforcing reduced density for the contact pads  7  on the devices. 
     Also, neither of the above prior techniques would work where a large number of devices are to be tested in parallel. In particular, the testing heads so provided cannot test more than two rows of devices, and are definitely incapable of testing a matrix array of devices. 
     SUMMARY OF THE INVENTION 
     Embodiments of this invention provide testing heads for microstructures, having a configuration which can facilitate the operations of sorting out probes and soldering them on a PC board, specifically densely clustered probes, and allowing an unlimited number of devices laid into plural rows to be tested. 
     One of the principles on which embodiments of the present invention stand is one of suitably shaping the resin holder attached to the probe backing ring in order to provide a clearer view of each probe in a cluster of probes during the soldering steps, while also maintaining an accurate control of the probe jutting sections or free lengths when a plurality of devices are under parallel test. 
     Presented is a testing head having cantilever probes and comprising a backing ring and a resin holder attached to the backing ring, as well as a plurality of contact probes held by the resin holder and formed with respective contact tips arranged to mechanically and electrically contact a plurality of contact pads of at least one device to be tested, the holder being formed with at least one suitably shaped outline to allow different probe rows to emerge in a cantilever manner. Additionally presented is a method of creating an electro/mechanical connection between a testing head and a test device. 
     The features and advantages of a testing head according to the invention will become apparent from the following description of embodiments thereof, given by way of non-limiting examples with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a testing head having cantilever probes, according to an embodiment of the prior art; 
     FIG. 2 is a sectional view of the testing head having cantilever probes according to the embodiment of FIG. 1; 
     FIG. 3 is a sectional view of a detail of the testing head having cantilever probes according to the embodiment of FIG. 1; 
     FIG. 4 is a sectional view of a detail of a testing head having cantilever probes according to a further embodiment of the prior art; 
     FIGS. 5A,  5 B and  5 C are schematic sectional views of a detail of the testing head having cantilever probes according to the embodiment of FIG. 4; 
     FIGS. 6A,  6 B and  6 C are schematic views of a testing head having cantilever probes according to a further embodiment of the prior art; 
     FIGS. 7A and 7B are schematic views of a testing head having cantilever probes according to a further embodiment of the prior art; 
     FIGS. 8A and 8B are schematic views of a testing head having cantilever probes according to a further embodiment of the prior art; 
     FIG. 9 is a schematic sectional view of a testing head having cantilever probes according to an embodiment of the invention; 
     FIG. 10 is a schematic sectional view of a testing head having cantilever probes according to the embodiment of FIG. 9; and 
     FIG. 11 is a schematic sectional view of the testing head having cantilever probes according to a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 9, a testing head  10  according to an embodiment of the invention, being employed to test at least one device, is shown. The testing head  10  comprises at least one backing ring  12  having a resin holder  13  suitable to hold a plurality of probes  14 . 
     The probes  14  are arranged in plural rows L 1 , L 2 , . . . , Ln in order to increase their density, it being possible for the probes to be laid in a pattern of either diagonal or straight or combination lines, depending on individual requirements. 
     Advantageously according to a preferred embodiment of the invention, the holder  13  has a suitably shaped outline  13   b  in correspondence of outer sections  19  of the probes  14 , i.e. in correspondence of a region of a PC board (not shown) where the probes  14  are soldered on the board. 
     In particular, the outline  13   b  shows a plurality of steps helping the detection of specific probes within the bunch of probes that emerge from the resin holder  13 , by virtue of the probe selection having been restricted to no more than one or a small number of probe rows. 
     In a preferred embodiment of the invention, the outline  13   b  includes a plurality of steps, all with the same width and height. In particular, the outline  13   b  has as many steps as many the rows of probes  14  are. 
     According to an alternative embodiment the outline  13   b  is provided with a number of steps which is a submultiple of the number of rows, with each step defining points of emergence of more than one row of probes  14 . 
     Assuming the probes  14  to be arranged in eight rows L 1 -L 8 , the outline  13   b  of the resin holder  13 , as shown in FIG. 11, has four steps, with each step defining the points of emergence of two rows of probes  14 . This is given by way of non-limiting example of a testing head according to an embodiment of the invention. 
     The number of rows per step may be varied, even within the same testing head, to meet specific requirements. It should be noted that one probe row might include probes placed at different heights above the plane containing the plurality of devices  11  to be tested. 
     In this way, soldering an unlimited number of probes, in different rows, can be made easier by providing the outline  13   b  with an appropriate number of steps. 
     Advantageously according to embodiments of the invention, the resin holder  13  has a larger size than the backing ring  12 , e.g. by adding final filling resin  20  to blend these elements to each other, as schematically shown in FIG.  10 . 
     Shown in FIG. 11 is testing head  10  according to a further embodiment of the invention. The testing head  10 , for application to a plurality of devices  11  to be tested, comprises at least one backing ring  12  having a resin holder  13  for holding a plurality of probes  14 . 
     In particular, the probes  14  have an end portion or contact tip  16  that is bent through a suitable angle from the probes to contact a plurality of contact pads  17  on said plurality of devices  11  to be tested. 
     Furthermore, the probes  14  are arranged in several rows L 1 , L 2 , . . . , Ln for increasing their density, the probe layout in front view being a pattern of diagonal or straight or combination lines, depending on individual requirements. 
     Good contacting of the contact pads  17  of the device to be tested by the probes  14  of the testing head  10  is conventionally ensured by the pressure of the testing head  10  on the device, the probes  14  vertically flexing in the opposite direction from the device direction of advance toward the testing head  10 . 
     Advantageously according to embodiments of the invention, the holder  13  has a suitably shaped outline  13   a  in correspondence of the jutting sections or free lengths  18  of the probes  14 , i.e. in correspondence of the devices to be tested. 
     In particular, the outline  13   a  shows a plurality of steps having shape and dimensions suitably selected to ensure an even pressure of the probes  14  on the contact pads  17  as the devices to be tested are brought to contact the testing head  10 . 
     In a preferred embodiment of the testing head  10 , schematically shown in FIG. 11, the holder  13  has a saw-tooth outline  13   a , establishing a common level of emergence for said probes  14 . In other words, the points  15  of emergence of the probes  14  from the resin holder  13  lie on a parallel plane to the plane containing the plurality of devices  11  to be tested, but are shifted in position from one another in order to reach contact pads  17  on different devices. 
     Advantageously according to a preferred embodiment of the invention, the probes  14  have all jutting sections or free lengths  18  that jut equally out of the emergence points  15 . Thus, a plurality of devices  11  laid in several parallel rows can be tested by using equally dimensioned probes formed out of the same materials, at the same time as the proper pressure can be maintained on the contact pads  17  of devices lying in different rows, as schematically shown in FIG.  11 . 
     It can be seen in FIG. 11 that the probes in touch with pads located in adjacent device rows spaced a predetermined distance apart, known as the “scribing line”, belong to different rows but to the same step. In this case, a variation in the jutting sections or free lengths  18  must be provided. However, this variation is far less than in conventional testing heads, and can be readily compensated by altering the probe dimensions, e.g. varying the shape of the probe tip. 
     In addition, the testing heads  10  according to embodiments of the invention obviate the shortcomings of conventional testing heads having cantilever probes, and allow a matrix array of devices laid serially in plural rows to be tested. 
     In particular, the density of those probes  14  which are to test devices placed farther inwards of the backing ring  12  is the same as the density of the probes  14  used for testing devices placed next to the backing ring  12 , thus providing for an equal density of contact pads  17  on the devices to be tested, regardless of their positions in a matrix array. 
     To summarize, the testing heads  10  according to embodiments of the invention allow probes  14  of equal size and formed out of the same material to be used for testing a matrix array of devices having an unlimited number of rows and columns. 
     Changes can be made to the invention in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all methods and devices that are in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined by the following claims.