Abstract:
Probes suitable for use with densely packed fine-pitch 2-D contact arrays are provided by use of an electrically insulating guide plate in connection with vertical probes, where the vertical probes have probe flexures that are either vertically folded sections, or coils having a horizontal axis. Preferably, the probes are configured such that the probe flexures are inside the guide plate holes, and the parts of the probes extending past the guide plate are relatively rigid. This configuration alleviates problems associate with probe shorting, because the probe flexures are enclosed by the guide plate holes, and are therefore unable to come into contact with flexures from other probes during probing.

Description:
FIELD OF THE INVENTION 
     This invention relates to probe arrays for electronic part testing. 
     BACKGROUND 
     Arrays of probes are often employed for electronic part testing, especially in connection with integrated circuit (IC) chips. Progress in IC chip technology tends to entail an increased number of contacts per IC chip and/or a decrease in the spacing between chip contacts. This spacing is known as the pitch of the contacts. A relatively recent development in IC technology is the use of chip contacts that are arranged in a fine-pitch densely packed 2-D array.  FIG. 1  shows an example of such a contact array  104  on a chip  102 . Previously, chip contacts have usually been arranged as linear arrays of contacts (i.e., 1-D arrays that are not densely packed in two dimensions). 
     This change from 1-D to 2-D arrays of chip contacts has significant implications for probe design, and approaches that are suitable for a 1-D contact array are generally unsuitable for use with 2-D arrays. An example of the difficulties that can arise in such cases is shown on  FIG. 2 . In this example, a vertical probe  204  has a flexure section  206  and passes through a guide plate  202  to control the position of the probe tip (at bottom of figure). Extending the probe concept of  FIG. 2  to a 1-D array of contacts has been done. For a 1-D probe array, it is possible to arrange the probes such that the flexure section  206  of each probe has room to move during operation without shorting to another probe. Such 1-D probe arrays can have fine pitch (i.e., 100 μm or less) between adjacent probes without shorting. 
     However, it is typically not possible to arrange probes as shown on  FIG. 2  in a fine pitch, densely-packed 2-D probe array without encountering insuperable probe shorting issues. This problem is further exacerbated by deformation of probes during normal operation of the probe array (i.e., the contact force between probes and chip is usually sufficient to elastically deform the probes). Thus, even if a non-shorting arrangement of probes is possible when the probes are not deformed, it is likely that probe deformation during use will lead to shorts. 
     Accordingly, it would be an advance in the art to provide probe arrays suitable for probing densely packed, fine-pitch 2-D contact arrays. 
     SUMMARY 
     Probes suitable for use with densely packed fine-pitch 2-D contact arrays are provided by use of an electrically insulating guide plate in connection with vertical probes, where the vertical probes have probe flexures that are either vertically folded sections, or coils having a horizontal axis. These kinds of probe flexures can be fabricated at the required probe dimensions (e.g., consistent with a pitch of 100 μm or less). Preferably, the probes are configured such that the probe flexures are inside the guide plate holes, and the parts of the probes extending past the guide plate are relatively rigid. This configuration alleviates the above-identified problem of probe shorting, because the probe flexures are enclosed by the guide plate holes, and are therefore unable to come into contact with flexures from other probes during probing. 
    
    
     
       BRIEF DESCRIPTION OF THE. DRAWINGS 
         FIG. 1  shows a dense packed array of probe contact points on an electronic part. 
         FIG. 2  shows a prior art probe concept. 
         FIGS. 3   a - c  show top views of probe guide plates suitable for use with embodiments of the invention. 
         FIGS. 4   a - b  show side views of embodiments of the invention having vertically folded probes. 
         FIGS. 5   a - c  show an embodiment of the invention having probes disposed pair wise in guide plate through holes. 
         FIGS. 6   a - b  show an embodiment of the invention having horizontally coiled probes. 
         FIGS. 7   a - b  show a probe having a reduce width skate on its tip. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 3   a - c  show top views of guide plates suitable for use with embodiments of the invention. In these examples, a guide plate  302  may have a hexagonal (or triangular) close packed 2-D array of through holes  308 , a guide plate  304  may have a square close packed 2-D array of through holes  310 , and a guide plate  306  may have a rectangular close packed 2-D array of through holes  312 . Optionally, a heat sink  320  may be coupled to the guide plate, e.g., as shown on  FIG. 3   a , to provide heat removal capability in the guide plate. Such heat removal can be important in cases where probe current carrying capacity (CCC) is thermally limited, which is often the case in practice. Such a heat sink may be mechanically coupled to the guide plate, or can be any other way of removing heat from the guide plate (e.g., forced air/gas flow). 
     The examples of  FIGS. 3   a - c  are representative, and the invention can be practiced with any arrangement of through holes in the guide plate. However, the hole pattern of the guide plate does need to match the contact pattern for the IC chip to be tested. For example, a guide plate as in  FIG. 3   b  or  3   c  would be unsuitable for use in connection with a chip having the contact pattern of  FIG. 1 . Instead, the guide plate of  FIG. 3   a  would be used. As indicated above, an important aspect of these contact patterns is their fine pitch. For one probe array design, the IC chip had a hexagonal close packed pattern of contacts on a 45 μm pitch, and the probes and guide plate were scaled accordingly. Thus, the guide plate controls the probe alignment. 
       FIGS. 4   a - b  show side views of an embodiment of the invention. In this example, probes  404 ,  406 , and  408  are shown disposed in through holes of guide plate  402 . The probes of this example have probe flexures that are vertically folded sections of the probes. Preferably, as shown on  FIGS. 4   a - b , the probe flexures are inside the through holes of the guide plate, which prevents probe shorting due to motion of the probe flexures. Guide plate  402  is an electrical insulator to avoid shorting probes. Suitable materials for the guide plate include but are not limited to: silicon nitride, aluminosilicate, and polyimide. Suitable materials for the probes include but are not limited to: NiCo, Cu alloys, composite metals (e.g., NiCo body with a Rh tip), and Tungsten. 
     Depending on the details of probe mechanical design, it may be useful to include more than one vertically folded section in the probe flexure. The example of  FIG. 4   a  shows a single folded section, while the example of  FIG. 4   b  shows two vertically folded sections in the probe flexure. Any number of vertically folded sections can be employed. Adding vertically folded sections to a probe design tends to scale the pitch and probe over-travel together, provided other details of probe design, such as material and lateral dimensions are held constant. Here, the probe over-travel is the vertical difference between probe base position when first contact is made with a chip, and probe base position in the chip testing configuration. In other words, the over-travel is the vertical distance compression experienced by the probe during normal probing of a device under test. 
     With vertically folded probes as shown in  FIGS. 4   a - b , the current path along the length of the probe is increased due to the vertically folded sections. This may be undesirable in situations where resistive heating of the probes significantly affects probe current carrying capacity. 
       FIGS. 5   a - b  shows an embodiment of the invention that addresses this potential issue. In this example, vertically folded probes are disposed in through holes of guide plate  402 . In contrast to the examples of  FIGS. 4   a - b , where each through hole had a single probe in it, here each through hole has two probes in it. In other words, the probes are disposed pair wise in the guide plate through holes, as opposed to individually. Preferably, all probes have the same shape (to expedite probe array fabrication), and the probes that share each through hole are rotated with respect to each other by 180 degrees about a probe vertical axis. Probes sharing the same through hole will make electrical contact with each other, so they should probe either the same contact on the device under test (DUT), or separate contact pads that are electrically connected on the DUT (e.g., as can occur for a power supply or ground contact). 
       FIGS. 5   b  and  5   c  show this relative rotation.  FIG. 5   b  shows a side view of probe  502  along line A of  FIG. 5   a , and  FIG. 5   c  shows a side view of probe  504  along line B of  FIG. 5   a . As shown on  FIGS. 5   b  and  5   c , the two probes sharing a through hole (i.e., probes  502  and  504 ) have the same shape, but one of the two probes is rotated 180 degrees about its vertical axis relative to the other probe. This provides a more direct current path from the probe base to the probe tip, because current can flow from probe  502  to probe  504  (and vice versa) as opposed to being limited to following the folded vertical sections of the probes. This probe configuration may also advantageously reduce the chance of probes jamming in the guide plate through holes as a result of normal probe deflection. More specifically, probe jamming can be reduced when the edges of the crosslink in the meandering probe shapes are not overlapping (i.e., the cross link of one probe falls on the straight section of the other probe). In the example of  FIGS. 5   a - c , straight sections  501  and  503  of probe  502  line up with cross links  505  and  507  of probe  504 . 
     Vertically folded probes as in the preceding examples have several significant design characteristics. This approach is scalable in pitch over a range that at least includes 30 μm pitch to 110 μm pitch. Probes in separate guide plate holes are permanently electrically isolated from each other, which removes the potential for probe shorting at ultra low probe pitches. For one design, probe length was 75 μm and probe over-travel was 1 μm. As indicated above, probe length and over-travel can be scaled together by incorporating multiple vertically folded sections into the probes. 
       FIGS. 6   a - b  show an alternative probe configuration where the probe flexure is a coil having a horizontal axis. Here probes  602 ,  604 , and  606  are disposed in through holes of a guide plate  402 . As indicated above, it is preferred that the probe flexures be within the guide plate through holes, and that parts of the probe extending beyond the guide plate be relative rigid compared to the probe flexures. Optionally, the probe coils can be filled with a material  610  to improve electrical and/or thermal conduction relating to the probe. For example, electrically conductive silicone rubber can be employed as material  610 . It is advantageous to electrically connect the coil loops for a single probe to each other to reduce probe resistance, and a flexible material such as silicone rubber will not interfere significantly with the probe mechanical design. It is further advantageous for material  610  to improve heat transfer from the probe to the guide plate, thereby providing improved heat removal from the probes, which can increase the probe current carrying capacity. 
     Probes with horizontal coils as in the example of  FIGS. 6   a - b  have several significant design characteristics. This approach is simple and provides low profile probes. Probes in separate guide plate holes are permanently electrically isolated from each other, which removes the potential for probe shorting at ultra low probe pitches. For one design, probe over-travel was &gt;50 μm. Greater over-travel is possible using longer horizontal coil probes. 
     In some cases, it is preferred for the probe tips to include a reduced-width skate, e.g., as shown on  FIGS. 7   a - b . In this example, a probe  702  includes a skate  704  at its tip.  FIG. 7   b  shows a close up side view along line A of  FIG. 7   a . Here the reduced width of skate  704  is apparent. Such probe skates can reduce contact resistance and provide more reliable contact between the probes and the DUT contacts. Skates can be employed in connection with vertically folded probes (as in the example of  FIGS. 7   a - b ), or in connection with probes having a horizontal coil. 
     Solder re-flow can be employed to reduce probe contact resistance. In this situation, the probe distal ends are solder reflowed. Here probe distal ends refers to ends of the probe that are away from (i.e., distal to) the contact point between probe and DUT. Such reflow can reduce resistance between the probes and the probe carrier the probes are connected to. This solder reflow approach can be employed in connection with vertically folded probes or with probes having horizontal coils. 
     Probes as described above can be made with conventional probe fabrication approaches, including but not limited to: electroforming, micro-electrical-mechanical systems (MEMS) fabrication technology, multilayer plating, and etching of metal foil (e.g. with deep reactive ion etching (DRIE)). 
     Vertically folded probes tend to be especially suitable for applications having very tight pitch requirements (e.g. &lt;60 μm), while probes having horizontal coils tend to be more suitable for applications that can have a relatively large pitch (e.g. pitch between 80 μm and 150 μm), but which also require significant probe over-travel (e.g. over-travel &gt;50 μm).