Electronic device with multiple bonding wires, method of fabrication and method of testing bonding wire integrity

Abstract: A device presenting split contact pad portions (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b), one for each multiple bonding wire (44a, 44b), and metal connecting strips, the end portion (37a, 37b, 38a, 38b, 39a, 39b, 40a, 40b) of which is also split into parallel strip segments, each connected to a respective contact pad, so as to form a plurality of parallel current paths between an externally connected node (22-25) and the contact pin (27-30). When current is supplied (59) along the connecting strip, therefore, this produces a voltage drop along each segment connected to a sound bonding wire, but not along the segments connected to broken or nonbonded wires; and by determining (50) the existence of a voltage difference between the contact pad portions, it is possible to detect the interruption of one or more of the bonding wires.

Description: The present invention relates to an electronic device with multiple bonding wires, a method of fabricating such a device and a method of testing device bonding wire integrity. As is known, electronic power devices frequently require the use of two or more bonding wires between the pin of the device package and the die in which the device is formed, when low resistivity is required and/or when the bonding wire is called upon to withstand strong current. In the case of multiple-wire connections, the problem arises of checking the integrity of all the wires at the final testing stage. To solve this problem, it has been proposed to split the contact pad to which one end of the bonding wires is bonded, as well as the component connected to the pad, and to connect each bonding wire to a respective portion of the split pad to test each wire separately by separately turning on each component connected to it. The above solution is shown schematically in Figures 1 and 2. Figure 1 shows, schematically, an electronic device 1 comprising an output stage component (here, a MOS transistor 2), one terminal of which is connected to a contact pad 3 in turn connected to a pin 4 by two wires 5. Figure 2 shows, schematically, the same device - here indicated at 10 - modified according to the above known solution. More specifically, component 2 is split into components 2a, 2b, the output terminals 6a, 6b of which are connected to respective pads 3a, 3b into which pad 3 is split; components 2a, 2b present separate control terminals 7a, 7b for operating them independently; and pin 4 is connected by one wire 5 to pad 3a, and by the other wire 5 to pad 3b. By turning components 2a, 2b on separately, the above solution therefore provides for determining correct connection of respective wire 5. Such a solution, however, inevitably complicates the drive stage (not shown) of the device, which must be capable of turning on each component 2a, 2b separately, and requires an additional control for enabling the test function and selecting the output stage to be activated. Moreover, splitting the output components and relative pads, the need for electrically isolating the split components, and the presence of additional components for performing the test functions, considerably increase the area of the electronic device. This drawback is further compounded when, instead of splitting a single output component, a bridge configuration is involved requiring splitting of all four bridge elements. US-A-5,101,154 discloses an open bond detector and a method of testing the integrity of the bonding wires of the device according to the preamble of claims 1 and 11. In this document, the detector is arranged between the respective bonding pad and the power supply conductor. It is an object of the present invention to provide an electronic device, a fabrication method, and a test method designed to overcome the drawbacks typically associated with known solutions, while at the same time ensuring reliable testing of the bonding wire integrity. According to the present invention, there are provided an electronic device with multiple bonding wires, as claimed in Claim 1; a method of fabricating such a device, as claimed in Claim 6; and a method of testing the integrity of the bonding wires of the device, as claimed in Claim 11. In practice, the present invention provides for only splitting the contact pads and part of the (metal) line connecting the externally connected terminal of the components to the split pads, so as to achieve a plurality of parallel current paths between the outside terminal of the device and the contact pin. When supplied with current, therefore, the connecting line produces a voltage drop along each of its split portions connected to a sound bonding wire, but not along the split portions connected to broken or nonbonded wires; so that any interruption of one or more of the bonding wires may be detected by measuring any voltage difference between the split pads. A number of preferred, non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a known electronic power device with a multiple bonding wire; Figure 2 shows a further known power device; Figure 3 shows a simplified circuit diagram of a device in accordance with the present invention; Figure 4 shows a detail of part of the layout of the Figure 3 device; Figure 5 shows a variation of a detail of the Figure 3 device; Figure 6 shows the electric diagram of the Figure 5 detail. In the Figure 3 embodiment, the device 15 according to the present invention comprises an output bridge 16, a drive section (not shown), and other electronic components (not shown). In the example shown, the bridge comprises four MOS transistors 17-20 connected to form four nodes 22-25, of which, typically, nodes 22, 23 are connected to the supply and to ground, and nodes 24, 25 form the output of the bridge; nodes 22-25 are all connected outside device 15 by respective pins 27-30; and a two-wire connection is used to ensure low resistivity and a high current carrying capacity, as indicated above. In the Figure 3 example, the output pads connected to nodes 22-25 are split to form four pairs of output pads 31a,31b, 32a,32b, 33a,33b, 34a,34b connected respectively to nodes 22, 23, 24, 25 by four connecting lines in the form of metal strips. More specifically, the connection between node 22 and output pads 31a, 31b is split and formed of metal line portions 37a, 37b; the connection between node 23 and output pads 32a, 32b is split and formed of metal line portions 38a, 38b; the connection between node 24 and output pads 33a, 33b presents a split metal line portion 39a, 39b (terminating at pads 33a, 33b) and a common metal line portion 39c (extending from node 24); and the connection between node 25 and output pads 34a, 34b presents a split portion 40a, 40b and a common portion 40c. Each pad is connected to the respective pin by a wire 44a, 44b. Device 15 also comprises four comparators 47-50, one for each pair of pads 31-34, and each presenting two inputs connected to the pads in a respective pair. The outputs of comparators 47-50 are connected externally of device 15 as shown in Figure 3 for comparator 50 only. For this purpose, the device presents a contact pad 53 connected to an own pin 54 by a bonding wire 55. Figure 3 also shows the connection to a test device 58 comprising a current source 59 connected to pin 30; and a fault detecting and signaling stage 60 connected to pin 54. Figure 4 shows a detail of Figure 3 illustrating the actual implementation of connecting lines 37-40 in the form of metal strips indicated using the same numbering system as in Figure 3. In Figure 4, also, transistors 17-20 are shown in the form of rectangles corresponding to the area in which they are integrated; and also shown are the connecting lines between transistors 17-20, formed of portions 38c, 40c for the connections to pads 33, 34, and portions 37c, 39c for the connections to pads 31, 32. In the Figure 3 and 4 example, the portions of the split metal connecting lines, the relative contact pads, and the respective bonding wires represent pairs of parallel paths. When current is forced along the paths, and if the bonding wires are sound and the paths balanced, the current is divided into half and produces substantially the same small voltage drop along both paths. In actual fact, the difference generally existing in the length of the bonding wires results in a voltage difference at the pads, so that a small difference in potential exists at the inputs of the comparator (50 in Figure 3) connected to the test connection. This difference, however, is generally below the sensitivity of the comparator, or the comparator may be designed with a trigger threshold higher than it, so that its output is zero (logic 0), and a sound connection is indicated by fault detecting and signaling stage 60. Conversely, if either one of bonding wires 44a, 44b is interrupted (e.g. wire 44a), current only flows, and produces a voltage drop, along one of the parallel paths (path 44b, 34b, 40b). In this case, pad 34b presents a different potential from pad 34a (equal to that at the junction between portions 40a, 40b, 40c); the comparator detects a voltage difference above its trigger threshold, and supplies an output voltage other than zero (logic 1 signal); and the output signal is detected by stage 60 which therefore signals a fault. The Figure 5 embodiment relates to a three-wire connection. Figure 5 shows only part of device 15, and a component 61 connected externally of device 15 by a metal connecting line 62, the end portion of which is divided into three parts 63a, 63b, 63c; three areas or pads 64a, 64b, 64c connect parts 63 of line 62 to a pin 65 by means of respective bonding wires 66a, 66b, 66c; and areas 64a, 64b, 64c are also connected to the inputs of a comparator 68 via a system of switches 69 (multiplexer) for selectively connecting areas 64a, 64b, 64c to comparator 68 and so permitting the comparator to measure, each time, the potential difference between areas 64a and 64b, areas 64a and 64c, and areas 64b and 64c. Figure 6 shows an electric diagram of the Figure 5 structure, in which switch system 69 and comparator 68 are not shown; bonding wires 64a, 64b, 64c are represented by resistors 70a, 70b, 70c; parts 63a, 63b, 63c are represented by resistors 71a, 71b, 71c of substantially the same resistance R a , R b , R c ; and the common portion of connecting line 62 and the current path through component 61 (and any other components) to ground is represented by resistor 73. Figure 6 also shows the current source 59 for generating current I which is divided into currents I a , I b , I c along the three parallel paths. If V a , V b , V c are the voltages with respect to ground of areas 64a, 64b, 64c, and if: ΔV a,b = V a -V b = I a *R a -I b *R b ΔV b,c = V b -V c = I b *R b -I c *R c ΔV a,c = V a -V c = I a *R a -I c *R c and all the connections are sound: I a = I b = I c so that: ΔV a,b = ΔV b,c = ΔV a,c = 0. Conversely, if wire 66a is interrupted (broken or improperly bonded), no current flows through resistors 70a, 71a, so that: ΔV b,c = 0; ΔV a,b = ΔV a,c < 0. In the case, for example, of a connecting line with a sheet resistance of R □ = 20 mΩ/□ , a length/width L/W ratio of 0.5, and whose terminal portion is divided into parts 63a, 63b, 63c so that they present a L/W ratio of 1.5, this gives R a = R b = R c = R □ x L/W = 30 mΩ . If current I equals 1 A, and only wire 66a is broken, I a = 0; and I b = I c = 0.5 A , so that ΔV a,b = ΔV a,c = -15 mV. . If two wires (e.g. 66a, 66b) are broken, I a = I b = 0 ; and I C = 1 A , so that ΔV a,c = ΔV b,c = -30 mV. On the basis of the voltage difference detected, it is therefore possible to determine if any and how many and which bonding wires are interrupted. Integrity of the bonding wires is typically checked during final testing of device 15, at the end of fabrication, by connecting device 58 and activating the relative procedure. For externally communicating the outcome of the comparison by the comparator/s, specific pads (and pins) may be provided, as shown in Figure 3, or one or more of the existing pads also used for other functions may be employed. In the latter case, provision must be made for activating the correct function by means of an appropriate input configuration (a special input specifying, instant by instant, the function to be activated) or by programming a device register (an obviously advantageous solution if the device already features memory registers for its functions, in which case, forming a specific register for memorizing and setting the desired function, or adding a dedicated bit in an existing register poses no problem). A command for enabling/disabling the test function may also be provided, as described above, by appropriately programming the inputs and/or by writing a code into one or more registers. The advantages of the device and test method described are as follows. In particular, they provide for clearly indicating any interruption of the bonding wires with no need for duplicating or multiplying the externally connected components or their control stages. Though the solution described involves an increase in occupied area for forming split or multiplied contact pads and accommodating the comparators, the increase in area due to the comparators may be reduced by providing one comparator, to the inputs of which all the pairs or groups of pads are connected selectively, in which case, however, a switching section is required. Alternatively, the increase in area due to the comparators may be eliminated by locating them outside the device and including them in test device 58. No such increase in area exists in the case of devices already featuring comparators for final testing. Clearly, changes may be made to the device and test method as described and illustrated herein without, however, departing from the scope of the present invention. In particular, the comparators (in addition to being internal or external) may be formed in any way, bearing in mind that the currents for biasing the input stages may be such as to require turnoff of some or all of the comparators to prevent excessive losses at the contact pads when the power stages (final components) are off. As stated above, provision may be made for one comparator for each pair of pads to be compared, or for a single comparator with a switching section. Moreover, an internal logic circuit may be provided for determining the output of the various comparators, and, in the event any one indicates a fault on any one of the bonding wires, for supplying a fault output signal. The current source may be external, as shown, or internal, and, for example, supply current through the final components.

Claims: An electronic device (15) including: a package having pins (27-30) for connection to the exterior; a die (15) having bonding pads (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b); a plurality of bonding wires (44) extending between said bonding pads and said pins; a component (17-20) in said die defining a circuit node (22-25) for connecting externally said component; an electrically conductive connecting line (37-40) on the die connecting said circuit node (22-25) to said bonding pads; at least a portion of said connecting line comprising a plurality of line portions (37a, 37b, 38a, 38b, 39a, 39b, 40a, 40b) each connecting said circuit node to a respective bonding pad (31-34), said line portions defining, together with said bonding wires (44), parallel current paths between said pins (27-30) and said circuit node (22-25); potential difference detecting means (47-50) for detecting interruption of said bonding wires, characterized in that said potential difference detecting means (47-50) have inputs connected to pairs of said bonding pads (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b) so as to determine the potential difference between said pairs of bonding pads and generate a fault output signal in presence of a substantially other than zero value of said potential difference, and in that it comprises current supply means (59) connected to said pins (27-30) and supplying substantially the same currents along said pairs of parallel current paths. A device as claimed in Claim 1, characterized in that said connecting line (37-40) comprises a metal strip presenting an end portion divided into a plurality of strip portions (37a, 37b, 38a, 38b, 39a, 39b, 40a, 40b) presenting substantially the same resistance. A device as claimed in Claim 2, characterized in that said strip portions (37a, 37b, 38a, 38b, 39a, 39b, 40a, 40b) are identical and extend parallel to one another. A device as claimed in any of Claims 1-3, characterized in that said detecting means comprise a comparator (47-50) having inputs connected to said pairs of bonding pads (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b). A device as claimed in any of Claims 1-4, characterized in that it comprises switching means (69) for selectively connecting different pairs of bonding pads (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b) to said potential difference detecting means (47-50). A method of fabricating an electronic device (15) as claimed in any one of the claims 1-5, comprising the steps of: forming a die (15) having bonding pads (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b); forming a component (17-20) in said die defining a circuit node (22-25) for connecting externally said component; forming a package having pins (27-30) for connection to the exterior; forming an electrically conductive connecting line (37-40) on the die connecting said circuit node (22-25) to said bonding pads; at least a portion of said connecting line comprising a plurality of line portions (37a, 37b, 38a, 38b, 39a, 39b, 40a, 40b) each connecting said circuit node to a respective bonding pad (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b), said line portions defining, together with said bonding wires (44), parallel current paths between said pins and said circuit node; bonding a plurality of bonding wires (44) extending between said bonding pads and said pins; forming potential difference detecting means (47-50) for detecting interruption of said bonding wires, and testing the integrity of said bonding wires (44); characterized in that said step of testing the integrity comprises the steps of: supplying equal currents to pairs of said parallel current paths; and determining the potential difference between the bonding pads of said pairs of said parallel current paths. A method as claimed in Claim 6, characterized in that said step of forming a plurality of line segments comprises the step of forming a metal strip presenting an end portion divided into a plurality of strip portions presenting a same resistance. A method as claimed in Claim 7, characterized in that said step of forming a metal strip comprises the step of forming identical strip portions extending parallel to each other. A method as claimed in any one of Claims 6-8, characterized in that said step of supplying current comprises the step of supplying current by means of an external current source connected to said external connecting pin. A method as claimed in any one of Claims 6-9, characterized in that said step of determining the potential difference comprises the steps of connecting pairs of said bonding pads to comparing means; determining the potential difference between said pairs of bonding pads; and generating a fault output signal in the presence of a substantially other than zero value of said potential difference. A method of testing the integrity of the bonding wires (44) of an electronic device (15) comprising a package having pins (27-30) for connection to the exterior; a die (15) having bonding pads (31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b); a plurality of bonding wires (44) extending between said bonding pads and said pins; a component (17-20) in said die defining a circuit node (22-25) for connecting externally said component; an electrically conductive connecting line (37-40) on the die connecting said circuit node to said bonding pads; at least a portion of said connecting line comprising a plurality of line portions (37a, 37b, 38a, 38b, 39a, 39b, 40a, 40b) each connecting said circuit node to a respective bonding pad, said line portions defining, together with said bonding wires (44), parallel current paths between said pins (27-30) and said circuit node (22-25); potential difference detecting means (47-50) for detecting interruption of said bonding wires, characterized by the steps of supplying equal currents to pairs of said parallel current paths; and determining the potential difference between the bonding pads of said pairs of said parallel current paths.

Inventors: CHRAPPAN SOLDAVINI FRANCESCO, SALINA ALBERTO, CHRAPPAN SOLDAVINI, FRANCESCO, SALINA, ALBERTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H01L21
IPC codes: H01L23, G01R31, H01L21, H01L27