Abstract:
A pin-assignment method is provided for use on an IC package to arrange pin connections. The pin-assignment method can allow an improvement in the electro-static discharge (ESD) protection capability for the IC chip packed in the IC package. Specifically, the pin-assignment method organizes the no-connect pins of the IC package into groups and then assigns each of the two pins that bound each no-connect pin group to be connected to a power bus of the IC chip. This allows for an increased ESD protective capability for the no-connect pins. Moreover, the pin-assignment method can simplify the wiring complexity of the IC package.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of Ser. No. 09/512,196, filed Feb. 24, 2000, for “Pin Assignment Method for Integrated Circuit Packages to Increase the Electrostatic Discharge Protective Capability Thereof”, now abandoned, which is in turn a continuation of Ser. No. 09/045,327, filed Mar. 20, 1998, for “Pin Assignment Method for Integrated Circuit Packages to Increase the Electrostatic Discharge Protective Capability Thereof”, now U.S. Pat. No. 6,107,681. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to a pin-assignment method for integrated circuit (IC) packages, which can allow an increase in the electro-static discharge (ESD) protective capability for the IC chip packed in the IC package. Specifically, the pin-assignment method organizes the no-connect pins of the IC package into groups and then assigns each of the two pins that bound each no-connect pin group to be connected to a power line, whereby the IC chip can be increased in its ESD protective capability and simplified in its wiring complexity. 
     2. Description of Related Art 
     Electrostatic discharge (ESD) is a movement of static electricity from a nonconductive surface, which usually causes damage to the semiconductors and various other circuit components in IC chips. A person walking on a carpet, for instance, can carry an amount of electrostatic charge up to several thousands of volts under high relative humidity (RH) conditions and over 10,000 volts under low relative humidity conditions. If such a person touches an IC chip by hand, the electrostaticity on his/her body would instantaneously be discharged to the IC chip, thus causing damage to the IC chip. The ESD damage is particularly common and severe on CMOS (complementary metal-oxide semiconductor) IC devices. 
     To protect IC chips against ESD damage, various solutions have been proposed. One solution suggests the provision of an ESD protective circuit between the internal circuit of the IC chip and each of the bonding pads.  FIG. 1  shows a conventional pin-assignment method used on an IC package. As shown, the IC package includes an IC chip on which an internal circuit  20  and a plurality of bonding pads  11 ,  13 ,  15  are formed. Further, the IC package includes a plurality of pins  10 ,  12 ,  14 ,  16 ,  17 ,  18 ,  19  on the periphery thereof, of which the pin  10  is a power pin (i.e., V DD  or V SS  pin) which is internally connected via a bonding wire  100  to the bonding pad  11 ; the pin  12  is an I/O pin which is internally connected via a bonding wire  120  to the bonding pad  13 ; the pin  14  is an input pin which is internally connected via a bonding wire  140  to the bonding pad  15 ; and the other pins  16 ,  17 ,  18 ,  19  are not in use (not wired) and thus are referred to as “no-connect pins”. To prevent ESD current from flowing via the bonding pads  11 ,  13 ,  15  into the internal circuit, each of the bonding pads  11 ,  13 ,  15  is connected to a ESD protective circuit (not shown). 
     A trend in IC packaging is to provide a larger number of pins on a single package so as to achieve the purpose of a high packing density of pins on the IC package. Since the IC package is very small in size, the increased number of pins will cause the gap (i.e., the pitch) between two adjacent pins, as indicated by the reference numeral G in  FIG. 1  between the pins  14  and  19 , to be further reduced. The reduction of the pin gap, however, causes a new problem in ESD protection for the IC package. This problem is described in a paper entitled “New Failure Mechanism due to No-Connect Pin ESD Stressing” which is authored by Matsumoto et al. and published in 1994 EOS/ESD Symposium, pp. 90-95. This paper reveals the fact that, when a human body model (HBM) ESD pulse is repeatedly applied to a certain no-connect pin on the IC package, any of its two neighboring pins, if wired to the internal circuit, would become particularly vulnerable to ESD damage. This is because that the electrostatic charge will accumulate in the resin around the no-connect pin, thus resulting in a large potential difference between the no-connect pin and its neighboring pins, thus significantly reducing the ESD resistant capability of the neighboring pins. 
     Taking the IC package of  FIG. 1  as an example, assume that the input pin  14  is able to withstand a maximum of ESD stress of 3 kV (kilovolt), then when an ESD stress of 1.5 kV is applied to the no-connect pin  19 , the electrostatic charge therefrom will accumulate in the resin around the no-connect pin  19 , eventually resulting in a large potential difference between the no-connect pin  19  and its neighboring pins (i.e.,  14 ,  18 ). When this potential reaches a large enough level, it would cause a sudden ESD current to flow through the gap G to the neighboring pin  14 . Said ESD current will then flow from the pin  14  via the bonding wire  140  and the bonding pad  15  to the internal circuit, whereby an ESD damage could occur. In short, when an ESD stress of 3 kV is being applied to the pin  14 , the internal circuit of the IC chip wired to the pin  14  would not be damaged thereby; however, the application of an ESD stress of 1.5 kV to the no-connect pin  19  would cause ESD damage to the internal circuit wired to the neighboring pin  14 . 
     Early types of IC packages have only a small number of pins thereon, so the above-mentioned proximity problem that would cause ESD damage is unobvious. However, newer types of IC packages, such as QFP (quad flat packages), MQFP, TQFP, etc., usually come with more than one hundred pins that are packed in plastic or resin compounds. With such a large number of pins on a small-size IC package, the above-mentioned proximity problem becomes a serious consideration. One conventional solution to this problem is to increase the ESD protective capability of the input and I/O pins of the IC package to a higher level, for example from 2 kV to 4-5 kV. This scheme can protect the input and I/O pins of the IC package against ESD damage when any of its neighboring no-connect pins is subjected to an ESD stress of 2 kV. One drawback to this solution, however, is that the ESD protective circuitry needed to provide such an ESD protective capability will take up more area on the IC chip, thus increasing the chip size. 
     On an IC package, those pins that are electrically and functionally engaged, such as input pins, output pins, I/O pins, and power pins, are referred to as active pins. Each active pin is electrically wired to a bonding pad and an ESD protective circuit. Typically, the circuit connected to the power buses on the IC chip has the highest ESD protective capability since the power pins are connected to the power bus V DD  or V SS . In addition to its high ESD capacity, each power bus has a capacitance of from 1 nF (nanofarad) to 10 nF formed between the N-well and P-well, or between the N-well and the substrate of the IC chip, which can absorb a great amount of charges from ESD. The input pins, I/O pins and output pins are inferior to the power pins in ESD protective capability. 
       FIGS. 2A and 2B  are schematic diagrams used to depict two conventional pin-assignment methods used for pin assignment on IC packages. 
     Referring to  FIG. 2A , the IC package shown here includes an IC chip  58  having a plurality of bonding pads  42 ,  44 ,  46 ,  48  formed thereon. Further, the IC-package includes a plurality of pins  30 ,  32 ,  34 ,  36 ,  38 ,  40 , which are respectively assigned as a V SS  power pin, an input pin, a first no-connect pin, a V DD  power pin, a second no-connect pin, and an I/O pin. The V SS  power pin  30 , the input pin  32 , the V DD  power pin  36 , and the I/O pin  40  are wired respectively via a plurality of bonding wires  50 ,  52 ,  54 ,  56  to the bonding pads  42 ,  44 ,  46 ,  48 ; while the first and second no-connect pins  34 ,  38  are unwired. Via the bonding pads  42 ,  44 ,  46 ,  48 , these active pins (i.e.,  30 ,  32 ,  36 ,  40 ) are functionally connected to the internal circuit of the IC chip  58 . 
     Referring to  FIG. 2B , the IC package shown here includes an IC chip  88  having a plurality of bonding pads  72 ,  74 ,  76 ,  78  formed thereon. Further, the IC package includes a plurality of pins  60 ,  62 ,  64 ,  66 ,  68 ,  70 , which are respectively assigned as a V SS  power pin, an input pin, a first no-connect pin, a second no-connect pin, an I/O pin, and a V DD  power pin. The V SS  power pin  60 , the input pin  62 , the I/O pin  68 , and the V DD  power pin  70  are wired respectively via a plurality of bonding wires  80 ,  82 ,  84 ,  86  to the bonding pads  72 ,  74 ,  76 ,  78 ; while the first and second no-connect pins  64 ,  66  are unwired. Via the bonding pads  72 ,  74 ,  76 ,  78 , these active pins (i.e.,  60 ,  62 ,  68 ,  70 ) are functionally connected to the internal circuit of the IC chip  88 . 
     It can be seen from  FIGS. 2A and 2B  that, by the conventional pin-assignment methods, the no-connect pins are arranged arbitrarily; in the case of  FIG. 2A , for example, the no-connect pins are arranged next to the input pin, the V DD  power pin, and the I/O pin; while in the case of  FIG. 2B , the no-connect pins are arranged next to the input pin and the I/O pin. These pin-assignment methods take no consideration of ESD protections. Therefore, when these pin-assignment methods are utilized on IC packages with a high density of pins, the arrangement of the no-connect pins next to ESD-sensitive pins could cause the problem of ESD damage. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide a pin-assignment method for an IC package, which can increase the ESD protective capability for the IC chip encased in the IC package and also simplify the complexity of the ESD protective circuitry needed to provide the ESD protective capability. 
     In accordance with the foregoing and other objectives of the present invention, a pin-assignment method capable of increasing the ESD protective capability of an IC chip is provided. The pin-assignment method organize said no-connect pins into at least one group including either one no-connect pin or a number of consecutive no-connect pins, and then assigns each of the two pins that bound each no-connect pin group to be connected to a power bus. Fundamentally, the pins of an IC package are organized in such a manner that the no-connect pins are set apart into a plurality of groups, each group containing one no-connect pin or a number of consecutive no-connect pins, and each of the two pins that bound each no-connect pin group is assigned to a power line. This allows for an increased ESD protective capability for the no-connect pins. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram used to depict the proximity problem which arises in an IC package having a high density of pins with a small pitch; 
         FIG. 2A  is a schematic diagram used to depict a first conventional pin-assignment method for pin assignment on an IC package; 
         FIG. 2B  is a schematic diagram used to depict a second conventional pin-assignment method for pin assignment on an IC package; 
         FIG. 3  is a schematic diagram used to depict a first preferred embodiment of the pin-assignment method according to the invention for pin assignment on an IC package; 
         FIG. 4  is a schematic top view of an IC package used to depict the definition of a consecutive group of no-connect pins in accordance with the pin-assignment method of the invention; and 
         FIG. 5  is a schematic diagram used to depict a second preferred embodiment of the pin-assignment method according to the invention for pin assignment on an IC package. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with the invention, the pins of an IC package are organized in such a manner that the no-connect pins are set apart into a plurality of groups or at least one group, with each group containing one single no-connect pin or a number of consecutive no-connect pins, and then each of the two pins that bound both sides of each no-connect pin group is assigned to be a power pin, such as a power pin V DD , a power pin V SS , or a ground pin GND, for electrically connect to a power bus V DD , a power bus V SS , or a ground bus GND of the IC chip. This arrangement is based on the fact that the pins that are connected to a power bus can better withstand the condition of its neighboring no-connect pin being subjected to an ESD-stress of 5 kV. Therefore, in this case, the ESD protective circuitry for the power pins needs not be expanded while nonetheless retaining good ESD protective capability. 
       FIG. 3  is a schematic diagram depicting the pin-assignment method according to the invention for pin assignment on an IC package. The IC package includes an IC chip  134  having a plurality of bonding pads  116 ,  118 ,  122 ,  124  formed thereon. Further, the IC package includes a plurality of pins  102 ,  104 ,  106 ,  108 ,  112 ,  114  which are assigned respectively as an input pin, a V SS  power pin, a first no-connect pin, a second no-connect pin, a V DD  power pin, and an I/O pin. The input pin  102  is internally wired via a bonding wire  126  to the bonding pad  116 ; the V SS  power pin  104  is internally wired via a bonding wire  128  to the bonding pad  118 ; the V DD  power pin  112  is internally wired via a bonding wire  130  to the bonding pad  122 ; the I/O pin  114  is internally wired via a bonding wire  132  to the bonding pad  124 ; and the no-connect pins  106 ,  108  are unwired. The two no-connect pins  106 ,  108  are consecutive in order on the IC package and therefore are considered as one group. In accordance with the invention, the two pins that bound both sides of this no-connect pin group, i.e., the pin  104  and the pin  112 , are each assigned as a power line; for example, in the case of  FIG. 3 , the pin  104  is assigned as a V SS  power pin, while the pin  112  is assigned as a V DD  power pin. With this arrangement, the ESD stress applied to the two no-connect pins  106 ,  108  would substantially cause no ESD damage to other pins. The no-connect pins  106 ,  108  therefore need not be connected to additional ESD protective circuits. 
       FIG. 4  is a schematic top view of an IC package used to depict the definition of “no-connect pin group” in accordance with the pin-assignment method of the invention. The IC package  200  here is a quad flat package having four sides and four corners. A no-connect pin group is defined as a single no-connect pin or a number of consecutive no-connect pins. If two no-connect pins are arranged on the same corner but on different sides, for example the pin  216  and the pin  218 , on the angle  214 , the pin  224  and the pin  226  on the angle  222 , and the pin  230  and the pin  236  on the corner  228 , they are still considered as consecutive and thus belong to the same group. 
     In the case of  FIG. 4 , for example, assume the following no-connect pin groups are arranged: the pins  216 ,  218 ,  220 ; the pin  202 ; the pins  224 ,  226 ; the pins  204 ,  206 ,  208 ; and the pins  234 ,  236 ,  230 ,  232 . In accordance with the invention, each no-connect pin group should be bounded by two power pins. Therefore, the two pins  238 ,  240  which bound the consecutive group of no-connect pins  216 ,  218 ,  220  are each assigned as a power pin; the two pins  242 ,  244  which bound the single no-connect pin  202  are each assigned as a power pin; the two pins  246 ,  248  which bound the consecutive group of no-connect pins  224 ,  226  are each assigned as a power pin; the two pins  250 ,  252  which bound the consecutive group of no-connect pins  204 ,  206 ,  208  are each assigned as a power pin; and the two pins  254 ,  256  which bound the consecutive group of no-connect pins  234 ,  236 ,  230 ,  232  are each assigned as a power pin. The power pins can be electrically connected to the power bus V DD , the power bus V SS , or the ground bus GND of the IC chip. 
     Further to the benefit of an increased ESD protective capability, the pin-assignment method of the invention can allow for easy bonding of wires onto those bonding pads that are connected to power pins. This benefit is described in the following with reference to FIG.  5 . 
     Referring to  FIG. 5 , the IC package shown here includes an IC chip  334  having a plurality of bonding pads  316 ,  318 ,  324  formed thereon. Further, the IC package includes a plurality of pins  302 ,  304 ,  306 ,  308 ,  312 ,  314  which are respectively assigned as an input pin, a first power pin, a first no-connect pin, a second no-connect pin, a second power pin, and an I/O pin. The input pin  102  is internally connected via a bonding wire  326  to the bonding pad  316 ; and the I/O pin  314  is internally connected via a bonding wire  332  to the bonding pad  324 . The pin-assignment method here differs from that shown in  FIG. 3  only in that the two power pins (i.e.,  304 ,  312 ) that bound the consecutive group of no-connect pins  306 ,  308  is wired to a common bonding pad (i.e., respectively via the bonding wire  328  and the bonding wire  330  to the same bonding pad  318 . This arrangement also can prevent the ESD stress applied to the two no-connect pins  306 ,  308  from causing ESD damage to other pins. Therefore, the bonding pads on the IC chip can be arranged in such a manner that at least one bonding pad used for power connection is disposed near each no-connect pin group. This allows the same bonding pad to be wired to the two power pins that bound the no-connect pin group, thus saving the number of bonding wires that can reduce the manufacturing cost. 
     The invention is characterized in that the pins of an IC package are organized in such a manner that the no-connect pins are set apart into a plurality of groups or at least one group, with each group containing one single no-connect pin or a number of consecutive no-connect pins, and then each of the two pins that bound both sides of each no-connect pin group is assigned as a power pin. The invention not only can provide an increased ESD protective capability, but also allow for a reduced wiring complexity and thus the manufacturing cost. The pin-assignment method of the invention is useful on IC packages with plastics or resin compounds, such as QFP, MQFP, and TQFP. Besides, the pin-assignment method of the invention is particularly useful to provide an increased ESD protective capability when used on IC packages having more than 100 pins, of which at least five are no-connect pins, such as 100-pin, 128-pin, and 160-pin IC packages having at least five no-connect pins, or on IC packages with more than 200 pins, of which at least 10 are no-connect pins, such as 208-pin IC packages having at least 10 no-connect pins. 
     The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.