Abstract:
The semiconductor module includes a heat spreader and at least two semiconductors coupled thereto. Each of the semiconductors comprises a die containing integrated circuitry and electrical connectors coupled to the die. The module also includes a flexible circuit having opposing first and second sides. The first side of the flexible circuit coupled to the heat spreader, while the second side is coupled to the electrical connectors. The module also includes a termination resistor electrically coupled to the integrated circuitry of at least one of the semiconductors.

Description:
[0001]     This application claims priority to and is a continuation of U.S. Ser. No. 11/398,458 filed on Feb. 7, 2002 entitled “Semiconductor Module With Serial Bus Connection To Multiple Dies”, which is a reissue of U.S. Pat. No. 6,833,984, which is a continuation in part of U.S. Ser. No. 09/564,064 filed on May 3, 2000 entitled “Semiconductor Module with Imbedded Heat Spreader”, all of which are incorporated by reference herein in their entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to semiconductor modules and in particular to a semiconductor module that allows for more efficient interconnection between the semiconductor module and a computing device&#39;s transmission channel.  
       BACKGROUND OF THE INVENTION  
       [0003]     The semiconductor industry is constantly producing smaller and more complex semiconductors, sometimes called integrated circuits or chips. This trend has brought about the need for smaller chip packages with smaller footprints, higher lead counts, and better electrical and thermal performance, while at the same time meeting accepted reliability standards.  
         [0004]     In recent years a number of microelectronic packages have been produced to meet the need for smaller chip packaging. One such package is referred to as a chip scale package (CSP). CSPs are so called because the total package size is similar or not much larger than the size of the chip itself. Typically, the CSP size is between 1 and 1.2 times the perimeter size of the chip, or 1.5 times the area of the die. One example of a CSP is a product developed by TESSERA® called “MICRO BGA” or μBGA. In a CSP, the semiconductor has a set of bond pads distributed across its surface. A first surface of an insulating, flexible film is positioned over the semiconductor surface. Interconnect circuitry is positioned within the film. Electrical connections are made between the interconnect circuitry and the semiconductor bond pads. Solder balls are subsequently attached to a second surface of the film in such a manner as to establish selective connections with the interconnect circuitry. The solder balls may then be attached to a printed circuit board.  
         [0005]     CSPs may be used in connection with memory chips. Memory chips may be grouped to form in-line memory modules. In-line memory modules are surface mounted memory chips positioned on a circuit board.  
         [0006]     As memory demands increase, so does the need for increased memory capacity of in-line memory modules. A need has also arisen for materials and methods that lead to increased performance by more closely matching the coefficient of thermal expansion of the materials used in these memory modules. Examples of such in-line memory modules are single in line memory modules or SIMMs and dual in-line memory modules or DIMMs. DIMMs have begun to replace SIMMs as the compact circuit boards of preference and essentially comprise a SIMM wherein memory chips are surface mounted to opposite sides of the circuit board with connectors on each side.  
         [0007]     A problem with in-line memory modules is that adding more chips to the circuit board spreads out the placement of the chips on the circuit card and therefore requires reconfiguration of the circuit card connectors and their associated connections on the motherboard, which means replacing the memory card and in some cases the motherboard.  
         [0008]     Another problem with current in-line memory modules is that a separate heat spreader must be positioned across a set of memory chips. The heat spreader adds cost to the assembly process and adds significant weight to the module.  
         [0009]     Existing Multi-Chip Modules (MCM&#39;s) typically connect the transmission channel to semiconductors via electrical contact points or ball-outs on the MCM. Each electrical contact point then connects to a semiconductor in the MCM via an electrical lead, so that a signal may be transmitted along the transmission channel to each semiconductor via that semiconductor&#39;s electrical lead. However, each successive electrical lead slightly degrades the signal, by placing a load on the signal. By the time the signal reaches the last semiconductor connected to a transmission channel, the signal may have degraded so as to be unusable.  
         [0010]     Modern MCM&#39;s, such as those disclosed in the U.S. patent application Ser. No. 09/564,064, disclose MCMs that include relatively long electrical leads. The longer the electrical lead, the more the signal degradation. This is because the speed of the signal is inversely related to the length of the electrical lead. Therefore, existing MCMs can only handle a maximum of approximately thirty two semiconductors connected to a single transmission channel before the signal has degraded to an unusable form.  
         [0011]     In view of the foregoing it would be highly desirable to provide a semiconductor module that overcomes the shortcomings of the abovementioned prior art devices.  
       SUMMARY OF THE INVENTION  
       [0012]     A semiconductor module is provided which includes a heat spreader, at least two semiconductors thermally coupled to the heat spreader, and a plurality of electrically conductive leads electrically connected to the semiconductors. At least one of the electrically conductive leads is common to both of the semiconductors. The semiconductor module also includes a termination resistor electrically coupled to at least one of the semiconductors.  
         [0013]     A method of making a semiconductor module is also taught, whereby a plurality of electrically conductive leads are provided. At least two semiconductors are electrically coupled to the plurality of electrically conductive leads, where at least one of the electrically conductive leads is common to both of the semiconductors. The semiconductors are then thermally coupled to a heat spreader. Subsequently, a termination resistor is electrically coupled to at least one of the semiconductors.  
         [0014]     The termination resistor coupled to the semiconductors substantially reduces any degradation of the signal caused by a load placed on the signal from electrical leads, as the signal is not being split as is the case with stubs in existing semiconductor modules. Furthermore, by incorporating the termination resistor into the semiconductor module, the need for a termination resistor on the printed circuit board is eliminated, thereby reducing the need for additional circuit board space, and deceasing circuit board layout complexity and cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0016]      FIG. 1  is a front view of a semiconductor module according to an embodiment of the invention;  
         [0017]      FIG. 2  is a side view of the semiconductor module shown in  FIG. 1 ;  
         [0018]      FIG. 3  is an underside view of the semiconductor module shown in  FIG. 1 ;  
         [0019]      FIG. 4  is a side view of a semiconductor module according to another embodiment of the invention;  
         [0020]      FIG. 5  is a side view of a semiconductor module according to yet another embodiment of the invention;  
         [0021]      FIG. 6  is a side view of a semiconductor module according to still another embodiment of the invention;  
         [0022]      FIG. 7  is a front view of a semiconductor module according to another embodiment of the invention;  
         [0023]      FIG. 8  is a front view of a semiconductor module according to yet another embodiment of the invention;  
         [0024]      FIG. 9  is a perspective view of multiple semiconductor modules installed on a printed circuit board;  
         [0025]      FIG. 10  is a side view of a semiconductor module according to another embodiment of the invention;  
         [0026]      FIG. 11  is a flow chart of a method of making a semiconductor module according to an embodiment of the invention;  
         [0027]      FIG. 12  is a side view of a semiconductor module according to yet another embodiment of the invention;  
         [0028]      FIG. 13  is a front view of a semiconductor module according to a further embodiment of the invention;  
         [0029]      FIG. 14  is a side view of the semiconductor module shown in  FIG. 13 ; and  
         [0030]      FIG. 15  is a flow chart of a method of making a semiconductor module according to another embodiment of the invention.  
     
    
       [0031]     Like reference numerals refer to corresponding parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]      FIG. 1  is a front view of a semiconductor module  100  according to an embodiment of the invention. A semiconductor  102  is electrically connected to a plurality of traces or electrically conductive leads  108  by any conventional method such as wire bonding or thermocompression bonding. The electrically conductive leads  108  may be incorporated into flexible circuitry or tape  104 , which preferably consists of copper traces within a thin dielectric substrate (such as polyimide, epoxy, etc.).  
         [0033]     As shown in  FIG. 1 , the flexible circuitry  104  may be bonded, with an epoxy or the like, directly onto the side of the heat spreader  106 . The heat spreader  106  is preferably made from a material with good heat dissipation properties, such as a metal.  
         [0034]     In a preferred embodiment, two semiconductors  102  are positioned on opposing sides of the heat spreader  106 . The leads  108  preferably run the length of each sides of the heat spreader  106 , culminating at electrical contact points  110  at the base of the heat spreader  106 . Electrical contact points  110  may for example comprise solder balls or bond pads. The semiconductors may further comprise of single dies or multiple stacked dies.  
         [0035]      FIG. 2  is a side view of the semiconductor module  200  shown in  FIG. 1 . This view shows the semiconductors  102  and the flexible circuit  104  attached to both sides of the heat spreader  106 . As can be seen, the flexible circuit  104  wraps around the side walls  202  and  204  and base  206  of the heat spreader  106 .  
         [0036]      FIG. 3  is an underside view of the semiconductor module shown in  FIG. 1 . This figure more clearly shows the array of electrical contact points  110 . Each lead  108  connects a semiconductor  102  to a distinct contact point  110 . However, certain of the contact points  112  are common to both semiconductors  102 . In this case, a single lead  108  connects both semiconductors  102  to a shared common contact point  112 . Common contact points  112  may include a common voltage supply node, a reference voltage node, or an electrical ground node. Shared contact points  112  reduce the overall number of leads  108  and contact points  110  needed and therefore reduces the footprint of the module. The contact points  110  may be implemented as solder bumps or balls, metal points, or any other electrical connection. An advantage of placing the contact points at the base of the heat spreader  106  is that the contact points  110 , being remote from the semiconductor  102 , do not experience major temperature variations and therefore have reduced thermal mismatch stress. Thermal mismatch stress is caused by the low thermal expansion of the semiconductor  102  relative to the typically much higher expansion of a printed circuit board.  
         [0037]      FIG. 4  is a side view of a semiconductor module  400  according to another embodiment of the invention. In this embodiment, semiconductors  402  on a flexible circuit  404 , are bonded directly to a heat spreader  406 . The bond may be by any means but is preferably made by gluing the semiconductors  402 , with an epoxy or the like, to the side of the heat spreader  406 . The glue is chosen to closely match the thermal expansion properties of the semiconductor  402 , heat spreader  406  and flexible circuit  404 . The glue should also have good thermal conduction properties. This embodiment, where the semiconductors  402  are bonded directly to the heat spreader,  406  is favored due to the direct conduction of heat from the semiconductors  402  to the heat spreader  406 .  
         [0038]      FIG. 5  is a side view of a semiconductor module  500  according to yet another embodiment of the invention. In this embodiment, the heat spreader  506  has a “u” shape defining a channel  508 . This embodiment provides the benefit of increasing the surface area of the heat spreader  506  exposed to the surrounding air, thus increasing the rate that heat generated by the semiconductors  502  is dissipated to the surrounding air. Either the heat spreader  506  may conform to the shape of the flexible circuit  504  and semiconductor  502 , or the flexible circuit  504  and semiconductor  502  may conform to the shape of the heat spreader  506 . Both of these configurations are shown in  FIG. 5 , at  510  and  512  respectively.  
         [0039]      FIG. 6  is a side view of a semiconductor module  600  according to still another embodiment of the invention. In this embodiment, the heat spreader  606  is in a “n” shape forming an interior channel  608 . This embodiment also provides the benefit of increasing the surface area of the heat spreader  606  exposed to the surrounding air, thus increasing the rate heat generated by the semiconductors  602  is dissipated to the surrounding air. In this embodiment, the heat dissipating external surfaces may further dissipate heat by being exposed to an external air circulation device (e.g. a fan).  
         [0040]     In the embodiments shown in FIGS.  1  to  5 , signal channels in an electronic device may enter and exit the semiconductor module at electrical contact points in one area or footprint at the base of the heat spreader, as shown at  110  of  FIG. 1 . In the embodiment shown in  FIG. 6 , however, signal channels in an electronic device enter the semiconductor module  600  at electrical contact points  610  and exit from electrical contact points  612 .  
         [0041]      FIG. 7  is a side view of a semiconductor module  700  according to another embodiment of the invention. In this embodiment, leads  708  fan out on the flexible circuitry  704 . That is, the leads  708  in the flexible circuitry  704  are closer together at the semiconductor  702  than at the array  710 , which is more spread out than that shown in  FIG. 1 . The fanned out leads  708  create a more dispersed array with contact points  710  spaced further from one another. This embodiment compensates for a constant size footprint should larger semiconductors  702  be incorporated into the module at a later stage.  
         [0042]      FIG. 8  is a side view of a semiconductor module  800  according to yet another embodiment of the invention. In this embodiment, two tape and semiconductor combinations  802  and  804  are placed on one heat spreader  806 . Thus, the apparatus of  FIG. 8  processes two or more separate signal channels with a single heat spreader  806 .  
         [0043]      FIG. 9  is a perspective view  900  of multiple semiconductor modules  908  installed on a printed circuit board (PCB). The semiconductor modules  908  may be placed directly onto channels  902  on a PCB  910  or other suitable substrate, such that each electrical contact point electrically connects with a channel  902 .  
         [0044]     The semiconductor modules  908  may be placed directly onto a PCB  910 , such as a motherboard, or alternatively onto an in-line memory module circuit card which in turn slots into another PCB, such as a motherboard. In this manner the footprint of an in-line memory module circuit card may remain constant even if additional semiconductor modules  908  are slotted onto the in-line memory module circuit card. As the footprint of the array is always constant, the in-line memory module circuit card does not have to be changed each time additional memory is required, thereby enhancing the upgradability of electronic devices. The invention provides a memory module with a small footprint. Adding further chips to the module does not effect the footprint.  
         [0045]     When in an aligned position, each electrical contact point electrically connects with a corresponding electrical contact on the substrate or PCB. Where the electrical contact points are solder bumps, the electrical connection between the semiconductor module and the PCB may be made by heating the solder bumps to cause reflow of the solder and allowing subsequent cooling, thereby fusing the semiconductor module  908  to the PCB  910 .  
         [0046]     Alternatively, or in addition, fastening mechanisms  904  and  906  may be provided for securely anchoring the semiconductor modules  908  onto the PCB  910 . Such fastening mechanisms  904  and  906  may include clamps, slots, or the like.  
         [0047]      FIG. 10  is a side view of a semiconductor module  1000  according to another embodiment of the invention. In this embodiment the semiconductor module  1000  connects to a pin grid array (PGA) socket or slot  1002 , which in turn connects to a PCB. This embodiment is especially useful when connecting a semiconductor module to PCB&#39;s with incompatible footprints. In this way, a semiconductor module  1000  with a footprint created by electrical contact points  110 , may be connected to a PCB with a different footprint, where electrical contacts  1004  on the PGA slot  1002  are arranged to correspond with the footprint on the PCB.  
         [0048]      FIG. 11  is a flow chart of a method  1100  of making a semiconductor module according to an embodiment of the invention. A plurality of electrically conductive leads are provided  1102 , preferably on a flexible circuit or tape. Two semiconductors are then electrically connected  1104  to the leads. The semiconductors are then thermally coupled  1106  to a heat spreader. This is preferably done by mounting  1108  the semiconductor directly to opposing walls of the heat spreader as shown in  FIGS. 4-6 . Alternately, the flexible tape may be used as the contact surface with the heat spreader as shown in  FIG. 2 . The leads may then be soldered  1110  to a PCB. The module may also be anchored  1112  to the PCB by means of a fastening mechanism as discussed above. Alternatively, the module may connect  1114  to a PGA as described in relation to  FIG. 10 . Anchoring  1112 , soldering  1110 , and connecting  1114  may occur simultaneously.  
         [0049]     In an alternative embodiment, a semiconductor package such as a CSP may have its solder balls attached to the flexible circuitry. The combination of the semiconductor package and the flexible circuitry is then bonded to the heat spreader. In this manner existing semiconductor packages may be used to manufacture the semiconductor module according to the invention.  
         [0050]     Another alternative embodiment may include shielding to protect the semiconductor from electromagnetic forces. In addition, adhesive may be placed between the tape and the base of the heat spreader to cushion the contact points and ensure contact between the contact points and the PCB.  
         [0051]     The semiconductor module of the invention eliminates the need for a separate heat spreader. The invention reduces overall cost and weight through shared common contact points or nodes. The common contact points also allow for a constant footprint to be maintained independent of the size or number of semiconductors used. Furthermore, the module is reliable as the semiconductors are not exposed to as high thermal stresses. The module also substantially improves heat dissipation by exposing greater surface areas to the surrounding air.  
       Multi-Chip Modules  
       [0052]     As explained above in the background section of this specification, many existing semiconductor modules position their embedded semiconductors relatively far from the circuit board to which they are attached. Each semiconductor in such semiconductor modules connects to a transmission channel via its own electrical lead. A signal passing along the transmission channel from lead to lead is degraded by a load placed on the signal by each successive lead. The longer the stub, the more the signal is degraded. Each successive lead further degrades the signal, until such time as the signal has been degraded so as to be useless. Most semiconductor modules also include a termination resistor at the end of each transmission channel on the printed circuit board. The present invention addresses the problem associated with signal degradation in semiconductor modules having relatively long electrical leads.  
         [0053]     Impedance matching of an electrical load to the impedance of a signal source and the characteristic impedance of a transmission channel is often necessary to reduce reflections by the load, back into the transmission channel. As the length of a non-terminated transmission line increases, reflections become more problematic. When high frequency signals are transmitted or passed through even very short transmission lines, such as printed circuit board (PCB) traces, a termination resistor may be inserted at the load to avoid reflections and degradations in performance.  
         [0054]     In the multi-chip modules of the present invention, termination resistors are preferably internal to the MCM&#39;s. The use of external termination resistors presents a number of drawbacks. The placement of a termination resistor outside an MCM results in an additional stub or short transmission line between the termination resistor and the integrated circuit device. External termination resistors also require significant circuit board space, and increase circuit board layout complexity and cost.  
         [0055]      FIG. 12  shows a side view of a semiconductor module  1200  according to yet another embodiment of the invention. A number of semiconductors  1204  are electrically coupled to a plurality of traces or electrically conductive leads  1202  (only one is shown) by any conventional method such as wire bonding or thermocompression bonding. The electrically conductive leads  1202  are preferably incorporated into a flexible circuit or tape  1210 , which preferably consists of copper traces within a thin dielectric substrate (such as polyimide, epoxy, etc.).  
         [0056]     The semiconductors  1204  on the flexible circuit  1210 , are preferably bonded directly to a heat spreader  1218 . Alternatively, as shown and described in relation to  FIG. 2 , the flexible circuit  1210  may be bonded directly to the heat spreader  1218 . The bond may be made by any means but is preferably made by gluing the semiconductors  1204  or flexible circuit  1210 , with an epoxy or the like, to the side of the heat spreader  1218 . The glue is chosen to closely match the thermal expansion properties of the semiconductor  1204 , heat spreader  1218 , and flexible circuit  1210 . The glue should also have good thermal conduction properties. This embodiment, where the semiconductors  1204  are bonded directly to the heat spreader  1218  is favored due to the direct conduction of heat from the semiconductors  1204  to the heat spreader.  
         [0057]     The heat spreader  1218  is preferably made from a material with good heat dissipation properties, such as a metal. In a preferred embodiment, the semiconductors  1204  are positioned on opposing sides of the heat spreader  1218 . The electrical leads  1202  connect the semiconductors  1204  to electrical contact points  1216  at the base of the semiconductor module  1200 . In use, electrical contact points  1216  may for example comprise solder balls or bond pads. The electrical contact points  1216  electrically couple the electrical leads  1202  to a transmission channel  1214  on a printed circuit board  1212 . Electrical signals are transmitted along the transmission channel  1214  to electrical contact points  1216 . The electrical signals are then passed from the electrical contact points  1216  through the electrical leads  1202  to each of the semiconductors  1204 .  
         [0058]     In this embodiment, the semiconductors  1204 , on opposing sides of the heat spreader  1218 , are connected to one another in series by the electrical lead  1202 . It should be noted that multiple (i.e., more than two) semiconductors  1204  may be connected together in series. The final semiconductor in the series, remote from the transmission channel, electrically couples to a termination resistor  1208 . The termination resistor  1208  is preferably thermally coupled to the heat spreader  1218  so that any heat built up in termination resistor  1208  can dissipate through the heat spreader.  
         [0059]     The termination resistor  1208  connected in series to the semiconductors  1204  substantially reduces any degradation of the signal caused by a load placed on the signal from the electrical leads  1210 , as the signal is not being split as is the case with stubs in existing semiconductor modules. A signal is transmitted from a signal source, along the transmission channel  1214 , along an electrical lead  1202 , to each semiconductor  1204  connected in series, and is terminated at the termination resistor  1208 . Furthermore, by incorporating the termination resistor  1208  into the semiconductor module  1200 , the need for a termination resistor on the printed circuit board  1214  is eliminated.  
         [0060]     This embodiment of the invention is particularly useful now that the memory capacity of individual semiconductors has increased to a point where only a few semiconductors are needed for many applications.  
         [0061]      FIG. 13  is a front view of the semiconductor module  1300  according to a further embodiment of the invention. This semiconductor module  1300  is identical to the semiconductor module  100  shown in  FIG. 1 , except for a termination resistor  1302  disposed on the heat spreader.  FIG. 14  is a side view of the same semiconductor module  1300  shown in  FIG. 13 . In this embodiment, the semiconductors  1304  are not connected in series, but rather each semiconductor connects to its own transmission channel. Likewise, each termination resistor  1302  connects to a single semiconductor. In use, a signal is transmitted along each transmission channel, to its respective semiconductor, after which it is terminated at a termination resistor  1402  to eliminate reflections.  
         [0062]     The resistance value of the termination resistor  1208  ( FIG. 2 ) or  1302  ( FIGS. 13 and 14 ) is selected such that its impedance substantially matches the impedance of the transmission channel and signal source to which it is connected. Furthermore, any form of termination may be used, such as parallel termination, Thevenin termination, series termination, AC termination, Schottky-diode termination or the like.  
         [0063]      FIG. 15  is a flow chart of a method  1500  of making a semiconductor module according to another embodiment of the invention. According to the method  1500  a plurality of electrically conductive leads are provided (step  1502 ). At least two semiconductors are electrically coupled (step  1504 ) to the plurality of electrically conductive leads, where at least one of the electrically conductive leads is common to both of the semiconductors. The semiconductors are then thermally coupled (step  1506 ) to a heat spreader. Subsequently, a termination resistor is electrically coupled (step  1508 ) to at least one of the semiconductors.  
         [0064]     The semiconductors may be electrically coupled in series, where the semiconductors are capable of being electrically coupled to a transmission channel. Moreover, an additional termination resistor may be electrically coupled to the semiconductor not already connected to the termination resistor, where each of the semiconductors is capable of being electrically coupled to a separate transmission channel.  
         [0065]     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the present invention as defined in the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and not limited to the foregoing description.