Stacked printed circuit board memory module and method of augmenting memory therein

[0002] This application is a continuation of prior U.S. patent application Ser. No. 09/231,613, entitled “STACKED PRINTED CIRCUIT BOARD MEMORY MODULE” which was filed on Jan. 14, 1999, and issued as U.S. Pat. No. 6,324,071 on Nov. 27, 2001.

[0003] The invention relates to high density memory systems for high-speed computer and network systems, and more particularly to an improved high density memory module.

[0004] With the introduction of network servers and work stations that can utilize memory in the gigabyte range and can operate at speeds of 100 Mhz or higher, fast and high density memory modules are needed to reach these memory capacities and speeds. Present day computer systems typically include hundreds of discrete components mounted on printed circuit boards (PCBs) interconnected with wiring on the board. The PCBs may also include sockets and connectors for receiving additional components, component modules and multichip modules, and connectors to other PCBs.

[0005] Computer memory often consists of one or more memory modules which plug into connectors on main printed circuit boards in computers (motherboards). The PCB memory module connector sockets are interconnected by a common set of address, data and control lines. Generally, there are several memory module connectors and when the memory requirements increase, additional modules may be added onto the motherboards. However, as computer system speeds and memory requirements have continued to increase and more integrated devices are incorporated onto PCBs, traditional memory packaging schemes have become inadequate. A constant goal in designing integrated circuit (IC) modules is to pack more integrated circuitry into the same or less space. This may be accomplished by physically scaling down the electrical components, such as decreasing transistor size at the substrate level, thereby increasing transistor density on semiconductor chips. Another possibility has been to increase the number of integrated circuits on the PCB. With present PCB technology, ICs may be mounted on both surfaces (front and back) of the PCB using surface mount techniques. However, the PCBs generally cannot be increased in length or height due to space limitations imposed by available areas on motherboards and within computer housings thereby limiting the amount of PCB real estate available for additional memory ICs.

[0006] As the density requirements of modules increase, solutions are needed to meet these requirements. There are currently three solutions to meet the requirements. First, the individual PCB can be made larger to accommodate more memory chips, this includes folding the PCB in half using a flex circuit. However, increasing the number of chips on individual boards consequently increases the length of the traces between chips and other PCBs. The increase in the trace length has cause a deviation from standards which require certain lengths to be maintained in order to prevent skew among clock, address, and data signals. Other transmission line problems occur when these high speed signals are transmitted over traces that are too long. Such problems include reflections, cross-talk, and electromagnetic induction. Therefore, placement of memory ICs on PCBs is critical to design considerations when trying to increase memory capacity and density.

[0007] The second solution to increase memory density is to decrease semiconductor die size to fit more memory in the same semiconductor package. However, decreasing die size while increasing memory density leads to greater costs. The industry norm is a 64 Megabit die. There have been increases to a 128 Megabit and 256 Megabit die but with a corresponding increase in cost of approximately five to six times.

[0008] The third solution to increasing memory density is to stack semiconductor die in the same package. While this solution increases the memory density, heat dissipation becomes a problem. Each of the individual ICs become hot and the heat cannot be properly dissipated from the PCB. The increased heat causes the performance of the memory module to decrease and often fail. As a result, the memory modules cannot be run at full performance. Often clock speeds and data transfers have to be decreased to reduce heat generated by the modules. Moreover, heat generation problems limit the number of memory modules that can be populated on a PCB, degenerating performance. Therefore, the number of memory ICs that can be placed on any given PCB memory module is limited due to heat dissipation and other considerations.

[0009] In an implementation of the invention, a memory module is provided that can stand alone as a primary board for insertion into a motherboard of a computer. The primary board has capability to receive additional daughter printed circuit boards on either surface. These additional daughter printed circuit boards provide additional memory to the computer without taking up an additional memory module socket. Additional daughter boards may be inserted to the daughter boards already connected to the primary board, without taking up any additional slots on the motherboard. The connectors between the primary board and each additional daughter board provide the electronic coupling necessary for the motherboard to send and receive data and address information. These connectors are placed so as to shorten the overall trace length of the memory module. Open air channels at the upper end of each of the primary and daughter boards aid in heat dissipation thereby increasing overall performance of the module.

[0010] Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings and the claims.

[0011] FIG. 1A is a front view of a primary circuit board.

[0012] FIG. 1B is a rear view of a primary circuit board.

[0013] FIG. 1C is a front view of a daughter circuit board.

[0014] FIG. 1D is a rear view of a daughter circuit board.

[0015] FIG. 2 is a perspective view of the stacked printed circuit board memory module showing the primary board (center) and two daughter boards (front and back).

[0016] FIG. 3A is a side view of a primary circuit board and three daughter circuit boards before insertion.

[0017] FIG. 3B is a side view of two stacked printed circuit board memory modules having a primary board and two daughter boards, each module inserted into a motherboard.

[0018] FIG. 4A is a front view of a primary circuit board showing a female connector.

[0019] FIG. 4B is a rear view of a primary circuit board having no connector.

[0020] FIG. 4C is a front view of a daughter circuit board having a male connector.

[0021] FIG. 4D is a rear view of a daughter circuit board having no connector.

[0022] FIG. 4E is a side view of a stacked printed circuit board memory module including a primary board and a daughter board.

[0023] FIG. 5A is a bottom view of a stacked printed circuit board memory module including a primary circuit board and three daughter circuit boards.

[0024] FIG. 5B is a bottom view of a stacked printed circuit board memory module including a primary circuit board and a daughter circuit board.

[0025] Referring to FIG. 1A, there is shown a front view of a primary circuit board 10 with the first surface 1afacing forward. Electronic components 2aare mounted on the first surface 1a.Only four individual electronic components 2aare shown in the figure for the purpose of clarity. Additional components may be added in other implementations of the invention. In an implementation of the invention electronic components 2amay be Synchronous Dynamic Random Access Memory (SDRAM) ICs. Vias 6 are present on the surface 1aof the primary circuit board 10 in order to provide access to couple the electronic components 2aon the first surface of the primary board 1ato the other side of the board. Only one via 6 is shown in the figure. Via conductors 7 provide the electric coupling to components on the other side of the board. Only via conductor 7 is shown in the figure. Primary connectors 3aare also mounted on first surface 1a.In an implementation of the invention, female-type primary connectors may be mounted on the first surface 1a.Only a few conductive leads are shown for the purpose of clarity. Connectors 3aare adapted to receive connectors on a daughter circuit board. Conductive leads 4 define control connectors of a control connector which run along a connecting edge 8 of the primary board in order to electrically couple with a motherboard. Conductive leads carry the signals from the control and address lines of a control motherboard. Connecting edge 8 is designed to be received by a motherboard for mechanical support. Conductive paths 5aare mounted on the surface 1ain order to couple the electronic components 2awith the primary connectors 3aand with the conductive leads 4. Only a couple of conductive paths 5aare shown in the figure.

[0026] Referring now to FIG. 1B, there is shown a rear view of primary circuit board 10. Additional electronic components 2bare mounted on the surface 1b.Via 6 gives access to the front surface 1aand electronic components 2aof the primary board 10. Via conductor 7 couples electronic components 2a,2b.Primary connectors 3bare adapted to receive daughter connectors from a daughter circuit board. Conductive paths 5bcouple electronic components 2bwith primary connectors 3band conductive leads 4.

[0027] Referring now to FIG. 1C, there is shown a front view of a daughter circuit board 11. Electronic components 13aare mounted on the surface 12a.Via 16 allows electric coupling access to the other side of the daughter circuit board 11. Via conductor 17 allows electric coupling to, electronic components on the other side of the daughter board 11. Daughter connectors 14aare mounted on the surface 12a.In an implementation of the invention, daughter connectors 14amay be male-type connectors. The make-type connectors 14aare adapted to mechanically and electrically couple with primary connectors 3a.Connectors 14amay also mechanically and electrically couple with primary connectors 3b.Conductive paths 15aelectrically couple daughter connectors 14awith electronic components 13a.

[0028] Referring now to FIG. 1D, there is a rear view of daughter circuit board 11. Electronic components 13bare mounted on the surface 12b.Via 16 allows electrical access to the first surface 12aof the daughter circuit board 11. Via conductor 17 allows electric coupling between electronic components 13band electronic components 13a.Daughter connectors 14bare mounted to the surface 12b.In an implementation of the invention, daughter connectors 14bare female-type connectors. Daughter connectors 14bare adapted to receive additional daughter connectors on addition daughter circuit boards. Conductive paths 15belectrically couple electronic components 13band daughter connectors 14b.

[0029] Although male/female-type connectors have been shown in the figures, other types of connectors to interconnect the primary board 10 and daughter board 11 would be suitable. For example, Zero Insertion Force (ZIF) connectors would be suitable to interconnect the primary board 10 and the daughter board 11.

[0030] Referring now to FIG. 2, there is shown a perspective view of a primary circuit board 10, daughter circuit board 11, and daughter circuit board 20. Daughter circuit board 20 has corresponding elements as daughter board 11. In an implementation of the invention, the connectors 3ashown as female-type connectors on primary board 10 receive the daughter connectors 14aon daughter board 20. In an implementation, the daughter connectors 14aon daughter board 20 are male-type connectors so as to couple with female-type primary connectors 3a.In the same implementation, male-type connectors 14aon daughter board 11 may be coupled with female-type connectors 3bon primary board 10.

[0031] Referring now to FIG. 3A, there is shown a side view of primary board 10, daughter board 11, daughter board 38 and daughter board 30. In an implementation, the female-type connector 3aon primary board 10 may be mechanically and electrically coupled with male-type connector 14aon daughter board 38. The female primary connector 3bmay be mechanically and electrically coupled with male-type connector 14aon daughter board 11. Female-type connector 14bon daughter board 38 may be coupled with male-type connector 14aon daughter board 39.

[0032] Referring now to FIG. 3B, there is shown a side view of two stacked printed circuit board memory modules 33a,34amounted on a motherboard 30. Module 33ais mounted on motherboard 30 by motherboard connector 31. Module 34ais mounted on motherboard 30 by motherboard connector 32. Module 33aincludes a primary board 33ccoupled with two daughter boards 33band 33d.Module 34aincludes a primary board 34ccoupled with two daughter boards 34band 34d.Female-type connector 14bon daughter board 33bmay receive an additional male-type connector from an additional daughter circuit board. Female-type connector 14bon daughter board 34dmay receive an additional male-type connector from an additional daughter circuit board. Upper air channels 35 run along the board between electronic components 13a,13bbetween daughter circuit boards 33b,33d,34b,34d,as well as between electronic components 2a,2bon primary circuit board 33c,34cand electronic components 14a,14bon daughter boards 33b,33d,34b,34d.Upper air channels 35 allow for better heat dissipation from the memory modules 33a,34athereby increasing overall performance.

[0033] The interconnection of the primary connectors 3a,3band the daughter connectors 14a,14b,as well as the interconnection between daughter connectors 14a,14ballows the daughter circuit boards 33b,33d,34b,34d,and primary boards 33c,34cto be electrically and mechanically coupled in a substantially spaced and parallel relation. The orientation of the daughter boards 33b,33d,34b,34dand primary boards 33c,34cis such that all daughter connectors l4a,14band all primary connectors 31, 3bare aligned in a straight line that runs perpendicular to daughter boards 33b,33d,34b,34dand primary boards 33c,34c.

[0034] The linear interconnection between daughter connectors 14a,14band primary connectors 3a,3bat a lower end of the modules 33a,34a,close to motherboard connectors 31 and 32 allow a decrease in the conductive path (trace length) that address, control and data signals must travel from motherboard 30 to modules 33a,34a.This decrease in the trace length decreases skew among clock, control and data signals, as well as other transmission line problems such as reflections, cross-talk, and electromagnetic induction.

[0035] In an implementation of the invention, primary board 33cmay be connected to motherboard connector 31 and primary board 34cmay be connected to motherboard connector 32 as standalone memory modules. In other implementations, daughter circuit boards 33b,33d,34b,34dmay be stacked on primary boards 33c,34c.In further implementations, additional daughter circuit boards (not shown) may be stacked onto daughter boards 33b,34dthrough daughter connectors 14a,14b.

[0036] Referring now to FIG. 4A, there is shown a front view of an implementation of a primary circuit board 40. Electronic components 42aare mounted on surface 41a.Primary connectors 43 are mounted to the surface 41a.In an implementation of the invention, only surface 41aof the primary circuit board has primary connectors 43. Conductive leads 44arun along connecting edge 44bat an edge of primary circuit board 40. Conductive paths 440 electrically couple electronic components 42a,primary connectors 43 and conductive leads 44a.Vias 400 provide electrical access to the other side of primary circuit board 40. Via conductors 410 provide electrical coupling to electronic components on the other side of primary circuit board 40.

[0037] Referring now to FIG. 4B, there is shown a rear view of primary circuit board 40. Electronic components 42bare mounted to the surface 41b.Vias 400 provide electrical access to the front side of the primary circuit board 40. Via conductors provide electrical coupling between electronic components 42aand 42b.In an implementation of the invention, the rear surface 41bhas no primary conductors mounted on it.

[0038] Referring now to FIG. 4C, there is shown a front view of a daughter circuit board 45. Electronic components 47aare mounted on surface 46a.Vias 420 provide electrical access to the other side of the daughter circuit board. Via conductor 430 provides electrical coupling to electronic components on the other side of the daughter circuit board 45. Daughter connectors 48 are mounted on the surface 46a.In an implementation of the invention, daughter connectors 48 are mounted only on the front surface 46a.Conductive paths 450 electrically couple electronic components 47aand daughter connectors 48.

[0039] Referring now to FIG. 4D, there is shown a rear view of daughter circuit board 45. Electronic components 47bare mounted to the surface 46b.Via 420 allows electrical access to the front side of the daughter board 45. In an implementation of the invention, no daughter connectors are mounted to surface 46b.

[0040] Referring now to FIG. 4E, there is shown a sideview of a stacked printed circuit board memory module 49. In an implementation of the invention, a primary connector 43 on a primary board is electrically and mechanically coupled to a daughter connector 48 on a daughter circuit board 45. There are no other connectors on either the primary or daughter circuit board. Upper air channel 460 runs along the top of the stacked printed circuit board memory module 49 and provides airflow between electronic components 42aon the primary board 40 and the electronic components 47aon the daughter board 45. Improved airflow in this manner improves overall performance of the module 49.

[0041] Referring now to FIG. 5A, there is shown a bottom view of a stacked printed circuit board memory module 50 including a primary board 10 and three daughter boards 11, 38, 39. Shown are various daughter connectors 14a,14band primary connectors 3a,3b.The bottom view of the stacked printed circuit board memory module 50 shows that lower air channels 51 are created when the primary connectors 3a,3binterconnect with the daughter connectors 14a,14b.Lower air channels 51 allow air to flow to and from lower air channels 51 to upper air channels 35. This airflow allows for improved heat dissipation in the memory module thereby increasing overall performance.

[0042] Referring now to FIG. 5B, there is shown a bottom view of an implementation of the stacked printed circuit board memory module 55. A primary board 40 and a daughter circuit board 45 are connected by daughter connectors 48 and primary connectors 43. A lower air channel 52 is created when daughter connectors 48 and primary connectors 43 are interconnected. The lower air channel 52 allows for airflow to and from lower air channel 52 and upper air channel 460. This airflow allows for improved heat dissipation in the memory module thereby increasing overall performance.

[0043] Other implementations are within the scope of the following claims.