SPECIFICATIONElectronic component moduleThis invention relates to electronic component modules, and to methods of assembling together electronic components, in particular, but not exclusively, integrated circuits, to form operational modules with high component densities and offering scope for automated manufacture.
According to one aspect of the present invention there is provided an electronic component module including a stack of elements, each element including at least one printed circuit at a surface of which is mounted at least one electronic component, and a spacer frame disposed at said surface and surrounding the at least one electronic component, electrical connection tracks on the printed circuit extending to the exterior of the spacer frame, and wherein electrical connection means between adjacent printed circuits in the stack are disposed externally of the respective spacer frames.
According to another aspect of the present invention there is provided a method of assembling together a plurality of electronic components whereby to form an operational module, comprising the steps of manufacturing a plurality of printed circuits each comprising a respective portion of the circuit of the module, mounting at least one of the electronic components to each of the printed circuits at a respective surface thereof, disposing a respective spacer frame at the said surface of each printed circuit to surround at least one electrical component thereon with electrical connection tracks of the printed circuit extending to the exterior of the spacer frame, forming a stack of the printed circuits with the components mounted thereon and electrically interconnecting the tracks of the stacked printed circuits as required to complete the circuit of the module.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which:Figs. 1 to 3 illustrate three alternative concepts for continuous production of flexible printed circuits;Fig. 4 illustrates one version of a spacer frame;Fig. 5 illustrates a flexible printed circuit mounted to a spacer frame, there being a number of electronic components surface mounted to the printed circuit;Fig. 6 illustrates a flexible printed circuit mounted to a stiffening board, a number of electronic components being surface mounted to the printed circuit;Fig. 7 illustrates a double-sided assembly;Fig. 8 illustrates a double-sided assembly with a heat sink;Fig. 9 illustrates a module assembly;;Fig. 10 illustrates a spacer frame which is notched for edge connections, andFig. 11 illustrates interconnection in a module assembly via conductors in edge notches.
One embodiment of a high density component package or module proposed by the present invention employs flexible printed circuits, such as printed circuits produced in a continuous flexible film strip. The film strip may include a standard size of frames 1 containing the printed circuits (not shown) as illustrated in Fig. 1 and include holes 2 associated with each frame for registration purposes, as for example during circuit printing, or continuous sprocket holes 3 as illustrated in the left-hand half of Fig. 3. For optimum use of the printed circuit base material all of the frames on one strip have a standard dimension across the strip width. However, the length of the frames may vary, as illustrated in Fig. 2 with registration holes and in the right-hand half of Fig. 3 with sprocket holes.The circuit patterns provided in the frames 1 may also vary along the length of the strip, although for maximum simplicity there would be one standard circuit pattern per film strip run.
For maximum compactness of a final module assembly it is considered that the electronic components, one or more per printed circuit, should be surface mounted to the flexible printed circuit, although other mounting systems may alternatively be employed. In the case of surface mounted components the components may be mounted to the flexible circuit whilst the latter is still in the continuous strip form or after cropping out individual frames therefrom. With surface mounted components the flexibility/resilience of the printed circuit base material will serve to relieve soldered joint stresses. If it is required to mount conventional through-boarded leaded components then the flexible film circuit may be bonded to an appropriate support media.
The flexible printed circuit frame to which the components are mounted requires a support and this may be achieved by mounting the printed circuit frames 1 onto a carrier which is of the form of a picture frame 4 (Fig.
4), for example as employed for 35mm photo- graphic transparency mounts. The support frame 4 may be developed to provide various additional functions, its thickness providing the electronic component height clearance as illustrated in Fig. 5 where a printed circuit 5 carrying components 6 is mounted to a support frame 7 which serves also as a spacer.
Alternatively, or additionally the printed circuit 5 carrying components 6 may be bonded to a stiffening board 8 (Fig. 6). The stiffening board 8 may be a purely supportive member, comprised for example of a plastics laminate, or it may provide additional functions, for example it may be of a suitable thermal conductivity so as to provide temperature equalisation. A degree of cushioned support can be provided over the entire mounting area in addition to any mounting support frame or board. Interposition of an elastomeric layer between the flexible film and any rigid support is known to reduce solder joint stresses on surface mounted components occurring during temperature cycling. Two circuits 9,10 may be supported back-to-back between two spacer/support frames 11,12 as indicated inFig. 7 to form a double sided assembly.
Alternatively, and not illustrated, two circuits may be bonded to opposing faces of a stiffening board. By using an aluminium or similar high thermal conductivity material as the stiffening board a direct thermal path is provided from the components to the outside ambient.
Such an aluminium plate may be extended to provide a heat dissipating fin 13 (Fig. 8) (electronic components omitted) for air or fluid cooling or, alternatively, for clamping to a heat sink. A single sided assembly may also be provided with a fin similarity to fin 13 of the double sided assembly of Fig. 8. It is considered that temperature predictions for this form of solid, heat finned construction will be easier than for arrangements in which air-flow is over a surface upon which the components are mounted irregularly.
A complete module assembly of electronic components consists of a series of supported circuits and spacer frames assembled as a solid stack, an example of which is illustrated schematically in Fig. 9. The stack is held together by, for example, adhesives, clipping or by being contained within an appropriate framework.
Interconnection between the various circuits of a stack is achieved at the edges of the individual circuits, conductive tracks of the printed circuits being arranged to extend to the exterior of the spacer frames, one method of interconnection takes advantage of the flexible nature of the circuits, which for this purpose preferably have a thermoplastic base.
A notched edge frame 14 as illustrated in Fig.
10 may be employed or alternatively a notched edge supporting (stiffening) board (not shown). The notches may be in the form of half circular holes. When mounting the flexible circuit onto such a board or frame it is folded over the edge and down into the half holes. Where this part of the flexible circuit carries a copper track there is thus provided a half "plated through" hole. Interconnections in the stack are then completed by joining these half holes lengthwise along the stack (as indicated in Fig. 11) using, for example, copper leads 15 or flow or dip soldering or selective plating up.
For clarity the inter-circuit connections have been omitted from the module shown in Fig.
9, however that figure does illustrate two possibilities for external connection of the module in use. The stiffening boards or frames may be used to provide edge board contacts 16 for direct plug-in to a one part connector or a mounting 17 for any two part connector.
One of the end plates 18 of a module may also be used for connector mounting. One of the end plates 18 may be provided with pins (not shown) when, in particular with short stacks, they may be inserted and soldered into a conventional printed board. Additionally both end plates 18 may be provided with pins or connectors allowing plug/socket connection. If some of the spacers or stiffening boards are extended to form guide runners 19, insertion into a library shelf is allowed.
The extended boards or spacers 20 comprise cooling fins.
The module provided by the invention offers four sides and two ends with a high degree of flexibility in the way in which they may be used. One side may provide heat sinking, whereas one side or end may be used for mounting and the remaining faces may be used for interconnections. The interconnection system may be used as part of the stack holding-together structure. For stability it may be necessary to restrict the maximum length of the stack to, for example, not more than twice the largest frame dimension. The printed circuit film widths may be of the order of 2 to 4 inches (5 to 10 cm).
The module thus provides for high density component mounting with scope for automated manufacture, employing continuous film production of printed circuits, exploitation of the low component height of surface mounted components, and use of a stacked three dimensional construction with good heat dissipation and short interconnection path lengths. By adopting a multiple number of small circuits it is considered that there is created a compact final assembly offering short interconnection path lengths in a construction suited to automatic assembly up to the completed module level. Since this approach involves using a large number of small boards, it will entail an increased number of electrical joints as compared with a large planar board. However, the overall advantages of the module are considered to outweigh this disadvantage.
An example of achievable component densities will now be described. Surface mounted components require generally 3mm component height. Thus assuming 1.6mm thickness for a stiffening board a double-sided "frame" could be provided in a total thickness of say 8mm (3 + 1.6 + 3mm), which in round numbers gives say 3 double-sided "frames" per inch (2.54 cm) of stack. A reasonable component (integrated circuit) density is 9 components on an approximately 3" X 3" (7.5 X 7.5 cm) single frame, or 18 components on a double-sided "frame". Thus at three frames per inch, a 3 inch stack would carry 162 components in 27 cubic inches i.e.
6 components per cubic inch (16.4 cm3). For comparison, a current telecommunication system board (nominally 11 1/4 inches by 13 1/2 inches on 0.8 inch mounting centres (28.6 cm by 34.3 cm on 2 cm mounting centres)) carries around 80 to a maximum of 140 integrated circuits, giving around 1 component per cubic inch (16.4 cm3). For another telecommunications system board, nominally 8.7 inches by 10 inches as 1 inch mounting centres (22.1 cm by 25.4 cm on 2.56 cm mounting centres), carrying an average of 110 integrated circuits, the component density is 1.25 per cubic inch (16.4 cm3). Thus the module of the present invention provides a greatly increased possible component density. It is also considered that the layout problems of the module are simpler than those of a large planar board, since in place of a large, single, totally interconnected set of components on the one planar board, there are employed relatively small boards which can readily be fully autorouted in CAD, plus reasonably simple board to board interconnections which can also be autorouted.