Integral Protective Enclosure for Tab or Similar Delicate Assembly Mounted Locally on a Flexible Printed Circuit Board

1. Field of the Invention

The present invention relates to a provision of mechanical protection for a Tape Automated Bonding (TAB) device mounted on a flexible printed circuit board (PCB) during assembly and life. For the purpose of the following description and claims, a TAB device refers to a TAB or similarly delicate electrical device assembly.

2. Prior Art

Such protection has additional heat conducting properties to assist heat dissipation from the TAB device. A problem associated with a flexible electrical assembly including a TAB device concerns mechanical failure of delicate, electrical between the flexible circuit board and the device. These connections can be easily damaged or broken during post manufacturing processes in which the flexible characteristic of the flexible electrical assembly is not restrained. Similar breakages can also occur where the flexible characteristics of such an assembly are exercised as part of the normal installation and operation of a product.

Another problem associated with a flexible electrical assembly including a TAB device concerns the removal of heat

energy generated by the TAB device whilst retaining the flexible characteristic of the flexible electrical assembly.

EPA103067 describes a method by which a multiplicity of integrated circuit modules mounted on flexible circuit board can be cooled. The modules are orientated so that the top surface of each makes contact with a common planar surface which is part of a cooling plate. A coefficient of thermal conductivity between the cooling plate and each module is minimised by maximising associated areas of contact with thermally conductive lubricant. A pressure means, acting upon the flexible circuit board is directed to press the modules and the flexible circuit board toward the common cooling plate.

Whilst application to TAB technology is feasible, a problem with this arrangement is that no protection against mechanical failure is provided for TAB connections during manufacture of an electrical assembly. Furthermore, the flexible characteristic of the circuit board cannot be realised in such an electrical assembly.

Another approach, described by EPA, 116,396, provides a similar cooling system whereby a slab like heatsink material has raised pillars which protrude through holes in the flexible circuit board and make thermal contact with the flat undersides of integrated circuit modules mounted on the circuit board.

Good thermal contact to TAB devices cannot however be achieved in this manner. Furthermore, as described in the previous example, a common clamping format is suggested for maintaining good thermal contact between the heatsink and the modules by which means the flexible characteristics of the flexible electrical assembly are restricted.

Accordingly, the present invention provides, a flexible electrical assembly comprising a TAB device mounted on a one side of a flexible circuit board, an array of printed electrical connections which may extend on either side of the circuit board in order to connect terminals of the device to terminals of other devices mounted on the circuit board, an individual heatsink having locating means for mounting the heatsink on a different side of the flexible circuit board to that on which the device is mounted, characterised in that the device is rigidly enclosed between the heatsink and a cover utilising the locating means of the heatsink for attachment to a side of the flexible circuit board on which the device is attached, in order to provide mechanical protection of the device. A mechanically protective enclosure for a TAB device mounted on a flexible circuit board is therefore provided comprising two portions in which a first portion is a punched, formed metal heatsink having locating means, for mounting the heatsink on the flexible circuit board, comprising a plurality of tabs, and a second, protective cover portion which is a resilient moulding having locating means which are receptive to said tabs.

The advantage of this arrangement is that the TAB device is bonded directly to the heatsink, to optimise thermal conductivity, by a process, involving only two mechanical parts, which subsequently become part of a mechanically protective assembly enclosing the TAB device. Furthermore the simplicity of this assembly is attractive for automated production and therefore low manufacturing cost. The heatsink has an open structure, for ease of cleaning, with no traps for contaminants which can lead to insulation resistance failure via dendritic growth between adjacent electrical connections.

In a particularly preferred embodiment, the area of the flexible circuit board occupied by the heatsink is similar to the area of the flexible circuit board occupied by the protective cover so that a plurality conductive wires which connect the device to the array of connections printed on the flexible circuit board are mechanically protected. This has the advantage that the flexible electrical assembly is thereby protected against failure modes which result from shock and vibrational forces.

By another aspect of the invention, the protective cover incorporates a preloaded member which defines a thickness of a film of thermally conductive adhesive compound interposed between the device and the heatsink during a bonding process. This has the advantage that there is no requirement for additional fixture complexity during the manufacture of a TAB electrical assembly in which the TAB device is bonded to a heatsink.

A particular embodiment of the invention will now be described with the aid of the following diagrams in which:

FIG. 1 is a cross sectional view of a flexible electrical assembly comprising TAB device, protective cover, and heatsink, mounted on a flexible circuit board.

FIG. 2 is a perspective drawing which further illustrates the flexible electrical assembly shown in FIG. 1.

FIG. 3 is a perspective view illustrating the opposite surface of the flexible printed circuit board, the other surface of which is shown in FIG. 2.

As shown in FIG. 1, a TAB device (3) includes a silicon chip (4), an encapsulant seal (5) to protect electrical connections to the chip, and an array of TAB outer leaf electrical connections (2) from the chip. These outer leaf connections are delicate and can be damaged during manufacturing processes.

A heatsink (9), compatible with the TAB device, initially punched out of a metal sheet. A suitable metal for this purpose is aluminium. The heatsink is subsequently formed to provide a thermal contact area (11), shown in FIG. 2, for making thermal contact with an exposed surface of the TAB device via a thin layer of thermally conductive adhesive (10). Furthermore, the heatsink includes heat dissipating fins for removing heat from the TAB device, and a set of crimping leaves (1) for securing the heatsink to both one side of a flexible circuit board (8) and a protective plastic cover (7).

The protective plastic cover includes a moulded-in spring (6) to pressurise the adhesive between the heatsink and the chip surface before the adhesive cures. This optimises a thermal bond between the heatsink and the chip.

FIG. 2 illustrates the order in flanges or tabs 14 and 16 of which the heatsink, TAB device, flexible circuit board and protective cover are assembled. Receptive holes (12) in the protective cover align with the crimping leaves of the heatsink. The flexible circuit board contains locating holes (13) through which the crimping leaves pass, prior to passing through the aforementioned receptive holes. The assembled combination of the heatsink, the flexible circuit material, and the protective cover enclosing the device is thereby secured by splaying the protruding crimping leaves against the protective cover.

FIG. 3 shows a schematic of the other surface of the printed circuit board 8. The other surface is identical to the surface shown in FIG. 2. The other surface includes locating holes 13a and printed circuit lands or conductors connected to the printed circuit terminals. Appropriate insulating areas (not shown) separate the printed circuit terminals to prevent short circuit conditions.