Printed circuit board assembly and a method for manufacturing the printed circuit board assembly

Printed circuit board assembly including an optical subassembly having a carrying face for carrying at least one optoelectronic component in a such way that light emitted from/directed to is transmitted through the optical subassembly; a printed circuit board having a supporting area for supporting the optical subassembly; wherein the printed circuit board supporting area includes a hole lodging at least one part of the optoelectronic component and wherein at least one part of the carrying face is fixed by flip-chip bonding to at least one part of the supporting area.

FIELD OF THE INVENTION

The instant invention relates to printed circuit board assembly and a method for manufacturing the printed circuit board assembly.

BACKGROUND OF THE INVENTION

Because of the ever increasing requirements in data rates in communication systems, due for example to the Internet, the limits of using electrical communications between printed circuit boards (PCB) are being reached. It has become difficult to guarantee good signal integrity when transferring information at high frequencies (e.g. 25 Gb/s or higher) through electrical lines between two electrical components such as a printed circuit board.

To respond to this bandwidth demand, high-speed systems now use optical waveguide light to transfer light-carried information.

Light enables to improve the transfer of information between two points since light is less sensitive to interference phenomenon. However, electronic infrastructures (such as telecom cabinet) still implement printed circuit boards which still use electricity-carried information. So, it is necessary to implement on the printed circuit board devices, designed for converting light to/from electricity and for directing light into/from the optical waveguide.

To this end, it has been proposed on the market devices such as optical transceiver and active optical cable which are capable to convert optical signal into electrical signal and vice versa. These devices comprise an active component also known as optical engine, whose function is to manage electrical/optical signal conversion.

An optical engine comprises a substrate, having electrical paths, which supports optoelectronic components configured for converting optical signal into electrical signal and vice versa.

In use these optical engine are mounted to a printed circuit board of the electronic system. For instance the printed circuit board can be a mother or daughter card of a backplane assembly or a printed circuit board of an Active Optical Cable.

It is known from WO2010/010395 an optical engine made of a transparent substrate (e.g. glass, glass ceramic, photoformable glass).

An object of the present invention is to propose an improved printed circuit board assembly in which the optical engine package is easy, effective and less costly.

SUMMARY OF THE INVENTION

To this aim, the printed circuit board assembly according to the invention comprises:an optical engine having a carrying face for carrying at least one optoelectronic component in a such way that light emitted from/directed to is transmitted through the optical engine; anda printed circuit board having a supporting face for supporting the optical engine.

The printed circuit board supporting face comprises a hole lodging at least one part of the optoelectronic component. At least one peripheral border of the carrying face is fixed by flip-chip bonding to at least one border of the supporting face, said border of the supporting face surrounding said hole.

With this feature, the optical subassembly is fixed to the printed circuit board.

Advantageously, the optoelectronic components are preserved from heat during the fixing of the subassembly to the printed circuit board.

Advantageously, a heat dissipator covers the optoelectronic components and protects them against pollution.

On the different Figures, the same reference signs designate like or similar elements.

DETAILED DESCRIPTION

Referring toFIG. 1, the printed circuit board assembly2according to the invention comprises an optical engine4for carrying at least one optoelectronic component6and a printed circuit board8also named mother board, configured for supporting the optical engine4.

The optical engine4comprises two staked layers10,12. These layers10,12are made of transparent material such as plastic, moulded glass or fused silica.

One principal face14of the lower layer12is provided with the optoelectronic components6as well as with electrical tracks16for interconnecting the optoelectronic components. This principal face14is named hereafter carrying face14.

The optoelectronic components6are electrically linked and mechanically fixed to the electrical tracks16by flip-chip bonding. In the present description, the face of the optoelectronic components6which is attached to the electrical tracks16and to the lower layer12is named assembling face18and the opposite face is named back face20.

The optoelectronic components6include among other light-emitting optoelectronic devices such as vertical-cavity surface emitting lasers (VCSEL) and light-receiving optoelectronic devices such as photo-diodes or photo-detectors. The optoelectronic component6may be electrically connected to an electronic control device22configured for driving them. The device22may also be mounted through flip-chip on the carrying face14of the engine4.

On the schematic representation ofFIG. 1, only one optoelectronic component6is visible. In reality, the optical engine4carries several optoelectronic components6arranged in rows and possibly in rows and columns.

Light beams coming from or going to the optoelectronic components6cross the optical engine4widthways.

At least one light-beam forming structure, typically a lens24can be formed for example by laser cutting in the optical engine4. This lens is, for example, adapted to collimate a light beam coming from an optoelectronic component6and going to, for example, a non represented optical device adapted to guide the light beam towards an optical fibre.

The printed circuit board8usually comprises a substrate26made of epoxy resin prepreg or glass fibre covered on one side or on both sides with a thin copper foil27coated with a solder mask. On the embodiment illustrated onFIG. 1, electronic components (not represented) are deposited on the top face30and on the bottom face32of the printed circuit board8. These electronic components are generally electrically linked to the optoelectronic components6and/or the control device22.

The printed circuit board8comprises a hole34adapted to lodge the optoelectronic components6. The optical engine4is fixed by flip-chip bonding to the printed circuit board8. In particular, one part of the carrying face14of the optical engine4is electrically and mechanically fixed by flip-chip bonding to one part of a supporting area31of the printed circuit board. According to the embodiment shown on this figure, the peripheral border36of the carrying face14is fixed by flip-chip bonding to the border38surrounding the hole34of the printed circuit board. The hole34has, for example, a rectangular or a square shape.

In variant, electrical tracks are deposited only on one face of the printed circuit board8. According to another variant, the printed circuit board is a multilayer circuit board.

In variant, only one side of the border36of the carrying face14is fixed by flip-chip bonding to one side of the border38of the hole.

According to the embodiment illustrated onFIG. 1, a heat dissipator40is fixed to the back face20of the optoelectronic component6. The attachment material41can be, for instance, tape, thermal grease, pressure sensitive adhesive, glue or epoxy. The heat dissipator40can be a heat sink or a heat spreader.

Advantageously, the heat dissipator40is adapted to cool the optoelectronic components6and the control device22during the flip-chip bonding of the border36of the engine4to the border38of the printed circuit board.

Advantageously, the heat dissipator40also cools the electronic components6and the control device22during the use of the printed circuit board assembly2.

Advantageously, the heat dissipator40covers the optoelectronic components6and protects them against pollution.

According to the embodiment illustrated onFIG. 1, the printed circuit board8comprises a cavity42for accommodating the heat dissipator40. This cavity42mouths in the hole34.

This cavity42has advantageously an area superior to the area of the hole34when considering that the area of the hole34and the area of the cavity is measured in planes parallel to the top face30plane. This embodiment allows the implementation of large heat dissipator40. In such case, the upper peripheral surface44of the heat dissipator40can furthermore be fixed to a shoulder46formed between the hole34and the cavity42.

In variant, the heat dissipator40is only fixed to the shoulder46and not to the back face20of the optoelectronic components6. In such case, the heat dissipator40is preferably in thermal contact with the optoelectronic component6to cool them.

In variant, the hole34is a through hole and the heat dissipator40is lodged in this though hole.

OnFIG. 1, the heat dissipator40is entirely housed in the cavity42such that the bottom face32of the printed circuit board is flat. In variant, only a part of the heat dissipator40is housed in the cavity42, the other part forming a protrusion with respect to the bottom face32of the printed circuit board.

FIG. 2schematically shows a second embodiment of the invention. Compared to the first embodiment, it mainly differs in that the printed circuit board8is a multilayer circuit board, i.e. it comprises a first printed circuit board48including a substrate layer26coated on both sides with a copper foil27, and a second printed board50including three copper foils27and a substrate layer26. A spot facing or shoulder52is formed in the hole34such that the top face54of the shoulder52is opposite to the carrying face14of the optical engine4. This top face54is the top face of a copper foil27. The border36of the optical engine4is fixed by flip-chip bonding to this shoulder52.

Advantageously, the depth between the shoulder52and the top face of the printed circuit board is chosen for being similar to the thickness of the optical engine4such that the printed circuit board assembly is flat. In reference toFIG. 3, the method of manufacturing a printed circuit board assembly according to the invention comprises the following steps:providing54an optical engine4having a carrying face14carrying at least one optoelectronic component6in a such way that the light emitted from/directed to the optoelectronic component6is transmitted through the optical assembly4;providing56a printed circuit board having a supporting area31configured for supporting the optical engine4;forming58a hole in the supporting face for lodging at least one part of the optoelectronic component6;fixing60by flip-chip bonding at least one peripheral border36of the carrying face14to at least one border38of the supporting area31, said border38surrounding said hole34;attaching62a heat dissipator40to a back face of the optoelectronic component6; the back face20being the face opposite to the assembling face18of the optoelectronic component6.

Advantageously, the step of fixing the peripheral border36of the carrying face14to the border38of the supporting area31is performed simultaneously to attach a heat dissipator40to the back face20of the optoelectronic component.

In variant, the method comprises a step of forming a shoulder52in the printed circuit board and the fixing step is a step of fixing one part of the carrying face14to one part of the shoulder52.

FIG. 4schematically shows a third embodiment of the invention. Compared to the second embodiment, it mainly differs in that the heat dissipator40comprises a protuberance53. This protuberance53has a flat face which is fixed to the electronic control device22. In particular, the electronic control device22has one face fixed to the optical engine4and an opposite face fixed to the heat dissipator. The protuberance's height is equal to the thickness difference between the optoelectronic component6and the electronic control device22.

In variant, the heat dissipator40presents a hole having a depth which is equal to the difference between the thickness of the optoelectronic component6and the thickness of the electronic control device; the optoelectronic component6being fixed to the back face of this hole.

FIG. 5schematically illustrates a fourth embodiment of the invention. This embodiment is similar to the second embodiment.

In this case, the thickness of the optoelectronic component6is inferior to the thickness of the electronic control device22so that the heat dissipator40is provided with a protuberance53positioned under the optoelectronic component6. The flat extremity face of the protuberance is fixed to the back face20of optoelectronic component6. The protuberance's height is equal to the thickness difference between the optoelectronic component6and the electronic control device22.

According to this embodiment, the optical engine4comprises only one layer10made of transparent material.

The optical engine4comprises a reflective arrangement64adapted to guide the light beam from/directed to the optoelectronic component6to/from a plane parallel to the top face30of the printed circuit board8. This reflective arrangement64is inclined at an angle of 45° with respect to the top face30of the printed circuit board. This reflective arrangement64can for example be made by laser cutting.

According to this embodiment, a lens24is not built up within the optical engine4. Instead, a lens66is fixed or formed on the carrying face14of the optical engine4. The lens66is configured for collimating the light beam output from the laser.

According to this embodiment, the printed circuit board assembly2also comprises a second heat dissipator68adapted to cool the optical engine4during the transmission and reception of light beams. This second heat dissipator68is fixed, on one hand, on a face70of the optical engine4opposite the carrying face14, and on the other hand, on the top face30of the printed circuit board. These fixations are achieved through, for example, tape, thermal grease, pressure sensitive adhesive, glue, epoxy, a clip or a screw.

The second heat dissipator68can be a heat spreader or a heat sink.

OnFIG. 5, the second heat dissipator6has a stair shape. However any other shape like an L shape would be suitable.

The fifth embodiment of the invention shown onFIG. 6is similar to the fourth one at the exception that the second heat dissipator68is provided with a protruding reflective arrangement72adapted to accomodate a corresponding cavity74of the optical engine4. The reflective arrangement72is inclined at an angle of 45° with respect to the top face30to guide the light beam from/directed to the optoelectronic component6to/from a plane parallel to the top face30of the printed circuit board8. The reflective arrangement72is for example made by surface treatment of an area of the second heat dissipator's protrusion.

The cavity74is formed in the optical engine4in front of the optoelectronic component6, for example by laser cutting.

The reflective arrangement72and the cavity74form also positioning elements, i.e. elements adapted to passively position the second heat dissipator68with respect to the optical engine4.

According to the embodiment illustrated onFIG. 6, the second heat dissipator68and the optical engine4comprise supplementary positioning elements76,78. For example, a pin or a bulge76can be built in the second heat dissipator68and a corresponding aperture78can be formed in the optical engine4.

Advantageously, these positioning elements allow a quick, simple and precise mounting of the second heat dissipator68to the optical engine4.