Power Amplifier System

Description

This invention relates to a power amplifier system.

Power amplifier systems for portable communications devices, such as mobile telephones, wireless communicators, radio-equipped laptop or notebook computers etc., typically include a power amplifier module forming part of a transmit chain of a radio device. The power amplifier module is located between a modulator circuit and an antenna connection. The power amplifier module typically draws more electrical power than any other component of the radio device. The power amplifier module typically also generates more heat than any other component.

Typically, power amplifier modules are mounted on to a laminate carrier, such as a circuit board, with wire bond or flip chip inter-connections to tracks on the carrier. Surface mount device (SMD) components are provided on the carrier around the power amplifier module, and an overmould encapsulates the power amplifier module. It is known also to use a low temperature co-fired ceramic (LTCC) type carrier for a power amplifier module.

It is known to embed integrated circuits within a printed circuit board (PCB). This technology is used extensively by Imbera (www.imbera.fi), for instance. However, embedded integrated circuit technology does not allow for the integration of power amplifier modules into a circuit board, because power amplifier modules generate too much heat. WO 2005/104635 discloses the use of thermal vias to conduct heat away from an embedded component. However, such vias are insufficient satisfactorily to remove the heat generated by a power amplifier module.

The present invention was made in this context.

According to the invention, there is provided a power amplifier system comprising: a power amplifier module, the module comprising an integrated circuit and having first and second main faces and a peripheral edge; and

a hardware module or printed wire board, the hardware module or printed wire board having first and second main exterior faces which are generally parallel to one another; wherein the power amplifier module is integrated into the board between the two main faces, the power amplifier comprising one or more thermally conductive vias arranged in the hardware module or printed wire board to connect the power amplifier module to one of the main exterior faces of the hardware module or printed wire board, each of the one or more thermally conductive vias being located fully or at least partly within a volume defined by: a first main face of the power amplifier module, a first main face of the hardware module or printed wire board, and imaginary surfaces which extend parallel to the plane of the major faces of the hardware module or printed wire board and which coincide with the peripheral edge of the power amplifier module.

The arrangement of the vias allows the removal of sufficient heat to allow a power amplifier module to be embedded, whereas previously such was not feasible. The vias may be high volume vias, which are able to conduct heat for a substantial distance from the power amplifier module through the hardware module or printed wire board to the exterior thereof. Alternatively, if the distance between the power amplifier module and the exterior of the hardware module or printed wire board is not particularly great, relatively low volume vias can be used.

The power amplifier module may be connected by vias to connectors on both of the first and second main faces of the hardware module or printed wire board.

The power amplifier module may include first and second power amplifier stages which are electrically connected together via a component or circuit located on one of the first and second main faces of the hardware module or printed wire board. The component or circuit may for instance be a filter or an impedance matching circuit. Since connections between the power amplifier module and the component or circuit are formed largely within the printed wire board or hardware module, the connections can be low inductance connections.

The first and second power amplifier stages may include separate ground connections. This provides better grounding, and thus provides numerous advantages.

Even if the power amplifier module does not include power amplifier stages which are electrically connected together via a component or circuit located on one of the main faces of the hardware module or printed wire board, the power amplifier module may includes first and second power amplifier stages which include separate ground connections. This provides better grounding, and thus provides numerous advantages.

The power amplifier system may comprise a generally planar shielding component located between the second main face of the power module and the second main face of the hardware module or printed wire board extending substantially parallel to the second main face of the hardware module or printed wire board. This has numerous advantages compared to the use of an external shielding component. The system may comprise a module including an integrated circuit located between the generally planar shielding component and the second main face of the hardware module or printed wire board. In this way, interfacing between the power amplifier module and other integrated circuits can be significantly simpler than is possible with conventional systems.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a hybrid schematic cross section through an embodiment of a power amplifier system according to the present invention;

Figure 2 is a hybrid schematic cross section through a second embodiment of a power amplifier system according to the present invention;

Figure 3 is a circuit diagram illustrating the circuit provided by Figure 2 power amplifier system.

Referring to Figure 1, a power amplifier system 10 is shown comprising generally a power amplifier (PA) module 11 integrated within a printed wire board 12. The PA module 11 may merely be an integrated circuit (IC) comprising a number of components interconnected in such a way as to provide power amplification functionality, or it may be a module, using the term in the conventional sense, incorporating such an IC.

The printed wire board 12 has an upper exterior face 13 and a lower exterior face 14. The printed wire board 12 is made up of a number of layers. The PA module 11 is located in an aperture in a first layer 15. The first layer 15 is above and adjoins a second layer 16. First and second signal lines 17, 18 are located on the uppermost surface of the first layer 15. The first signal line 17 connects with an interconnect on the PA module 11 at one end and with a first microvia 19 at its other end. The first microvia 19 extends through the first and second layers 15, 16 to a third signal line 20, which is located on the lowermost surface of the second layer 16. The third signal line 20 is further connected, but these further connections are not shown in the Figure. The second signal line 18 is connected at one end to an interconnect on the PA module 11 and at the other end to a second microvia 21. The second microvia 21 extends through the first and second layers 15, 16 to a fourth signal line 22, which is located on the bottommost surface of the second layer 16. The fourth signal line 22 is connected at an end opposite the second microvia 21 to a third microvia 23. The third microvia 23 is formed in a third layer 24, which is immediately below the second layer 16. The third microvia is connected also to a fifth signal line 25, which is located on the bottommost surface of the third layer 24. The fifth signal line 25 is connected via a fourth microvia 26 which is formed in a fourth layer 27. The fourth layer 27 is immediately below the third layer 24. The lowermost surface of the fourth layer 27 constitutes the lower exterior face 14 of the printed wire board 12. The fourth microvia 26 connects the fifth signal line 25 to a first metallised part 28. The first metallised part 28 is electrically connected to a first solder ball 29, which forms part of a ball grid array 30 mounted on the lower exterior face 14 of the printed wire board 12. The result of these components is to connect an interconnect on the uppermost surface of the PA module 11 to the ball

grid array 30 on the lower exterior surface 14 of the printed wire board 12. This allows electrical signals to be carried in to and/or out of the PA module 11.

A further connection is made between a point on the uppermost surface of the PA module 11 and the ball grid array 30 on the lower exterior surface 14 of the printed wire board 12 as follows. A fifth microvia 35 is connected directly to an interconnect on the uppermost surface of the PA module 11. The fifth microvia 35 is formed in a fourth layer 36 of the printed wire board 12. The fourth layer 36 is immediately above the first layer 15. Also connected to the fifth microvia 35 is a sixth signal line 37, which is connected at its other end to a sixth microvia 38, which is also formed in the fourth layer 36. The sixth microvia 38 is connected to a seventh signal line 39 which is interposed between the first and fourth layers 15, 36. The sixth signal line 39 also is connected to a seventh microvia 40, which extends through the first and second layers 15, 16 to a seventh signal line 41 interposed at the boundary between the second and third layers 16, 24. The seventh signal line 41 is electrically connected to an eighth microvia 42, which extends through the third layer 24 to an eighth signal line 43, interposed at the boundary between the third and fourth layers 24, 27. A ninth microvia 44 connects the eighth signal line 43 to a second metallisation part 45, which is formed on the lower exterior surface 14 of the printed wire board 12. A second solder ball 46 is connected to the second metallisation part 46, thereby to connect the ball grid array 30 to another interconnect on the uppermost surface of the PA module 11.

The first to ninth microvias 19, 21, 23, 26, 35, 38, 40, 42 and 44 have the primary function of carrying electrical signals between the PA module 11 and the ball grid array 30 on the lower exterior surface 14 of the printed wire board 12.

The power amplifier system 10 also includes some vias having a primary function of conducting heat from the PA module 11 to the ball grid array 30. Two such thermal vias 50, 51 are shown in the Figure. Each of these vias 50, 51 is connected directly to the lowermost surface of the PA module 11 at one end, extends through the second, third and fourth layers 16, 24 and 27 and meets with a third metallised part 52 on the lower exterior face 14 of the printed wire board 12. As can be seen from

the Figure, the thermal vias 50, 51 are considerably larger than the microvias. In particular, the thermal vias 50, 51 have a diameter around two or three times the diameter of the microvias. As with the microvias, the thermal vias 50, 51 are generally frustoconical in shape. The thermal vias 50, 51 are constituted of a material having a high thermal conductivity, for instance copper. . Each of the thermal vias 50, 51 has a longitudinal axis which is perpendicular to the plane of the lower exterior face 14 of the printed wire board 12 and to the plane of the PA module 11. Thus, the thermal vias 50, 51 are located wholly within a volume defined by the lower exterior face 14 of the printed wire board 12, the lower face of the PA module 11 and imaginary surfaces which extend parallel to the plane of the exterior faces 13, 14 of the printed wire board 12 and coincide with peripheral edges of the PA module 11.

The thermal vias 50, 51 preferably contact the PA module 11 at locations very close to the locations on the PA module 11 where the largest amount of heat is generated. Since the thermal vias 50, 51 are high volume vias which connect via the shortest path between the PA module 11 and the ball grid array 30, the thermal vias 50, 51 are able to conduct a significant quantity of heat away from the PA module 11. The effect of this is maximised if the thermal vias 50, 51 connect to the PA module 11 at the places where the most amount of heat is generated.

As shown, the thermal vias 50, 51 extend through several layers 16, 24, 27. Tolerances for thermal vias are not as critical as tolerances for signal vias, including the microvias.

The third metallisation part 52 is connected to a number of solder balls some of which are shown in the drawing as 53, 54 and 55. As well as conducting heat away from the PA module 11 , these solder balls 53, 54, 55 provide a connection to electrical ground.

Since the PA module 11 is integrated within the first layer 15, the distance between the lower surface of the PA module 11 and the ball grid array 30 is greater than the

distance between the uppermost surface of the PA module 11 and the upper exterior surface 13 of the printed wire board 12.

Electrically connected to an interconnect on the uppermost surface of the PA 11 is a tenth microvia 60, which is formed in the fifth layer 36. A ninth signal line 61 connects the tenth microvia 60 to eleventh and twelfth microvias 62, 63 which are formed in a sixth layer 64. The ninth signal line 61 is interposed at the junction between the fifth and sixth layers 36, 64. An uppermost surface of the sixth layer 64 constitutes the upper exterior face 13 of the printed wire board 12. On the upper exterior face 13 of the printed wire board 12, a fourth metallisation part 65 is connected to both of the eleventh and twelfth microvias 62, 63. The fourth metallisation part 65 constitutes a ground conductor. Thus, the effect of the tenth, eleventh and twelfth microvias 60, 62 and 63, the ninth signal line 61 and the fourth metallisation part 65 is to connect electrically an interconnect on the upper surface of the PA module 11 to ground potential. These components have the additional effect of providing a path for the conduction of heat from the PA module 11 to the fourth metallisation part 65. Since the fourth metallisation part 65 is exposed to atmosphere, these components provide a conduit for the transfer of heat from the PA module 11 to atmosphere. Since the distance between the top surface of the PA module 11 and the upper exterior face 13 of the printed wire board 12 is less than the distance between the lower surface of the PA module 11 and the lower exterior face 14 of the printed wire board 12, high volume thermal vias are not necessarily needed. The effectiveness of the components in conducting heat away from the PA module 11 depends in part on the length of the path between the PA module 11 and the fourth metallisation part 65. By arranging for the eleventh and twelfth microvias 62, 63 to be directly above the tenth microvia 60, the length of the path can be minimised, and thus the rate at which heat can be conducted away can be maximised. It will be appreciated that it is not essential that the eleventh and twelfth microvias 62, 63 are directly above the tenth microvia 60, although the greater the distance between the top of the tenth microvia 60 and the bottoms of the eleventh and twelfth microvias, the less effective is the arrangement at conducting heat away from the PA module 11.

Also connected to the uppermost surface of the PA module 11 is a thirteenth microvia 70. A ninth signal line 71 connects the thirteenth microvia 70 to a fourteenth microvia 72. The thirteenth microvia 70 is located within the fourth layer 36, and the fourteenth microvia 72 is located within the sixth layer 64. A fifth metallisation part 73 is formed on the upper exterior face 13 of the PA module 11 and contacts the fourteenth microvia 72, to form an electrical path between the firth metallisation part 73 and the interconnect on the PA module 11. Similarly, a further electrical path is provided between the PA module 11 and a sixth metallisation part 75 via a sixteenth microvia 76, a tenth signal line 77 and a seventeenth microvia 78. A filter 80 is connected to the fourth, fifth and sixth metallisation parts 65, 73 and 75, and thus is connected to the PA module 11 , via a number of solder balls 81 , 83.

Thus, the filter 80 is included in a circuit implemented on the PA module 11. Furthermore, since the length of the paths between the PA module 11 and the filter 80 are relatively short, the inductance experienced by signals passing between the PA module 11 and the filter 80 can be minimised.

To fabricate the power amplifier system 10, the first layer 15 is formed, with tracks on its surfaces in appropriate locations. A cavity is then formed in the first layer, and the PA module 11 is emplaced and embedded in the cavity. The second and fourth layers then are provided, with microvias and conduction paths added as appropriate. The remaining layers are then built onto these layers.

Referring now to Figure 2, a second embodiment of a power amplifier system is shown. The power amplifier system 100 includes generally a PA module 101 and a secondary module 102 embedded within a printed wire board 103.

In particular, the PA module 101 is located within a first layer 104. A second layer 105 is formed immediately above the first layer 104. The first and second layers 104 and 105 are relatively thick. Immediately above the second layer 105 is a third layer 106, which is thin compared to the first and second layers 104, 105. Immediately below the first layer 104 and the PA module 101 is a fourth layer 107, which is relatively thin. Beneath the fourth layer is formed a fifth layer 108, which is also

relatively thin. The bottommost surface of the fifth layer constitutes a lower exterior surface 109 of the power amplifier system 100.

A number of electrical paths between the lowermost surface of the PA module 101 and a ball grid array 110 are formed on the lower exterior surface 109 of the power amplifier system 100. In particular, a first microvia 111 is formed in the fourth layer 107 and provides a connection between an interconnect on the lower surface of the PA module 101 and a first signal line 112. A second microvia 113 connects the first signal line 112 to a metallisation part 114 formed on the lower exterior surface 109. A first solder ball 115 forming part of the ball grid array 110 is connected to the first metallisation part 114. Furthermore, third and fourth microvias 116, 117 are connected in parallel between the lower surface of the PA module 101 through the fourth layer 107 to a second signal line 113, which is interposed between the fourth and fifth layers 107, 108. Fifth, sixth and seventh microvias 119, 120, 121 are connected in parallel between the second signal line 118 and a second metallisation part 122, which is formed on the lower exterior surface 109. The fifth, sixth and seventh microvias 119, 120, 121 extend through the fifth layer 108. Although not visible in the Figure, the fifth, sixth and seventh microvias 109, 120, 121 have longitudinal axes which are nearby to longitudinal axes of the third and fourth microvias 116, 117. In this way, the length of the path between the part of the PA module 101 which is contacted by the third and fourth microvias 116, 117 to the second metallisation part 122 is minimised. The amount of heat that is able to flow from the PA module 101 to the second metallisation part 122 is a function of the number of thermal conduction paths therebetween, the conductivity of those paths and the length of those paths. Thus, the use of microvias, instead of high volume thermal vias, is possible whilst still allowing for the conduction of sufficient heat away from the PA module 101 because the lengths of the paths are relatively short and because there are plural thermal conduction paths in parallel. As with the Figure 1 PA system, it is preferred that the microvias which contact the PA module 101 are located close to parts of the PA module 101 that generate the greatest amount of heat.

Further thermal paths between the PA module 101 and the ball grid array 110 are provided, as shown in the Figure.

In this example, the second metallisation part 122 is a ground-connected terminal. However, this is not essential. Some of the paths from the lower surface of the PA module 101 to the lower exterior surface 109 of the printed wire board 103 may be used for carrying electrical signals with content, or may be used solely for conducting heat away from the PA module 101. To maximise the conductivity of heat away from the PA module 101, the thermal paths constituted by microvias and signal lines should be as short as is possible. It is advantageous if the paths, or at least one or some of the paths, are located wholly within a volume defined by the lower exterior surface 109, the lower surface of the PA module 101 and imaginary surfaces which extend parallel to the plane of the lower exterior surface 109 and which coincide with the peripheral edge of the PA module 101.

Located within the second layer 105 are eighth and ninth microvias 125, 126. The eighth microvia 125 is connected via a third signal line 127 to a first through via 128. A second solder ball 129 is connected to the first through via 128 by way of a third metallisation part 130, a tenth microvia 131, a fourth signal line 132, an eleventh microvia 133 and a fifth signal line 134. These components provide an electrical connection between the second solder ball 129 and an interconnect on the upper surface of the PA module 101.

Immediately above the third layer 106 is a planar shielding component 140. This comprises a metallic layer, and extends for almost the whole of the area of the printed wire board 103. At least, the shielding component 140 is larger than and completely overlaps the PA module 101.

Immediately above the shielding component 140 is a sixth layer 141 , a seventh layer 142, an eighth layer 143 and a ninth layer 144. The sixth and seventh layers 141 , 142 are relatively thick in comparison to the eighth and ninth layers 143, 144. Embedded within the seventh layer 142 is the second module 102. The second module 102 and the PA module 101 are generally in alignment with one another,

although they may be, as shown in the Figure, differently sized. The PA module 101 and the second module 102 are electrically shielded from one another by the shielding component 140. Thus, the modules 101, 102 are unable to interfere with one another. The use of the shielding component 140 to provide shielding of modules is particularly important with radio frequency circuits. It is the shielding component 140 which allows the PA module 101 to be embedded within the printed wire board 103 along with other active components within close proximity.

The second module 102 is electrically connected to SMD components 150, 151 and 152, which are located on an upper exterior surface 153 of the printed wire board 103. A filter 155 is mounted on the upper exterior surface 153 by solder balls 156, through which an electrical connection can be made to the second module 102 and/or the SMD components 150, 151 and 152. The filter 155 and the SMD component 152 also are electrically connected to the PA module 101 via respective paths including vias 160, 161 extending through the shielding component 140. The vias 160, 161 are electrically insulated from the shielding component 140. Furthermore, the secondary module 102 is connected to the PA module 101.

The second module 102 may be a switched mode power supply (SPMS). Conventionally, avoidance of interference between an SPMS and a PA module imposed significant constraints on system design. However, the features of this invention provides a simply interference avoidance solution.

The power amplifier system 100 is constructed in a manner similar to the power amplifier system 10.

With this technology the amplifier chip can be shielded around and no external shield is needed for the PWB 13.

An additional effect of the invention is that the interconnections do not need to be located on the periphery of the PA module 11, as is necessary with wire bond technology. This leads to a number of advantages. Firstly, the number of connection points can be increased, compared to conventional PA modules.

Significantly, connections can be made to be low inductance, whereas the bond wires needed to connect conventional PA modules to other components or circuits necessarily provide relatively high impedance connections. Furthermore, the placement of the connections can be selected to simplify interconnections and to optimise electrical performance.

This in turn provides a significant advantage in that it allows the PA module to use an off-chip interstage circuit. Particularly, a matching network or a filter is connectible between two subcircuits provided by the PA module. This is illustrated in Figure 3.

Referring to Figure 3, a transmit chain circuit 200 is shown comprising in sequence a first amplifier stage 201, a first filter 202, a second power amplifier stage 203, a matching circuit 204 and a second filter 205.

The first and second amplifier stages 201, 203 are implemented by the PA module 101. The first filter is implemented by the filter 155. The matching circuit 204 and the second filter 205 are implemented by two of the SMD components 150, 151 and 152.

In an alternative embodiment, the first and second amplifier stages are implemented by separate PA modules or ICs, embedded separately in the printed wire board 103.

The use of the first filter stage 202 between the first and second amplifier stages 201, 203 results in less noise than corresponding filtering performed after the second amplifier stage 203. Furthermore, the filtering can be performed by a low power level circuitry. The insertion loss of the filter does not result in substantial power loss in the radio system. Thus, the total efficiency of the radio system can be improved. Consequently, the power amplifier system is more stable than a corresponding prior art system, and it has a lower noise level. Thus, for a given noise level, the gain of the power amplifier system can be higher than is possible with the corresponding prior art system. These advantages are achieved by the

invention because the use of wire bonds to connect an interstage component can be avoided.

The microvias and the signal lines of the Figures 1 and 2 embodiments preferably ate copper components. Although gold or aluminium typically is used in wire bond technologies, it is possible to use copper with embedded technology, which gives rise to the usual advantages of copper interconnects.

A power amplifier system constructed according to the invention can be smaller than the corresponding prior art power amplifier system. This is particularly advantageous where volume is desired to be minimised, such as in mobile devices such as mobile telephones, personal digital assistants, etc.

The invention also allows for improved grounding of the circuits of the PA module. Better grounding is possible in part because ground connections can be made from both sides of the PA module. Better grounding results because chip inductance is reduced and because ground current coupling is reduced. This also results in a reduction of stability problems, compared to a corresponding prior art amplifier system.

By providing a separate ground for each of the amplifier stages 201, 203, ground current coupling between the amplifier stages 201, 203 is reduced. This also helps to provide stability to the PA stable, by avoiding feedback loops via grounding inductances between different PA stages. Significantly, improving ground connections allows a higher gain for a given system performance.

Referring again to Figure 2, the path commencing with the microvia 125 and ending with the solderball 129 comprises a ground connection for the first amplifier stage 201, and the path starting with the microvias 116 and 117 comprises a ground connection for the second amplifier stage 203.

Although not shown in the Figures, the invention also provides a system including a PA module with plural signal paths, in which a one of the paths is selected for use

at a particular time by suitable control of a switch arrangement. In this way, the PA module can be used in different configurations. For instance, plural matching networks provides at an output, an input or interstage (between amplifier stages) are provides. A different matching network is switched into the circuit depending on operating parameters. For instance, different matching circuits may be used at different power levels or different frequency bands.

The invention is not limited to the above described embodiments, and numerous alternatives and substitutions are possible.

For instance, although the embodiments utilise PWBs as the carrier for the PA module, a hardware module may instead be used.

Although the metallisation for the signal lines is copper in the embodiments, other electrically conductive materials may be used instead.

In place of microvias, blind vias or drilled vias could be used.

The PA module 11, 101 may include voltage controlled oscillator, modulator and/or digital to analogue conversion circuitry as well as power amplifier circuitry.

The PA systems 10, 100 are included in respective mobile communications devices (not shown). The PA systems 10, 101 are particularly suitable for use in mobile telephones, smartphones, personal digital assistants and the like.

The scope of the invention is limited only by the appended claims and their equivalents.