PRINTED CIRCUIT BOARD

Abstract: In a printed circuit board comprising a track pattern (L1) and a substrate (DP), an additional layer (L3) of copper is provided, which is electrically insulated from the track pattern by a layer of an insulating material (L2). Both these layers (L2, L3) are situated between the track pattern (L1) and the substrate (DP). During operation of a circuit obtained by making use of the printed circuit board, the additional layer (L3) serves as a heat spreader, resulting in an efficient transport of the heat generated by components (C1, C2) in the circuit.

Description: Printed circuit board The invention relates to a printed circuit board comprising a support plate of an electrically insulating material which is provided with a first and a second surface, the first surface being at least partly covered with a first layer of an electroconductive material. The invention also relates to a circuit device for feeding a lamp, and to a compact lamp. A printed circuit board as mentioned in the opening paragraph is known from Japanese patent application 08204293 A. A track pattern is provided, generally by etching, in the first layer, which track pattern electroconductively interconnects electric components of a circuit provided on the printed circuit board. Some of these components generate comparatively much heat during operation of the circuit. Generally the circuit is provided in an apparatus in such a manner that said heat must be largely dissipated via the support plate. As the support plate is made of an electrically insulating material, said support plate generally is a poor heat conductor in practice. This hampers the dissipation of the heat generated by the component via the support plate. When use is made of the printed circuit board in accordance with the prior art, this problem can be partly obviated by leaving the first layer substantially intact (i.e. not providing a track pattern) at locations proximate to components which generate relatively much heat during operation of the circuit. As the first layer is substantially intact at locations proximate to the component, the heat generated by the component can easily spread over these locations. As a result, the surface area over which the heat generated by the component is transferred to the support plate is larger than the surface area of the part of the component contacting the printed circuit board. As a result, the heat transfer from the component to the support plate is improved to a limited extent. The same applies to the heat transfer by the support plate since a larger part of the support plate is involved in the heat transport. However, the consequence of this improved heat transfer is that a part of the surface of the printed circuit board cannot be provided with a track pattern and components, as a result of which the printed circuit is inefficiently used. In other words, for a given circuit use has to be made of a relatively large printed circuit board. This is an important drawback, particularly if circuits are used whose miniaturization is pursued. It is an object of the invention to provide a printed circuit board whose surface can be efficiently used to form a track pattern and to place components, which printed circuit board also enables the heat generated by components to be satisfactorily transferred to the support plate and by the support plate. To achieve this, a printed circuit board as mentioned in the opening paragraph is characterized in that a second layer of a heat-conducting material and a third layer of an electrically insulating material are situated between the first surface and the first layer, said third layer being situated between the first and the second layer. In a printed circuit board in accordance with the invention, the first surface is completely, or substantially completely, covered by the second layer. It is possible to choose the thickness of the third layer to be such that a good electrical insulation and a reasonable or good heat conduction between the first and the second layer is brought about. The heat generated by a component provided on the surface of the printed circuit board is relatively rapidly transferred via the third layer to the second layer. Upon arrival in the second layer, this heat spreads substantially immediately over the entire second layer, which can be attributed to the fact that said second layer is made of a heat-conducting material. As a result, the heat transfer from the second layer to the support plate takes place via the entire first surface of the support plate. Consequently, the entire support plate contributes to the heat transfer by the support plate instead of only a part of the support plate in the immediate proximity of the component. By virtue of the presence of the second layer, the heat generated by components placed on the printed circuit board is effectively distributed over the entire second layer, resulting in an efficient heat dissipation, enabling the surface of the printed circuit board to be efficiently used for realizing a circuit. Good results have been obtained using examples of a printed circuit board in accordance with the invention, wherein the first layer comprises copper, and the second layer comprises a metal selected from the group formed by copper and iron. These metals are good conductors of both heat and electricity. It has also been found that satisfactory results are obtained with examples of a printed circuit board in accordance with the invention, wherein the thickness of the first layer and the third layer is chosen to be in the range between 10 micrometer and 200 micrometer, preferably between 20 micrometer and 50 micrometer. In a corresponding manner, it has been found that the thickness of the third layer should preferably be chosen in the range between 50 micrometer and 100 micrometer. In a printed circuit board in accordance with the invention, the support plate serves to provide the printed circuit board with a certain mechanical sturdiness. In practice it has been found that the thickness of the support plate should preferably be chosen in the range between 50 micrometer and 1.6 mm, preferably between 0.3 mm and 1.6 mm. It is to be noted that the support plate frequently comprises an adhesive layer having a thickness of several tens of micrometers for bonding the second layer. If a printed circuit board in accordance with the invention is used as a clip-on p.c.b. which is provided on a motherboard, and the motherboard is provided with a "heat sink" or a "heat spreader", it is advantageous to bring the second layer of the printed circuit board into thermal contact with the "heat sink" or the "heat spreader" of the motherboard. As a result, the heat generated by components on the clip-on p.c.b. is efficiently dissipated via the motherboard. A printed circuit board in accordance with the invention can very suitably be used in a circuit device for feeding a lamp, wherein a track pattern is provided in the first layer of the printed circuit board, and electric components are provided on the printed circuit board, which are electrically interconnected by the track pattern. A printed circuit board in accordance with the invention enables the heat generated by the components to be efficiently dissipated, so that the circuit can be relatively small, which is desirable in many lamp applications. If the circuit device is accommodated in a housing having an inner wall, preferably at least a part of the second surface of the support plate is connected via a fourth conducting layer to a part of the inner wall of the housing. In this manner, it is achieved that the heat transport from the support plate to outside the housing takes place efficiently. If the electric potential of the second layer is maintained at a predetermined level during feeding the lamp, the second layer does not only play a part in the removal of heat but also as a shield providing protection against electromagnetic interference generated by the components and the track pattern. A comparatively small circuit device for feeding a lamp, wherein heat is efficiently dissipated is desirable, in particularly, if the circuit device is used in the ballast circuit of a compact lamp comprising a light-transmitting discharge vessel comprising a filling containing an inert gas, - means I for maintaining a discharge in the discharge vessel during operation of the lamp, a lamp housing secured to the discharge vessel, a lamp cap provided with electric contacts and secured to the lamp housing, and a ballast circuit for generating a supply voltage, during lamp operation, to feed the discharge vessel, which ballast circuit is coupled between the electric contacts and the means I. In such a compact lamp, an efficient heat disposal can be brought about, more particularly, if the lamp housing and the lamp cap are provided with an inner wall, and a part of the second surface of the printed circuit board is connected via a fourth layer of a heat-conducting material to a part of the surface of said inner wall. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 diagrammatically shows a conventional printed circuit board on which several components are mounted; Fig. 2 diagrammatically shows an example of a printed circuit board in accordance with the invention on which several components are mounted, and Fig. 3 shows an example of a compact lamp in accordance with the invention. In Fig. 1, DP denotes a support plate of an electrically insulating material provided with a first surface 01 and a second surface O2. LI forms a first layer of an electroconductive material. In this example, copper is used for the electroconductive material. In the layer LI, there is provided a track pattern, and Cl and C2 denote components which form a circuit in combination with the track pattern. If this circuit is in operation, both the component Cl and the component C2 generate heat. As the first layer LI is largely etched away to form the track pattern, the heat-conducting capacity of the first layer is comparatively small. For this reason, only a small part of the heat generated by the components is spread over the first layer. The bulk of the heat is dissipated by the part of the support plate which is in the immediate proximity of the component. In Fig. 1 this is shown by means of arrows indicating the heat transport. As the heat is transported by a comparatively small part of the support plate, this heat transport is inefficient and insufficient for many applications. In Fig. 2, corresponding parts bear the same reference numerals as in Fig. 1. In Fig. 2, DP denotes a support plate of an electrically insulating material provided with a first surface 01 and a second surface 02. A second layer L2 of a heat-conducting material is provided directly on the surface 01 of the support plate DP. In this example, copper is used for the heat-conducting material. A third layer L3 of an electrically insulating material is provided on a surface of the second layer facing away from the support plate DP. A first layer LI of an electroconductive material is provided on a surface of the third layer facing away from the second layer. In this example, the electroconductive material is copper. A track pattern is provided in the layer LI, and Cl and C2 are components which form a circuit in combination with said track pattern. The second layer is connected to ground and hence serves as a shield providing protection against electromagnetic interference generated by the components. If this circuit is in operation, both component Cl and component C2 generate heat. As the first layer LI is largely etched away to form the track pattern, the heat- conducting capacity of the first layer is comparatively small. For this reason, only a small part of the heat generated by the component is spread over the first layer. The bulk of the heat is transferred to the second layer via the third layer. As the second layer is made from a heat- conducting material, the heat supplied via the third layer spreads over the second layer. In this manner, the transfer of this heat to the support plate takes place via the entire surface 01, and the entire support plate contributes to the dissipation of the heat generated by the components Cl and C2. As a result, the heat transport is efficient and even sufficient in situations where very small dimensions of the circuit are desired. In Fig. 3, reference numeral 8 denotes a light-transmitting discharge vessel provided with a filling containing an inert gas and with two electrodes (not shown). In this example, the filling also contains mercury. In this example, these electrodes form means I for maintaining a discharge in the discharge vessel during lamp operation. A luminescent layer is applied to the wall of the discharge vessel. Reference numeral 6 denotes a lamp housing which is secured to the discharge vessel 8, and reference numeral 3 denotes a lamp cap provided with electric contacts (1 and 2), which is secured to the lamp housing. The printed circuit board PP and the components C1-C4 diagrammatically represent a ballast circuit for generating, during lamp operation, a supply voltage for feeding the discharge vessel. This ballast circuit is coupled between the electric contacts 1 and 2, via the conductors E, and the electrodes, via conductors 9. The printed circuit board PP is connected with its surface 02, via a fourth layer L4 of a heat conducting material, to the inner wall of the lamp housing 6 and the lamp cap 3. During operation of the compact lamp, the heat generated by the components C1-C4 is transported to the second layer via the third layer and spreads over this second layer. This heat is transferred, via the whole surface 01 of the support plate, to the support plate which contributes entirely to the transport of the heat to the fourth layer L4. As the heat transport takes place via the entire support plate, said heat transport to the fourth layer L4 takes place in an efficient manner. As L4 is made of a heat-conducting material and is in contact with the inner wall of the lamp housing and the lamp cap, the heat transport from the fourth layer to the lamp housing and the lamp cap and, subsequently, to outside the compact lamp also takes place in an efficient manner. By virtue thereof, it is possible to embody the ballast circuit of the compact lamp and hence the compact lamp itself so as to be comparatively small. In a practical embodiment of the example of a printed circuit board as shown in Fig. 2, the first and the second layer are made of copper. Both layers are 35 μm thick. The third layer consists of an adhesive with a thermal-conduction coefficient of 0.5 W/mK and has a thickness of 50 micrometer. The support plate is 1.6 mm thick and also has a thermal- conduction coefficient of 0.5 W/mK. The heat transfer coefficient of the support plate is approximately 50 W/m K. The ambient temperature is 0 °C. A 1W dissipator is in contact with 1 cm 2 of the first layer. For this practical embodiment, it was found that the temperature of the first layer at the location of the dissipator was approximately 50 °C. For a correspondingly constructed printed circuit board, yet without the second layer, it was found that under corresponding conditions the temperature of the first layer was approximately 230 °C at the location of the dissipator.

Claims: CLAIMS: 1. A printed circuit board comprising a support plate of an electrically insulating material which is provided with a first and a second surface, the first surface being at least partly covered with a first layer of an electroconductive material, characterized in that a second layer of a heat-conducting material and a third layer of an electrically insulating material are situated between the first surface and the first layer, said third layer being situated between the first and the second layer. 2. A printed circuit board as claimed in claim 1, wherein the first layer comprises copper, and the second layer comprises a metal selected from the group formed by copper and iron. 3. A printed circuit board as claimed in claim 1 or 2, wherein the thickness of the first layer and the third layer is chosen to be in the range between 10 micrometer and 200 micrometer, preferably in the range from 20 micrometer to 50 micrometer. 4. A printed circuit board as claimed in claim 1, wherein the thickness of the third layer is chosen to be in the range between 50 micrometer and 100 micrometer. 5. A printed circuit board as claimed in claim 1, wherein the thickness of the support plate is chosen to be in the range between 50 micrometer and 1.6 mm, preferably between 0.3 mm and 1.6 mm. 6. A circuit device for feeding a lamp, comprising a printed circuit board as claimed in claim 1, wherein a track pattern is provided in the first layer of the printed circuit board, and electric components are provided on the printed circuit board, which are electrically interconnected by the track pattern. 7. A circuit device as claimed in claim 6, wherein the electric potential of the second layer is maintained at the predetermined level during feeding the lamp. 8. A circuit device as claimed in claim 6 or 7, wherein the circuit device is provided with a housing having an inner wall, and a fourth layer of a heat conducting material connects a part of the second surface of the support plate to a part of the inner wall. 9. A compact lamp provided with a light-transmitting discharge vessel comprising a filling containing an inert gas, means I for maintaining a discharge in the discharge vessel during operation of the lamp, - a lamp housing secured to the discharge vessel, a lamp cap provided with electric contacts and secured to the lamp housing, and a ballast circuit for generating a supply voltage, during lamp operation, to feed the discharge vessel, which ballast circuit is coupled between the electric contacts and the means I, characterized in that the ballast circuit comprises a circuit device as claimed in claim 6. 10. A compact lamp as claimed in claim 9, wherein the lamp housing and the lamp cap are provided with an inner wall, and a part of the second surface of the printed circuit board is connected via a fourth layer of a heat conducting material to a part of the surface of the inner wall.

Inventors: MIES CORNELIS J, MIES, CORNELIS, J.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H05K1
IPC codes: F21S2, F21V23, F21Y103, H01J61, H05K1