Field of the Invention
The invention relates to a semi-finished product for the production of a printed circuit board with at least one recessed electronic component having at least one conductive layer structured to provide a connector pad for an electronic component, fan-out lines connected to the connector pad and further to provide at least one laser-stop device encompassing the connector pad, wherein the laser-stop device has at least one passage for passing-through the fan-out lines as well as a method for producing a printed circuit board with at least one recessed component
Description of the Related Art
Printed circuit boards, also referred to as printed wire boards are panels carrying and electrically connecting electronic components such as transistors and the like and, hence, form vital parts of electronic products.  Printed Circuit board have a more or less complex structure depending on the specific application.  In general a printed circuit board has a plurality of alternating conductive layers and insulating layers bonded together by hardening panels of glass fibres impregnated with organic resin, said panels forming the insulating layers.  Such panels for use in the production of printed circuit boards are widely known in the industry as “prepregs” (preimpregnated fibres), which are delivered and processed in an uncured, hence viscous state of the organic resin.  The actual insulating layer of the printed circuit board results when the organic resin has cured.  The insulating layers carry conductive layers, usually formed of copper foil, the conductive layers being appropriately structured to form wirings to electrically connect the electronic components.  Modern printed circuit boards allow for a high degree of integration of electronic components and their appropriate wiring. 
Starting from traditional printed circuit boards having the electronic components mounted on top of the board, today's electronic components have reached a degree of miniaturisation that allows for their accommodation within inner layers of the printed circuit board.  To this end, the conductive layer that is destined to carry such a component or even many components, which layer is called target layer in the context of the present invention, is structured to provide a connector pad for the electronic component, such as for example a ball grid array.  The connector pad is connected to so-called fan-out lines which are wirings connected to contacts of the connector pads leading away from the connector pad in order to connect the connector pad to the other wirings of the printed circuit board.  In the course of the production of a highly integrated, multi-layered printed circuit board, the just described conductive layer is then covered with a plurality of further insulating layers and further conductive layers to form the multiple layers of the printed circuit board.  Said further conductive layers are normally structured to provide wirings and can, of course, also be structured to provide additional connector pads for further recessed components.  In the context of the present invention, the term “recessed component” means an electronic component received in a recess of a printed circuit board.  At a later production step, the covered connector pads have to be uncovered or decapped in order to be able to place the component on the connector pad.  This is usually done by laser-cutting.  To this end, a CO2 laser is deployed to cut through the insulating layers above the target layer.  A CO2 laser is advantageous for its nature not to cut through copper.  In this context it is known in the art to provide an appropriate laser stop device for decapping a connector pad, the laser stop device being an area of copper surrounding the connector pad completely to be able to cut around the connector pad to completely liberate the at least one cap layer for subsequently removing it to reveal the connector pad.  As a consequence, the fan-out lines were designed to stop right before the laser stop device since the laser stop device, also being made of conductive material, would otherwise bypass the fan-out lines if they crossed the laser stop device in the same conductive layer.  The actual wiring of the fan-out lines to the rest of the printed circuit board was, hence, carried out in a conductive layer situated above the cap layer so that the laser stop device could be crossed.  This, obviously, adds to the overall thickness of the printed circuit board.  To overcome this problem, it is known to provide a passage in the otherwise closed laser stop device for passing-through the fan-out lines.  Consequently, in the area of the passage in the laser stop device, cutting with laser is not possible, since the laser, in absence of a copper trace in the area of the passage, would cut through the underlying insulating layer(s).  The cap layer, or the plurality of cap layers had to be cut otherwise or simply torn in the area where laser cutting could not have been performed due to the underlying passage in the laser stop device. 
There is, however, a constant need for further miniaturisation in the electronic industry in order to provide consumers and professionals with ever smaller yet more capable electronic devices and installations which require more performance-oriented electronic components to be packaged in a smaller space.  Such components require fan-out lines on all of its sides so that more uncut areas would remain in the at least one cap layer.  However, the more uncut segments in the overlying insulating layer, i.e. the cap layer, the more difficult it becomes to decap the electronic component.