Patent Publication Number: US-2011051384-A1

Title: Printed circuit board element having at least one component embedded therein and method for embedding at least one component in a printed circuit board element

Description:
The invention relates to a printed circuit board element comprising at least one prefabricated electric or electronic component, in particular a chip, embedded between a base and a cover layer. 
     Furthermore, the invention relates to a method for embedding at least one electric or electronic component, in particular a chip (semiconductor component) in a printed circuit board element, wherein the component is adhered to a base, whereupon a cover layer is applied on top of the base including the component by pressure. 
     Apart from externally equipping printed circuit boards with electric and/or electronic components and, in particular, active electronic components or integated circuits, respectively, which commonly are termed chips, it is an increasing desire in printed circuit board technology to embed such components, in particular chips, also in the interior of the respective printed circuit board element, wherein the components are enclosed between individual layers of the printed circuit board element. A technique therefor has been disclosed in U.S. Pat. No. 6,396,153 B, e.g., wherein on a side which later comes to lie in the interior, a connecting layer of insulating material is coated with a polymer adhesive, to which then a chip is glued. Subsequently, a substrate layer is applied to the adhesive layer and shaped around the chip, e.g. by injection moulding or by compression, so that, finally, the chip will be embedded between this substrate and the connecting layer through which the chip will be contacted. A similar application of chips to substrates by means of an adhesive layer has been disclosed in DE 4 433 833 A, EP 611 129 A and U.S. Pat. No. 5,564,181 A. This technology is complex, and also the adhesive layer applied facewise as an additional layer, also present externally of the chip, is disturbing. 
     On the other hand, it has been known (cf. e.g. DE 196 42 488 A and DE 199 54 941 A) to fasten chips to printed circuit board elements by merely locally applied adhesive, in particular in the course of embedding chips in printed circuit board elements. For embedding in printed circuit board elements, however, chips that are as thin as possible should be used, and in particular so-called “thinned” chips, i.e. chips with a substrate side that has been considerably ground off, of course without negatively affecting the circuit contained in the chip, chip thicknesses e.g. in the order of 50 μm or 70 μm being attained, whereas standard chips have a thickness of 700 μm, e.g. Such thinned chips naturally are highly flexible so that they are bent when they are pressed onto amounts of adhesive applied locally in the form of droplets, since these amounts of adhesive are not plane, but have a cambered shape. Moreover, with the thinned chips, a facewise distribution of the adhesive previously applied to the substrate or to the lower side of the chip by pressing on, as desired, is not possible, so that—apart from a bending of the chip—also an insufficient gluing-adhesion may be the consequence. A further disadvantage is that here, in the case of bent chips, their subsequent contacting, after their inclusion between printed circuit board layers is a problem since the positions of the contact sites on the chip have shifted relative to the set position due to the bending of the chip. 
     It is now an object of the invention to remedy this situation and to propose a technique by which the components, in particular also thinned chips, can be fastened by gluing to the respective base when embedding them in printed circuit board elements, wherein the components are treated gently and their positions can be exactly defined. Furthermore, the invention aims at enabling the gluing fastening of the components in a particularly economical manner, with an adhesive being also merely locally applied. 
     To achieve this object, the invention provides a printed circuit board element, and a method, respectively, as defined in the independent claims. Advantageous embodiments and further developments are indicated in the dependent claims. 
     Thus, according to the invention, adhesive film sections, or adhesive strip sections, respectively, are used to fix the components on the respective bases by adhering. These sections are separated, e.g. by cutting or punching, from prefabricated, in particular strip-shaped adhesive films which, preferably, are provided in the form of rolls, and the sections have a predetermined, uniform thickness, e.g. in the order of from 10 μm to 15 μm, and preferably they have a thickness of approximately 12 μm. These adhesive films, or strips, respectively, in particular are thermo-setting, the adhesive, e.g., being pre-cured by infrared irradiation after a separate attachment of the respective section on the substrate and finally, after attachment of the component, being completely cured in an oven. The adhesive film sections are separated substantially in the dimensions of the components, e.g. cut out or punched out, and, according to an advantageous embodiment, separately before attachment of the components on the base. In this instance, the adhesive film sections are pressed on during their attachment after they have been positioned and, to a certain degree, pre-cured—optionally also by using a heated tool. One possibility may further consist in that the adhesive film sections are applied to the base and pressed on, the base being heated from the opposite side thereof to thereby cure the adhesive film sections to a limited extent, i.e. pre-cure them, wherein, optionally, also the components are heated. Pre-curing has the purpose of attaching the adhesive film section to the base with sufficient strength, yet the adhesive film section shall remain sufficiently sticky so as to adhere the respective component thereto by compression. 
     The adhesive films which, in particular, are strip shaped, may have a per se conventional structure, such as a structure comprising a carrier film coated on both sides with an adhesive, wherein cover films may in turn be applied over these glue layers. Another possibility consists also in omitting the carrier film and only providing an adhesive layer, e.g. likewise between two cover films. The cover film enables an increase in stability, and it may, e.g., consist of polyimide. As the cover films, e.g. polyethylene films or polyethylene therephthalate films (PE films or PET films) may be used. As the adhesive, a per se conventional polymer glue, such as with a low elasticity module, preferably is used in combination with an epoxy resin as well as fillers. Also a modified polyimide may be used in combination with an epoxy resin. 
     When using adhesive film sections with cover foils on both sides thereof, it is preferably proceeded such that the one, lower cover film, eg. a PE film, is pulled off before the adhesive film section is applied to the base so as to thereby adhere the adhesive film section to the base, and that the other, upper cover film, e.g. a PET film, is pulled off only shortly before the component is applied so as to protect the adhesive layer up to that point of time. 
     An advantageous possibility also consists in previously attaching the adhesive film sections to the component to that side thereof which is to be connected to the base and to thus attach the respective component including the adhesive film section on the base. In this instance, it may advantageously be proceeded such that an adhesive film is previously attached to a wafer containing a plurality of components, whereupon the components together with the adhesive film sections are separated from each other and are each attached to the associated base. 
     For complete curing of the glue of the respective adhesive film section, the base together with the component glued thereto is suitably introduced into an oven and heated, e.g. to a temperature of from 130° C. to 150° C. or to 170° C. In this instance, it may also be advantageous to carry out this complete curing of the glue in a reactive or in an inert atmosphere, in particular in a nitrogen atmosphere. 
     After the respective component has been glued to the base, a resin-copper cover layer, such as a so-called RCC film (RCC resin coated copper film)), can be applied over the base including the component by pressure, and subsequently contact holes can be applied in this cover layer by laser drilling. Subsequently, a metallization by electroplating will be effected in the region of the contact holes so as to contact the embedded components or conductive layers, whereupon, finally, patterning will be effected on the outer conductive layer (copper layer) by photolithography. 
    
    
     
       The invention will be explained in more detail hereinafter by way of preferred exemplary embodiments to which, however, it shall not be restricted, and with reference to the drawings. In the drawings, in detail, 
         FIG. 1  schematically shows a cross-section through a part of a multilayer-printed circuit board element comprising an embedded component, e.g. a thinned chip; 
         FIGS. 2A to 2E  show consecutive steps in the production of such a printed circuit board element, with a component being embedded, in schematic partial cross-sections; 
         FIG. 3  schematically shows—side by side—various options for applying a component on a base of a printed circuit board element in perspective illustrations; 
         FIG. 3A  shows a schematic section through an embodiment of an adhesive film section; 
         FIGS. 4A to 4C  show an advantageous option for applying a wide, strip-shaped adhesive film material on a chip wafer ( FIG. 4A ) and the subdivision of the wafer including the adhesive film into chips with adhesive film sections ( FIG. 4B ) as well as the application of the chips including the adhesive film sections fixed thereto to a substrate or to a base of a printed circuit board element ( FIG. 4C ); and 
         FIGS. 5A to 5D  show a modified embodiment of the method for applying chips (or, generally, components) on a base by using adhesive film sections, wherein the sections are previously separated from an adhesive tape, either by means of a cutting tool ( FIG. 5A ) or by means of a punching tool ( FIG. 5D ), then are separately applied to the base ( FIG. 5B ) and, finally, the chips are glued to these adhesive film sections ( FIG. 5C ). 
     
    
    
     In  FIG. 1 , a part of a printed circuit board element  1  is schematically illustrated, which contains an embedded electric component in the form of a thinned chip  2 . To simplify matters, in the following reference will always be made to such a chip  2  by way of example, within the scope of the invention, however, embedding of other electric or electronic components, in particular passive components, such as resistors, capacitors, ESD (Electro static discharge) protection elements, laser diodes, photodiodes etc. in a printed circuit board element  1  in the present manner is also provided. These are discrete components, wherein preferably the thickness of these components—either right from the beginning, by the production process, such as with capacitors, or by a subsequent rear-side thinning—is approximately from 50 μm to 70 μm. By embedding, such thin components can be safely enclosed by the resin so as to protect them from moisture or from mechanical wear, e.g., a corresponding dielectric thickness then being present above the component in this instance and a plane surface being attained. This is important for laser drilling processes, on the one hand, yet also for other processes which require a plane surface. 
     The component, or chip  2 , respectively, is glued to a base  4 , e.g. a common FR 4  base material (resin core, in particular with conductive layer) or substrate, as is conventionally used in printed circuit board technology, by means of a section  3  of a strip-shaped adhesive film, termed adhesive film section  3  hereinafter. Accordingly, the printed circuit board element  1  may comprise a conducting track  5 , e.g. in the form of a patterned Cu layer, on the base  4  in a per se common way, which conducting track leads to further electric components not further illustrated in the printed circuit board element  1 ; above this conducting track  5  as well as above the chip  2 , a cover layer  6  of resin having an upper copper coating  7  is applied, which, e.g., may be a common RCC film (resin coated copper film, i.e. a resin-copper film laminate). Furthermore, for contacting the chip  2  as well as the conducting track  5 , laser bores, or micro-vias  8 ,  9 , respectively, are provided in this cover layer  6 , which micro-vias have galvanic metallizations  10  and  11 , respectively, on their side walls. 
     In  FIG. 2A , the application of a component, or chip  2 , respectively, on such a base  4  is shown in detail. As can be seen, at first a clearance in the conductive coating, i.e. in the conducting track  5 , is provided on the base  4 , i.e. on the printed circuit board core, in a region  12 , and in this region  12 , an adhesive strip section  3  has already been positioned and glued on by pressing on and also pre-cured to a predetermined extent so that its adhesion to the base  4  is ensured. The chip  2  is now applied to this adhesive strip section  3  by pressing it thereto, as indicated by arrow  13 , and adhered. The adhered state of the chip  2  can be seen in the lower part of  FIG. 2B . On its upper side, the chip  2 , moreover, includes contact regions  14 ,  15  which will be cantacted later on by means of the laser bores  8  already mentioned by way of  FIG. 1  made through the cover layer  6 . 
     After attachment of the respective chip  2  on the base  4 , complete curing of the adhesive material of the adhesive strip section  3  will occur which may take place in an oven in a nitrogen atmosphere at a temperature of, e.g., from 130° C. to 150° C. or also from 150° C. to 170° C., depending on the type of materials used. 
     With the help of the adhesive strip section  3 , a precise, secure application and adhesive attachment of the chip  2  is possible, even if these chips  2  are so-called thinned chips, which, as mentioned, due to grinding off a substantial part of the chip substrate may have a thickness of merely approximately 50 μm, e.g., more generally from 10 μm to 70 μm (instead of approximately 700 μm, e.g.), and which therefore are highly flexible and sensitive. The conventional application of a liquid glue in the form of drops would be a problem for such thinned chips, since with the thinned chips, there could be no planar distribution of the glue drops when the chips are pressed onto the latter, apart from the fact that extraordinarily slight amounts of glue would be possible so that dosing would be difficult and, thus, in this standard procedure a precise and reliable fastening of the chips by gluing to the base  4  would not be possible. On the other hand, with the help of the adhesive fastening described by using adhesive strip sections  3 , an exact application and a good adhesion of the chip  2  is rendered possible within the scope of the mechanical properties required. 
     After the application of the adhesive strip section  3  on the printed circuit board base  4 , and of the chip  2  on the adhesive strip section  3 , the base  4 , preferably on a panel with a plurality of similarly equipped printed circuit board bases, will be introduced into an oven, as mentioned before, so as to completely cure the adhesive. This may be in a reactive or also in an inert atmosphere, e.g. nitrogen. 
     Subsequently, in the course of a conventional pressing with an RCC film  6  which has an upper copper layer  7  and which may consist of an epoxy resin, e.g., embedding of the chip  2  in the interior of the preliminary printed circuit board element can be performed by the application of temperature and pressure (cf. the arrows  16  in  FIG. 2B ). Again, depending on the materials used, the temperature may be 200° C. or 220° C., and the pressure during this pressing is 20 bar, or 30 bar, e.g. 
     In the multilayer printed circuit board element thus produced, the bores  8 ,  9  are then made by means of laser beams for contacting the inwardly located components, in particular chips  2 , as well as the conducting tracks  5 ; the result of this method step is shown in  FIG. 2C . 
     Subsequently, as shown in  FIG. 2D , the contact sites  14 ,  15  of the components  2  as well as the conducting tracks  5  in the interior of the multilayer are contacted by galvanic metallization, wherein the walls of the bores  8 ,  9  are coated with copper so that the metallizations  10 ,  11  will be obtained. Subsequently, by means of a conventional photolithographic process, the upper copper layer  7  is patterned, so that finally the printed circuit board element  1  as shown in  FIG. 2E  is obtained. 
     In  FIG. 3 , three possible ways of fixing a component with the assistance of adhesive film sections  3  are schematically indicated, side by side for the sake of simplicity, and in combination with a single substrate core as base  4 . 
     In detail, on the left-hand side it is shown that an adhesive film section  3  merely consisting of adhesive material—with possible lower and upper cover films already pulled off—has been previously positioned on the base  4 , pressed thereto and pre-cured. Subsequently, with the assistance of a tempered punch-suction tool  17 , a component  2  held on the tool  17  by means of vacuum is put into position under the control of a computer and pressed onto the adhesive film section  3  according to arrow  13  (cf. also  FIG. 2A ). As has been mentioned, the thin or thinned component  2  may, e.g., have a thickness in the order of merely approximately 50 μm, cf. the measure D in  FIG. 3 , and the adhesive film section  3  may, e.g., have a thickness d in the order of merely from 8 μm to 15 μm. 
     In the central part of  FIG. 3 , a comparable attachment of a thinned or thin component  2  by means of a tool  17  is shown, yet here an adhesive film section  3  is shown which has a carrier film  3   a  between two layers of adhesive  3   b ,  3   c , see here also  FIG. 3A . Moreover, in its original state, this adhesive film section  3  may have a lower cover film  3   d  and an upper cover film  3   e , as illustrated in  FIG. 3A , these cover films  3   d ,  3   e  being pulled off when applying the adhesive film section  3 , or the component  2 , respectively, as schematically indicated in  FIG. 3A . In detail, of course the lower cover layer  3   d  will at first be pulled off, whereupon the adhesive film section  3  is pressed onto the printed circuit board base  4 . The upper cover film  3   e  preferably is pulled off only directly before attachment of the component  2  so as to protect the upper adhesive layer  3   c  as long as possible. 
     Quite generally, in case of an adhesive film material having a single adhesive layer (see  FIG. 3 , left-hand side), i.e. without a carrier film, a thickness of between, e.g., 10 μm and 50 μm (without cover film) is given, whereas an adhesive film section with adhesive layers  3   b ,  3   c  on either side of a carrier film  3   a  may very well be somewhat thicker, such as 12 μm to 200 μm. The upper cover film may, e.g, be made of PET and have a thickness of 50 μm, whereas the lower cover film to be pulled off at first may consist of PE material, and may have a thickness of 25 μm. 
     As the adhesive material, a polymer adhesive combination is suitable, which may also contain an epoxy resin and which, e.g., may be produced on the basis of a modified polyimide or of a polymer having a low module of elasticity. 
     Finally, on the right-hand side in  FIG. 3 , a further possibility for fastening a component is shown, wherein the adhesive film section  3  previously has been attached at first to the lower side of the respective component  2 , and the component  2  including the adhesive film section  3  is attached and adhered in the desired position on the base  4  by means of the tool  17 . 
     Such a previous attachment of adhesive film sections  3  on the components  2  results advantageously if, as shown in  FIGS. 4A ,  4 B and  4 C, an appropriately wide adhesive strip, i.e. strip-shaped adhesive film material  19 , is attached to the lower side of a wafer  18  containing a plurality of chips  2  or the like components. In  FIG. 4A , at  20  it is schematically indicated how the adhesive film material  19  is laminated to the wafer  18  by means of a roller, after a possible lower PE cover foil has been pulled off. The upper PET cover film which in the attachment according to the illustration on the right-hand side of  FIG. 3  will then come to lie on the lower side, may also be pulled off immediately, or it may be pulled off directly before applying the chip  2  on the printed circuit board base  4 . 
     After the wafer rear side has been laminated in this way with adhesive film material  19 , as illustrated in  FIG. 4A , the wafer  18  is subdivided into the individual chips  2  according to  FIG. 4B , which has been quite schematically indicated by means of a cutting tool  21 . 
     Subsequently, the chips  2  with the adhesive film sections  3  thus obtained directly on the lower side of the chip  2  will be attached to the printed circuit board base  4  for which the temperature-adjusted suction tool  17  already explained by way of  FIG. 3  can be used. 
     The base  4  may be heated from its lower side by means of a heating block  22 , just as a heated block  23  kept, e.g., at a temperature of approximately 80° C. can be used during the application of the adhesive film material  19  on the wafer  18  according to  FIG. 4A  so as to already partially cure the adhesive material in each case. 
     In  FIGS. 5A to 5D , as an alternative to  FIGS. 4A to 4C , a technique is illustrated in which the individual adhesive film sections  3  previously have been separated by transverse cutting from a strip-shaped adhesive film material  19  by means of a cutting tool  24 , e.g. In this case, the one cover film, i.e. the PE cover film, has been pulled off previously, and then the individual adhesive film sections  3  are cut and attached by means of the tool  17  on the printed circuit board base  4 , cf.  FIG. 5B , wherein the printed circuit board base  4  again may lie on a heated block  22  so as to already preliminarily cure the adhesive material of the adhesive film sections  3  to a certain extent. 
     As an alternative, according to  FIG. 5D  a punching tool  25  can be used for punching and directly pressing the adhesive film sections  3  onto the base  4 . In this instance, the base  4  may rest on a modified heating block  22 ′ kept, e.g., at a temperature of 140° C. 
     Subsequently, no matter how the adhesive film sections  3  have been applied to the base  4 , the components  2  on the adhesive film sections  3  are attached to the base  4  by means of the tool  17 . As mentioned above, a possibly present upper PET cover foil will previously be pulled off the adhesive film sections  3 . 
     As shown in  FIG. 4A  or  5 A and  5 D, respectively, the adhesive film material  19  may be provided as a rolled-up material and may be pulled off the roll so as to obtain the individual adhesive film sections  3 . 
     Before attaching the adhesive film sections  3  (or the component  2  together with the adhesive film sections  3  previously attached thereto) on the base  4 , this base may also be irradiated by an infrared lamp in the attachment positions and pre-heated, such heating period being from 10 to 20 seconds, depending on the output of the infrared lamp. In tests, the force applied when pressing on the adhesive film sections  3 , or the components  2 , respectively, was 37N. The curing temperature for the adhesive of the adhesive film sections  3  was 150° C., and 170° C., respectively.