referring first to fig1 to 27 , a die - up , flip - chip integrated circuit package substrate according to the invention is made by a sequential build up that starts with a thin stainless steel plate 10 ( 0 . 062 ″ thick ) that has been prepared by surface oxidation . copper is flash plated to steel plate 10 as shown in fig2 to form a thin ( e . g ., 10 - 20 micron , especially 15 micron ) copper layer 11 . the adhesion of the copper layer 11 to plate 10 is relatively weak , such that plate 10 can be pulled off later . a thicker layer 12 of dry film dielectric photoresist , such as a polyclad aqueous photodevelopable resist , is coated onto layer 11 and a pattern is developed therein in a manner well known in the art . this pattern preferably corresponds to the locations of electrical contacts to be located on the outside of the finished ic package . an optional second conductive metal 13 such as tin is flashed into the resulting channels 14 , then flashed over with a thin layer ( e . g ., 3 microns ) of copper , followed by plating to form a copper layer 16 in accordance with the location of the electrical contacts . the second metal layer 13 is applied for purposes of the manufacturing process as described hereafter but forms no part of the finished substrate . the resist layer 12 is then stripped by conventional methods , leaving the plate with copper layer 11 , and a first circuit pattern 15 of including tin and copper layers 13 , 16 . a resin - impregnated fiber laminate 17 , known commercially as pre - preg , is then applied with a light tack to the back of the assembly as shown in fig8 . an aramid - fiber epoxy pre - preg laminate 17 is preferred . as shown , the resin of laminate 17 flows and embeds circuit pattern 15 on three sides . a laser such as an esi 5200 yag laser or hitachi co 2 laser is then used at a power level sufficient to burn through the beta - stage pre - preg laminate 17 but not sufficient to bum through the underlying copper . holes or vias 21 are drilled at the location of each interconnect required by the circuit design . a conductive paste 22 of resin and conductive metal powder is then filled into holes 21 . as paste 22 , a sinterable material available commercially as ormalink made by ormet corporation is preferred . other preferred materials are described in u . s . pat . nos . 5 , 716 , 663 , 5 , 376 , 403 , 5 , 853 , 622 and 5 , 922 , 397 , the contents of which are incorporated by reference herein . ormalink contains copper powder in a resin base . the heat supplied during the fusing of one pre - preg laminate to another as described hereafter is effective to sinter the metal particles in the conductive adhesive to achieve electrical conductivity . after filling of vias 21 , the mylar release liner 18 of laminate 17 is peeled off , and a steel plate assembly 23 having a steel plate 10 a , copper coating 11 a , and tin and copper layers 13 a , 16 a forming a second circuit pattern 15 a thereon is inverted and positioned over filled vias 21 ( fig1 ). assembly 23 may be made in the same manner as described leading up to fig7 but with a different circuit design . assembly 23 is pressed into face - to - face contact with the underlying layers so that connection points of copper layers 16 a contact and bond to the exposed tops of the filling material 22 , which forms a conductive pathway through to the underlying circuit pattern 15 . this is carried out with a circuit press known in the art that uses multiline tooling to precisely align the assembly . steel plate 10 a is then removed by means of the weak adhesion between plate 10 a and copper layer 11 a , and layer 11 a is then removed by flash copper etching ( fig1 ), leaving layers 13 a , 16 a intact to form circuit pattern 15 a superposed on the contact pattern 15 . tin layer 13 a ( if applied ) is then electrolytically stripped , leaving copper layer 16 a exposed . in the alternative , if the etching of a thin outer copper layer such as 11 or 11 a can be precisely controlled , the intervening tin layer 13 , 13 a , etc . can be omitted , and the outer copper layer removed without stripping the underlying copper circuit layer . a second piece of pre - preg laminate 27 is placed over copper layer 16 a , resin side down , embedding copper layer 16 a as shown in fig1 . a second set of vias 28 are then drilled through laminate 27 at desired interconnect points , the laser again being stopped by the underlying copper . additional conductive paste 29 such as ormalink is filled into vias 28 , and the outer pre - preg liner 31 is removed . for the sake of uniformity , each of the pre - preg laminates used are preferably identical in type and dimensions . the resulting structure is now ready for final pairing . a second circuit assembly 33 is prepared having the same layer structure as shown in fig1 except for differences in the specific circuit pattern . assembly 33 is formed with second and third circuit patterns 15 b and 15 c on opposite sides of a third pre - preg laminate 35 . assembly 33 is inverted and brought into precise alignment with the underlying structure so that contact portions of copper layer 16 b of assembly 33 are brought into registration with the filler material 29 in vias 28 ( fig2 ). the respective steel plates 10 , 10 b and copper release layers 11 , 11 b are then successively removed from opposite sides of the resulting paired assembly 34 , leaving the optional tin layers 13 , 13 c exposed . copper layers 11 , 11 b can be removed using a chemical copper etchant which does not remove the underlying tin . these tin layers 13 , 13 c are then electrolytically stripped in the same manner as layer 13 a , leaving copper layers 16 , 16 c exposed . a soldermask layer 36 is then applied to both sides , then imaged and developed to expose desired contact points 37 on opposite sides of the assembly . the contacts 37 are then surface finished with a precious metal 38 such as silver , resulting in a circuit board substrate 41 having the structure shown in fig2 . in this example four circuit layers 15 , 15 a , 15 b and 15 c are formed , with 15 and 15 c being disposed on the outside beneath soldermask layers 36 , and circuits 15 a , 15 b being disposed on the inside , embedded between the associated pieces of pre - preg which have been fused together . to minimize the number of holes laser drilled through the prepreg , it is preferred to run the signal lines along the opposite outer sides of the assembly as circuits 15 , 15 c . circuits 15 a and 15 b are preferably designed as power and ground planes , respectively . these planes are larger ( wider , more planar ) in comparison with the signal lines , and embedding them inside the pre - preg laminate isolates them from each other and the signal lines . die pad pads 42 on one side for flip - chip die attachment as well as an outer row or rows of solder ball pads 43 on the other side , each connected by a conductive line 44 of one of circuits 15 or 15 c ( lines 44 on both sides of the substrate are shown in fig2 for purposes of illustration .) fig2 illustrates a ground plane 45 with current passing through the sintered conductive paste material in the vias when moving between a plane 45 and a pad 42 or 43 . solder ball pads 43 may be given an osp , silver or tin finish , and die pad pads 42 may have a copper / osp or solder finish . connections for lines 44 are similar , except that a single via including three stacked “ cones ” of conductive filler material penetrates the entire thickness of the assembly , either at the location of pad 42 or 43 . if desired , the intermediate steps used to form the intermediate circuit 15 a can be omitted ( from fig1 to 20 ). in such a case the ground and power planes could be formed at offset locations in the same embedded circuit layer . similarly , the procedure shown in fig1 - 20 may be repeated if needed to build in more than two embedded circuit layers . in addition , vias may be provided and filled for conducting heat away from the die , if needed . fig3 - 34 illustrate an alternative embodiment of a ic package substrate 51 of the invention which may have substantially the same layer structure as described for substrate 41 . however , a soldermask layer 46 on the die side is reduced in size ( length and width ), so that a square or rectangular heat sink 52 , which also acts as a stiffener for the assembly , can been adhered to one side of the layer structure by an adhesive or direct bonding , preferably by means of a layer 48 of a low cure adhesive that minimizes mechanical stress , such as of the tovay yef series . heat sink 52 includes a relatively thick copper plate clad 53 with an outer finish layer 54 such as nickel . a square central opening 56 forms a die cavity . solder mask 46 may optionally be slightly set back from the inner edge of heat sink 52 , following boundary 57 as shown in fig3 . exposed surfaces of signal lines in this area are covered later during encapsulation of the die . a central area 61 of the die side , which area 61 will underlie the die when installed and is of smaller length and width than the die , is configured with an array of power and ground pads 62 which are connected back to enlarged , depthwise interconnects 63 by conductive lines 64 . lines 64 often connect together several pads 62 and interconnects 63 . pads 62 are generally arranged at regular intervals , and are positioned according to the die manufacturer &# 39 ; s requirements . central area 61 also includes a number of signal pads 66 which have associated interconnects 67 . a square peripheral area 71 surrounding central area 61 contains exposed signal pads 72 which connect directly to rows of pins along the periphery of the die . a further square area 73 surrounding area 71 contains signal lines 74 which run from pads 72 towards the periphery of the device , disappearing beneath the inner edge of heat sink 52 , and then penetrating through the thickness of the device to ball pads 89 on the opposite side . as shown in fig3 - 34 , the solder ball side 81 of the substrate 51 includes a central group of ball pads 82 ( 8 by 8 in this example ) for power and ground connections which are connected back to interconnects 63 and power and ground pads 62 on the die side . some of these may be joined by conductive lines 83 if it is necessary to reposition a ball pad location . surrounding pads 82 is a square central area 86 free of pads through which conductive lines 93 on the ball side run , and around area 86 is a square outer area 88 in which a large number of signal ball pads 89 are positioned . these are generally arranged in rows and columns , in this example , nine pads deep on each side , with an innermost row 91 being 18 by 18 pads and the outermost row 92 being 34 by 34 pads , less pads omitted on the four corners . as shown in fig3 , some of the outer ball pads 89 a are connected by means of conductive lines 93 disposed just beneath the associated soldermask layer to depthwise interconnects 67 positioned near and among the central ball pads 82 , whereby pads 89 a are electrically connected to signal pads 66 on the die side . other pads 89 b have adjacent interconnects 96 that emerge underneath heat sink 52 and connect to each of lines 74 on the opposite side , leading to the outer signal pads 72 . it is most convenient to locate the power and ground pads on the side of the substrate directly opposite the die connections , but of course such connections could be routed to the side in the same manner as the signal connections . similarly , having conductive lines 74 , 93 on both sides of the device permits up to double the number of signal connections at a given spacing as compared to a single - sided construction . the foregoing embodiments also conserve space by vertical stacking of vias to form the interconnects . conventional manufacturing methods with less precise registration between adjacent layers generally require use of offset vias , greatly increasing the amount of space required . a substrate according to the invention can save up to 50 % in required space as compared to a conventional package substrate with the same number of connections . once formation is complete , a circuit board substrate 41 or 51 of the invention is singulated , tested , inspected and packed for shipment . final processing is carried out by the end user , who applies the die to the die pads and solder balls to the solder ball pads , then encapsulates the die in the conventional manner to form the finished integrated circuit package . such a circuit board substrate according to the invention provides numerous advantages over other known ic package designs . the flip - chip die connection eliminates the expense of connecting bonded wires and avoids the need to form a cavity for the die . the invention further provides a highly effective technique for forming conductive vias and achieving registration of fine features such as electrical contact points during the production process even when a large number of substrates are formed at the same time as a panel and later singulated by cutting . the use of bonded pre - preg laminates makes circuit board substrate 41 highly cost effective as compared to substrate packages requiring milled copper supports / heat sinks . the invention permits fine resolution of small features . for example , a substrate of dimensions 35 by 35 mm with a minimum line width and line spacing of 35 microns can accommodate an i / o count of 816 , bump pad diameter 125 microns , via pad diameter 90 microns , signal capture pad diameter 125 microns , signal via diameter 90 microns , staggered bump pad pitch 160 microns , solder ball pitch of 1 . 00 mm , with 2 signal layers and 2 power / ground layers . the filled vias , due to the laser used to burn the vias , tend to be conical , for example , 90 microns diameter on the outside , 75 microns diameter on the inside , with a depth of about 80 microns ( equals the thickness of the surrounding dielectric layer .) the sinterable conductive paste material , once sintering occurs , forms a conductive network of metal particles that is highly effective for conducting current . in subsequent sintering cycles , as needed when the second or subsequent pre - preg laminates are added , the previously sintered material in the vias does not re - melt and retains its superior electrical conductivity . the method of the invention keeps each half of a laminate pair bonded to a rigid backing or support ( the steel plate ) during assembly . this provides an enormous advantage in obtaining accurate registration of interconnections that cannot be matched by attempting to laminate a piece of pre - preg or adhesive tape onto a circuitized piece of pre - preg . both halves of the assembly should be rigidly secured to the support during pairing to ensure a superior product . according to a further aspect of the invention , the end user encapsulates the die in a novel manner in order to maximize heat dissipation through the heat sink . unlike in other formation processes where the die is mounted adjacent to the core or heat sink , in the present invention the die 100 as shown in fig3 is spaced from the heat sink 52 . the encapsulant used to cover the die is typically an epoxy that is non - conductive of both heat and electricity . according to a further aspect of the present invention , the die is placed in contact with a heat - conductive bridge that conducts heat from the die to the heat sink to a degree substantially better than a conventional encapsulant . this could be done , as shown in fig3 , by use of a layer 101 of an encapsulant having heat - conductive filler particles 102 distributed therein ( e . g ., aluminum or the like ) surrounding die 100 which would improve the heat conductivity to the heat sink 52 preferably without creating any electrical conductivity . a further layer 103 of conventional encapsulant lacking the conductive filler 102 may be filled in over layer 101 if needed . such an arrangement may become essential as integrated circuits are designed to operate at ever - increasing speeds . the heat conductive bridge is not limited to the filler particles 102 shown . for example , after the space around the die is filled with encapsulant to the level of the die , a backing plate made of copper , similar to the heat sink itself , can be inserted like a lid behind the die 100 . the backing plate has the same length and / or width as recess 56 and thereby acts as a heat bridge . a further layer of encapsulant is filled in behind the heat conductive plate , which could be perforated or cut - away ( e . g ., x - shaped ) so that the encapsulant could be filled in both above and below in one step after placement of the plate . the invention described herein is not limited to the applications described above and can be used for making circuit boards other than those for mounting an ic die . while the invention has been described with reference to the illustrated embodiment , it is not intended to limit the invention but , on the contrary , it is intended to cover such alternatives , modifications and equivalents as may be included in the spirit and scope of the invention .