Abstract:
An electrical sub-circuit assembly is secured to a printed circuit board by providing a printed circuit board with a basin, the printed circuit board having electrical pathways leading to the top or side or on the bottom of said basin, and positioning an electrical sub-circuit assembly in the basin, the sub-circuit assembly having dimensions corresponding to the size of the basin so as to be a close fit therein and having electrical connections which locate adjacent corresponding electrical pathways on the printed circuit board. The sub-circuit assembly is secured in the basin, and the electrical connections on the sub-circuit assembly are connected to the corresponding pathways on the printed circuit board.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates to securing electrical sub-circuit assemblies to printed circuit boards.  
       BACKGROUND OF INVENTION  
       [0002]     Within a main circuit system, there may frequently be a number of individual sub-circuits which are each contained within a separate assembly, which may be a packaged assembly. Such sub-circuits may for example be transistor dies, integrated circuit dies or passive components. Both the physical attachment of such sub-circuit assemblies to a main circuit package and the electrical interconnections therebetween may be unreliable in terms of their rigidity and power handling capabilities and often produce poor electrical impedance matching results thereby producing high insertion loss between the interconnections. Prior art methods of providing electrical conductivity between a main circuit and a sub-circuit using bond wires or bump connections often result in mismatch of the electrical circuits, problems with high power capabilities, increased insertion loss and costly manufacturing processes. Conventional bond wires can also cause parasitic inductances, antenna effects and signal cross talk.  
         [0003]     It is therefore an object of this invention to provide an improved method of securing an electrical sub-circuit assembly to a printed circuit board.  
       SUMMARY OF INVENTION  
       [0004]     The present invention provides a method for coplanar attachment and interconnection of electronic devices and assemblies by forming a cavity or “basin” feature within the main circuit platform. This basin accommodates the shape of the sub-circuit assembly, with the depth of the basin being equal to the thickness of the sub-circuit assembly, with suitable adjustment to accommodate any needed adhesives or solder necessary to fix the sub-circuit assembly to the main circuit platform. The basin can be formed for example by machining a finished laminate, such as by a mechanical milling process or by laminating two or more suitably shaped planar pieces together.  
         [0005]     This invention provides a low cost and reliable circuit assembly method by permitting a sub-circuit assembly to be placed directly on a main circuit platform by an automated machine, then physically fastened with proper adhesive or solder paste dispensed by an automated machine and then electrically connected with the aid of an automatic machine.  
         [0006]     This invention renders it possible to both physically and electrically attach either an individual component or a more sophisticated sub-circuit assembly to a main circuit platform such that one surface of the sub-circuit assembly is coplanar with the surface of the main circuit platform. Simple planar electrical connection techniques can then be applied between the sub-circuit and main circuit assemblies, thereby eliminating the need for bond wires and improving impedance match and current carrying capabilities and reducing insertion loss between the sub-circuit assembly and the main circuit platform.  
         [0007]     This invention also eliminates the need for a package to house the sub-circuit assembly and the wire bonds or bump connectors which are used to connect within a package. The elimination of such a package permits the deposition of biological or other chemical layers without the intrusion of wires or package geometries.  
         [0008]     This invention can be used in a wide range of electronic products. Particular utility can be found in the following:  
         [0009]     (a) Attachment of sensors: this invention is useful in attaching a sub-circuit die to a platform assembly where the active surface of the die must face upwardly and be exposed. The active surface may include elements such as biological and chemical sensors, MEMS devices and optical devices. These sensors would function in either the vapour or liquid phase. The active surface of the die and the platform assembly, being coplanar, will produce less aerodynamic or fluid-dynamic turbulence or impediments in the vicinity of the active surface when compared with conventional bond wire interconnections.  
         [0010]     (b) Radio frequency (RF) products: the coplanar nature of interconnections in accordance with the invention minimizes RF parasitic effects and impedance mismatch, leading to higher operating efficiency and better performance. In a cell phone for example, this means better sensitivity and longer battery life. At a more detailed level, this interconnection technique in accordance with the invention permits direct microstrip launching from the main circuit board platform into a sub-circuit assembly which may house a RF semiconductor device. This invention enables the removal of the high frequency barrier inherent to conventional bond wires at microwave and millimetre-wave applications and the coplanar feature assists in extending the operating frequency range.  
         [0011]     This invention may also permit significant cost savings by eliminating expensive coaxial or other interconnection methods between circuit boards. Interconnection is particularly difficult with radio frequency circuitry. Over a broader range of electronic products, this invention provides an alternative to the prevailing motherboard/daughterboard paradigms in which daughterboards (sub-circuit assemblies) are positioned either parallel or perpendicular to motherboards (main circuit assemblies) and interconnected by bulky and expensive connectors or cables.  
         [0012]     In radio frequency products, the ability to combine dissimilar platform materials may lead to cost reduction. For example, a sub-circuit assembly comprising a small amount of higher cost low-loss material can be coplanar mounted into a larger cheaper yet higher-loss main circuit platform. The noise figure of a receiver can be enhanced by providing the sub-circuit LNA portion on a small low-loss material such as a ceramic material, while the main circuit can benefit from the low cost and mechanical strength of regular epoxy-fibreglass (FR4) circuit board material.  
         [0013]     (c) Miniaturization and low profile design: this invention makes it possible to make “flatter” circuit assemblies. By embedding the sub-circuit assemblies within the main circuit board, parts will not project above the main circuit board, thus resulting in potentially lower overall thickness of an assembly, and the better protection of components.  
         [0014]     Typically, packaged sub-circuits which rely on bond wires to complete electrical connections between the components and package are limited to the size of the bond wire and the inductance and resistive impedance characteristics of the bond wire. It is difficult to provide packaged sub-circuits which utilize conductive bump techniques with good inductance and resistance characteristics due to the small cross-sectional area at the point of electrical contact within the bump. The present invention eliminates the costly package and electrical connectivity requirement between the component and package with bonding or conductive bump processes. An improvement in current carrying capabilities, less insertion loss and impedance matching may also be a direct result of the present invention. Instead of having electrical signals flow through very small diameter bond wires or the narrow cross-sectional area of conductive bumps, electrical signals with the present invention can flow through properly dimensioned electrical paths to maintain current carrying, insertion loss and impedance match integrity.  
         [0015]     Interconnections which utilize small diameter bond wires typically have a higher inductance and resistance values than for interconnections utilizing solder, conductive adhesive or SMT components. The inductance of a wire can be approximated using equation 1 (from “RF Circuits Design”, C. Bowick, Howard W. Sams Company, ISBN: 0-672-21868-2).  
               L   ⁢           ⁢   wire     =     0.002   ⁢     (   l   )     ⁢     (     2.3   ⁢     Log   ⁡     (         4   ⁢     (   l   )       D     -   0.75     )         )               (   1   )             
 
         [0016]     For a wire of 0.001 inches in diameter (D) and 1 mm in length (1) a value of 1.01 nH is derived from equation (1). This value compares favourably with 3D bond wire electrical modeling data published by the Electrical Packaging Group of Amkor Technology. A value of 0.606 nH is derived from Amkor&#39;s 3D modeling approximation found in equation (2). 
 
 L 11=1.0703( l )−0.4641   (2) 
 
         [0017]     The approximate inductance of a 0402 defined SMT resistance component of zero ohms using equation (1) is 0.150 nH and is derived by replacing the width of the 0402 SMT with the diameter D of equation (1). This is a lower value than for a single bond wire of the same length. However, several bond wires in parallel would reduce the bond wire inductance to a lower value than that of the SMT component&#39;s inductance but with much more additional complexity in the wire bonding process.  
         [0018]     The comparison of the resistance values of the bond wires versus the SMT 0402 component is slightly more complicated as a phenomena called the skin effect occurs. As the frequency of the signal passing through a conductive block is increased, the current tends to flow near the surface of the block and not through its centre. Equation (3) describes how the depth of this current carrying layer is calculated.  
             δ   =       2   ωμσ               (   3   )             
 
         [0019]     Where, ω is the frequency component, μ is the permeability of the current carrying block and σ is the conductivity of the current carrying block. Typically, it can be assumed that the current flows in several layers each δ thick for a total of nδ thickness where n is the number of layers.  
         [0020]     The calculation of the resistance for a wire of 0.001 inches in diameter (D) and 1 mm in length (l) is illustrated in equation (4), where area is the cross-sectional area of the bond wire where the current flows. This value is then calculated as 0.129 Ω.  
             R   =     l     σ   ⁡     (   Area   )                 (   4   )             
 
         [0021]     This value again compares favourably with 3D bond wire electrical modeling data published by the Electrical Packaging Group of Amkor Technology. A value of 0.127 Ω is derived from Amkor&#39;s 3D modeling approximation found in equation (5). 
 
 R 11=128.7( l )−2.0149   (5) 
 
         [0022]     The approximate resistance of a 0402 defined SMT resistance component of zero ohms using equation (4) and accounting for skin depth δ is 0.000925 Ω and is again derived by replacing the width of the 0402 SMT with the diameter D for the calculation of the area of equation (4).  
         [0023]     This large difference of resistance values between bond wires and the SMT 0402 component would impact the current carrying capabilities of the bond wire along with an increase in losses when compared with the conductive adhesive and SMT 0402 component. Similar results would also apply to the narrow cross-sectional area associated with a conductive bump.  
         [0024]     The present invention is specially useful with respect to sensor devices utilizing surface acoustic wave (SAW) or other acoustic wave (AW) devices.  
         [0025]     Example of use of acoustic wave devices in conjunction with RF components are well described in:  
         [0026]     U.S. Pat. No. 5,488,866, TIME-INTERLEAVED METHOD FOR EFFICIENT OPERATION OF AN ACOUSTIC WAVE SENSOR ARRAY, and in Hunt&#39;s publication, W. D. Hunt, D. D. Stubbs and S. H. Lee, “Time-dependent Signatures of Acoustic Wave Biosensors,”  IEEE Proceedings,  Vol. 91, no. 6, pp. 890-901, June 2003.  
         [0027]     The present invention is an improvement over Yatsuda&#39;s method which incorporates conductive bumps but still requires a separate package, described in:  
         [0028]     U.S. Pat. No. 5,252,882, SURFACE ACOUSTIC WAVE DEVICE AND ITS MANUFACTURING METHOD”, and his publications;  
         [0029]     H. Yatsuda, H. Iijima, K. Yabe and O. Iijima “FLIP-CHIP STW FILTERS IN THE RANGE OF 0.4 TO 5 GHZ,” IEEE UFFC Symposium, October, 2002. Japan Radio Co., Ltd., Kamifukuoka-shi, Saitama. 356-0011 Japan  
         [0030]     Hiromi Yatsuda, Taira Horishima, Takeshi Eimura, and Takao Ooiwa Miniaturized “SAW Filters Using a Flip-Chip Technique,” IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 43, NO. 1, pp. 125-130, January 1996.  
         [0031]     Another area in which this invention provides improvement is the simpler refinement of the full-wave analysis using a finite-element simulation between the package and bond wires. An example of this modeling is shown in:  
         [0032]     C. Finch, Xiaomin Yang, T. X. Wu and B. Abbott, “RF Saw Filter Package Design For Wireless Communications,” Microwave And Optical Technology Letters, Vol. 39, No. 5, Dec. 5, 2003.  
         [0033]     The elimination of the bond wires will greatly reduce the complexity of the connectivity model.  
         [0034]     Other attempts to eliminate the wire bonds require soldering techniques to complete the connectivity between the component die and the electrical substrate as shown in:  
         [0035]     U.S. Pat. No. 6,731,000 B1, Haque, FOLDED-FLEX BONDWIRE-LESS MULTICHIP POWER PACKAGE.  
         [0036]     Haque et al requires as described in this patent the placement of connectivity studbumps on the die. These three dimensional studbumps allow for sufficient contact within a soldering process and may not function with a flat contact as described within this Patent. Coplanar design techniques would still be difficult with Haque&#39;s method.  
         [0037]     The present invention is also well suited for sensor applications in which a biologically coated acoustic wave device is submerged in water for the detection of target molecules. The coplanar feature of die attachment provides a simpler method of placing a potting medium to cover the electrical portion of the die and not cover the biological area within the die. The submerged device would only allow an interface between the exposed biological layer and the water. Since there are no bonding wires protruding from the die which may be harmed during the potting process, the coplanar method and structure of the present invention is a significant improvement over the prior art.  
         [0038]     A coplanar bridge connection in accordance with the invention may connect a transmission line microstrip structure on the motherboard to a corresponding structure on the sub-circuit assembly (daughterboard). The bottom of the basin can be a groundplane which is contiguous between the sub-circuit assembly and the motherboard. The benefit of such an interconnection is that it is receptive to RF signals, which is particularly important with ever-increasing computer central processor operating frequencies and with the increasing miniaturization and increasing operating frequencies of wireless devices.  
         [0039]     Thus, this invention improves the method and structure of the sub-circuit attachment process onto a main circuit electrical platform. The sub-circuit assembly being attached may be a discrete packaged or unpackaged die, semiconductor device (transistor, diode, etc.), an integrated circuit in die (or bumped die) form, quartz devices such as crystals, filters, resonators, surface acoustic wave (SAW), surface transverse wave (STW), bulk acoustic wave (BAW), piezoelectric transducers, micro mechanical systems (MEMS) devices, chemical sensors, biological sensors, light emitters and light sensors. A sub-circuit assembly is generally attached to an electrical platform which is a specially prepared printed circuit board which could be made from any common laminate material, including epoxy fibreglass, phenolic, PTFE, alumina, etc. This invention is particularly effective for attaching sub-circuit assemblies which are incompatible with conventional soldering techniques used to attach components to circuit boards. Such incompatibility may be due to temperature sensitivity, or sensitivity to chemicals such as fluxes used in the soldering procedure within the electronics manufacturing process. This invention is also particularly useful for attaching sub-circuit assemblies to main circuit electrical platforms in cases where the surface of the component used for electrical connections must be facing upwards and/or exposed to ambient environment.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0040]     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which:  
         [0041]      FIG. 1  is a diagrammatic perspective view of an acoustic wave device of a bare die;  
         [0042]      FIG. 2  is a similar view of a main circuit platform comprising a multi-layer printed circuit board in accordance with one embodiment of the invention, showing basin details and electrical pathways;  
         [0043]      FIG. 3  is a diagrammatic plan view showing a SAW die placed in the basin in the multi-layer printed circuit board;  
         [0044]      FIG. 4  is a similar view showing conductive adhesive dots placed on both the die and electrical traces of the printed circuit board and a non-conductive adhesive to secure the die;  
         [0045]      FIG. 5  is a similar view showing zero Ohm SMT resistors located on the conductive dots placed on both the dies and electrical traces of the printed circuit board;  
         [0046]      FIG. 6   a  is a diagrammatic perspective view of an acoustic wave device on a bare die;  
         [0047]      FIG. 6   b  is a similar view showing a suitable coating for handling by an SMT machine, and a window in the coating for exposing sensitive material; and  
         [0048]      FIG. 7  is a plan view of the product shown in  FIG. 6   b  with a conformal coating layer over the main circuit platform for protection against vapours and liquids. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0049]     Referring to the drawings a surface acoustic wave (SAW) die assembly  100  is shown in  FIG. 1 . A first interdigital transducer (IDT)  121  is suitably located on a piezoelectric substrate  110  of thickness T 115. The IDT  121  transforms an electrical signal into an acoustic wave and, being a reciprocal mechanism, also transforms an acoustic wave into an electrical signal. A second IDT  122  is suitably located along the propagation path of the acoustic waves  140 . An IDT has inherent impedance which is a function of the physical geometry of the IDT. The IDT electrical connections  130  can thus be designed to match the impedance value of the adjacent IDTs  121 ,  122 . The electrical connections  130  are similar to microstrip geometries and are suitably scaled in dimensions to also achieve an impedance equal to the inherent impedance of the IDTs  120 ,  122 . The electrical connections  130  are located to bring the electrical interconnections of the circuit on the die substrate  110  to the periphery thereof.  
         [0050]     Other sub-circuit die assemblies with similar physical geometries are also within the scope of this invention. The main criteria is that the electrical connections  130  bring to the physical periphery of the sub-circuit assembly and the die substrate  110  an electrical connection which matches the impedance of the equivalent internal circuitry  121 ,  122 . Sub-circuit die assemblies may also have electrical connections  130  positioned on either the top or bottom or both layers of the assembly.  
         [0051]     Although the best benefit can be achieved if bare die sub-circuit assemblies are attached to a main circuit electrical platform, it is also within the scope of the invention to attach packaged semiconductor devices to electrical platforms, such as circuit board assemblies with the actual packaged semiconductor devices being placed upsidedown. Packaged radio-frequency sub-circuits can be attached to a main circuit in this way with minimal impedance mismatch. The connectors of the packaged sub-circuit are equivalent to the electrical connections  130  shown in  FIG. 1 .  
         [0052]     The SAW die component shown in  FIG. 1  is placed within an electrical platform  200  of the main circuit assembly shown in  FIG. 2 . A multi-layer printed circuit board (PCB) is used as an example to explain the method and structure of coplanar die attachment. The first layer  211  with physical thickness T 205 has electrical pathways  220  which provide electrical interconnections between components of other electrical connections. Within the first layer  211 , a basin region  240  is located at the physical position where the sub-circuit assembly such as the piezoelectric die substrate  110  is to be located. The reservoirs  230  which will receive excess adhesive material are a result of cutting reliefs during the milling process and are suitably placed so as not to interfere with the electrical pathways  220 . In this embodiment, the reservoirs  230  are placed at the four comers of the basin region  240 . The basin region  240  exposes a second layer  212  of the electrical platform structure  200  along with the electrical pathways  225  of the layer  212 . The electrical pathways  225  of layer  212  are physically expanded to allow the formation of a metal ground plane suitable for microstrip geometries. Other layers, up to layer n  213  complete the structure of the electrical platform  200 .  
         [0053]     The basin region  240  can be formed either by machining a finished laminate, by mechanical milling or other means of material removal, or by laminating two or more planar pieces together. For milling, the basin region  240  is milled within the first layer  211  to a depth equal to the thickness of a sub-circuit assembly to be attached with an adjustment to accommodate any needed adhesives and adjusted in thickness geometries to accommodate any needed adhesives or solder used to affix the sub-circuit assembly within the basin region  240 . For lamination, two or more layers of laminate are joined together. The first layer  211  is the same thickness as the sub-circuit assembly with an adjustment to accommodate any needed adhesives or solder used to affix the sub-circuit assembly. The first layer  211  has a portion of its material removed before the lamination process to produce an opening which is the same shape as the perimeter of the sub-circuit assembly. For a simple embodiment, the second layer  212  would be the bottom piece and be void of any basins, and the first layer  211  and the second layer  212  would be laminated together, forming the basin  240 .  
         [0054]     The physical placement of a sub-circuit assembly such as a SAW die assembly  302  within a basin region  340  located within an electrical platform  300  is shown in  FIG. 3 . The perimeter dimensions of the basin region  340  are such that the SAW die assembly  302  can be placed within the basin region  340  to minimize the gap between the SAW die assembly  302  and the top surface of the first layer  311 . The dimensions of the basin region  340  are also such that electrical pathways  320  situated on the surface of the first layer  311  align themselves with electrical connections  322  of the SAW die assembly  302 . The first surface layer  311  is coplanar with the surface of the SAW die assembly  302  such that both have a minimum difference in height and gap between them. For other assembly components, the dimensions and coplanar positioning of a sub-circuit assembly would also be such that the electrical pathways  320  and the second layer  212  shown in  FIG. 2  would align themselves with the die assembly electrical connections  322  located on either or both sides of the SAW die assembly  302 . The reservoirs  330  are positioned so as not to interfere with any electrical pathways  320  or electrical connections  322 . The reservoirs  330  are constructed such that a part of the geometry of the die  302  either extends into each reservoir  330  or lies directly adjacent to it.  
         [0055]     The physical attachment of a sub-circuit SAW die assembly  402  to a main circuit electrical platform  400  is shown in  FIG. 4 . Physical attachment of the SAW die assembly  402  to the main circuit electrical platform  400  is required to eliminate any possible lateral movement of the SAW die assembly  402  during the electrical connection processing or post manufacturing of the main circuit electrical platform  400 . The SAW die assembly  402  or other equivalent sub-circuit assembly is retrieved from a tray or reel and placed by a surface mount technology (SMT) machine into a basin region  440 . A non-conductive adhesive  435  is then properly dispensed either manually or with an automated process utilizing an SMT machine, stenciling, pin transfer or syringe-dispensing into the reservoirs  430 . The viscosity of the non-conductive adhesive  435  is chosen so that it makes contact with the SAW die assembly  402 , the geometric surfaces of the basin region  440  and the top surface of the second layer  212 . For some non-conductive adhesive products, a temperature curing process would then improve the rigidity of the non-conductive adhesive  435  to prevent further movement of the SAW die  402 . Curing the non-conductive adhesive can be achieved for example with ultraviolet light, moderate temperature baking, normal solvent evaporation or chemical reaction.  
         [0056]     The electrical attachment of the SAW die assembly  402  to the main circuit electrical platform  400  can be accomplished by several methods. A first method involves continuous conductive adhesive tracks  427  which are dispensed either manually, or preferably with a placement/dispensing machine, beginning on the electrical pathways  420  of the electrical platform  400  and continuing with a continuous conductive track onto the closely adjacent electrical connections  422 . The continuous nature of the conductive adhesive track  427  completes the electrical connection between the electrical pathways  420  and the adjacent electrical connections  422 .  
         [0057]     A second method involves the placement of dots  425  of conductive material on certain areas of the electrical pathways  420  and electrical connections  422 , the conductive material being in the form of conductive adhesive or solder paste. The conductive material can be dispensed either manually of with an automated process utilizing a placement/dispensing machine, stenciling, pin transfer or syringe dispensing methods onto the electrical pathways  420  located on the electrical platform  400  and separately onto the closely adjacent electrical connection  422  of the SAW die assembly  402 . The resultant conductive dots  425  are positioned adjacent each other at the end extremities of the electrical pathways  420  and electrical connections  422 .  
         [0058]     This method is further illustrated in  FIG. 5 . The completion of the electrical circuits between the SAW die assembly  502  and other circuitry located on the electrical platform  500  and connected by electrical pathways  520  is accomplished by either a standard SMT zero ohm resistor or other value SMT component  528 , or piece of conductive material, dispensed either manually or with an automated process onto the conductive dots  525 . For example, both interconnection and AC coupling can be achieved simultaneously by using a chip capacitor SMT component  528 . Also, active devices such as SMT packaged transistors or integrated circuits including ball grid arrays (BGAS) or flip-chips can be used instead of the SMT component  528 . For some conductive material products, a temperature curing process can improve both the physical structure and the conductivity between the conductivity adhesive material  525  and the SMT component  528 . Curing can be achieved with ultraviolet light, moderate temperature baking, normal solvent evaporation or chemical reaction. In the case of more robust devices which are temperature tolerant, the dots  525  of solder paste conductive material can be processed by any common soldering method, including convection reflow, vapour phase reflow, hot-plate conduction, hand-soldering, wave-soldering and laser or xenon beam selective soldering. For other sub-circuit assemblies with similar physical geometries and with electrical connections such as a ground plane on the bottom face of a die, an initial step must be performed before the die is placed within the basin. Referring to  FIG. 2 , a conductive material is suitably dispensed either manually or with an automated process onto the electrical pathways  225  of the second layer  212  before the die is place within the basin.  
         [0059]     An important issue which arises during placement of the sub-circuit assembly on the main circuit electrical platform is the potential damage to the exposed circuitry on the surface of the sub-circuit assembly during automated placement. By way of example,  FIG. 6   a  shows a SAW die assembly  602  fashioned onto a piezoelectric substrate  610 . Interdigital transducers such as first and second IDTs  621 ,  622  and electrical connections  630  are typically fabricated using thin film techniques. The thin film could be damaged as a vacuum chuck or other handling apparatus of an SMT machine imposes pressure on the die assembly  602 . One way of protecting the die assembly  602  is to selectively coat part of the surface of the die assembly with a protective handling layer  660  as shown in  FIG. 6   b.  The protective handling layer  660  should be applied as early as possible in the manufacturing process of the die and, if possible, before the die assembly is separated from its original wafer or equivalent form. The protective handling layer  660  can be arranged to provide correct exposure of the electrical connections  630  and thereby enable proper placement of the conductive adhesive  525 , conductive adhesive tracks  527  and the standard SMT components  528  shown in  FIG. 5 .  
         [0060]     Such processing may also involve the provision of a window  670  in the protective handling layer  660 . The window  670  permits the post processing of a suitable sensitive material  680 , such as a biological, polymer or other interface material, to be applied to enhance the SAW die assembly  602  in its role as a sensor. The sensitive material  680  is attached to the exposed surface of the die  602  and operates to transform measurable constituent vapour or liquids which can be detected by the circuit parameters of the sub-circuit assembly. The window  670  exposes the part of the die assembly circuitry, for example the first and second IDTs  621 ,  622  shown in  FIG. 6   a  and which produce the required sub-circuit assembly parameter change.  
         [0061]     Another important issue which then arises is the potential damage to the circuitry and PCB when certain vapours or liquids come into contact with areas surrounding the window  670 .  FIG. 7  illustrates how a conformal coating  700  can cover the electrical platform  500 , die assembly  502 , electrical pathways  520 , electrical connections  522 , conductive adhesive dots  525 , conductive adhesive tracks  527  and the SMT components  528 , as shown in  FIG. 5 . This permits the sensitive material  780  and a partial protective handling layer area  760  to be exposed to a vapour or liquid, with the remainder of the circuit and components shown in  FIG. 5  being isolated therefrom.  
         [0062]     The advantages of the invention will now be readily apparent to a person skilled in the art from the foregoing description of preferred embodiments. Other embodiments will also now be readily apparent, the scope of the invention being defined in the appended claims.