Abstract:
A method of applying encapsulant to wire bonds between a die and conductors on a supporting substrate, by forming a bead of the encapsulant on a profiling surface, positioning the profiling surface such that the bead contacts the die and, moving the profiling surface relative to the die to cover the wire bonds with the encapsulant. Wiping the encapsulant over the wire bonds with a profiling surface provides control of the encapsulant front as well as the height of the encapsulant relative to the die. The movement of the profiling surface relative to the die can closely controlled to shape the encapsulant to a desired form. Using the example of a printhead die, the encapsulant can be shaped to present an inclined face rising from the nozzle surface to a high point over the wire bonds. This can be used by the printhead maintenance facilities to maintain contact pressure on the wiping mechanism. However, it will be appreciated that the encapsulant can be shaped to have ridges, gutters, grooves and so on by using a particular shape of profiling surface and relative movement with the die.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of integrated circuit packaging. In particular, the encapsulation of the wire bonds between a circuit board and the contact pads on the integrated circuit die. 
       CO-PENDING APPLICATIONS 
       [0002]    The following applications have been filed by the Applicant simultaneously with the present application: 
         [0003]    MPN007US MPN008US MPN009US MPN010US 
         [0000]    The disclosures of these co-pending applications are incorporated herein by reference. The above applications have been identified by their filing docket number, which will be substituted with the corresponding application number, once assigned. 
       CROSS REFERENCES TO RELATED APPLICATIONS 
       [0004]    Various methods, systems and apparatus relating to the present invention are disclosed in the following US patents/patent applications filed by the applicant or assignee of the present invention: 
         [0000]                                                        6405055   6628430   7136186   10/920372   7145689   7130075   7081974       7177055   7209257   7161715   7154632   7158258   7148993   7075684       7158809   11/225172   11/474280   11/635482   11/635526   11/650545   11/653241       11/653240   11758648   7241005   7108437   6915140   6999206   7136198       7092130   7249108   6566858   6331946   6246970   6442525   09/517384       09/505951   6374354   7246098   6816968   6757832   6334190   6745331       7249109   10/203559   7197642   7093139   10/636263   10/636283   10/866608       7210038   10/902833   10/940653   10/942858   11/706329   11/757385   11/758642       7170652   6967750   6995876   7099051   11/107942   7193734   11/209711       11/599336   7095533   6914686   7161709   7099033   11/003786   7258417       11/003418   11/003334   11/003600   11/003404   11/003419   11/003700   7255419       11/003618   7229148   7258416   11/003698   11/003420   6984017   11/003699       11/071473   11748482   11/778563   11/779851   11/778574   11/853816   11/853814       11/853786   11/856694   11/003463   11/003701   11/003683   11/003614   11/003702       11/003684   7246875   11/003617   11/764760   11853777   11/293800   11/293802       11/293801   11/293808   11/293809   11/482975   11/482970   11/482968   11/482972       11/482971   11/482969   11/097266   11/097267   11/685084   11/685086   11/685090       11/740925   11/763444   11/763443   11/518238   11/518280   11/518244   11/518243       11/518242   11/084237   11/084240   11/084238   11/357296   11/357298   11/357297       11/246676   11/246677   11/246678   11/246679   11/246680   11/246681   11/246714       11/246713   11/246689   11/246671   11/246670   11/246669   11/246704   11/246710       11/246688   11/246716   11/246715   11/246707   11/246706   11/246705   11/246708       11/246693   11/246692   11/246696   11/246695   11/246694   11/482958   11/482955       11/482962   11/482963   11/482956   11/482954   11/482974   11/482957   11/482987       11/482959   11/482960   11/482961   11/482964   11/482965   11/482976   11/482973       11/495815   11/495816   11/495817   6227652   6213588   6213589   6231163       6247795   6394581   6244691   6257704   6416168   6220694   6257705       6247794   6234610   6247793   6264306   6241342   6247792   6264307       6254220   6234611   6302528   6283582   6239821   6338547   6247796       6557977   6390603   6362843   6293653   6312107   6227653   6234609       6238040   6188415   6227654   6209989   6247791   6336710   6217153       6416167   6243113   6283581   6247790   6260953   6267469   6588882       6742873   6918655   6547371   6938989   6598964   6923526   6273544       6309048   6420196   6443558   6439689   6378989   6848181   6634735       6299289   6299290   6425654   6902255   6623101   6406129   6505916       6457809   6550895   6457812   7152962   6428133   7216956   7080895       11/144844   7182437   11/599341   11/635533   11/607976   11/607975   11/607999       11/607980   11/607979   11/607978   11/735961   11/685074   11/696126   11/696144       11/696650   11/763446   10/407212   7252366   10/683064   10/683041   11766713       11/841647   11/482980   11/563684   11/482967   11/482966   11/482988   11/482989       11/293832   11/293838   11/293825   11/293841   11/293799   11/293796   11/293797       11/293798   11/124158   11/124196   11/124199   11/124162   11/124202   11/124197       11/124154   11/124198   11/124153   11/124151   11/124160   11/124192   11/124175       11/124163   11/124149   11/124152   11/124173   11/124155   7236271   11/124174       11/124194   11/124164   11/124200   11/124195   11/124166   11/124150   11/124172       11/124165   11/124186   11/124185   11/124184   11/124182   11/124201   11/124171       11/124181   11/124161   11/124156   11/124191   11/124159   11/124176   11/124188       11/124170   11/124187   11/124189   11/124190   11/124180   11/124193   11/124183       11/124178   11/124177   11/124148   11/124168   11/124167   11/124179   11/124169       11/187976   11/188011   11/188014   11/482979   11/735490   11/853018   11/228540       11/228500   11/228501   11/228530   11/228490   11/228531   11/228504   11/228533       11/228502   11/228507   11/228482   11/228505   11/228497   11/228487   11/228529       11/228484   11/228489   11/228518   11/228536   11/228496   11/228488   11/228506       11/228516   11/228526   11/228539   11/228538   11/228524   11/228523   11/228519       11/228528   11/228527   11/228525   11/228520   11/228498   11/228511   11/228522       111/228515   11/228537   11/228534   11/228491   11/228499   11/228509   11/228492       11/228493   11/228510   11/228508   11/228512   11/228514   11/228494   11/228495       11/228486   11/228481   11/228477   11/228485   11/228483   11/228521   11/228517       11/228532   11/228513   11/228503   11/228480   11/228535   11/228478   11/228479       6087638   6340222   6041600   6299300   6067797   6286935   6044646       6382769   10/868866   6787051   6938990   11/242916   11/242917   11/144799       11/198235   11/766052   7152972   11/592996   6746105   11/763440   11/763442       11/246687   11/246718   11/246685   11/246686   11/246703   11/246691   11/246711       11/246690   11/246712   11/246717   11/246709   11/246700   11/246701   11/246702       11/246668   11/246697   11/246698   11/246699   11/246675   11/246674   11/246667       11/829957   11/829960   11/829961   11/829962   11/829963   11/829966   11/829967       11/829968   11/829969   7156508   7159972   7083271   7165834   7080894       7201469   7090336   7156489   10/760233   10/760246   7083257   7258422       7255423   7219980   10/760253   10/760255   10/760209   7118192   10/760194       10/760238   7077505   7198354   7077504   10/760189   7198355   10/760232       10/760231   7152959   7213906   7178901   7222938   7108353   7104629       11/446227   11/454904   11/472345   11/474273   7261401   11/474279   11/482939       11/482950   11/499709   11/592984   11/601668   11/603824   11/601756   11/601672       11/650546   11/653253   11/706328   11/706299   11/706965   11/737080   11/737041       11/778062   11/778566   11782593   11/246684   11/246672   11/246673   11/246683       11/246682   60/939086   7246886   7128400   7108355   6991322   10/728790       7118197   10/728784   10/728783   7077493   6962402   10/728803   7147308       10/728779   7118198   7168790   7172270   7229155   6830318   7195342       7175261   10/773183   7108356   7118202   10/773186   7134744   10/773185       7134743   7182439   7210768   10/773187   7134745   7156484   7118201       7111926   10/773184   7018021   11/060751   11/060805   11/188017   7128402       11/298774   11/329157   11/490041   11/501767   11/499736   7246885   7229156       11/505846   11/505857   11/505856   11/524908   11/524938   7258427   11/524912       11/592999   11/592995   11/603825   11/649773   11/650549   11/653237   11/706378       11/706962   11749118   11/754937   11749120   11/744885   11/779850   11/765439       11842950   11/839539   11/097308   11/097309   7246876   11/097299   11/097310       11/097213   11/210687   11/097212   7147306   7261394   11/764806   11/782595       11/482953   11/482977   11/544778   11/544779   11/764808   09/575197   7079712       6825945   09/575165   6813039   6987506   7038797   6980318   6816274       7102772   09/575186   6681045   6728000   7173722   7088459   09/575181       7068382   7062651   6789194   6789191   6644642   6502614   6622999       6669385   6549935   6987573   6727996   6591884   6439706   6760119       09/575198   6290349   6428155   6785016   6870966   6822639   6737591       7055739   7233320   6830196   6832717   6957768   09/575172   7170499       7106888   7123239   11/066161   11/066160   11/066159   11/066158   11/066165       10/727181   10/727162   10/727163   10/727245   7121639   7165824   7152942       10/727157   7181572   7096137   10/727257   10/727238   7188282   10/727159       10/727180   10/727179   10/727192   10/727274   10/727164   10/727161   10/727198       10/727158   10/754536   10/754938   10/727227   10/727160   10/934720   7171323       11/272491   11/474278   11/488853   11/488841   11749750   11749749   10/296522       6795215   7070098   7154638   6805419   6859289   6977751   6398332       6394573   6622923   6747760   6921144   10/884881   7092112   7192106       11/039866   7173739   6986560   7008033   11/148237   7222780   11/248426       11/478599   11/499749   11/738518   11/482981   11/743661   11/743659   11/752900       7195328   7182422   11/650537   11/712540   10/854521   10/854522   10/854488       10/854487   10/854503   10/854504   10/854509   7188928   7093989   10/854497       10/854495   10/854498   10/854511   10/854512   10/854525   10/854526   10/854516       10/854508   7252353   10/854515   10/854506   10/854505   10/854493   10/854494       10/854489   10/854490   10/854492   10/854491   10/854528   10/854523   10/854527       10/854524   10/854520   10/854514   10/854519   10/854513   10/854499   10/854501       7266661   7243193   10/854518   10/854517   10/934628   7163345   11/499803       11/601757   11/706295   11/735881   11748483   11749123   11/766061   11775135       11772235   11/778569   11/829942   11/014731   11/544764   11/544765   11/544772       11/544773   11/544774   11/544775   11/544776   11/544766   11/544767   11/544771       11/544770   11/544769   11/544777   11/544768   11/544763   11/293804   11/293840       11/293803   11/293833   11/293834   11/293835   11/293836   11/293837   11/293792       11/293794   11/293839   11/293826   11/293829   11/293830   11/293827   11/293828       11/293795   11/293823   11/293824   11/293831   11/293815   11/293819   11/293818       11/293817   11/293816   11/838875   11/482978   11/640356   11/640357   11/640358       11/640359   11/640360   11/640355   11/679786   10/760254   10/760210   10/760202       7201468   10/760198   10/760249   7234802   10/760196   10/760247   7156511       10/760264   7258432   7097291   10/760222   10/760248   7083273   10/760192       10/760203   10/760204   10/760205   10/760206   10/760267   10/760270   7198352       10/760271   10/760275   7201470   7121655   10/760184   7232208   10/760186       10/760261   7083272   11/501771   11/583874   11/650554   11/706322   11/706968       11/749119   11779848   11/855152   11855151   11/014764   11/014763   11/014748       11/014747   11/014761   11/014760   11/014757   11/014714   7249822   11/014762       11/014724   11/014723   11/014756   11/014736   11/014759   11/014758   11/014725       11/014739   11/014738   11/014737   11/014726   11/014745   11/014712   11/014715       11/014751   11/014735   11/014734   11/014719   11/014750   11/014749   7249833       11/758640   11/775143   11/838877   11/014769   11/014729   11/014743   11/014733       11/014754   11/014755   11/014765   11/014766   11/014740   11/014720   11/014753       7255430   11/014744   11/014741   11/014768   11/014767   11/014718   11/014717       11/014716   11/014732   11/014742   11/097268   11/097185   11/097184   11/778567       11852958   11852907   11/293820   11/293813   11/293822   11/293812   11/293821       11/293814   11/293793   11/293842   11/293811   11/293807   11/293806   11/293805       11/293810   11/688863   11/688864   11/688865   11/688866   11/688867   11/688868       11/688869   11/688871   11/688872   11/688873   11/741766   11/482982   11/482983       11/482984   11/495818   11/495819   11/677049   11/677050   11/677051   11/014722       10/760180   7111935   10/760213   10/760219   10/760237   7261482   10/760220       7002664   10/760252   10/760265   7088420   11/446233   11/503083   11/503081       11/516487   11/599312   11/014728   11/014727   7237888   7168654   7201272       6991098   7217051   6944970   10/760215   7108434   10/760257   7210407       7186042   10/760266   6920704   7217049   10/760214   10/760260   7147102       10/760269   7249838   10/760241   10/962413   10/962427   7261477   7225739       10/962402   10/962425   10/962428   7191978   10/962426   10/962409   10/962417       10/962403   7163287   7258415   10/962523   7258424   10/962410   7195412       7207670   11/282768   7220072   11/474267   11/544547   11/585925   11/593000       11/706298   11/706296   11/706327   11/730760   11/730407   11/730787   11/735977       11/736527   11/753566   11/754359   11/778061   11/765398   11/778556   11/829937       11/780470   11/223262   11/223018   11/223114   11/223022   11/223021   11/223020       11/223019   11/014730   7154626   7079292   11/604316                    
The disclosures of these co-pending applications are incorporated herein by reference.
 
       BACKGROUND OF THE INVENTION 
       [0005]    Integrated circuits fabricated on silicon wafer substrates are electrically connected to printed circuit boards by wire bonds. The wire bonds are very thin wires—around 25 to 40 microns in diameter—extending from contact pads along the side of the wafer substrate to contacts on the printed circuit board (PCB). To protect and strengthen the wire bonds, they are sealed within a bead of epoxy called encapsulant. The wires from the contact pads to the PCB are made longer than necessary to accommodate changes in the gap between the PCB and the contact pads because of thermal expansion, flex in the components and so on. These longer than necessary wires naturally form an arc between the contact pads and the PCB. The top of the wire arc is often about 300 microns above the contact pads although some wire bonding may extend even higher. As the name suggests, the encapsulant needs to encapsulate the full length of the wire so the encapsulant bead will extend 500 microns to 600 microns proud of the contact pads. 
         [0006]    The integrated circuit fabricated on the silicon wafer is often referred to as a ‘die’. For the purposes of this specification, the term die will be used as a reference to an integrated circuit fabricated on a wafer substrate using lithographic the well known etching and deposition techniques commonly used in semiconductor fabrication. If the die is purely an electronic microprocessor, there is little need to keep close control of the encapsulant bead dimensions. However, if the die is a micro-electro mechanical systems (MEMS) device with an active upper surface, it may be necessary or desirable to bring the active surface of the die onto close proximity with another surface. One such situation applies to inkjet printheads. The proximity of the print media to the nozzle array influences the print quality. Similarly, if a cleaning surface is wiped across the nozzles, the bead of encapsulant can hamper the wiping contact. 
         [0007]    Another problems arises because of sides of the encapsulant bead are not straight. One commonly used technique for depositing the encapsulant involves extruding it from a needle directly onto the line of wire bonds. The encapsulant volume and placement on the die is not very accurate. Variations in the pressure from the pump or slight non-uniformities in the speed of the needle cause the side of the bead contacting the active surface to be reasonably crooked. As the side of the bead is not straight, it has to be generously spaced from any active parts on the active surface to comfortably accommodate the perturbations. Spacing the electrical contacts away from the active portions (say for example, inkjet nozzles) of the active surface uses up valuable wafer real estate and reduces the number of dies that can be fabricated from a wafer disc. 
         [0008]    In light of the widespread use of inkjet printheads, the invention will be described with specific reference to its application in this field. However, the ordinary will appreciate that this is purely illustrative and the invention is equally applicable to other integrated circuits wire bonded to a PCB or other support structure. 
       SUMMARY OF THE INVENTION 
       [0009]    According to a first aspect, the present invention provides a microprocessor device comprising: 
         [0010]    a support structure having a chip mounting area and a conductor mounting area; 
         [0011]    a die supported on the chip mounting area, the die having a back surface in contact with the chip mounting area and an active surface opposing the back surface, the active surface having electrical contact pads; 
         [0012]    a plurality of electrical conductors at least partially supported on the conductor mounting area; and, 
         [0013]    a series of wire bonds extending from the electrical contact pads to the plurality of electrical conductors supported on the conductor mounting area; wherein, 
         [0014]    the chip mounting area is raised relative to the conductor mounting area. 
         [0015]    By raising the chip mounting area relative to the rest of the PCB, or at least the conductors connected to the PCB end of the wire bonds, the top of the arc formed by the layer is much closer to the active surface of the die. This, in turn, allows the bead of encapsulant to have a lower profile relative to the active surface. With a lower encapsulant bead, the active surface can be brought into closer proximity with another surface without making contact. For example, the nozzle array on a printhead IC can be 300 microns to 400 microns from the paper path. 
         [0016]    Preferably, the chip mounting area is raised more than 100 microns relative to the conductor mounting area. Preferably, the support structure has a step between the chip mounting area and the conductor mounting area. 
         [0017]    Preferably, the plurality of conductors are incorporated into a flexible printed circuit board (flex PCB) with a line of bond pads along an edge closest the die, the bond pads being more than 2 mm from the contacts pads on the die. 
         [0018]    Preferably, the wire bonds are formed from wire with a diameter less than 40 microns and extend less than 100 microns above the active surface of the die. 
         [0019]    Preferably, the wire bonds are plastically deformed such that they extend less than 50 microns above the active surface of the die. 
         [0020]    Preferably, the active surface has functional elements spaced less than 260 microns from the contacts pads of the die. In a particularly preferred form, the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected. In some embodiments, the support structure is a liquid crystal polymer (LCP) molding. 
         [0021]    Preferably, the wire bonds are covered in a bead of encapsulant, the bead of encapsulant extending less than 200 microns above the active surface of the die. 
         [0022]    Preferably, the wire bonds are covered in a bead of encapsulant, the bead of encapsulant having a profiled surface that is flat, parallel to and spaced less than 100 microns from the active surface. 
         [0023]    Preferably, the wire bonds are covered in a bead of encapsulant, the bead of encapsulant having a profiled surface that is flat and inclined relative to the active surface. 
         [0024]    Preferably, the wire bonds are covered in a bead of encapsulant, the encapsulant being an epoxy material that is thixotropic when uncured. 
         [0025]    Preferably, the wire bonds are covered in a bead of encapsulant, the encapsulant being an epoxy material has a viscosity greater than 700 cp when uncured. 
         [0026]    In a particular embodiment, the printhead IC is mounted in a printer such that during use the nozzles are less than 100 microns from the paper path. 
         [0027]    According to a second aspect, the present invention provides a method of profiling a wire bond between a contact pad on a die, and a conductor on a supporting structure, the method comprising the steps of: 
         [0028]    electrically connecting the contact pad on the die to the conductor on the supporting structure with a wire bond, the wire bond extending in an arc from the contact pad to the conductor; 
         [0029]    pushing on the wire bond to collapse the arc and plastically deform the wire bond; and, 
         [0030]    releasing the wire bonds such that the plastic deformation maintains the wire bond in a flatter profile shape. 
         [0031]    The strength of the wire bond is known to be relatively small; of the order of 3 to 5 grams force. However, the Applicant&#39;s work has found that the wire bond structure is robust enough to withstand a certain degree of work hardening from plastic deformation. The arc of the wire bond can be deformed into a flatter profile without compromising the electrical connection with the PCB. 
         [0032]    Preferably, the die has an active surface that has functional elements, the contacts pad being formed at one edge of the active surface, the wire bond has a diameter less than 40 microns and the arc extends more than 100 microns above the active surface of the die. 
         [0033]    Preferably, the wire bond is plastically deformed such that it extends less than 50 microns above the active surface of the die. 
         [0034]    Preferably, the wire bond is pushed by engagement with a blade having a rounded edge section for contacting the wire bond. 
         [0035]    Preferably, the method further comprises the steps of: 
         [0036]    applying a bead of encapsulant over the wire bond; and, 
         [0037]    moving a profiling surface over the active surface to flatten the bead of encapsulant. 
         [0038]    Preferably, the bead of encapsulant having a profiled surface that is flat, parallel to and spaced less than 100 microns from the active surface. 
         [0039]    Optionally, the bead of encapsulant having a profiled surface that is flat and inclined relative to the active surface. 
         [0040]    Preferably, the encapsulant being an epoxy material has a viscosity greater than 700 cp when uncured. In a particularly preferred form, the encapsulant being an epoxy material that is thixotropic when uncured. 
         [0041]    Preferably, the method further comprises the steps of: 
         [0042]    positioning the profiling surface adjacent and spaced from the active surface to define a gap; and, 
         [0043]    applying the bead of encapsulant onto the contact pads such that one side of the bead contacts the profiling surface and a portion of the bead extends into the gap and onto the active surface. 
         [0044]    Preferably, the active surface has functional elements spaced less than 260 microns from the contacts pads of the die. In a particularly preferred form, the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected. In some embodiments, the printhead IC is mounted in a printer such that during use the nozzles are less than 100 microns from the paper path. 
         [0045]    Preferably, the support structure has a chip mounting area and a conductor mounting area, the die is supported on the chip mounting area, and a plurality of electrical conductors at least partially supported on the conductor mounting area wherein, the chip mounting area is raised relative to the conductor mounting area. 
         [0046]    Preferably, the chip mounting area is raised more than 100 microns relative to the conductor mounting area. Preferably, the support structure has a step between the chip mounting area and the conductor mounting area. In some embodiments, the plurality of conductors are incorporated into a flexible printed circuit board (flex PCB) with a line of bond pads along an edge closest the die, the bond pads being more than 2 mm from the contacts pads on the die. 
         [0047]    Preferably, the support structure is a liquid crystal polymer (LCP) molding. 
         [0048]    According to a third aspect, the present invention provides a method for profiling a bead of encapsulant extending along an edge of a die mounted to a supporting structure, the method comprising the steps of: 
         [0049]    depositing a bead of encapsulant onto wire bonds along the edge of the die; 
         [0050]    positioning a profiling surface over the die at a predetermined spacing from the die; 
         [0051]    moving the profiling surface across the bead before the bead of encapsulant has cured to reshape the bead profile; and, 
         [0052]    curing the bead of encapsulant. 
         [0053]    The invention has found that the encapsulant can be effectively shaped by a profiling surface without stripping the encapsulant from the wire bonds. The normally convex-shaped upper surface of the encapsulant bead can be pushed to one side of the bead with the profiling surface. With a lower encapsulant bead, the active surface can be brought into closer proximity with another surface without making contact. For example, the nozzle array on a printhead IC can be 300 microns to 400 microns from the paper path. By collapsing or flattening the wire bond arcs before applying and profiling a bead of encapsulant, the nozzle array on the printhead IC can be less than 100 microns from the paper path. 
         [0054]    Preferably, the wire bonds extend in an arc from respective contact pads on the die to corresponding conductors on the support structure and the method further comprises the steps of: 
         [0055]    pushing on the wire bonds to plastically deform the wire bonds; and, 
         [0056]    releasing the wire bond such that plastic deformation maintains the wire bond in a flatter profile shape. 
         [0057]    Preferably, the die has an active surface that has functional elements, the contacts pad being formed at one edge of the active surface, the wire bond has a diameter less than 40 microns and the arc extends more than 100 microns above the active surface of the die. 
         [0058]    Preferably, the wire bond is plastically deformed such that it extends less than 50 microns above the active surface of the die. 
         [0059]    Preferably, the wire bond is pushed by engagement with a blade having a rounded edge section for contacting the wire bond. 
         [0060]    Preferably, the bead of encapsulant has a profiled surface that is flat, parallel to and spaced less than 100 microns from the active surface. 
         [0061]    Preferably, the bead of encapsulant has a profiled surface that is flat and inclined relative to the active surface. 
         [0062]    Preferably, the encapsulant being an epoxy material has a viscosity greater than 700 cp when uncured. 
         [0063]    Preferably, the encapsulant being an epoxy material that is thixotropic when uncured. 
         [0064]    Preferably, the method further comprises the steps of: 
         [0065]    positioning the profiling surface adjacent and spaced from the active surface to define a gap; and, 
         [0066]    applying the bead of encapsulant onto the contact pads such that one side of the bead contacts the profiling surface and a portion of the bead extends into the gap and onto the active surface. 
         [0067]    Preferably, the active surface has functional elements spaced less than 260 microns from the contacts pads of the die. In a further preferred form, the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected. In some embodiments, the printhead IC is mounted in a printer such that during use the nozzles are less than 100 microns from the paper path. 
         [0068]    Preferably, the support structure has a chip mounting area and a conductor mounting area, the die is supported on the chip mounting area, and a plurality of electrical conductors at least partially supported on the conductor mounting area wherein, the chip mounting area is raised relative to the conductor mounting area. 
         [0069]    Preferably, the chip mounting area is raised more than 100 microns relative to the conductor mounting area. In a particularly preferred form, the support structure has a step between the chip mounting area and the conductor mounting area. 
         [0070]    Preferably, the plurality of conductors are incorporated into a flexible printed circuit board (flex PCB) with a line of bond pads along an edge closest the die, the bond pads being more than 2 mm from the contacts pads on the die. 
         [0071]    Preferably, the support structure is a liquid crystal polymer (LCP) molding. 
         [0072]    According to a fourth aspect, the present invention provides a method of applying encapsulant to a die mounted to a support structure, the method comprising the steps of: 
         [0073]    providing a die mounted to the support structure, the die having a back surface in contact with the support structure and an active surface opposing the back surface, the active surface having electrical contact pads; 
         [0074]    positioning a barrier proximate the electrical contact pads and spaced from the active surface to define a gap; and, 
         [0075]    depositing a bead of encapsulant onto the electrical contact pads such that one side of the bead contacts the barrier and a portion of the bead extends into the gap and onto the active surface. 
         [0076]    Placing a barrier over the active surface so that it defines a narrow gap allows the geometry of the encapsulant front (the line of contact between the encapsulant and the active surface) can be more closely controlled. Any variation in the flowrate of encapsulant from the needle tends to cause bulges or valleys in the height of the bead and or the PCB side of the bead. The fluidic resistance generated by the gap between the barrier and the active surface means that the amount of encapsulant that flows into the gap and onto the active surface is almost constant. The reduced flow variations make the encapsulant front closely correspond to the shape of the barrier. Greater control of the encapsulant front allows the functional elements of the active surface of the die to be closer to the contact pads. 
         [0077]    Preferably, the barrier is a profiling surface and the method further comprises the steps of: 
         [0078]    moving the profiling surface over the active surface to flatten the bead of encapsulant. 
         [0079]    Preferably, the method further comprises the steps of: 
         [0080]    prior to depositing the bead of encapsulant, electrically connecting the contact pads on the die to respective conductors on the support structure with wire bonds, the wire bonds each extending in an arc from the contact pad to the conductor; 
         [0081]    pushing on the wire bonds to collapse the arc and plastically deform the wire bond; and, 
         [0082]    releasing the wire bonds such that plastic deformation maintain the wire bonds in a flatter profile shape. 
         [0083]    In a further preferred form, the active surface that has functional elements, the contacts pad being formed at one edge of the active surface, the wire bond has a diameter less than 40 microns and the arc extends more than 100 microns above the active surface of the die. 
         [0084]    Preferably, the wire bond is plastically deformed such that it extends less than 50 microns above the active surface of the die. In another preferred form, the wire bond is pushed by engagement with a blade having a rounded edge section for contacting the wire bond. 
         [0085]    Preferably, the bead of encapsulant has a profiled surface that is flat, parallel to and spaced less than 100 microns from the active surface. 
         [0086]    Optionally, the bead of encapsulant has a profiled surface that is flat and inclined relative to the active surface. 
         [0087]    Preferably, the encapsulant being an epoxy material has a viscosity greater than 700 cp when uncured. 
         [0088]    Preferably, the encapsulant is an epoxy material that is thixotropic when uncured. 
         [0089]    Preferably, the active surface has functional elements spaced less than 260 microns from the contacts pads of the die. In a particularly preferred form, the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected. Preferably, the printhead IC is mounted in a printer such that during use the nozzles are less than 100 microns from the paper path. 
         [0090]    Preferably, the support structure has a chip mounting area and a conductor mounting area, the die is supported on the chip mounting area, and a plurality of electrical conductors at least partially supported on the conductor mounting area wherein, the chip mounting area is raised relative to the conductor mounting area. In a particularly preferred form, the chip mounting area is raised more than 100 microns relative to the conductor mounting area. In preferred embodiments, the support structure has a step between the chip mounting area and the conductor mounting area. In particularly preferred embodiments, the plurality of conductors are incorporated into a flexible printed circuit board (flex PCB) with a line of bond pads along an edge closest the die, the bond pads being more than 2 mm from the contacts pads on the die. 
         [0091]    Preferably, the support structure is a liquid crystal polymer (LCP) molding. 
         [0092]    According to a fifth aspect, the present invention provides a method of applying encapsulant to wire bonds between a die and conductors on a supporting substrate, the method comprising the steps of: 
         [0093]    forming a bead of the encapsulant on a profiling surface; 
         [0094]    positioning the profiling surface such that the bead contacts the die; and, 
         [0095]    moving the profiling surface relative to the die to cover the wire bonds with the encapsulant. 
         [0096]    Wiping the encapsulant over the wire bonds with a profiling surface provides control of the encapsulant front as well as the height of the encapsulant relative to the die. The movement of the profiling surface relative to the die can closely controlled to shape the encapsulant to a desired form. Using the example of a printhead die, the encapsulant can be shaped to present an inclined face rising from the nozzle surface to a high point over the wire bonds. This can be used by the printhead maintenance facilities to maintain contact pressure on the wiping mechanism. This is illustrated further below with reference to the drawings. However, it will be appreciated that the encapsulant can be shaped to have ridges, gutters, grooves and so on by using a particular shape of profiling surface and relative movement with the die. 
         [0097]    Preferably, the method further comprises the steps of: 
         [0098]    dipping the profiling surface into a reservoir of the encapsulant material to form a the bead of encapsulant material on the profiling surface. 
         [0099]    Optionally, the profiling surface is a blade with a straight edge and the method further comprises the steps of: 
         [0100]    orienting the blade such that the straight edge is lowest and dipping the straight edge into the encapsulant material to form the bead of encapsulant along the straight edge. 
         [0101]    Preferably, the die has an active surface with functional elements and a plurality of contacts pad being formed along one edge for connection with the wire bonds such that the wire bonds extend in an arc from the contacts pads to each of the conductors respectively, the wire bonds having a diameter less than 40 microns and the arc extends more than 100 microns above the active surface of the die. 
         [0102]    Preferably, the method further comprises the steps of: 
         [0103]    prior to encapsulation, pushing on the wire bonds to collapse the arc and plastically deform the wire bonds; and, 
         [0104]    releasing the wire bonds such that plastic deformation maintains the wire bonds in a flatter profile shape. 
         [0105]    Preferably, the wire bond is plastically deformed such that it extends less than 50 microns above the active surface of the die. Preferably, the wire bond is pushed by engagement with a blade having a rounded edge section for contacting the wire bond. 
         [0106]    Preferably, the encapsulant covering the wire bonds has a profiled surface that is flat, parallel to and spaced less than 100 microns from the active surface. 
         [0107]    Preferably, the bead of encapsulant having a profiled surface that is flat and inclined relative to the active surface. 
         [0108]    Preferably, the encapsulant being an epoxy material has a viscosity greater than 700 cp when uncured. 
         [0109]    Preferably, the encapsulant is an epoxy material that is thixotropic when uncured. Preferably, the functional elements are spaced less than 260 microns from the contacts pads of the die. In a further preferred form, the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected. Optionally, the printhead IC is mounted in a printer such that during use the nozzles are less than 100 microns from the paper path. 
         [0110]    Preferably, the support structure has a chip mounting area and a conductor mounting area, the die is supported on the chip mounting area, and a plurality of electrical conductors at least partially supported on the conductor mounting area wherein, the chip mounting area is raised relative to the conductor mounting area. In a particularly preferred form, the chip mounting area is raised more than 100 microns relative to the conductor mounting area. In another preferred form, the support structure has a step between the chip mounting area and the conductor mounting area. In a preferred embodiment, the plurality of conductors are incorporated into a flexible printed circuit board (flex PCB) with a line of bond pads along an edge closest the die, the bond pads being more than 2 mm from the contacts pads on the die. In some embodiments, the support structure is a liquid crystal polymer (LCP) molding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0111]    Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: 
           [0112]      FIG. 1  is a schematic representation of a common prior art technique for applying a bead of encapsulant to wire bonds; 
           [0113]      FIG. 2  is a schematic representation of a die mounted to a supporting structure with a chip mounting area raised relative to the flex PCB mounting area; 
           [0114]      FIGS. 3A ,  3 B and  3 C are schematic representations of the encapsulant bead being profiled into a desired shape using a moveable blade; 
           [0115]      FIGS. 4A to 4D  are schematic representations of wire bonds being profiled by plastic deformation; 
           [0116]      FIGS. 5A and 5B  show the encapsulant bead height reductions for plastically deformed wire bonds; 
           [0117]      FIGS. 6A to 6C  show the encapsulant bead being applied to the wire bonds using the profiling blade; and, 
           [0118]      FIGS. 7A and 7B  show the profiling blade being used to control the encapsulant bead front on the surface of the die. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0119]      FIG. 1  shows a common technique used for applying a bead encapsulant to wire bonds. A die  4  is mounted to a supporting structure  6  adjacent the edge of a flex PCB  8  (flexible printed circuit board). The die  4  has a line of contact pads  10  along one edge and the flex PCB  8  has corresponding bond pads  12 . Wire bonds  16  extend from the bond pads  10  to the bonds pads  12 . Power and data is transmitted to the die  4  via conductive traces  14  in the flex PCB  8 . This is a simplified representation of the dies mounted within many electronic devices. The printhead IC dies mounted to the LCP (liquid crystal polymer) molding to receive print data from an adjacent flex PCB, as described in U.S. Ser. No. ______ (Our Docket RRC001US) incorporated herein by cross reference, is one example of this type of die mounting arrangement. The ordinary worker will appreciate that the die may also be mounted directly to a hard PCB with traces formed thereon. 
         [0120]    The wire bonds  16  are covered in a bead on encapsulant  2  to protect and reinforce the bonds. The encapsulant  2  is dispensed from a discharge needle  18  directly onto the wire bonds  16 . Often the encapsulant bead  2  is three separate beads—two beads of so-called ‘dam’ encapsulant  20 , and one bead of ‘fill’ encapsulant  22 . The dam encapsulant  20  has a higher viscosity than the fill encapsulant  22 , and serves to form a channel to hold the fill encapsulant bead. The height H of the bead  2  above the die  4  is usually about 500-600 microns. In most electronic devices, this does not pose a problem. However, if the die has an active surface that needs to operate in close proximity to another surface, this bead can be an obstruction. 
       Elevating the Die Relative to the Flex PCB 
       [0121]      FIG. 2  shows a stepped support structure  6  that has raised the chip mounting area  26  relative to the PCB mounting area  24  (or at least the area mounting the PCB bonds pads  12 ). With the die  4  on a raised chip mounting area  26 , the arc of the wire bonds  16  are lower relative to active surface  28  of the die  4 . In fact, the end of the wire bond  16  attached to the contact pad  10  can be the apex of the arc (bearing in mind that the wire bond arc is intended to accommodate some relative movement of the die and PCB). When the wire bonds  16  are covered with encapsulant  2 , the bead has a reduced height H above the active surface  28  of the die  4 . If the bead of encapsulant  2  uses two beads of dam encapsulant  24  and a fill encapsulant  22 , the positions, volumes and viscosities of the beads need to take the step into account. Bead heights less than 100 microns are easily achievable, and with additional measures, such as wire arc collapsing and bead profiling (discussed below), bead height of less than 50 microns are possible. 
         [0122]    With the die  4  raised above the flex PCB  8  by 410 microns, the height of the wire bonds  16  above the die is about 34 microns. With the die raised 610 microns above the flex PCB, the wire bond height is around 20 microns. Raising the die even further has shown little or no further reduction in wire bond height with a step of 710 microns having a wire bond height of around 20 microns. 
         [0000]    Shaping the Encapsulant Bead with a Profiling Blade 
         [0123]      FIGS. 3A to 3C  show the encapsulant  2  being profiled with a profiling blade  30 . The support structure  6  is again stepped to reduce the height of the wire bonds  16  above the die  4 . Before the epoxy encapsulant  2  has cured, the profiling blade  30  moves across the die  4  and wire bonds in a predetermined path. As shown in  FIG. 3B , the blade  30  displaces the top of the bead  30  to its flex PCB side to form a flat top surface  32  that is at a significantly reduced height H above the die  4 . 
         [0124]    The encapsulant bead  2  may be a plurality of separate beads as shown in  FIGS. 1 and 2 , or a single bead of one material. However, for close dimensional control of the profiled encapsulant, the encapsulant materials used should be thixotropic—that is, once deposited from the discharge needle, or profiled by the blade  30 , the material should not flow under its own weight, but rather hold its form until it cures. This requires the epoxy to have an uncured viscosity greater than about 700 cp. A suitable encapsulant is DYMAX 9001-E-v3.1 Chip Encapsulant produced by Dymax Corporation with a viscosity of approximately 800 cp when uncured. The blade  30  may be ceramic (glass) or metal and preferably about 200 microns thick. 
         [0125]    It will be appreciated that the relative movement of the blade  30  and the die  4  can be precisely controlled. This allows the height H to be determined by the tolerance of the wire bonding process. As long as H is greater than the nominal height of the wire bond arc above the die, plus the maximum tolerance, the encapsulant  2  will cover and protect the wire bonds  16 . With this technique, the height H can be easily reduced from 500-600 microns to less than 300 microns. If the heights of the wire bond arcs are also reduced, the height H of the encapsulant bead can be less than 100 microns. The Applicant uses this technique to profile encapsulant on printhead dies down to a height of 50 microns at its lowest point. As shown in  FIG. 3C , the lowest point is at the encapsulant front and the blade  30  forms an inclined face  32  in the top of the bead  2 . The inclined face is utilized by the printhead maintenance system when cleaning the paper dust and dried ink from the nozzle face. This illustrates the technique&#39;s ability to not just reduce the height of the encapsulant bead, but to form a surface that can perform functions other than just encapsulate the wire bonds. The edge profile of the blade and the path of the blade relative to the die can be configured to form a surface that has a multitude of shapes for a variety of purposes. 
       Plastic Deformation of the Wire Bond Arcs 
       [0126]      FIGS. 4A to 4C  show another technique for lowering the profile of wire bonds.  FIG. 4A  shows the die  4  connected to the flex PCB  8  via the wire bonds  16 . While the stepped support structure  6  has lowered the height of the wire bond arcs compared to a flat supporting structure, the wire bonds still have a natural tendency to bow upwards rather than downwards towards the corner of the step. The wires  16  are typically about 32 microns in diameter and have a pull force of about 3 to 5 grams force. The pull force is the tensile load necessary to break the connection to the contact pad  10  or the bond pad  12 . Given the fragility of these structures (one of the reasons encapsulant is applied), conventional wisdom is to avoid any contact between the wire bond arcs and other solid surfaces. 
         [0127]    As shown in  FIG. 4B , the arc of the wire bonds  16  can be collapsed by a wire pusher  34 . The wire pusher  34  displaces the wire bond  16  enough to elastically and plastically deform the arc. The Applicants have shown that contact with the wire pusher  34  can cause localized work hardening in the wire, but as long as the pushing force is not excessive, it does not break. The end of the wire pusher  34  is rounded to avoid stress concentration points. The wire pusher may be a stylus for engaging single wire bonds or a blade that pushes on multiple wire bonds simultaneously. 
         [0128]    Referring now to  FIG. 4C , the wire pusher  34  is retracted and the wire springs back toward its original shape to relieve the elastic deformation. However, the plastic deformation remains and the wire bond height above the die  4  is much reduced. Testing has shown that an initial wire bond loop height of 200 microns can be reduced to about 45 microns using this technique. Tests have also shown that the pull strength of the plastically deformed wires remains at about 3 to 5 grams force. 
         [0129]    The collapse of the wire bonds is uncontrolled and leaves the wire bonds somewhat randomly deformed. However, pushing the wire bonds closer to the die provides more uniformly shaped collapsed wire bonds. The Applicant&#39;s work has shown that engaging the wires about 200 to 300 microns for the die provides the best results. 
         [0130]    As shown in  FIG. 4D , the die  4  and the flex PCB  8  are mounted to a flat support structure  6 . As discussed above, this means the original loop height of the wire bond arc is much higher—approximately 400 microns above the die  4 . Consequently, the wire has more plastic deformation when the loop is collapsed by the wire pusher. Even so, the Applicants results show that the residual loop height after pushing is about 20-50 microns. 
         [0131]      FIGS. 5A and 5B  show the collapsed wire bonds  16  covered with an encapsulant bead  2 . Even without bead profiling prior to curing, the height H of the bead above the die is much less than the bead necessary to encapsulate the original undeformed wire loops. 
         [0000]    Applying Encapsulant with Profiling Blade 
         [0132]      FIGS. 6A ,  6 B and  6 C show the application of the encapsulant bead using the profiling blade  30  instead of a discharge needle (see  FIGS. 1 and 2 ). As previously discussed, the flowrate of encapsulant from the discharge needle can vary and this gives rise to large variations on the position of the encapsulant front on the active surface of the die  4 . Consequently, any functional elements in the active surface of the die need to be sufficiently spaced from the contacts pads  10  to allow for the meandering encapsulant front. 
         [0133]    Applying the encapsulant with the profiling blade avoids the problems caused by the flowrate fluctuations from the discharge needle. As shown in  FIG. 6A , the bead of encapsulant  40  can be formed on the profiling blade  30  by simply dipping it into a reservoir of uncured encapsulant epoxy. Of course, the bead  40  may also be formed by any other convenient method, such as running the discharge needle along one end of the blade  30 . 
         [0134]      FIG. 6B  show the blade  30  having been lowered to touch the bead  40  onto the die  4 . When the encapsulant material touches the die surface, it wets and wicks along the surface while remaining pinned to the edge of the blade. The blade  30  is held at a predetermined height above the die  4  and moved over the bead  2  to flatten and lower its profile. The encapsulant displaced from the top of the bead  2  by the blade  30 , spreads over the PCB side of the bead  2 . It is not relevant if the encapsulant spreads further over the PCB than necessary. As long as the wire bonds  16  and the bonds pads  12  are covered, any additional encapsulant on the PCB  8  surface is not detrimental. 
         [0135]    In  FIG. 6C , the wire bond  16  height has been reduced by collapsing the arc in accordance with the techniques discussed above. As previously discussed, the bead  2  deposited by the discharge needle need not be as big to cover the wire bond  16  once it has been collapsed. Furthermore, the blade  30  can be brought closer to the die  4  without contacting wire bonds  16  when profiling the encapsulant  2 . Hence the bead profile in  FIG. 6C  is substantially lower than that of  FIG. 6B . 
       Encapsulant Front Control 
       [0136]    When the encapsulant material is dispensed from the discharge needle, minor variations in the flowrate can cause the bead to bulge at points of higher flow. Consequently, the side of the bead that contacts the active surface of the die is not straight, but has significant perturbations. These perturbations have to be accommodated between the contact pads and any functional elements on the active surface. The spacing between the contacts pads and the functional elements consumes valuable ‘chip real estate’. The Applicant has previously developed printhead dies with a spacing of 260 microns between the contact pads and the first row of nozzles. Better control of the encapsulant front reduces the space between the contacts and operational elements, and so the overall dimensions of the die. Hence the design can be more compact and more chips fabricated from the original wafer disc. 
         [0137]    As shown in  FIGS. 7A and 7B , the profiling blade  30  is used to control the front  36  of the bead of encapsulant  2 . The blade  30  is positioned over the die  4  to define a gap  42  between its lower edge and the active surface  28 . As the discharge needle  18  dispenses the encapsulant material  44 , it flows onto the active surface, one side of the blade and a fillet of the material extends through the gap  42 . Because of the flow restriction created by the gap, flow variations have a reduced effect on the dimensions of the fillet that flows through the gap. Therefore the encapsulant front  36  closely corresponds to the line of the lower edge of the blade  30 . 
         [0138]    As shown in  FIG. 7B , the profiling blade  30  is already in position to profile the encapsulant bead  2  once it has been dispensed from the discharge needle. The blade  30  simply moves over the die  4  in a direction away from the nozzles  38 . This keeps the encapsulant front  36  in place and flattens the profile of the encapsulant bead  2  over the wire bonds  16 . 
         [0139]    The invention has been described herein by way of example only. The ordinary will readily recognize many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.