PATENT DOCUMENT

Publication Number: US-9338832-B2
Application Number: US-201313889225-A
Country: US
Kind Code: B2

Title: Induction activated thermal bonding

Abstract:
The described embodiment relates generally to the field of inductive heating. More specifically an inductive heater designed for use in assembling electronics is disclosed. A number of methods for shaping a radio-frequency (RF) receiver structure are disclosed for the purpose of completing an inductive bonding process without causing harm to adjacent electrical components.

Claims:
What is claimed is: 
     
       1. A method for bonding a first device component to a second device component within an electronic device, the method comprising:
 directing an alternating magnetic field at a driving frequency to a stack structure having an electrically conductive receiver structure located between a first thermal adhesive layer and a second thermal adhesive layer while the stack structure is positioned between the first and second device components, the receiver structure having a first metal at a first receiver structure portion and a second metal at a second receiver structure portion, wherein the first and second device components are not substantially heated thereby and the receiver structure is tuned such that a substantial portion of the alternating magnetic field is converted to heat at the receiver structure; 
 allowing the heat at the receiver structure to transmit conductively into the first and second thermal adhesive layers in varying amounts of heat at different locations in the stack structure, wherein the second thermal adhesive layer has a thickness that is thicker than the first thermal adhesive layer; and 
 ceasing direction of the alternating magnetic field after sufficient heat has been transmitted into the first and second thermal adhesive layers to activate the first and second thermal adhesive layers and bond the first and second device components. 
 
     
     
       2. The method of  claim 1 , wherein heat transfer into the first and second thermal adhesive layers results in an asymmetric thermal gradient across the first and second thermal adhesive layers. 
     
     
       3. The method of  claim 2 , wherein the asymmetric thermal gradient results in the first device component receiving more heat than the second device component. 
     
     
       4. The method of  claim 1 , wherein temperatures at the receiver structure reach or exceed about 140° C. while temperatures at the first and second device components remain at or below about 85° C. 
     
     
       5. The method of  claim 1 , wherein other electronic device components outside and proximate the first and second device components are not exposed to damaging heat levels. 
     
     
       6. The method of  claim 1 , wherein heat transfer into the first and second thermal adhesive layers is symmetric despite the thickness difference in the thermal adhesive layers. 
     
     
       7. The method of  claim 1 , wherein the first metal is steel and the second metal is aluminum. 
     
     
       8. The method of  claim 1 , wherein the receiver structure comprises a first wire mesh having a first pitch at a first receiver structure portion, a second wire mesh having a second pitch at a second receiver structure portion, and a third wire mesh having a third pitch at a third receiver structure portion. 
     
     
       9. A method for bonding adherent components, the method comprising:
 directing an alternating magnetic field at a driving frequency to an electrically conductive receiver positioned proximate at least one thermal adhesive component and between first and second adherent components, the electrically conductive receiver having a first metal at a first electrically conductive receiver portion and a second metal at a second electrically conductive receiver portion, wherein the electrically conductive receiver has asymmetric thermal properties thereacross and is tuned such that a substantial portion of the alternating magnetic field is converted to heat at the electrically conductive receiver; and 
 allowing a sufficient amount of heat at the electrically conductive receiver to transmit conductively into the at least one thermal adhesive component in order to activate the at least one thermal adhesive component and thereby bond the first and second adherent components, wherein the heat is transmitted asymmetrically. 
 
     
     
       10. The method of  claim 9 , wherein the varying structure of the electrically conductive receiver comprises a layered structure having a magnetic field absorbing material on one side and a low heat conducting material on the other side. 
     
     
       11. The method of  claim 10 , wherein the low heat conducting material on the other side is a ceramic material. 
     
     
       12. The method of  claim 9 , wherein the varying structure of the electrically conductive receiver comprises a first wire mesh having a first pitch at a first electrically conductive receiver portion, a second wire mesh having a second pitch at a second electrically conductive receiver portion, and a third wire mesh having a third pitch at a third electrically conductive receiver portion. 
     
     
       13. A method for bonding adherent components, the method comprising:
 directing a first alternating magnetic field at a first driving frequency to an electrically conductive receiver positioned proximate at least one thermal adhesive component and between first and second adherent components, the electrically conductive receiver comprising a first portion having a first set of thermal properties and a second portion having a second set of thermal properties different from the first set of thermal properties, wherein the first portion of the electrically conductive receiver is tuned such that the first alternating magnetic field at the first driving frequency is converted to a substantial amount of heat at the first portion of the electrically conductive receiver, and the second portion of the electrically conductive receiver is tuned such that the first alternating magnetic field at the first driving frequency is not converted to a substantial amount of heat at the second portion of the electrically conductive receiver; 
 allowing a sufficient amount of heat at the first portion of the electrically conductive receiver to transmit conductively into a first portion of the at least one thermal adhesive component in order to activate the first portion of the at least one thermal adhesive component; 
 directing a second alternating magnetic field at a second driving frequency to the electrically conductive receiver, wherein the first portion of the electrically conductive receiver is tuned such that the second alternating magnetic field at the second driving frequency is not converted to a substantial amount of heat at the first portion of the electrically conductive receiver, and the second portion of the electrically conductive receiver is tuned such that the second alternating magnetic field at the second driving frequency is converted to a substantial amount of heat at the second portion of the electrically conductive receiver; and 
 allowing a sufficient amount of heat at the second portion of the electrically conductive receiver to transmit conductively into a second portion of the at least one thermal adhesive component in order to activate the second portion of the at least one thermal adhesive component. 
 
     
     
       14. The method of  claim 13 , wherein the electrically conductive receiver further comprises a third portion having a third set of thermal properties different from the first and second sets of thermal properties, the method further comprising:
 directing a third alternating magnetic field at a third driving frequency to the electrically conductive receiver, wherein the first and second portions of the electrically conductive receiver are tuned such that the third alternating magnetic field at the third driving frequency is not converted to a substantial amount of heat at the first and second portions of the electrically conductive receiver, and the third portion of the electrically conductive receiver is tuned such that the third alternating magnetic field at the third driving frequency is converted to a substantial amount of heat at the third portion of the electrically conductive receiver; and 
 allowing a sufficient amount of heat at the third portion of the electrically conductive receiver to transmit conductively into a third portion of the at least one thermal adhesive component in order to activate the third portion of the at least one thermal adhesive component. 
 
     
     
       15. The method of  claim 14 , wherein the first portion of the electrically conductive receiver comprises a first wire mesh having a first pitch, the second portion of the electrically conductive receiver comprises a second wire mesh having a second pitch, and the third portion of the electrically conductive receiver comprises a third wire mesh having a third pitch. 
     
     
       16. The method of  claim 13 , wherein the first portion of the electrically conductive receiver comprises a first metal and the second portion of the electrically conductive receiver comprises a second metal. 
     
     
       17. The method of  claim 13 , wherein the first portion of the electrically conductive receiver comprises a layer formed from a low heat conducting material and the second portion of the electrically conductive receiver does not comprise a layer formed from a low heat conducting material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Prov. Pat. Appl. No. 61/721,443, entitled “INDUCTION ACTIVATED THERMAL BONDING,” by Derek W. Wright, et al. filed on Nov. 1, 2012, and U.S. Prov. Pat. Appl. No. 61/801,777, entitled “INDUCTION ACTIVATED THERMAL BONDING,” by Derek W. Wright, et al. filed on Mar. 15, 2013, the contents of which are hereby incorporated herein by reference, in their entirety, for all purposes. 
     BACKGROUND 
     1. Technical Field 
     The described embodiment relates generally to the use of focused energy in electronics manufacturing. More particularly, devices and methods for using a radio-frequency (RF) alternating magnetic field to thermally bond adjoining components in an electronic assembly are described. 
     2. Related Art 
     A pressure sensitive adhesive (PSA) is an adhesive that bonds when pressure is applied to marry the adhesive with the adherent. An advantage to using PSA is that no solvent, water, or heat is required for activation since, as indicated by the name, a sufficient force is required to apply the adhesive to the surface. In some cases, though, an increased force may not increase adhesion. However, surface factors, such as smoothness, surface energy, and presence of contaminants may have a substantial influence on the ultimate bond strength and reliability. Moreover, PSA generally forms a reliable bond at room temperatures. As the temperature changes, however, the properties of the bond can change. For example, at reduced temperatures, pressure sensitive adhesives can experience reduced (or even loss) tack, whereas at high temperatures pressure sensitive adhesives can experience a reduced shear strength. 
     Therefore, in situations where bonded parts experience temperature variations that can adversely affect the PSA bond, a thermal bond film can be more desirable to use. Thermal bond films generally provide stronger and more reliable bond than PSAs. Also, thermal bond films may be desirable when narrow bond lines are used. However, in order to form a bond between the thermal bond film and the adherent, the thermal bond film must be exposed to sufficient heat for proper activation. The ability to deliver sufficient heat can be adversely affected by a number of extraneous factors, such as thermal properties of materials being bonded together, as well as materials in the thermal path between the heat source and the thermal bond film. The heat transfer rate from a heat source to a thermal bond film is inversely related to the thermal path resistance between the heat source and the thermal bond film. The thermal path resistance can be related to the thermal coefficients of the components within the thermal path, which when added together provide an overall resistance to the flow of heat from the heat source to the thermal bond film. Thus, thermal resistance can impose a much higher temperature at the thermal source than would otherwise be required. Furthermore, to achieve faster curing of a thermal bond film, a higher temperature may be required to achieve a desired thermal gradient. Having exceedingly hot elements in a bonding assembly can adversely affect components in the vicinity of the thermal path that are sensitive to high temperature conditions (such as plastics having a low melting point, or anodized aluminum susceptible to cracking). 
     Therefore what is desired is a method and apparatus for thermally activating an adhesive with a focused energy delivery. 
     SUMMARY OF THE DESCRIBED EMBODIMENTS 
     According to a first embodiment an apparatus for focused activation of an adhesive including an energy source is provided. The energy source may include an alternate current (AC) source configured to provide an AC at a driving frequency. In some embodiments a capacitor circuit selects the driving frequency. The apparatus may further include a conducting device coupled to the capacitor circuit and configured to use the AC to generate an alternating magnetic field. The conducting device may also direct a portion of the alternating magnetic field to a receiver structure proximal to the conducting device. Accordingly, the receiver structure is configured to absorb portions of the alternating magnetic field and to convert the absorbed alternating magnetic field to heat. In some embodiments the receiver structure is thermally coupled to an adhesive layer. 
     According to a second embodiment a receiver structure is provided. The receiver structure may include a magnetic absorption layer made of an electrically conductive material. Further, the magnetic absorbing layer is thermally coupled to an adhesive layer adjacent to an adherent component in a bonding assembly. 
     According to a third embodiment, a method for focused adhesive activation in a bonding assembly is presented. The method may include generating an alternating magnetic field using an induction heater. The method may also include directing the alternating magnetic field at a driving frequency to a receiver structure in the bonding assembly and converting the received alternating magnetic field to heat. In some embodiments the method includes thermally coupling an adhesive portion to the receiver structure and conductively transmitting the heat from the receiver structure to the adhesive portion. Finally, the method may include ceasing operation of the induction heater after sufficient heat has been transmitted to the adhesive portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. These drawings in no way limit any changes in form and detail that may be made to the described embodiments by one skilled in the art without departing from the spirit and scope of the described embodiments. 
         FIG. 1  is a block diagram of an induction heater system in accordance with some embodiments. 
         FIG. 2  shows a perspective view of an inductive thermal bonder incorporating an induction heating system according to some embodiments. 
         FIG. 3  shows a cross sectional view of a magnetic receiver structure according to some embodiments. 
         FIG. 4  shows a plan view of a magnetic receiver structure according to some embodiments. 
         FIG. 5  shows a plan view of a magnetic receiver structure according to some embodiments. 
         FIG. 6A  shows a magnetic field used for a focused adhesive activation and bonding, according to some embodiments. 
         FIG. 6B  shows a cross sectional view of a focused adhesive activation and bonding, according to some embodiments. 
         FIG. 7  shows a flowchart detailing a process in accordance with the described embodiments. 
         FIG. 8  shows a flowchart detailing a process in accordance with the described embodiments. 
         FIG. 9  shows a plan view of an RF receiver structure according to some embodiments. 
         FIG. 10  shows a plan view of an RF receiver structure according to some embodiments. 
     
    
    
     In the figures, elements referred to with the same or similar reference numerals include the same or similar structure, use, or process, as described in the first instance of occurrence of the reference numeral. 
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     A representative apparatus and application of methods according to the present application are described in this section. These examples are being provided solely to add context and aid in the understanding of the described embodiments. It will thus be apparent to one skilled in the art that the described embodiments may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the described embodiments. Other applications are possible, such that the following examples should not be taken as limiting. 
     In the following detailed description, references are made to the accompanying drawings, which form a part of the description and in which are shown, by way of illustration, specific embodiments in accordance with the described embodiments. Although these embodiments are described in sufficient detail to enable one skilled in the art to practice the described embodiments, it is understood that these examples are not limiting; such that other embodiments may be used, and changes may be made without departing from the spirit and scope of the described embodiments. 
     In state of the art applications, pressure sensitive adhesives (PSA) are convenient when thermally activating an adhesive may result in damage to surrounding structures and materials. Some PSAs may have relatively low activation temperatures, from about room temperature (25° C.) to little above room temperature (such as about 50° C.). However, use of PSAs becomes challenging for assembling devices having reduced dimensions. The challenges arise from the difficulty to apply uniform pressure in areas having small form factors and detailed features, as is common in handheld and portable electronic devices. This may be the case for example for bonding a lens to a lens mount, especially when the mount is embedded in a complex structure, or the lens includes more than one optical element. Thermal bond films may be desirable in applications where a PSA film is challenged. For example, a PSA is typically weaker than liquid adhesives or thermal bond film adhesives. Also, application of PSAs may be difficult in portions of the substrate having narrow bends. A thermal bond film as used in some embodiments may be selectively activated at a high temperature, such as 140° C., or even more, at a localized area. Embodiments as disclosed herein avoid damage that high temperature of adhesive activation may produce to surrounding elements by precisely focusing the area of heat delivered to the adhesive. In some embodiments, elements surrounding the adhesive activation area may remain at temperatures close to, or below about 85° C., while the adhesive is activated at temperatures higher than about 140° C. in the vicinity. Thus, systems and methods for focused activation of an adhesive as disclosed herein maintain structural and functional integrity of the bonding assembly during the bonding process. 
     Furthermore, systems and methods as disclosed herein may provide a symmetric thermal gradient across different components in a bonding assembly. A symmetric thermal gradient may be desirable in configurations where different components have different thermal properties. Furthermore, systems and methods as disclosed herein may provide an asymmetric thermal gradient in order to protect devices and components proximal to the bonding assembly. 
     Heater systems and methods consistent with the present disclosure may provide heat from within a bonding assembly. Thus, systems and methods consistent with the present disclosure avoid having to overheat an exterior element to provide heat to an adhesive layer embedded in the bonding assembly. This substantially reduces the risk of damaging delicate components in the proximity of the bonding area. 
     Embodiments according to the present disclosure result in a focused heat transfer to a selected adhesive activation area. To achieve focused heat transfer, some embodiments transmit an alternating magnetic field to a receiver structure in a bonding assembly. A receiver structure according to some embodiments may include a metallic material that produces a magnetic hysteresis placed within a bonding structure. In that regard, a conductive material which is also ferromagnetic (or ‘ferrous’) may provide enhanced magnetic hysteresis properties. Eddy currents generated in metallic materials by the incident magnetic field combined with magnetic hysteresis losses of ferrous materials enable a focused heating of an adhesive layer. For example, some embodiments include a steel film or another ferrous material between bond film layers to receive the alternating magnetic field in a selected area. Thus generating a focused heat flow into an adhesive film layer. Embodiments as disclosed herein may be used with substrates having low thermal conductivity, or substrates susceptible to damage under high temperatures (low melting T). For example, under high temperature some materials may present discoloration, surface blemishes, warping, or undesirable out-gassing, among other problems. In such circumstances, a focused heat source placed in a selected area provides a high adhesive activation temperature without risking damage to surrounding elements. For example, the receiver structure acting as a heat source may be located in close proximity to the adhesive, or embedded within. Thus, elements distant from the heat source receive a limited amount of heat. 
     Embodiments consistent with the present disclosure can focus a heat flow in parts of devices that are difficult to access with conventional tools. For example, in a bonding assembly adherent components may be obstructed by a surrounding element, or occluded by a portion of a substrate, a chassis, or a frame. In such configurations, it may be difficult to reach and provide heat to an adhesive portion adjacent to the adherent components. Moreover, it may be difficult to hold an adherent component in order to maintain structural support for the bonding assembly, while the adhesive is curing. Embodiments consistent with the present disclosure allow the heating of adhesive portions in bonding assemblies that are hard to reach otherwise. Also, embodiments as disclosed herein can apply a force to an adherent component to maintain structural support for the bonding assembly, while the adhesive is cured. 
     Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction. An induction heater can include an electromagnet, through which a high-frequency alternate current (AC) is passed. Electromagnetic induction may be generated by an alternating magnetic field incident on the object. Electromagnetic induction generates eddy currents within a metal, and the natural resistance to current flow leads to Joule heating of the metal. Heat may also be generated by magnetic hysteresis losses in materials that have significant magnetic permeability. The frequency of AC used depends on the object size, material type, coupling (between the conducting device and the object to be heated) and the penetration depth. 
     Accordingly, an apparatus and method for thermally activating a thermal bond layer in situ using alternating magnetic field induced heating is described. The apparatus includes a focused energy source configured to generate an alternating magnetic field at a selected frequency or range of frequencies, such as a frequency bandwidth. In some embodiments the focused energy source includes at least an AC current source configured to drive an conducting device with an AC at a driving frequency. The energy converter can take the form of an induction heater system configured to convert the AC at the driving frequency to an alternating magnetic field that oscillates at the driving frequency. The driving frequency may be a radio-frequency (RF) within a selected RF band. In some embodiments, an RF band may be between about a hundred Megahertz (MHz, 1 MHz=10 6  Hz) up to a few Gigaherz (GHz, 1 GHz=10 9  Hz). Some embodiments may use electromagnetic radiation with frequencies lower than 100 MHz, such as a few tens of MHz, or even lower. On the other hand, some embodiments may use electromagnetic radiation with frequencies higher than a few GHz, such as 100 GHz, or even higher. The alternating magnetic field is emitted by a transmitter that directs at least a portion of the alternating magnetic field to a receiver structure. 
     The receiver structure can be part of an assembly stack that includes a thermal bond layer in thermal contact with the receiver structure and at least one adherent structure. In one embodiment, the receiver structure can be part of an assembly stack in the bonding assembly. The receiver structure converts the alternating magnetic field to heat used to activate the thermal bond layer. In some embodiments, the activated thermal bond layer can secure the receiver structure to the assembly stack. In this way, components can be used as both a structural component and as an inductive heating component. Accordingly, the receiver structure can be tuned in such a way that a substantial portion of the received alternating magnetic field is converted to heat that is deposited directly in the receiver structure. The deposited heat is then transported from the receiver structure by way of thermal conduction to a thermal bond layer. The heat thermally activates the thermal bond layer. In one embodiment, the alternating magnetic field source can be embodied within a press head used to apply a predetermined force to the thermal bond layer and adherents, in combination with an alternating magnetic field. In one embodiment, the induction heater system can include a press to apply pressure to the assembly stack as well as direct an alternating magnetic field to the receiver structure in the assembly stack. Embodiments including a press are able to exert a specific amount of pressure for a period of time, at a selected temperature. Thus, a uniform coupling can be made between the thermal bond layers and the adherent components in a bond assembly. 
     It should be noted that the receiver structure can take many forms, according to embodiments disclosed herein. For example, the receiver structure can take the form of a metal foil, a metallized plastic film, a conductive mesh, or in some cases, conductive particles within a non-conductive substrate. In this way, the ability to tune the alternating magnetic field to specific receiver shapes and composition can be advantageously used to deliver specific amounts of heat at specific locations. This may also enable optimization of adhesive placement for maximum adhesive strength. Embodiments consistent with the present disclosure provide a controlled and focused heat deposition. Using a focused adhesive activation, some embodiments consistent with the present disclosure are desirably used for lens mounting applications. In a lens mounting procedure, the area where the adhesive is desirably activated is typically reduced in comparison with surrounding components, such as the lens and the mounting. The lens material is typically glass, and the mounting can be aluminum or some other metal or rigid material. To avoid damage or stress to the glass, or glass coating, and misalignment of the lens relative to the mount while the adhesive cures, it is desirable to have a focused heat application, such as provided in embodiments disclosed herein. 
     In another embodiment, methods and systems consistent with the present disclosure may be applied for bonding a cover glass to a product housing. The product housing may be used in a portable electronic device such as a handheld electronic device such as a cellular phone, a tablet computer, a laptop computer, or the like. 
       FIG. 1  is a block diagram of an embodiment of an induction heater system  100  in accordance with the described embodiments. As shown, induction heater system  100  can include power supply  102  that can operate at an alternating frequency. Accordingly, induction heater system  100  may include a transducer converting ‘direct current’ (DC) into an alternating magnetic field. The current is converted into a magnetic field by a conducting device  106 . In some embodiments, conductive device  106  may be an induction coil made of an electrically conductive material such as copper. The shape of conductive device  106  may define an area where the magnetic field is applied, and thus the heat generation is localized to that area. The magnetic field may be alternating at a selected working frequency. Accordingly, the magnetic field may change direction by approximately 180° at the selected working frequency. In some embodiments, the alternating magnetic field may be super-imposed to a constant magnetic field offset pointing in a specific direction. For example, the constant magnetic field offset may be desirable to adjust a ferromagnetic member of the bonding assembly during the bonding process. 
     In some instances, a particular power supply frequency can have particular induction heating characteristics geared for a specific shape of conductive device  106  and a given bonding component shape. Thus, selecting a particular alternating frequency can be advantageous for a given component, component size, etc. In some embodiments, the output of power supply  102  can be controlled by power supply controller  104 . Power supply controller  104  couples power supply  102  to conducting device  106  through a capacitor circuit  105 . In some embodiments, power supply  102  can include an adjustable alternating frequency. In such embodiments, a single power supply can be used to provide power at varying frequencies to conductive device  106 . Thus, the amount of alternating magnetic field as well as the penetration depth of the alternating magnetic field can be desirably controlled. 
     In some embodiments, induction heater system  100  may include capacitor circuit  105  to select the driving frequency for operating conducting device  106 . Capacitor circuit  105  may include any number of electronic components, including capacitors, transistors, inductors, or any other components used for frequency operation of an electronic device. Embodiments as disclosed herein may be useful when disparate adherent components are included in a bonding assembly. In such configurations, it may be desirable to use different adhesive layers adjacent to each of the different adherent components. 
       FIG. 2  shows a perspective view an embodiment of an inductive thermal bonder  200  incorporating induction heating system  100 , according to some embodiments. In particular, induction thermal bonder  200  can include conducting device  106  incorporated into head structure  202 . It should be noted that while conducting device  106  is shown shaped as a tight parallel coil, any number of other shapes are also possible as alternate configurations. For example, in some embodiments the distribution and strength of the magnetic field may be adapted to properly match the induction heating target. As shown in  FIG. 2 , conducting device  106  can be shaped in such a way that an alternating magnetic field induces eddy currents (and thus induction heating) in a focused area where heat deposition is desired. Induction thermal bonder  200  can be used to direct the alternating magnetic field at assembly stack  204 . Assembly stack  204  can include adherent components  206  and  208 . Adherent components  206  and  208  are bonded together using adhesive layers  210  (and  212 ) thermally coupled to receiver structure  214 . Adhesive layers  210  and  212  may include a thermal bond film, according to some embodiments. 
     The particular choice of Cartesian coordinates XZ in  FIG. 2  is arbitrary and shown for illustrative purposes only. 
     In the described embodiments, receiver structure  214  is configured to receive the alternating magnetic field from conducting device  106 . In this way, the alternating magnetic field energy provided by conducting device  106  can induce eddy currents within magnetic receiver structure  214 . The eddy currents heat receiver structure  214  and the heat is transported by way of conduction to adhesive layers  210  and  212 . In other words, eddy currents can be induced at a distance from conducting device  106  without physical contact between conducting device  106  and components  206  and  208 . The magnetic field emanating from conducting device  106  can also be designed so that heating of surrounding components is limited to selected areas of assembly stack  204 . In this way, areas of assembly stack  204  can remain relatively cool and not be subjected to heating. In some embodiments, receiver structure  214  can have asymmetric thermal properties (as well as asymmetric absorption properties). In this way, heat can be generated or asymmetrically transmitted in accordance with specific configurations of assembly stack  204 . For example, magnetic receiver structure  214  can take the form of a mesh, a shaped metal foil, or a layered structure having a magnetic field absorbing material on one side and a low heat conducting material (such as a ceramic or a different adhesive layer) on the other. The mesh may have metallic wires woven to form a first pitch. In this way, heat generated by the alternating magnetic field received is transported to one side of magnetic receiver structure  214  faster than to the opposite side of magnetic receiver structure  214 . This may result in a more even thermal gradient across assembly stack  204  when the heat is transported faster to a portion of the stack having lower thermal conductivity. In some embodiments, head structure  202  can exert a force onto assembly stack  204 . The applied force can help flatten, align, and properly position adherent components  206  and  208  with respect to adhesive layers  210  and  212 . 
     It should be noted that receiver structure  214  can take many forms. For example, receiver structure  214  can take the form of a metal foil, a metallized plastic film, a conductive mesh, or in some cases, conductive particles within a non-conductive substrate. In this way, the ability to tune the alternating magnetic field to specific receiver shapes can deliver specific amounts of alternating magnetic fields at specific locations. This in turn provides a controlled and localized heat deposition. 
     In some embodiments, it is desirable that adhesive layers  310  and  312  be made of a liquid material. Liquid adhesives are convenient because they are gap filling and are generally stronger after curing. While curing times for liquid adhesives is typically long, curing time may be substantially reduced by applying localized heat, as in embodiments disclosed herein. 
       FIG. 3  shows a cross sectional view of a magnetic receiver structure  314  according to some embodiments. As shown in  FIG. 3 , adhesive layers  310  and  312  may be adjacent to magnetic receiver structure  314 . Adhesive layer  310  may have a first thickness  320 , and adhesive layer  312  may have a second thickness  322 . Accordingly, first thickness  320  may be different from second thickness  322 . Varying the thickness of adhesive layers  310  and  312  may result in a heat transfer that is symmetric along the Z-axis. This may provide the same heating temperature to adhesive layers  310  and  312  that may have different chemical composition and thus different thermal properties. In some embodiments, it may be desirable to create an asymmetric thermal gradient along the Z-axis. For example, the bonding assembly may be proximal to delicate circuitry in the bottom, or at the top, of magnetic receiver structure  314 . In such embodiments, having first thickness  310  different from second thickness  322  may be desirable to create an asymmetric thermal gradient. 
     The particular choice of Cartesian coordinates XZ in  FIG. 3  is consistent with that of  FIG. 2 . 
       FIG. 4  shows a cross sectional view of a magnetic receiver structure  414  according to some embodiments. Magnetic receiver structure  414  includes a low heat conductivity layer  421 , and a magnetic receiving layer  422 . As shown in  FIG. 4 , low heat conductivity layer  421  may have different thickness ‘t 1 ’, and width ‘w 1 ’, as compared to magnetic absorption layer  422  (t 2 , and w 2 , respectively). Low thermal conductivity layer  421  may include a dielectric material such as glass, plastic or ceramic, or a polymer material having a high dielectric constant, such as polyimide. In some embodiments, low thermal conductivity layer  421  may include an optical component, such as a lens, a mirror, a prism, or a portion of an optical fiber. Magnetic absorption layer  422  may include a metal or an electrically conductive material such as copper, aluminum, tin, or gold. 
     Accordingly, magnetic absorption layer  422  absorbs an alternating magnetic field and generates heat. The generated heat is transmitted to portions of the bonding assembly on one side of structure  414  through low thermal conductivity layer  421 . In some embodiments low thermal conductivity layer  421  includes materials so that elements in the assembly stack adjacent to layer  421  receive a lesser amount of heat from magnetic absorption layer  422 . This may reduce the risk of damaging delicate elements in the bonding assembly proximal to the bonding surface. In some embodiments, elements on the side of magnetic receiving structure  414  adjacent to layer  421  may have a high thermal conductivity that compensates the low thermal conductivity of layer  421 . In such embodiments, having low thermal conductivity layer  421  may enable having a symmetric thermal gradient across the Z-axis in the bonding assembly. 
     The particular choice of Cartesian coordinates XZ in  FIG. 4  corresponds to that of  FIG. 2 . 
       FIG. 5  shows a plan view of a magnetic receiver structure  514  according to some embodiments. Magnetic receiver structure  514  has a mesh or grid shape formed of conductive wires. In some embodiments magnetic receiving structure  514  may be formed by weaving a plurality of conductive wires. The mesh in receiver structure  514  has a graded pitch. The graded pitch defines regions  521 ,  522 , and  523 , such that each region has a consistent first pitch diameter  531 , second pitch diameter  532 , and third pitch diameter  533 , respectively. Accordingly, heat transfer into the adhesive layers has different efficiency in regions  521 ,  522 , and  523 . For example, the temperature reached by the adhesive layer overlapping portion  523  may be lower than the temperature reached by the adhesive layer overlapping portion  521 . 
     The shape and size of regions  521 ,  522 , and  523  may be determined according to different heat desirability for the adhesive layer overlapping the regions. To obtain uniform bonding the adhesive may be desirably heated at the same temperature across a certain area of a substrate, according to some embodiments. However, the substrate portion overlapping an adhesive layer may have different heat conductivities in different portions. For example, in some embodiments a first substrate portion may include steel, and in some embodiments a second substrate portion may include aluminum. A graded grid having regions  521 ,  522 , and  523  transmits heat with different efficiency in each of the regions. Thus, magnetic receiving structure  514  provides a differentiated amount of heat to the adhesive in the vicinity of each of regions  521 ,  522 , and  523 . In embodiments where the substrate has different thermal properties, the adhesive layer (e.g., layers  210  and  212 , cf.  FIG. 2 ) can reach approximately the same temperature over different substrate portions. This results in uniform bonding across an area overlapping regions  521 ,  522 , and  523 . 
     In some embodiments, it may be desirable that different substrate portions of be heated at different temperatures due to different bonding requirements. For example, a portion of a substrate overlapping region  521  may include an adhesive layer that is activated at a first temperature. And a portion of a substrate overlapping region  522  may include an adhesive layer that is activated at a second temperature. The first temperature may be different from the second temperature. For example, the first temperature may be lower than the second temperature. In such configuration, pitch  531  in region  521  may be selected to transmit heat at a lower rate than region  522 . Thus, the amount of heat provided by region  521  is lower than the amount of heat provided by region  522 , resulting in a lower first temperature, compared to the second temperature. 
     The number of mesh regions illustrated in  FIG. 5  is illustrative only, and should not be limiting in any respect. Accordingly, one of ordinary skill in the art will recognize that different shapes and areas of different mesh regions may be combined. For example, the pitch  531 ,  532 , and  533  in adjacent regions may not gradually change in size. Thus, in some embodiments a region such as region  521  including pitch  531  may be adjacent to region  523 , including pitch  533 . And region  523  including pitch  533  may be adjacent to region  522 . Thus, in some embodiments regions  521  and  522  may not be adjacent to each other. Moreover, the grid in magnetic receiving structure  514  can have a square lattice shape, as shown, and any other shape, such as: triangular, hexagonal, rhomboid, or irregular shape. It should be noted that a similar result may be achieved with a magnetic receiver structure  514  including conductive particles having a varying particle density in regions  521 ,  522 , and  523 . Likewise, magnetic receiver structure  514  may include a conductive film having a varying area overlapping regions  521 ,  522 , and  523 , according to embodiments consistent with the present disclosure. Moreover, in some embodiments magnetic receiving structure  514  may include a conductive film having a varying thickness overlapping regions  521 ,  522 , and  523 . 
     The particular choice of Cartesian coordinates XY in  FIG. 5  corresponds to a ‘right-handed’ XYZ coordinate system consistent to that of  FIG. 2 . 
       FIG. 6A  shows a magnetic field  600  used for a focused adhesive activation and bonding, according to some embodiments.  FIG. 6A  illustrates a time plot of the magnitude of magnetic field  600 . A constant offset magnetic field component  601  of magnetic field  600  is shown. And an oscillating magnetic field component  602  of magnetic field  600  is also shown. In some embodiments, component  601  may be different from zero. Component  601  may be provided by using a non-zero ‘direct current’ (DC) to conducting device  106  (cf.  FIG. 1 ). Component  602  is associated to a heating process of a receiver structure according to embodiments consistent with the present disclosure. As mentioned above, the heating process may be the result of joule heating through the resistance to eddy currents induced in the receiver structure. In some embodiments, the heating process may be complemented by magnetic hysteresis, as the induced magnetization changes with the changing magnetic field. On the other hand, component  601 , being non-zero, may be used for a mechanical action on adherent components during the bonding process (e.g., components  206  and  208 , cf.  FIG. 2 ). This will be described in more detail below, with reference to  FIG. 6B . 
       FIG. 6B  shows a cross sectional view of a focused adhesive activation and bonding using magnetic field  600 , according to some embodiments.  FIG. 6B  shows adherent components  620  and  630 , and adhesive portion  610 . Adherent component  620  may include steel, or some other ferromagnetic material. Thus, applying magnetic field  600  as shown in  FIG. 6B  has two desirable effects, as follows. While magnetic field component  601  creates a force that keeps adherent component  620  adjacent to adherent component  630  in a desired position, alternating magnetic field component  602  generates a heat flux  605  to cure adhesive portion  610 . Embodiments of a focused adhesive activation and bonding process such as illustrated in  FIG. 6B  are desirable when access to adherent components  610  and  630  is difficult. For example, adherent component  610  may be a mount for a lens (e.g., adherent component  630 ), where the lens is embedded in a complex casing, or may be part of a multi-element optical component. In such configurations, components surrounding the bonding assembly may preclude using contact forces to maintain adherents  620  and  630  in place, during activation and curing of adhesive portion  610 . 
       FIG. 7  is a flowchart describing process a  700  in accordance with the described embodiments. Process  700  may include a focused adhesive activation and bonding for adherent components to be used in a handheld electronic device. Process  700  may include an induction heater system configured to provide an alternating magnetic field to a receiving structure embedded within adhesive layers adjacent to the adherent components (e.g., induction heater system  100 , Magnetic receiver structure  214 , adherent components  206  and  208 , and adhesive layers  210  and  212 , cf.  FIGS. 1 and 2 ). Step  702  includes generating an alternating magnetic field using the induction heater system. Step  702  may also include the induction heater system directing an alternating magnetic field at a predetermined frequency to the receiver structure incorporated within a bonding assembly. The bonding assembly may include at least an adhesive layer thermally coupled to an adherent component. Accordingly, the bonding assembly in step  702  may be an assembly stack (e.g., assembly stack  204 , cf.  FIG. 2 ). 
     Step  704  includes converting the absorbed alternating magnetic field to heat. Accordingly, in some embodiments step  704  can be performed by the receiver structure embedded within the bonding assembly. Step  706  includes conductively transmitting heat from the receiver structure to an adhesive layer. Step  708  includes ceasing the bonding operation. Step  708  may include stopping or turning the induction heater ‘off’. Accordingly, step  708  may include determining that sufficient heat has been absorbed by the adhesive layer, for activation. In some embodiments, step  708  can include measuring a temperature of the adhesive layer to determine that sufficient heat has been absorbed. In some embodiments, step  708  can include allowing a pre-selected amount of time while the induction heater is ‘on’, to lapse. 
       FIG. 8  is a flowchart describing a process  800  in accordance with the described embodiments. Process  800  may include a focused adhesive activation and bonding for adherent components to be used in a handheld electronic device. Process  800  may include an induction heater system configured to provide a magnetic field to a bonding assembly including adherent components (e.g., induction heater system  100 , receiver structure  214 , magnetic field  600 , adherent components  620  and  630 , and adhesive portion  610 , cf.  FIGS. 1, 6A and 6B ). The magnetic field may include a constant offset component and an oscillating component (e.g., component  601  and oscillating component  602 , cf.  FIG. 6A ). 
     Step  801  may include directing a constant magnetic field at an adherent component in the bonding assembly. Accordingly, step  801  may include applying the constant offset component of the magnetic field provided by the induction heater in the proximity of the adherent component. Thus, in embodiments consistent with the present disclosure step  801  may include applying a magnetic force to the adherent component in the bonding assembly. In some embodiments, step  801  may include applying a constant force to align the adherent component to the bonding assembly. Step  802  may be as described in detail above regarding step  702  in process  700  (cf.  FIG. 7 ). For example, in step  802  the induction heater may direct a magnetic field alternating at a predetermined frequency to the adherent component incorporated within the bonding assembly. The adherent component in step  802  may include a ferromagnetic material, or a material having a magnetic permeability that absorbs the alternating component of the magnetic field and generates heat. The bonding assembly may include at least an adhesive portion thermally coupled with the adherent component. Accordingly, the bonding assembly in step  802  may be an assembly stack (e.g., assembly stack  204 , cf.  FIG. 2 ). Steps  804  and  806  may be as described in detail above regarding steps  704  and  706  in process  700 , respectively (cf.  FIG. 7 ). Also, step  806  may include conductively transmitting heat from the heat generating adherent component of step  802  to an adhesive portion. Step  808  may be as described in detail above regarding step  708  in process  700  (cf.  FIG. 7 ). 
     In some embodiments, induction heater system  100  may be configured to provide an RF radiating energy to an RF receiving structure embedded within a bonding assembly. Accordingly, in such embodiments conducting device  106  may be configured as an RF antenna emitting RF radiation within a frequency band centered at a driving frequency. In such configurations the RF receiving structure may be an RF receiving antenna adapted to receive the RF radiation at the driving frequency. This will be described in more detail below, with reference to  FIGS. 9 and 10 , as follows. 
       FIG. 9  shows a plan view of an RF receiver structure  914  according to some embodiments. RF receiver structure  914  may be placed within a bonding assembly, consistent with embodiments disclosed herein. For example, RF receiver structure  914  may be placed as magnetic receiver structure  214  in bonding stack  204  (cf.  FIG. 2 ). In embodiments consistent with an RF receiver structure  914 , the driving frequency of an conducting device  106  may include a desired frequency band within an RF spectrum. 
     Receiver structure  914  has a shape and a dimension configured to effectively capture and absorb an alternating magnetic field in a pre-selected spectral region. In that regard, in some embodiments receiver structure  914  may have a varying shape in different areas overlapping the adjacent adhesive layers (e.g., layers  210  and  212 , cf.  FIG. 2 ). Likewise, in some embodiments receiver structure  914  may have portions having a varying thickness overlapping the adjacent adhesive layers. Receiver structure  914  includes an RF absorption strip  950  having a width  921  W. RF absorption strip  950  coils around to have a length, L. The width, W, and the length, L, of RF absorption strip  950  can be selected to determine the RF spectral region for absorption of an incoming alternating magnetic field. Other parameters that may determine the spectral absorption region may be the relative separation  922  between portions of RF absorption strip  950 . In some embodiments, RF absorption strip  950  may be formed of an electrically conductive material such as a metal. Further according to some embodiments, RF absorption strip  950  may be adjacent to a low heat conductive layer (e.g., layer  421 , cf.  FIG. 4 ). For example, in some embodiments receiver structure  914  may be formed by depositing a metallic material over a layer of polyimide in a first step. The metallic layer thus formed may be etched away in certain portions, to achieve a coiled structure as shown in  FIG. 9 . 
     In some embodiments, the transmitter and the receiver structure may be an antenna configured to transmit and receive electromagnetic radiation in any given region of the spectrum. The electromagnetic spectral region of choice may depend on the specific application, including the RF spectral region and the microwave region, or the terahertz (THz) frequency region (1 THz=10 12  Hz). 
     The particular choice of Cartesian coordinates XY in  FIG. 9  corresponds to a ‘right-handed’ XYZ coordinate system consistent to that of  FIG. 2 . 
       FIG. 10  shows a plan view of an RF receiver structure  1014  according to some embodiments. Accordingly, a first driving frequency may be selected for activation of a first adhesive layer, or a first portion of an adhesive layer, as desired. Likewise, a second driving frequency may be selected for activation of a second adhesive layer, or a second portion of an adhesive layer. Thus, system  100  (cf.  FIG. 1 ) can be configured to deliver RF radiation to each adhesive layer separately, by tuning the driving frequency accordingly. 
     RF receiver structure  1014  has a mesh or grid shape formed of conductive wires. In some embodiments RF receiver structure  1014  may be formed by weaving a plurality of conductive wires. The mesh in RF receiver structure  1014  has a graded pitch. The graded pitch defines regions  1021 ,  1022 , and  1023 , such that each region has a consistent pitch diameter  1031 ,  1032 , and  1033 , respectively. Accordingly, the mesh in RF receiver structure  1014  acts as a frequency selective antenna, wherein RF radiation is absorbed with different efficiency in regions  1021 ,  1022 , and  1023 . For example, an RF radiation in a short wavelength spectral region may be more efficiently absorbed in portion  1033  than it is in portion  1031 . Likewise, a long wavelength spectral region may be more efficiently absorbed in portion  1031  than it is in portion  1033 . Thus, a different amount of heat is generated from portions  1021 ,  1022 , and  1023  in RF receiver structure  1014  upon receiving RF radiation with a selected bandwidth. 
     The shape and size of regions  1021 ,  1022 , and  1023  may be determined according to different heat desirability for a substrate portion overlapping the regions. To obtain uniform bonding the adhesive may be desirably heated at the same temperature across a certain area of a substrate, according to some embodiments. However, the substrate portion overlapping an adhesive layer may have different heat conductivities in different portions. For example, in some embodiments a first substrate portion may include steel, and in some embodiments a second substrate portion may include aluminum. A graded grid having regions  1021 ,  1022 , and  1023  absorbs RF energy with different efficiency in each of the regions. Thus, RF receiving structure  1014  provides a differentiated amount of heat to the adhesive in the vicinity of each of regions  1021 ,  1022 , and  1023 . And each of regions  1021 ,  1022 , and  1023  focuses different amounts of heat according to the heat conductivity of the substrate portion proximal to that region (e.g., steel or aluminum, as described above). In such embodiments, the adhesive layer (e.g., layers  210  and  212 , cf.  FIG. 2 ) can reach approximately the same temperature, resulting in uniform bonding across an area overlapping regions  1021 ,  1022 , and  1023 . 
     In some embodiments, it may be desirable that different portions of the substrate be heated at different temperatures, due to different bonding requirements. For example, a portion of a substrate overlapping region  521  may include an adhesive layer that is activated at a first temperature. And a portion of a substrate overlapping region  1022  may include an adhesive layer that is activated at a second temperature. The first temperature may be different from the second temperature. For example, the first temperature may be lower than the second temperature. In such configuration, pitch  1031  in region  1021  may be selected to absorb an amount of RF radiation lower than the amount of RF radiation absorbed by region  1022 . Thus, the amount of heat provided by region  1021  is lower than the amount of heat provided by region  1022 , resulting in a lower first temperature, compared to the second temperature. 
     The number of mesh regions illustrated in  FIG. 10  is illustrative only, and should not be limiting in any respect. Accordingly, one of ordinary skill in the art will recognize that different shapes and areas of different mesh regions may be combined. For example, the pitch  1031 ,  1032 , and  1033  in adjacent regions may not gradually change in size. Thus, in some embodiments a region such as region  1021  including pitch  1031  may be adjacent to region  1023 , including pitch  1033 . And region  1023  including pitch  1033  may be adjacent to region  1022 . Thus, in some embodiments regions  1021  and  1022  may not be adjacent to each other. Moreover, the grid in RF receiving structure  1014  can have a square lattice shape, as shown, and any other shape, such as: triangular, hexagonal, rhomboid, or irregular shape. 
     The particular choice of Cartesian coordinates XY in  FIG. 10  corresponds to a ‘right-handed’ XYZ coordinate system consistent to that of  FIG. 2 . 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a computer readable medium for controlling manufacturing operations or as computer readable code on a computer readable medium for controlling a manufacturing line. The computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, DVDs, magnetic tape, and optical data storage devices. The computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of specific embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the described embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20130507
Publication Date: 20160510
Grant Date: 20160510
Priority Date: 20121101
Inventors: WRIGHT DEREK W.
UTTERMANN ERIK A.
MCCLURE STEPHEN R.
Assignee: APPLE INC
CPC Classifications: [{"code": "H05B6/105", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B6/14", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B6/105", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B6/14", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 50546046