Electrical Interconnection Of Circuit Elements On A Substrate Without Prior Patterning

Abstract

A method for producing electronic devices includes fixing a die that includes an electronic component with integral contacts to a dielectric substrate. After fixing the die, a conductive trace is printed over both the dielectric substrate and at least one of the integral contacts, so as to create an ohmic connection between the conductive trace on the substrate and the electronic component.

Claims

1. A method for producing electronic devices, the method comprising: fixing a die that includes an electronic component with integral contacts to a dielectric substrate; and after fixing the die, printing a conductive trace over both the dielectric substrate and at least one of the integral contacts, so as to create an ohmic connection between the conductive trace on the dielectric substrate and the electronic component.

2. The method according to claim 1, wherein printing the conductive trace comprises ejecting droplets of a conductive material onto both the dielectric substrate and the at least one of the integral contacts.

3. The method according to claim 2, wherein ejecting the droplets comprises directing a pulsed laser beam to impinge on a donor film comprising the conductive material, whereby the droplets are ejected by laser-induced forward transfer (LIFT).

4. The method according to claim 3, wherein ejecting the droplets comprises directing the pulsed laser beam so that the droplets are ejected toward the die at an oblique angle relative to a surface of substrate.

5. The method according to claim 3, wherein ejecting the droplets comprises directing the pulsed laser beam to impinge on a first donor film comprising a first metal having a first melting temperature so as to form an adhesion layer on the dielectric substrate along a track of the conductive trace, and then directing the pulsed laser beam to impinge on a second donor film comprising a second metal having a second melting temperature, higher than the first melting temperature, so as to build up the conductive trace over the adhesion layer.

6. The method according to claim 2, wherein the die has opposing upper and lower sides, wherein the lower side is fixed to the dielectric substrate, and wherein the at least one of the integral contacts over which the conductive trace is printed is on the upper side.

7. The method according to claim 6, wherein ejecting the droplets comprises building up a pillar from the ejected droplets alongside the electronic component from the dielectric substrate to the at least one of the integral contacts.

8. The method according to claim 2, wherein the die has opposing upper and lower sides with lateral sides therebetween, wherein the lower side is fixed to the dielectric substrate, and wherein the at least one of the integral contacts over which the conductive trace is printed is on one of the lateral sides.

9. The method according to claim 2, and comprising prior to ejecting the droplets, forming an array of holes in the dielectric substrate so as to inhibit recoil of the droplets from the dielectric substrate.

10. The method according to claim 1, and comprising printing adhesive dots on the dielectric substrate by ejecting droplets of an adhesive material onto the dielectric substrate by laser-induced forward transfer (LIFT), wherein fixing the die comprises placing the die on the adhesive dots.

11. The method according to claim 1, wherein the die has opposing upper and lower sides, wherein the lower side is fixed to the dielectric substrate, and wherein the method comprises, after the die has been fixed to the dielectric substrate, ejecting droplets of an adhesive material onto the upper side of the die by laser-induced forward transfer (LIFT).

12. The method according to claim 1, wherein the conductive trace is printed over a locus on the dielectric substrate along which there was no conductive material prior to fixing the die.

13. The method according to claim 12, wherein printing the conductive trace comprises automatically sensing a location of the at least one of the integral contacts, and printing the conductive trace responsively to the automatically-sensed location.

14. The method according to claim 1, wherein the dielectric substrate comprises a flexible foil.

15. The method according to claim 14, wherein the flexible foil comprises a material selected from a group of materials consisting of polymers, papers and fabrics.

16. A system for producing electronic devices, the system comprising: a placement station, which is configured to fix a die, which includes an electronic component with integral contacts, to a dielectric substrate; and a printing station, which is configured to print a conductive trace over both the dielectric substrate and at least one of the integral contacts of the die that has been fixed to the substrate, so as to create an ohmic connection between the conductive trace on the substrate and the electronic component.

17. The system according to claim 16, wherein the printing station is configured to print the conductive trace by ejecting droplets of a conductive material onto both the dielectric substrate and the at least one of the integral contacts.

18. The system according to claim 17, wherein the printing station comprises a laser, which is configured to emit a pulsed laser beam to impinge on a donor film comprising the conductive material, whereby the droplets are ejected by laser-induced forward transfer (LIFT).

19. The system according to claim 18, wherein the printing station is configured to direct the pulsed laser beam so that the droplets are ejected toward the die at an oblique angle relative to a surface of the dielectric substrate.

20. The system according to claim 18, wherein the printing station comprises optics, which are configured to direct the pulsed laser beam to impinge on a first donor film comprising a first metal having a first melting temperature so as to form an adhesion layer on the dielectric substrate along a track of the conductive trace, and then to direct the pulsed laser beam to impinge on a second donor film comprising a second metal having a second melting temperature, higher than the first melting temperature, so as to build up the conductive trace over the adhesion layer.

21. The system according to claim 17, wherein the die has opposing upper and lower sides, wherein the lower side is fixed to the dielectric substrate, and wherein the at least one of the integral contacts over which the conductive trace is printed is on the upper side.

22. The system according to claim 21, wherein the printing station is configured to build up a pillar from the ejected droplets alongside the electronic component from the dielectric substrate to the at least one of the integral contacts.

23. The system according to claim 17, wherein the die has opposing upper and lower sides with lateral sides therebetween, wherein the lower side is fixed to the dielectric substrate, and wherein the at least one of the integral contacts over which the conductive trace is printed is on one of the lateral sides.

24. The system according to claim 17, and comprising a surface preparation station, which is configured to form an array of holes in the substrate prior to ejecting the droplets so as to inhibit recoil of the droplets from the dielectric substrate.

25. The system according to claim 16, wherein the printing station is configured to print adhesive dots on the dielectric substrate by ejecting droplets of an adhesive material onto the dielectric substrate by laser-induced forward transfer (LIFT), and wherein the placement station is configured to place the die on the adhesive dots.

26. The system according to claim 16, wherein the die has opposing upper and lower sides, wherein the lower side is fixed to the dielectric substrate, and wherein the printing station is configured to eject droplets of an adhesive material onto the upper side of the die by laser-induced forward transfer (LIFT) after the die has been fixed to the dielectric substrate.

27. The system according to claim 16, wherein the conductive trace is printed over a locus on the dielectric substrate along which there was no conductive material prior to fixing the die.

28. The system according to claim 27, and comprising a sensor, which is configured to automatically sense a location of the at least one of the integral contacts, wherein the printing station is configured to print the conductive trace responsively to the automatically-sensed location.

29. The system according to claim 16, wherein the dielectric substrate comprises a flexible foil.

30. The system according to claim 29, wherein the flexible foil comprises a material selected from a group of materials consisting of polymers, papers and fabrics.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is schematic side view and block diagram of a system for electronic circuit assembly, in accordance with an embodiment of the present invention;

[0018] FIGS. 2A-2D are schematic pictorial illustrations showing successive stages in the production of an electronic circuit, in accordance with an embodiment of the invention;

[0019] FIG. 3 is an image captured by a scanning electron microscope (SEM) of an electronic component assembled into a circuit in accordance with an embodiment of the invention; and

[0020] FIGS. 4A-C are schematic side views of an electronic component on a substrate, showing successive stages in connection of a circuit trace to the electronic component, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0021] In traditional processes for manufacture of electronic circuits, a pattern of conductive traces is first deposited on a dielectric substrate (such as a rigid or flexible printed circuit substrate). The circuit components, such as semiconductor dies and other active and passive elements, are then fixed to the substrate and connected to the traces, typically by solder or wire bonding. This is the accepted order of operations in circuit manufacture and assembly.

[0022] In modern electronic products, however, there is an increasing demand to integrate electronic circuits onto other sorts of substrates, both rigid and flexible, which may not be compatible with traditional printed circuit technology. For example, some applications call for electronic devices to be mounted and interconnected on flexible substrates, such as plastic foils, paper or fabric. Adapting traditional processes to such substrates can be challenging and costly.

[0023] Embodiments of the present invention that are described herein address this problem by reversing the accepted order of operations: First a die, which includes an electronic component with integral contacts, is fixed to the dielectric substrate, for example by a suitable adhesive. After fixing the die, a conductive trace is printed over both the dielectric substrate and at least one of the integral contacts of the die, thus creating an ohmic connection between the conductive trace on the substrate and the electronic component. Other active and passive components can be fixed to the substrate and interconnected in like manner.

[0024] Conductive traces can be printed in this manner over a locus on the dielectric substrate along which there was no conductive material prior to fixing the die. In other words, the disclosed techniques require only a single process step to perform both deposition of the traces and connection of the traces to the circuit components. To ensure accurate printing of the conductive traces, an automatic sensing system, such as a computer vision system, can be used to sense the locations of the contacts of the die after the die has been fixed to the substrate, and printing of the traces can then be guided to the sensed locations.

[0025] In the disclosed embodiments, the conductive traces are printed using a LIFT process, in which droplets of a conductive material are ejected onto both the dielectric substrate and the integral contacts of the die. (These embodiments assume that the lower side of the die is fixed to the dielectric substrate, and the integral contacts over which the conductive trace is printed are on the upper side or lateral sides of the dies.) As described in detail hereinbelow, LIFT printing of metal droplets is able to overcome large height gapson the order of hundreds of micronsbetween the substrate and the contacts on the upper side of the die, while at the same time producing very fine traces. Such traces may be printed by LIFT with line widths down to 10-20 m, and may be trimmed (for example, by post-printing laser treatment) to even narrower widths if desired. Furthermore, LIFT printing of metals and highly viscous pastes is relatively indifferent to substrate properties, such as porosity, liquid absorbance and surface energy, and can thus print fine traces with high accuracy even on porous, absorbent substrates such as paper and fabric. This process can be used to create circuits that will afterwards withstand (within limits) bending and stretching of the substrate.

[0026] Alternatively or additionally, however, in some applications, other techniques that are known in the art for direct writing of conductive materials can be applied in creating at least some of the conductive traces.

[0027] FIG. 1 is a block diagram and side view of a system 20 for electronic circuit assembly, in accordance with an embodiment of the invention. In the pictured example, a placement station 26 places a component, such as a semiconductor die 24, on a circuit substrate 22. Placement station 26 may comprise, for example, a pick-and-place machine, in which a robotic arm 28 places components on substrate 22 under instructions from a control unit 32 in accordance with a predefined circuit layout. Typically, substrate 22 is mounted on a positioning assembly 30, such as a translation stage, to enable arm 28 to reach all desired locations. The operation of placement station 26 differs from pick-and-place machines that are known in the art, however, in that die 24 is placed on substrate 22 prior to printing of the conductive traces that will subsequently connect to the die.

[0028] Optionally, to ensure the accuracy of subsequent operations carried out in system 20, a sensing station 34 detects the actual position at which die 24 has been placed on substrate 22. Sensing station 34 comprises, for example, a camera 36, which captures images of substrate 22. Control unit 32 processes these images in order to ascertain the exact positions of the components on the substrate. Although sensing station 34 is shown in FIG. 1, for the sake of conceptual clarity, as a separate unit, in practice such sensors, such as camera 36, and sensing functionality may be integrated with the other stations in system 20. The application of sensing station 34 in system 20 is particularly advantageous in overcoming the limited accuracy of pick-and-place tools that are known in the art, as well as possible distortions that may occur in the shape of a flexible substrate in the course of the assembly process. Precise sensing of the positions of die 24 and other components enables accurate printing of very fine interconnecting traces (for example, with widths of 20 m or less) after the components have been fixed to substrate 22.

[0029] More generally speaking, control unit 32 may adapt the traces that are to be printed depending on the actual positions of the dies in relation to other circuit components. This sort of adaptation is particularly useful, for example, when there is high density of circuit components or many input/output pins on a given die, as well as narrow lines, one should take into consideration the final (actual) position of the die in relation to the rest of the circuit. In such cases, control unit 32 selects the trace locations so as to conform with electrical design rules and avoid circuit defects. This approach can be used in creating circuits with component density as great as or even greater than is achievable using conventional manufacturing processes.

[0030] Depending on the material properties of substrate 22, a surface preparation station 38 can be used to treat the surface of the substrate 22 in order to enhance adhesion of the traces thereon. For example, when the surface is very smooth, surface preparation station 38 may create arrays of small holes in the surface, typically several microns deep and several microns across, which will roughen the surface and reduce the recoil of metal droplets from the surface in the subsequent LIFT printing stage. The holes may be made specifically in the loci of the conductive traces that are to be printed on substrate 22, or alternatively over wider areas of the substrate 22. The holes may be drilled by a laser, for example, or impressed mechanically in the surface of the substrate 22.

[0031] A printing station, for example a LIFT station 40, prints conductive traces 56 on substrate 22, in ohmic connection with the contact pads on die 24. LIFT station 40 comprises an optical assembly 41, in which a laser 42 emits pulsed radiation. Optics 46 focus laser beam 48 onto a transparent donor substrate 50 having at least one donor film 52 deposited thereon. Positioning assembly 30 positions donor substrate 50 and/or circuit substrate 22 so that the donor substrate 50 is in proximity to the upper surface of the acceptor substrate, with donor film 52 facing toward the circuit substrate and a small gap in between (typically no more than a few hundred microns, and possibly less). Although positioning assembly 30 is shown in FIG. 1, for the sake of simplicity, as a basic X-Y-Z stage below circuit substrate 22, in practice the positioning assembly 30 in LIFT station 40 additionally or alternatively may be capable of positioning donor substrate and/or elements of optical assembly 41, as will be apparent to those skilled in the art.

[0032] Donor substrate 50 typically comprises a glass or plastic sheet or other suitable transparent material, while donor film 52 comprises a suitable donor material, such as one or more pure metals and/or metal alloys. Typically, the thickness of the donor films is no more than a few microns.

[0033] Optics 46 focus beam 48 to pass through the outer surface of donor substrate 50 and to impinge on donor film 52, thereby causing droplets 54 of a fluid to be ejected from donor film 52 and fly across the gap to land on circuit substrate 22. The fluid comprises a molten form of the material in donor film 52, which then hardens on the acceptor surface to form solid pieces having a shape defined by the printing pattern. A scanner 44, such as a rotating mirror and/or an acousto-optic beam deflector under control of control unit 32, scans laser beam 48 so as to irradiate different spots on donor film 52 and thus create conductive traces 56 in the appropriate locations on circuit substrate 22.

[0034] Laser 42 comprises, for example, a pulsed Nd:YAG laser with frequency-doubled output, which permits the pulse amplitude to be controlled conveniently by control unit 32. Typically, control unit 32 comprises a general-purpose computer, with suitable interfaces for controlling and receiving feedback from optical assembly 41, positioning assembly 30, and other elements of system 20. The inventors have found that for good LIFT deposition results, the optimal pulse duration is in the range of 0.1 ns to 1 ns, but longer or shorter pulses may be used, depending on application requirements. Optics 46 are similarly controllable in order to adjust the size of the focal spot formed by the laser beam on donor film 52.

[0035] The size of droplets 54 is determined, inter alia, by the laser pulse energy, duration, and focal spot size and the thickness of the donor film. The above-mentioned PCT International Publication 2015/181810, for example, describes LIFT techniques and parameters that can be applied so that each laser pulse causes a single, relatively large droplet to be ejected from the donor film. These techniques and parameters can be applied advantageously in LIFT station 40, since the droplets are ejected toward circuit substrate 22 with accurate directionality, making it possible to hold donor film 52 at a distance of at least 100 m from the acceptor substrate during ejection of the droplets and to create the desired structures with precision.

[0036] In some embodiments, LIFT station 40 causes droplets 54 of two or more different fluids, having different compositions, to be ejected from different donor films 52. For example, the different fluids can be ejected by directing beam 48 from laser 42 to impinge on different areas of donor films 52 (either on the same donor substrate 50 or different donor substrates) containing different materials. The different fluids may be ejected sequentially toward the same location on circuit substrate 22 and/or toward different locations, in order to print the desired traces 56.

[0037] For example, to enhance adhesion of traces 56 to certain types of circuit substrates, such as polymer substrates, a first donor film 52 comprising a metal with a low melting temperature, such as tin or a tin alloy, is used in printing an initial adhesion layer along the loci of the traces. The low-temperature metal droplets cool quickly while at the same time locally melting the polymer, thus forming a layer to which subsequent laser droplets with higher temperature and energy will readily adhere. (This adhesion layer need not be dense or fully coat the substrate. A low-density adhesion layer, covering no more than 20-30% of the intended area of trace 56, is often sufficient to anchor the subsequent structural layer.) Thereafter, a second donor film 52 with a structural metal, such as copper (and/or aluminum or nickel, for example) is printed over the adhesion layer in order to reach the desired dimensions, connectivity and conductivity of the trace. As noted earlier, control unit 32 controls scanner 44 and other elements of optical assembly 41 so as to write donor material from films 52 to the appropriate locations on circuit substrate 22 and to make multiple passes, as needed, in order to build up the deposited volume of the donor material to the desired thickness.

[0038] FIGS. 2A-2D are schematic pictorial illustrations showing successive stages in the production of an electronic circuit, in accordance with an embodiment of the invention. As a preliminary step, shown in FIG. 2A, substrate 22 is coated with an adhesive layer 60 or, alternatively, a suitable pattern of adhesive, such as an array of adhesive dots (not shown), which may also be printed by a LIFT process. (LIFT printing of adhesive may also be used in printing dots of adhesive, when needed, on top of die 24 and other circuit components.) Die 24 is then fixed in position, for example by placement station 26, with the lower side of the die 24 resting on adhesive layer 60, as illustrated in FIG. 2B. After all components have been placed on substrate 22, adhesive layer 60 may be cured (for example by application of ultraviolet radiation or heat) in order to hold the components securely in place.

[0039] When necessary, surface preparation station 38 prepares tracks 62 on substrate 22, as shown in FIG. 2C, over which traces 56 will subsequently be printed. As explained earlier, such preparation is advantageous for materials that are not inherently sufficiently rough to capture and hold the droplets printed by LIFT station 40. Tracks 62 may contain, for example, an array of shallow holes with a diameter that is one to two times the diameter of droplets 54 and depth of roughly 3-6 m. Alternatively or additionally, LIFT station may be operated to create an initial metal adhesion layer, as explained above. On the other hand, surface preparation before LIFT printing may not be needed on substrate materials that are sufficiently porous.

[0040] Finally, conductive trace 56 is printed by LIFT station 40 over both substrate 22 and the integral contacts on the upper side of die 24, as shown in FIG. 2D. Trace 56 is built up over the edges of die 24 in order to connect with the contacts of the die. (Details of an implementation of this latter step are shown in FIGS. 4A-C.) Although LIFT station 40 is capable of printing very fine traces, as explained above, in some cases wider and/or thicker conductive traces may be printed in order to sink heat away from the circuit components in operation.

[0041] FIG. 3 is an image captured by a scanning electron microscope (SEM) of an actual die 24 assembled onto substrate 22 in accordance with an embodiment of the invention. Substrate 22 in this example is a polyethylene terephthalate (PET) foil, while die 24 comprises an LED in a surface-mount package. Traces 56 are built up from LIFT droplets and extend over the shoulders of the die to reach the contact pads on the upper side. Application of a suitable voltage between traces 56 on the opposing sides of die 24 caused the LED to emit light.

[0042] FIGS. 4A-C are schematic side views of die 24 on substrate 22, showing successive stages in connection of a circuit trace to contacts 70 on the upper side of the die 24, in accordance with an embodiment of the invention. After initial placement of die 24, as shown in FIG. 4A, LIFT station 40 deposits successive droplets 72 to create a pillar 74 up to contact 70, as shown in FIGS. 4B-C. The inventors have created pillars of this sort with widths of 10-20 m by building up piles of single droplets 54. As the height of pillar 74 grows, positioning assembly 30 may move donor substrate 50 away from circuit substrate in order to maintain a sufficient gap between the donor and acceptor surfaces.

[0043] It is desirable to build a small tilt into the pillar (on the order of 3) to prevent gaps from opening between the pillar and the die contacts. Additionally or alternatively, angled LIFT printing, in which droplets are ejected from donor film 52 at an oblique (non-perpendicular) angle relative to the surface of substrate 22, may be applied in order to conformally coat the lateral sides of the die with metal without gaps between the pillar and the die, and thus enhance the mechanical robustness of the traces and connections. Angled LIFT printing techniques that can be used in this regard are described, for example, in PCT International Publication WO 2016/116921, whose disclosure is incorporated herein by reference.

[0044] Pillar 74 takes the place of wire-bonding, which is used to creating this sort of contact in electronic devices that are known in the art. The use of LIFT to create such contacts both enhances robustness of the device against mechanical shock and can reduce overall size, since there is no need to leave space around the components for connection of the wire bonds.

[0045] Although the examples shown in the figures and described above relate to particular types of components, the principles of the present invention may be applied in assembling circuits comprising a wide variety of different sorts of components, including (but not limited to): [0046] Semiconductor dies. [0047] Active devices, such as batteries, photovoltaics, sensors, optoelectronics, thin-film devices, and MEMS devices. [0048] Pre-assembled hybrid devices, such as rigid or flexible foils with components already mounted on them. [0049] Surface-mount devices, such as discrete transistors, capacitors, inductors and resistors.

[0050] It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.