Methods for low-temperature bonding of electronic components

Abstract

Method for bonding an electronic unit (31) with at least one electrically or thermally conductive contact (32, 34) to a carrier substrate (52) with at least one associated electrically or thermally conductive contact (55), arranged in a bonding area, comprising: Coating each of the contacts (32, 34) of the electronic unit (31) with a layer of bonding material (54); Arranging a reactive multilayer film (51) between each of the contacts (32, 34) of the electronic unit (31) and the associated contacts (55) of the carrier substrate (52); Applying pressure to the contacts (32, 34) of the electronic unit (31) in the bonding area; and Triggering an exothermic reaction in the reactive multilayer film (51), wherein the exothermic reaction results in the formation of a bonding point containing the bonding material (54) and residues of the reactive multilayer film (51) between the contact (32, 34) of the electronic unit (31) and the associated contact (55) of the substrate (52), wherein the reactive multilayer film (51) which is arranged between each of the contacts (32, 34) of the electronic unit (31) has an area which is smaller than the area of ​​the associated contact (55) of the substrate (52) which is defined by a solder mask (57) which surrounds the associated contact (55) of the substrate (52).

Description

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to methods for bonding semiconductor devices to substrates and methods for solder bonding temperature-sensitive devices, such as light-emitting diode (LED) assemblies, at low temperatures to a substrate containing a heat sink, without exposing the entire semiconductor device or the adjacent substrate to damaging heat.

[0002] Light-emitting diodes, or LEDs, are semiconductor lighting components that are increasingly replacing incandescent lamps in many lighting applications. As in Fig.As shown in Figure 1, a typical prior art LED assembly consists of an LED chip (not shown) enclosed in a protective housing 31 (LED package). Two or more connecting leads 34 with pins 36 extend from the sides of the package 31. A thermal coupling block 32, generally made of copper, is located at the base of the LED package 31 and coated with a wetting layer 33 to enable bonding to a substrate with a heat sink, such as a metal core printed circuit board (MCPCB). Fig. (1 not shown). A lens 35 can be mounted on the top of the LED housing 31 to direct emitted light. An exemplary LED housing 31, without the lens 35, can have dimensions of approximately 5 mm width x 5 mm length x 2 mm height.

[0003] For cost-effective manufacturing, the LED package 31, the lens 35, and the MCPCB substrate preferably comprise polymer or plastic materials. The MCPCB substrate is typically a printed circuit board containing a metal core within one or more layers of dielectric material. Copper traces and contacts are located outside the dielectric material, and a solder mask is arranged to surround the copper contacts and cover the traces. Fig. Figure 2 represents a typical contact point pattern, which corresponds to the connecting lines 34 and the block 32 of the in Fig. The LED housing 31 shown in Figure 1 fits together. For operation of the LED housing 31, the block 32 is bonded to contact point 55b, while the two lines 34 are each bonded to a corresponding contact point 55a.

[0004] In a conventional bonding process, the various bond points between the LED package 31 and the MCPCB contact points 55a, 55b are formed by a solder melting process at high temperatures. However, the solder melting process can damage the polymer lenses 35 by exposing them to the high temperatures reached during solder melting, and especially to the high temperatures used in lead-free solder melting procedures. Similarly, if large heat sinks are required, the plastic capping and the LED semiconductor electronics can be damaged by the melting process, as the thermally active mass of the heat sinks requires an excessively long solder melting cycle at an elevated temperature. If the solder melting cycle is not long enough, poor bonding can occur as a result of insufficient solder melting.Furthermore, the high melting temperature can negatively affect the brightness of the LED or the lifespan of the component.

[0005] Alternatively, the connecting leads 34 can be individually soldered using laser or thermocouple soldering, and the central block 32 can be attached with a thermally conductive adhesive. An electrical connection between the central block 32 and the contact point 55b is not required; however, since the block 32 serves to transfer heat from the LED package 31 to the metal core PCB, the thermal resistance between the central block 32 and the MCPCB must be low. Thermally conductive adhesives have acceptable but limited thermal conductivity and experience inconsistent viscosity during application, which can lead to varying bond line thicknesses. If the thickness of the bond line between the central block 32 and the contact point 55b is too great, the thermal resistance of the bond point increases.In extreme cases, the thickness of the bond point on the central block (32) can be so great that the legs 36 of the connecting leads 34 do not touch the contact points 55a, making laser soldering difficult or impossible. Consequently, conventionally manufactured LED assemblies often fail to achieve their full potential in terms of thermal, electronic, and optical performance.

[0006] The same problems occur with other electronic components and semiconductor devices with integrated contacts or thermal management blocks that must be bonded to an external heat sink and are sensitive to high bonding temperatures. Such components and devices include, but are not limited to, electrolytic capacitors, power amplifiers, and photovoltaic devices. Many components designed for eutectic lead-tin solder are damaged at the higher melting temperatures required for the use of lead-free solder.

[0007] Accordingly, it would be advantageous to provide a uniform and reliable method for bonding electronic components, such as semiconductor devices and LEDs, which are sensitive to extreme bonding temperatures, to a substrate. It would also be advantageous to provide such a bonding method that does not generate excessive heat outside the bonding area and that results in bond joints with a uniform thickness and strength, capable of conducting electrical current and thermal energy.

[0008] US Patent 2001 / 0038029 A1 discloses a method for bonding an electronic unit having at least one electrically or thermally conductive contact to a substrate having at least one associated electrically or thermally conductive contact arranged in a bonding area, comprising: coating each of the contacts of the electronic unit with a layer of bonding material; placing a reactive multilayer film between each of the contacts of the electronic unit and the associated contacts of the substrate; applying pressure to the contacts of the electronic unit in the bonding area; and initiating an exothermic reaction in the reactive multilayer film. The exothermic reaction results in the formation of a bond junction, containing the bonding material and remnants of the reactive multilayer film, between the contact of the electronic unit and the associated contact of the substrate.

[0009] US 2005 / 0 082 343 A1, AU 2004 / 256 020 A1, US 2001 / 0 050 370 A1 and US 6 517 218 B2 also disclose methods for bonding an electronic unit. BRIEF SUMMARY OF THE INVENTION

[0010] In one embodiment, the present invention provides a method for bonding an electronic unit with the features of claim 1. According to the invention, the exothermic reaction results in the formation of a bond point, containing the bonding material and remnants of the reactive multilayer film, between the contact of the electronic unit and the corresponding contact of the substrate. The reactive multilayer film, which is arranged between each of the contacts of the electronic unit, has an area that is smaller than the area of ​​the corresponding contact of the substrate, which is defined by a solder mask surrounding the corresponding contact of the substrate. BRIEF DESCRIPTION OF THE INDIVIDUAL VIEWS OF THE DRAWINGS Fig. Figure 1 is a schematic representation of a state-of-the-art light-emitting diode housing; Fig. Figure 2 is a schematic top view of the contact points on a state-of-the-art printed circuit board for mounting an LED housing. Fig. 1; Fig. Figure 3 is a representation of a reactive multilayer bonding process; Fig. Figure 4 is a schematic representation of an LED package bonded to a PCB using the methods of the present disclosure; Fig. Figure 5 illustrates a simplified cross-section of a bonding point formed by the method of the present disclosure and comprising residues of a reactive multilayer film; Fig. 6A and Fig. Figure 6B shows various conductor geometries of state-of-the-art LED packages; Fig.Figure 7 represents an unpackaged LED package bonded to a substrate; and Fig. Figure 8 represents an arrangement for testing the shear strength of a bonded LED package. DETAILED DESCRIPTION

[0011] The following detailed description illustrates the invention by way of example, without limiting it. The description enables a person skilled in the art to manufacture and use the present disclosure, and it describes several embodiments, adaptations, modifications, alternatives and applications of the present disclosure, including what is currently considered to be the best embodiment of the present disclosure.

[0012] In general, the present disclosure discloses a method for bonding various contacts between electronic units, such as LED packages, semiconductor devices or photovoltaic devices, and their substrates, such as printed circuit boards with a metal core, using pieces or segments of reactive multilayer films to provide sufficient thermal energy to melt and flow bonding materials, such as solder, without damaging adjacent heat-sensitive components in the electronic units or substrates.Although the numerous descriptions and examples throughout this specification relate to LED packages, it is understood that they are not limited to these but can be applied equally to a wide variety of electronic components, and especially those susceptible to damage from excessive heat or thermal energy. Additional types of electronic units that can be suitably bonded using the methods of this disclosure include flip chips, printed circuit boards with bonded chips, unpackaged chips, chip-sized packages, package-on-package packages, solder ball matrix components, leadless fine-pitch packages, leadless chip carriers, square flat packages, plastic chip carriers with leads, flat leads, SO packages, DPAKs, and D2PAKs.

[0013] The reactive multilayer foils used in the bonding processes of this disclosure are typically formed by magnetron sputtering and consist of thousands of alternating nanoscale layers of materials such as nickel and aluminum. The layers react exothermically when an external energy pulse initiates atomic diffusion between them, releasing a rapid burst of heat in a self-propagating reaction. When the reactive multilayer foils are sandwiched between layers of a bonding material, such as a solder or brazing alloy, the heat released by the exothermic reaction of the reactive multilayer foils can be used to melt these layers of bonding material, as described in [reference to relevant information]. Fig.Figure 3 illustrates this. The resulting bond layer comprises a solder or brazing alloy layer containing the reaction products of the reactive multilayer film. By controlling the properties of the reactive multilayer films, the amount of heat released by the reactive multilayer films during the exothermic reaction can be adjusted to ensure that sufficient heat is present to melt the bonding material layers while maintaining the mass of the adjacent components at or around room temperature. Further details regarding reactive multilayer films, bonding with them, and their reaction products are found in U.S. Patent No. 6,736,942, which is incorporated herein by reference.

[0014] Now with reference to Fig.Figure 3 shows an arrangement 9 for carrying out the general process for the reactive multilayer bonding of two components 10A and 10B. A discrete sheet, piece, or segment of reactive composite material 12, such as a reactive multilayer film, is placed between two layers or sheets of fusible bonding material 14A and 14B, usually solder or brazing alloy, which are in turn sandwiched between contact surfaces (not shown) of components 10A and 10B. The multilayer unit is then compressed, as symbolized by the vise 16, and the reactive multilayer film is ignited by a suitable means, as illustrated by a match and a flame 18. The reaction spreads rapidly through the reactive multilayer film 12 and melts the fusible bonding material 14A and 14B, which subsequently cools, thereby bonding components 10A and 10B together at the contact surfaces.The bonding surfaces provided herein can be from 0.25 mm. 2 up to many square centimeters.

[0015] With reference to Fig. 4. An electronic component, such as an LED package 31, with a central block 32 and at least one connecting lead 34, is bonded to one or more contacts on a printed circuit board 52 with a metal core using a bonding process that employs reactive multilayer films. For example, the LED package 31, with a plurality of contacts defined by a central block 32 and a plurality of connecting leads 34, is bonded to corresponding contacts of a printed circuit board (PCB) defined by contact points 55a and 55b using a reactive multilayer bonding process.

[0016] As in Fig.As shown in Figure 4, the PCB 52 comprises a metal core 56, typically aluminum, a dielectric layer 53, and contact points 55a and 55b. A polymer solder mask 57 surrounds and partially covers the contact points 55a and 55b. The contact points 55a and 55b may be formed of copper, and they may optionally be coated with a surface layer 54 of bonding material, such as tin, applied by hot air solder levelling (HASL) to form a tin layer typically 4 µm to 8 µm thick. It is apparent to the average person that the surface layer 54 can be selected from any of a variety of conventional surface treatments, including electroless tin plating, electroless nickel plating / electroless gold plating (ENIG), and electroless nickel plating.electroless nickel / immersion gold), lead-tin HASL, Sudsilber, chemical palladium / gold, electrolytic nickel / gold, molten tin / lead or any other surface treatment known in the field.

[0017] Advantageously, the connecting leads 34 and the central block 32 of the LED package are similarly coated with a layer of bonding material, such as tin, with a thickness of 4 µm to 30 µm. As with the surface layer 54, the layer of bonding material on the connecting leads 34 and the central block 32 of the LED package 31 can be any suitable bonding material, such as solder, solder alloy, or brazing alloy, and it can be applied by electroplating, dipping, or any other method generally known in the field. Such soldering and brazing materials include, but are not limited to, pure metals and alloys of indium, silver, and lead, as well as commercially available alloys such as Georo® and Incusil®.

[0018] Traditionally, pure tin has generally not been used as a bonding material due to its high melting point, but the process of the present disclosure enables the use of pure tin as a soldering or bonding material, thereby providing the advantage of higher ductility compared to other bonding materials, such as lead-free solder alloys.

[0019] To bond the central block 32 and the connecting leads 34 to the contact points 55, one or more pieces of a reactive multilayer film 51 are arranged between the contacts, i.e., between the connecting lead legs 36 and their adjacent contact points 55a, and between the central block 32 and the adjacent contact point 55b. A load, designed to hold the contact surfaces against the reactive multilayer films 51, is applied to each connecting lead leg 36 and the LED housing 31 to apply a load to the central block 32. Simultaneously, an energy pulse is applied to each piece of reactive multilayer film 51, sufficient to trigger an exothermic reaction within it.The resulting exothermic reaction of each piece of reactive multilayer film 51 generates sufficient heat energy to melt the surrounding bonding material or surface layer 54, which solidifies upon cooling, bonding the leads 34 and the central block 32 of the electronic component to their respective contact points 55 on the substrate or printed circuit board 52. The heat energy generated by the reaction of the reactive multilayer film is insufficient to raise the temperature of the adjacent LED package or the substrate (printed circuit board 52) significantly above room temperature.

[0020] Advantageously, the discrete segments of reactive multilayer film 51a and 51b are dimensioned to fit within the polymer solder mask 57, which surrounds each contact point 55 on the substrate. Preferably, the applied load for the exemplary LED package 31 described above and shown in the figures results in a pressure of 10 MPa to 100 MPa on each connecting lead 36 and of 1 MPa to 9 MPa on the LED package 31 (to apply a load to the central block 32). If the applied pressures are too low, the bond points resulting from the exothermic reaction of the reactive multilayer film 51 may be too weak. If the applied pressures are too high, the printed circuit board 56, the connecting leads 34 or the LED housing 31 may be damaged, and molten bonding material may be extruded from the bonding area during the exothermic reaction of the reactive multilayer films 51.

[0021] The resulting bond point, which is formed, for example, between the adjacent contact surfaces of the connecting conductor leg 36 and the contact point 55a, as shown in Fig. Figure 5 shows the residues 51r of the reactive multilayer film 51, which typically comprises a nickel aluminide. During the exothermic reaction, the reactive multilayer film 51 usually ruptures across its thickness to form smaller pieces, and the bonding material and surface layer 54, such as tin or another fusible material, flows through the ruptures to fill them, resulting in a characteristic microstructure shown in Fig. 5 is shown schematically.

[0022] The substrate 52 or printed circuit board (PCB) can also be a different type of printed circuit board, including, but not limited to, glass fibers, FR4, polybutadiene, or PTFE. The substrate can comprise multiple layers of dielectric and copper contact pads. Alternatively, the electronic component or LED package 31 can be bonded to a metal heat sink or a flexible circuit material.

[0023] It is apparent to the average professional that the connecting leads 34 of the electronic component or LED housing can each have a variety of shapes, including articulated leads, as in Fig. 6a shows a “connection-free” configuration, as in Fig.Figure 6b shows J-terminals or flat terminals, but these may include, but are not limited to, components with a solder ball matrix for connecting the electronic component or LED housing 31. Components with a solder ball matrix for connecting the electronic component or LED housing 31 can also be bonded using the methods of the present invention. Preferably, a large part of the top or contact surface of each connecting lead leg 36 is parallel to a corresponding connection point 55 on the carrier substrate 52, as shown in Figure 6b. Fig. Figure 4 shows that a large area is in contact with the reactive multilayer film 51 to establish a strong bond. Advantageously, the area is chosen to be larger than 50% of the area of ​​the connecting lead leg 36.

[0024] During the bonding process, the load on the electronic component 31 and the connecting leads 34 is preferably applied by a compliant element, such as a spring, an elastomer, or a foam, configured to maintain at least 80% of the initial load during and after the initiation of the exothermic reaction in the segments of reactive multilayer film 51. The compliant element advantageously responds to any increase or decrease in the thickness of the reactive multilayer film 51, bonding materials, and surface layers 54 while the materials are melting, in order to ensure that sufficient contact is maintained throughout the entire bonding process.

[0025] In a related embodiment of the present disclosure, only a portion of the bond points between the electronic component or LED package 31 and the support substrate 52 are formed using the exothermic reaction of segments of reactive multilayer film 51, and the remaining bond points are formed using any conventional bonding process. For example, the central block 32 can be bonded by reactive multilayer bonding, and the connecting leads 36, which may be offset from potentially heat-sensitive components of the electronic component 31, can be bonded by another method known in the field, such as laser soldering, gap welding, or thermomode soldering.In a similar embodiment, the connecting conductor legs 36 can be bonded by reactive multilayer bonding, while the central block 32 is bonded by another method known in the field, such as with thermally conductive epoxy.

[0026] In related embodiments, the contacts, such as the contact points 55, the connecting leads 34, or the central block 32, are not coated with a tin alloy 4 µm to 30 µm thick, but instead with a wettable surface layer, such as gold, silver, or palladium. For this embodiment, a fusible bonding material, such as a tin-based solder alloy, is advantageously combined with the pieces of reactive multilayer film 51 by magnetron sputtering, hot lamination, electroplating, or other methods known in the field. Preferably, a layer of fusible bonding material is bonded to one or both main surfaces of a piece of reactive multilayer film 51. Alternatively, the fusible bonding material can be in the form of a freestanding preform.The fusible bonding material can be placed on one or both sides of the reactive multilayer film 51. The fusible bonding material is positioned adjacent to the wettable surface layer, and the bonding process is carried out, as previously mentioned, by the exothermic reaction of the reactive multilayer film 51. For good adhesion and strength, the total thickness of the fusible bonding material in a bonded joint, including any bonding material layer on the surfaces of the connecting leads 34, the central block 32, and the copper contact points 55, should be at least 6 µm. More fusible bonding material can compensate for surface roughness or unevenness and improve reliability.The practical upper limit for the thickness of the fusible bonding material is determined by the amount of heat provided by the exothermic reaction of the reactive multilayer film 51 (since insufficient heat will prevent the fusible bonding material from melting and forming a bond), or by the escape of fusible bonding material from the bonding surface (which increases with increasing thickness of the fusible bonding material). This escape is undesirable, as it can lead to short circuits or damage to the LED package or the substrate. Therefore, the thickness of the fusible bonding material should be kept below 100 µm and preferably below 50 µm.

[0027] Fusible bonding materials suitable for combination with reactive multilayer film include, but are not limited to, pure metals and alloys of tin, indium, silver, gold, and lead. In a related embodiment, the fusible bonding material can be a higher-melting-point alloy, such as Georo® or Incusil®, which is advantageous when a ceramic substrate or printed circuit board is used instead of the previously mentioned metal-core PCBs (MCPCBs) for high-temperature applications.

[0028] In another related embodiment, the contact points 55, the conductors 34, the central block 32, or any combination thereof, can be coated with a tin alloy or other fusible bonding material, and a fusible bonding material can also be present on the surfaces of the pieces of reactive multilayer film 51 prior to the bonding process. The addition of tin or solder layers to the reactive multilayer film 51 enables good bonding at lower pressures than are required without such layers.

[0029] In another embodiment, which in Fig.As illustrated in Figure 7, the LED component 31 is not enclosed in a ceramic or polymer package with a copper block 32, but instead is in the form of an unpackaged chip 71 comprising silicon or another semiconductor material. The contact surfaces 72 to be bonded are nickel- and gold-plated, and the silicon is bonded to a printed circuit board or a substrate 73 by the reactive multilayer bonding process of the present disclosure using pieces of reactive multilayer film 74 with a layer of fusible material 75 on the side facing the unpackaged LED chip 71. If the substrate 73 does not have a suitable fusible bonding material in the bond contact areas, the reactive multilayer film 74 may further comprise a second layer of fusible bonding material 76 on the side facing the substrate 73.

[0030] In another embodiment, the LED housing 31 contains several LED chips 71. Each LED chip 71 may or may not have its own copper block 32 and its own connecting lines 34.

[0031] The following examples describe exemplary results for bonding electronic components to substrates using the methods of the present disclosure. It should be noted that these examples serve only for illustrative purposes and are not intended to limit the scope of the present invention in any way.

[0032] In the first example, the LED packages 31 (without lenses 35) and MCPCBs 52 were bonded by the methods of the present disclosure, as described in Fig.Figure 4 shows the connecting leads 34 and the central blocks 32. The connecting leads 34 and the central blocks 32 were each electroplated with a 5 µm tin layer, and the copper contact points 55 were similarly coated with 5 µm to 7 µm tin by hot air tinning. The connecting leads 34, as shown, had the articulated configuration as shown in Figure 4. Fig. Figure 6a shows that to improve the contact between the leads 34 and the contact points 55a, some leads 34 were conditioned by methods known in the field to bring the leg 36 to ±2° from the horizontal and in the same plane as the base of the LED housing 31, similar to Fig.1. Segments of reactive multilayer foil 51, comprising nickel and aluminum layers, with a 1 µm thick bonding material layer of Incusil® hard solder alloy on each outer surface and with a total thickness of approximately 40 µm, were used to create the bond points between the connecting leads 34, the central block 32, and the contact points 55. The central blocks 32 had a diameter of 4 mm, and the copper contact points 55 had a diameter of 4.3 mm. Each connecting lead leg 36 was approximately 2 mm x 2 mm after conditioning, and the associated contact points 55a were 3 mm x 3 mm. For a housing, discrete reactive multilayer foil pieces 51a with dimensions of 2 mm × 2 mm were arranged between the contact points 55a and the connecting lead legs 36, while a reactive multilayer foil disk 51b with a diameter of 4 mm was arranged between the contact point 55b and the central block 32.Pressure was applied to the upper surface of the LED housing 31 by a spring-loaded piston, igniting the reactive multilayer film disc 51b to initiate the exothermic reaction. Pressure was then applied to a connecting lead 34 by a spring-loaded piston, igniting the reactive multilayer film piece 51a. Similarly, pressure was then applied to the other connecting lead 34 by a spring-loaded piston, igniting the other reactive multilayer film piece 51a. The exothermic reaction of each reactive multilayer film piece generated sufficient heat energy to melt the surrounding bonding materials, which subsequently solidified to form the intended bond points between the respective contacts.

[0033] Similar bond points were produced, with only the central copper block 32 being bonded to the contact point 55b, and the shear strength was tested. Table 1 below shows the influence of the pressure and thickness of the reactive multilayer film on the bond strength. Table 1 Foil thickness (µm) Foil shape Connection pressure (MPa) Medium shear strength (MPa) Standard deviation from shear strength 1 40 4 mm × 4 mm square 1,38 32,78 8, 60 2 30 4 mm × 4 mm square 1,38 13,78 7, 96 3 30 4 mm × 4 mm square 2,76 18,58 8,43 4 40 Disc 3.9mm 1,38 34,30 4,80 diameter

[0034] Similar centered copper block bonds were fabricated using MCPCBs coated with a 1 µm layer of tin at the 55b junctions by a tinning process. Table 2 below shows the effect of pressure on bond strength when only 1 µm of tin is applied to the 55b junctions. Reactive multilayer foil discs with 1 µm of Incusil on each side and with 1 µm of Incusil plus 4 µm of pure tin on each side, applied by electroplating, were used. For these tests, reactive multilayer foil discs with a diameter of four millimeters were used. Table 2 Foil thickness (µm) surface layer of the film Connection pressure (MPa) Medium shear strength (MPa) Standard deviation from shear strength 1 40 1 µm Incusil 1,38 3,47 1,37 2 40 1 µm Incusil 4,14 12,2 4,98 3 40 1 µm Incusil 7,09 38,3 3,86 4 40 4 µm Sn 2,13 47,6 1,03 5 40 4 µm Sn 7,09 49,1 1, 79

[0035] In a separate set of tests, the connecting leads 34 were bonded to the contact points 55a and subjected to a shear test. The connecting leads 34 originally had the articulated configuration as shown in Fig. Figure 5a shows the film, but it was conditioned as previously described to provide an enlarged contact area. The reactive multilayer film 51 used was 40 µm thick with 1 µm of Incusil on each side, as previously described. The shear test is shown in Fig.Figure 8 illustrates this, where a load 61 is applied to the center of the side of the LED housing 31 and is balanced by forces 62 on the bond points of the two connecting leads. The following Table 3 shows the shear strength for a lead bond point under three conditions: firstly, in the wing-shaped configuration; secondly, in a conditioned configuration; and thirdly, in a conditioned configuration with twice the bond pressure. Table 3 lines Connection pressure (MPa) Average soil area 2 / mm 2 Average shear breaking load (lbs / kg) Medium shear strength (MPa) Standard deviation from shear strength Folding wing 5, 7 0,00120 / 0,774 3,33 / 1,51 9, 95 5,44 Conditioned 3, 5 0,00394 / 2,54 7,35 / 3,33 6,58 0,96 Conditioned 7, 3 0,00380 / 2,45 7,57 / 3,43 6,57 1,31

[0036] As can be seen from Table 3 above, conditioning the connecting leads 34 to increase the contact surface area significantly increased the mean shear failure load, but not the mean shear strength, since the bond area was considerably larger when the leads were conditioned. However, the uncertainty of the shear strength decreased considerably when the leads were conditioned, indicating greatly improved consistency and repeatability. Doubling the load from the second case to the third case made no significant difference to either the shear stress or the shear strength.

[0037] It is now apparent that the present disclosure, in one aspect, presents an improved method for bonding an electronic component, such as a light-emitting diode (LED) package 31, to a support substrate 52, such as a printed circuit board. The method comprises providing the electronic component or LED package 31, which includes at least two electrical leads 34 and a thermal path (e.g., the copper block) 32 for transferring heat from the component 31 to the support substrate 52, and further comprising providing the support substrate 52 for mounting the component or LED package 31 and for extracting heat from it. The support substrate 52 preferably includes a heat sink, connection points 55a for electrical connection to the leads 34 of the package, and a connection point 55b for thermal coupling the heat sink to the thermal path.The thermal path of the electronic component or LED package 31 is thermally coupled to the heat sink by the process of arranging one or more layers of bonding material, such as solder or brazing alloy, together with a piece of reactive multilayer film 51 between the thermal path and the heat sink, and subsequently initiating an exothermic reaction in the reactive multilayer film 51 to generate sufficient heat energy to melt the bonding material and form a bond between the thermal path and the connection point that is connected to the heat sink. Advantageously, the connection leads 34 of the package are also electrically connected to the contact points 55a of the substrate by layers of bonding material, such as solder or brazing alloy, which is melted by initiating an exothermic reaction in an interposed piece of reactive multilayer film 51.The thermal and electrical connections are therefore achieved without a significant increase in the temperature of the electronic unit or thermal damage to the LED semiconductor electronics or other heat-sensitive materials, such as polymers, in the electronic unit 31 or the carrier substrate 52.

[0038] In another aspect, the present disclosure provides an improved light-emitting diode (LED) assembly comprising an LED package, a substrate, and a bonding layer between the LED package and the substrate. The LED package includes an LED component, at least two electrical leads to the LED component, and a thermal path (e.g., the copper block) for transferring heat from the LED to the heat sink in the substrate. The substrate for supporting the LED package includes the heat sink, connection points for electrical connection to the package leads, and a contact point for thermal connection of the heat sink to the thermal path.The bond layers, which are arranged between each of the electrical conductors, the heat path and the associated contact points on the substrate, each comprise a layer of solder or hard solder and the reaction products resulting from the exothermic reaction of a piece of reactive multilayer film.

[0039] In summary, the present disclosure, in one embodiment, describes a method for bonding an electronic unit having at least one electrically or thermally conductive contact to a substrate having at least one associated electrically or thermally conductive contact arranged in a bonding area, comprising: coating each of the contacts with a layer of bonding material; arranging a reactive multilayer film between each of the contacts of the electronic unit and the associated contacts of the substrate; applying pressure to the contacts in the bonding area; and initiating an exothermic reaction in the reactive multilayer film, wherein the exothermic reaction results in the formation of a bond joint, containing the bonding material and the remnants of the reactive multilayer film, between the contact of the electronic unit and the associated contact of the substrate.

[0040] In an alternative embodiment, the present disclosure presents a bonded electronic assembly comprising: an electronic unit with at least one electrically or thermally conductive contact line coated with a first bonding material; a substrate with at least one electrically or thermally conductive contact point coated with a second bonding material; and the remnants of a reactive multilayer film arranged within at least one bonding point formed by the first and second bonding materials between the contact line and the contact point.

[0041] In an alternative method, the present disclosure discloses a method for bonding a temperature-sensitive electronic unit with at least one contact to a receiving contact of a support substrate, arranged in a bonding area, which comprises: arranging a reactive multilayer film between the contact of the electronic unit and the associated receiving contact of the support substrate; exerting a controlled pressure on the layer of reactive multilayer film by the contacts; and initiating an exothermic reaction in the reactive multilayer film, wherein the exothermic reaction results in the formation of a bonding point, which contains the residues of the reactive multilayer film, between the contact of the electronic unit and the associated receiving contact of the substrate.

[0042] It is evident that various aspects of the present disclosure can be realized in part in the form of computer-implemented processes and devices for carrying out these processes. These may include, but are not limited to, a computer-controlled application of pressure to the various contact areas during the bonding process. The present disclosure can also be realized in part in the form of computer program code containing instructions and implemented on physical data carriers such as magnetic disks, CD-ROMs, hard disks, or other computer-readable storage media. When the computer program code is loaded onto such media and executed by an electronic device, such as a computer, a microprocessor, or a logic circuit, the device becomes a device for carrying out the present disclosure.

[0043] The present disclosure can also be implemented in part in the form of computer program code, whether, for example, stored on a storage medium, loaded onto a computer and / or executed by it, or transmitted via any transmission medium, such as electrical wiring or cabling, fiber optics, or electromagnetic radiation. When the computer program code is loaded onto a computer and executed by it, the computer becomes a device for carrying out the present disclosure. When implemented in a general-purpose microprocessor, the computer program code segments configure the microprocessor to generate specific logic circuits.

[0044] Since various modifications can be made to the previously described designs without deviating from the scope of the disclosure, the entire subject matter of the invention, as contained in the foregoing description or illustrated in the accompanying drawings, is to be interpreted as illustrative and not in a restrictive sense.

Claims

[1] Method for bonding an electronic unit (31) with at least one electrically or thermally conductive contact (32, 34) to a carrier substrate (52) with at least one associated electrically or thermally conductive contact (55) arranged in a bonding area, comprising: Coating each of the contacts (32, 34) of the electronic unit (31) with a layer of bonding material (54); Arranging a reactive multilayer film (51) between each of the contacts (32, 34) of the electronic unit (31) and the associated contacts (55) of the carrier substrate (52); Applying pressure to the contacts (32, 34) of the electronic unit (31) in the bonding area; and Triggering an exothermic reaction in the reactive multilayer film (51), wherein the exothermic reaction results in the formation of a bonding point containing the bonding material (54) and residues of the reactive multilayer film (51) between the contact (32, 34) of the electronic unit (31) and the associated contact (55) of the substrate (52), wherein the reactive multilayer film (51) which is arranged between each of the contacts (32, 34) of the electronic unit (31) has an area which is smaller than the area of ​​the associated contact (55) of the substrate (52) which is defined by a solder mask (57) which surrounds the associated contact (55) of the substrate (52).

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