Process for soldering shape memory alloys

Abstract

A method of brazing a shape memory alloy (SMA) element onto a component, the method comprising: tinning an end face of the SMA element with a solder material and a predetermined solder paste material, wherein the solder paste material contains concentrated phosphoric acid or tin fluoride and the solder material has a melting temperature not exceeding 260°C (500°F), wherein the predetermined solder paste material contains the concentrated phosphoric acid with a mass of at least 80% phosphoric acid; positioning the tinned face of the SMA element relative to a surface of the component; the direct soldering of the tinned face of the SMA element to the surface of the component using the solder material; and Regulating the amount of heat penetrating into the depth of the SMA element while tinning and directly soldering the SMA element, thereby protecting the shape memory capabilities of the SMA element.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for soldering shape memory alloys. BACKGROUND

[0002] Shape memory alloys (SMAs) are a class of materials that exhibit pseudoelasticity as well as shape memory. The deformation of an SMA element, such as an SMA wire, is temporary and reversible by applying an external stimulus, such as heat or an electrical signal. The shape memory capabilities of an SMA element are largely based on a temperature- and stress-dependent solid-state phase change that occurs due to cooperative atomic rearrangement.

[0003] Certain mechatronic applications utilize SMA elements to support and transmit load and / or displacement, such as SMA wire-based control actuators. However, solder materials commonly used to connect conductive wires in electronic devices do not bond well to SMA materials, such as nickel-titanium. Therefore, current methods for connecting SMA elements to a component sometimes involve crimping a metal front end assembly onto the distal ends of the SMA element and subsequently affixing the crimped front end assembly to a surface of the component. However, crimping an SMA element introduces certain performance limitations, including potential slippage or fatigue over time at or adjacent to the crimped front end assembly.Such and comparable methods for soldering one component onto another component are described, for example, in DE 694 23 405 T2 and US 8 021 311 B2. SUMMARY

[0004] The present invention is based on the object of providing an improved method for soldering a shape memory alloy (SMA) element to a component and a method for soldering a nickel-titanium (NiTi) shape memory alloy (SMA) wire to a component.

[0005] This object is achieved by the subject matter of the independent patent claims. Advantageous embodiments of the present invention are described in the dependent claims.

[0006] Disclosed herein is a method for soldering a shape memory alloy (SMA) element to a component. The method, including specific steps to protect the critical shape memory properties of the SMA element, enables the direct soldering of an end face of an SMA element, such as an SMA wire, to a component. For example, the SMA element can be soldered directly to a surface mount pad or through-hole of a printed circuit board assembly, or directly to other SMA elements. The present approach is intended to help solve the manufacturing problem where conventional solder and solder paste combinations do not bond well with the construction materials of typical SMA elements, such as nickel-titanium (NiTi).The method disclosed herein, which allows for direct soldering of the SMA element without the use of conventional end crimps or any other structure between the SMA element and the surface to which the SMA element is soldered, is specifically intended to minimally impact the shape memory capabilities of the SMA element.

[0007] In a specific embodiment, a method for soldering an SMA element to a component includes tinning an end face of the SMA element with a predetermined solder paste and solder material, positioning the tinned end face of the SMA element with respect to a surface of the component, and soldering the tinned end face of the SMA element directly to the surface of the component. If the SMA element does not have an oxide layer, the solder material has a melting temperature not exceeding 500°F. Higher temperatures may be used if an oxide layer is present; in these cases, the solder material may be leaded.The solder material may be tin-based in one exemplary embodiment, although other materials may be used within the intended scope of the invention, including low-level tin-lead blends, indium blends with lead, silver, or tin, or various other exemplary material combinations provided below. The method involves controlling an amount of heat penetrating a depth of the SMA element while tinning and directly brazing the SMA element, thereby protecting the shape memory capabilities of the SMA element.

[0008] The method may include soldering the tinned face of the SMA element to the surface of the component using a lead-free solder material if the oxide layer is not present.

[0009] The process may optionally involve immersing the SMA element in an acid bath for a calibrated time sufficient to produce a clean SMA element. The bath may also be a mixture of hydrofluoric acid and nitric acid. The bathed SMA element may be rinsed in a water bath to remove any acid residue.

[0010] In various non-limiting exemplary embodiments, the solder material may contain at least 3.5% elemental silver by mass, such as KAPP ZAPP 3.5, or 5% elemental antimony by mass, or 9% to 15% zinc by mass. Other materials may be used to achieve the required melting temperature.

[0011] The SMA element can be constructed from nickel titanium and in some embodiments can be formed as an SMA wire.

[0012] The above features and advantages, as well as other features and advantages of the present disclosure, will become apparent from the following detailed description of the best mode for carrying out the disclosure when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of an exemplary soldering station for soldering shape memory alloy (SMA) elements to a component according to a disclosed method. Fig. Figure 2 is a flowchart describing an exemplary method for soldering SMA elements onto a component. Fig. Figure 3 is a flowchart describing an automated version of the process for soldering SMA elements onto a component. DETAILED DESCRIPTION

[0013] With reference to the drawings, in which like reference numerals correspond to the same or similar components in the different figures, Fig. 1 an exemplary soldering station 10 is shown schematically. As described below with reference to Fig. 3, the disclosure is not limited to manual operations. The soldering station 10 of Fig. 1 is for soldering a shape memory alloy (SMA) element 12 to a surface of a component 14. For example, the soldering station 10, as shown, may be used to solder the SMA element 12 to a contact surface 14C, e.g., to an electrical contact or actuator surface of an exemplary printed circuit board assembly. Useful applications for SMA materials extend well beyond the field of electronics, as is well known in the art, and therefore the printed circuit board assembly is merely illustrative. The component 14 may optionally be formed as a control board, with the SMA element 12 selectively connected to apply a force or load to the control board, again without being limited to such an application.

[0014] The soldering station 10 of the Fig. 1 includes a control unit 16 in conjunction with a soldering iron 18 having a thermally regulated soldering pin 19. A clip-on heat sink 11 may be used to secure the SMA element 12 and to serve as a heat sink, as indicated below. Furthermore, a fume hood 15 may be used to remove fumes that may be generated in the vicinity of the operator during the soldering process. Furthermore, the soldering station 10, as indicated below, includes the necessary materials and equipment for successfully soldering the SMA element 12 according to a Fig. 2. The required materials include the solder material 21 and the solder paste material 22 of the specific compositions described below. Conventional tip tinning material 24 may be used to help clean oxides and debris from the solder pin 19 prior to the soldering process.

[0015] As mentioned above, conventional solder materials are largely or completely ineffective when applied to SMA materials, particularly nickel-titanium SMA wires. Therefore, the specific material compositions, welding temperatures, and methods disclosed herein are intended to enable the direct soldering of the SMA elements 12 to the component 14. As used herein, the terms "direct" and "direct" require the absence of any structure between the SMA element 12 and the surface to which the SMA wire 12 is to be soldered. For example, conventional approaches involve crimping a metallic termination to an end face of an SMA wire and then soldering or attaching the metallic termination to a surface. Such a method is considered indirect because the SMA wire material itself is not soldered to the surface.

[0016] SMA wires and other SMA elements 12, when manufactured, have the ability to shrink, for example, up to 6% in length at a calibrated activation temperature, allowing the SMA element 12 to be used as an actuator. If the SMA element 12 is overheated during the soldering process, which could occur using conventional soldering methods, the shape memory capabilities of the SMA element 12 could degrade or be lost entirely. Therefore, all soldering steps of the method 100 described below require the application of regulated amounts of heat to a localized area of ​​the SMA element 12, in part by controlling the amount of heat at the tip 19T. This allows the solder paste material 22 to sufficiently wet the SMA element 12 so that the solder material 21 can bond tightly to the SMA element 12.The heat is then removed as quickly as possible to prevent heat penetration into the depth of the SMA element 12. In other words, the heat is retained locally so that the SMA element 12 does not lose its shape memory capabilities. Generally speaking, with specific examples disclosed below, the method 100 utilizes heat sinks, such as those shown in FIG. Fig. 1, a solder material 21 with a low melting temperature, and specific solder paste materials 22 to minimize the effect on the shape memory properties.

[0017] The control unit 16 of the Fig. 1 includes an input device 13 for setting a desired soldering temperature. For example, a temperature control knob, as shown, may be rotated to a desired temperature setting, which, for the purposes of the present method 100, may range from about 375°F to about 700°F, with soldering temperatures of about 450°F to about 550°F generally being suitable, and higher temperatures possibly being used when, as discussed below, an oxide layer is present on an outer surface of the SMA element 12. Other embodiments of the temperature control knob 16 may be digital in design and operation. Setting a desired soldering temperature via the input device 13 results in resistive heating of a tip 19T of the solder pin 19 to the desired temperature.

[0018] In the context of procedure 100 of the Fig. 2 and the automated procedure 200 of the Fig. 3, the solder paste material 22 may be an active or acidic compound with a low melting temperature. As is known in the art, the term "melting temperature" refers to the temperature above which a given material is in a completely liquid state. For the purposes of the present disclosure, the term "low melting temperature" refers, in a particular embodiment, to a temperature of no more than about 500°F. The solder paste material 22 is used to remove bright layers of surface oxides from the SMA element 12. The use of certain chemicals in the solder paste material 22, such as stannous fluoride (SnF2), may aid in removing oxide layers. Another exemplary mixture of the solder paste material 22 is a sufficiently concentrated form of phosphoric acid, e.g., a mixture containing at least 80% phosphoric acid.The SMA elements 12 are tinned with the solder material 21 and the solder paste material 22 on the end faces or other areas of the SMA element 12, which are finally soldered directly to the component 14.

[0019] The solder material 21 used in methods 100 and 200 may be a suitable lead-free solder material which, when oxide layers are not present on the surfaces of the SMA element 12 or are present at such low levels that the immediate bond to the component 14 is not unduly impaired. A lead-containing tube solder material 21 containing, for example, elemental tin and fluoride may optionally be used when oxide layers are present at sufficiently high levels relative to a calibrated threshold. When using a leaded tube solder form of the solder material 21, a higher relative soldering temperature, such as about 600°F to 700°F, may be selected via the temperature input device 13. To minimize the need for leaded solder material, methods 100 and 200 could include, in a separate manufacturing phase, the removal of oxide layers from the SMA element 12, such asby grinding or by means of an electrochemical bath and / or by the use of solder paste material 22 containing tin and fluoride.

[0020] With respect to exemplary embodiments of the solder material 21, a tin-based mixture may be used. For example, in some embodiments, a mixture of at least 85% elemental tin (Sn) is suitable. For the purposes of this particular example, a material mixture in the range of about 85% to 96.5% Sn may be used, with the remainder of the mixture being comprised of a suitable material, such as elemental zinc (Zn), silver (Ag), or antimony (Sb). Within the stated ranges, effective exemplary mixtures may include a mixture of 95% Sn and 5% Sb, i.e., Sn95Sb5, Sn96.5Ag3.5, Sn91Zn9, and Sn85Zn15. In further embodiments, zinc chloride or zinc fluoride may be used, particularly when surface oxides are present.Those skilled in the art will recognize that various other solder materials 21 may be provided having a low melting temperature threshold of not more than 500°F within the intended scope of the invention, including, but not limited to, Sn 95.5Cu4Ag0.5, Sn 90Zn 7Cu3, Pb 70Sn 30 to Pb 55Sn 45, Sn 50Pb 50, Sn 50Pb 48.5Cu1.5, Sn 60Pb 40 to Sn 95Pb 5, Sn 60Pb 38Cu2, Sn 60Pb 39Cu1, Sn 63Pb 37P0.0015-0.04, Sn 62Pb 37Cu1, Pb 80Sn 18Ag2, Sn 43Pb ​​43Bi 14, Sn 46Pb 46Bi 8, Bi 52Pb 32Sn 16, Bi46Sn34Pb20, Sn62Pb36Ag2, Sn62.5Pb36Ag2.5, In97Ag3, In90Ag10, In75Pb25, In70Pb30, In60Pb40, In50Pb50, In50Sn50, In70Sn15Pb9.6Cd5.4, Pb75In25, Sn70Pb18In12, Sn37.5Pb37.5In25, Pb54Sn45Ag1, Sn61Pb36Ag3, Sn56Pb39Ag5, Sn98Ag2, Sn65Ag25Sb10, Sn96.5Ag3.0Cu0.5, Sn95.8Ag3.5Cu0.7, Sn95.6Ag3.5Cu0.9, Sn95.5Ag3.8Cu0.7, Sn95.25Ag3.8Cu0.7Sb0.25, Sn95.5Ag3.9Cu0.6, Sn95.5Ag4Cu0.5, and Sn96.5Ag3.5.

[0021] The soldering temperature can be selected depending on the type of SMA element 12 and the presence or absence of surface oxide layers. Generally, the soldering temperature ranges between approximately 450°F and 700°F, which is sufficiently higher than the low melting temperature of the solder material 21. Higher temperatures within the example range may be used for oxide-coated SMA elements 12, although a lower temperature is more appropriate for SMA elements 12 lacking an oxide layer and for minimal impact on the shape memory properties of the SMA element 12. Since the soldering temperature exceeds the melting point of the solder material 21, pure tin is generally soldered at temperatures above 450°F. Adding 3.5% elemental silver, for example, reduces this temperature to approximately 430°F.

[0022] Referring to Fig. 2, in an exemplary embodiment, the method 100 begins, as shown, with step S102, where the SMA element 12 is first cut to a desired length and then inspected for the presence of an oxide surface layer or film. For example, the SMA element 12 may be viewed under a high-power microscope or subjected to other testing or inspection methods that may reveal oxide layers on the outer surfaces of the SMA element 12. The method 100 then proceeds to step S104.

[0023] In step S104, the oxide values ​​determined in step S102 can be compared with a calibrated threshold. The method 100 then proceeds to step S107 if the measured oxide values ​​are below the calibrated threshold, and to step S105 or step S106 if the determined oxide values ​​exceed the calibrated threshold.

[0024] In particular, step S105 may be performed within a process in which the oxide layers are removed separately as part of the method 100 prior to direct soldering, while step S106 may be performed when direct soldering of the oxide-containing SMA elements 12 is appropriate.

[0025] In optional step S105, the oxide layers may be carefully removed from the surface of the SMA element 12, for example, by chemical or acid etching, gentle grinding, or other suitable process step. For example, the SMA element 12 may be immersed in an acid oxide removal bath for a calibration time suitable for removing the oxide layers, followed by a thorough rinsing of the SMA element 12 with a bath of clean, liquid water or another suitable cleaning solution. In one possible embodiment, an oxide-coated SMA element 12 may be bathed in a mixture of hydrofluoric acid (HF) and nitric acid (HNO3). One such mixture that may be used is a mixture of 5% concentrated HF, i.e., at least 48% HF, and 15% concentrated HNO3, i.e., at least 70% HNO3.Step S105 may additionally or alternatively include manually abrading the surface of the SMA element 12 with fine-grain sandpaper or other abrasive material to carefully remove the oxide film without abrading the underlying surface of the SMA element 12. The method 100 proceeds to step S107 after the oxide layers have been removed to a level below the threshold applied in step S104.

[0026] Step S106 involves setting the soldering temperature of the control unit 16 of the Fig. 1. to a threshold temperature suitable for brazing the oxide-coated SMA element 12. The brazing temperature may exceed 600°F if, as noted below, a solder alloy or leaded solder material 21 is used. Examples of suitable solder alloys include a mixture of approximately 70% to 80% lead (Pb), 10% to 20% tin (Sn), and 1% to 5% silver (Ag), ALU-SOL 45D, or other suitable lead mixtures. Depending on the configuration of the control unit 16 and the temperature input device 13, it may be necessary in step S106 to rotate a knob or select a digital setting via a keypad. At each step of the process 100, the amount of heat penetrating into a depth of the SMA element 12 is carefully controlled via the attachable heat sinks 11 and by using the specific materials and oxide removal processes, particularly during tinning and direct soldering of the SMA element.This is done to protect the shape memory capabilities of the SMA element. The method 100 then proceeds to step S108.

[0027] Step S107 involves setting the temperature of the control unit 16 to a temperature threshold suitable for soldering relatively clean, oxide-free SMA elements 12. The execution of step S107 is based on a decision in step S104 that the determined oxide levels are sufficiently low to begin direct soldering with a lead-free solder material 21, such as KAPP ZAPP 3.5, or the removal of oxide layers in step S105 and achieve said sufficiently low oxide levels. The soldering temperatures in step S107 may be approximately 450°F to 550°F, with the temperature set as indicated in step S106 above. The method 100 then proceeds to step S109.

[0028] In step S108, the solder material 21 may be applied to the SMA element 12. A leaded tube solder version of the solder material 21, for example, with approximately 70% elemental lead mass, may be used for this purpose. The method 100 then proceeds to step S110.

[0029] In step S109, an end face of the SMA element 12 is tinned with the solder paste material 22, which in this case is a tube solder material 22 and solder material 21, so that the SMA element 12 is sufficiently coated with solder material 21. The tip 19T of the solder pin 19 can be cleaned with the tip tinning material 24 as needed. As is known in the art, tinning is a process in which solder paste material 22 and solder material 21 are applied to an end face of the SMA element 12 to ensure that a bond of sufficient integrity is formed between the solder material 22 and the SMA element 12 prior to attaching the SMA element 12 to the component 14. The method 100 then proceeds to step S111.

[0030] Step S110 involves cleaning the tip 19T with the tip tinning material 24, which in this case should be lead-free. A suitable lead-free tip tinning material 24 is, for example, a mixture of tin, ammonium phosphate, and diammonium phosphate. The method 100 then proceeds to step S111.

[0031] Step S111 involves immersing a face of the SMA element 12, which was previously tinned, into the solder material 21, wherein the composition of the solder material 21 depends on whether oxide layers were present in step S102 or were not removed in step S105. That is, if oxides are detected, a leaded tube solder version of the solder material 21 can be used to help remove the oxide and tin from the SMA element 12. If no oxides are detected, the solder material 21 may be lead-free.

[0032] Step S111 may further include clamping the tinned end face to the clip-on heat sinks 11. For example, alligator clips may serve as wire holders for the SMA element 12, but also as a suitable heat sink, further protecting the shape memory effect of the SMA element 12. The tip 19T of the solder pin 19 is coated with a solder material 21 of the types mentioned above, such that a small solder pool of molten solder is present on the tip 19T. The solder pool on the tip 19T is then brought into contact with the SMA element 12, where the solder paste material 21 coats the SMA element 12, with the SMA element 12 being passed through the solder pool of molten solder in an appropriate shape to tin the SMA element 12. The method 100 then proceeds to step S113.

[0033] Step S113 includes tinning with the tip tinning material 24 from Fig. 1 of all contact surfaces of the component 14 to which the SMA element 12 is soldered. For example, step S113 may involve tinning a contact point 14C of the Fig. 1. The method 100 then proceeds to step S115.

[0034] In step S115, the solder pin 19 is held against the contact pad 14C of the component 14 while a small bead of solder material 21 is applied to the contact pad 14C. The clean SMA element 12 is moved into the solder bath while the solder material 21 remains molten. The solder pin 19 is removed once the SMA element 12 is properly positioned. The SMA element 12 is held in place until the solder material 21 has cooled, typically in only a few seconds. The heat sink 11 can be released. The process 100 repeats with each solder joint formed.

[0035] Further embodiments of the method 100 from Fig. 2 are easily conceivable for an expert in the field. For example, the procedure 200 can be Fig. 3 may include an automated or semi-automated wave soldering process in which the SMA element 12 is first subjected to a series of baths in step 202. In an exemplary embodiment, in a step analogous to step S105 of Fig. 2, a first acid bath may be used to remove the oxide layers, while a second bath of clean water removes any residual acid. Additional baths may be used to apply the solder paste material 22 to coat the face of the SMA element 12 and to immerse the coated face of the SMA element 12 into the solder material 21.

[0036] In step 204, the tinned face of the SMA element 12 may be brought into contact with a previously tinned contact pad 14C of the component 14 and heated via a heat gun, oven, or other heat source to melt the solder 21. In step 206, the SMA element 12 and the tinned contact pad 14C may be pressed together in a clamp or press and allowed to cool. Alternatively, the SMA element 12 may be placed in a pick-and-place machine of a type known in the art and temporarily bonded to the contact pad 14C.

[0037] In step 208, the SMA element 12 and component 14 may be subjected to another series of baths of solder paste material 22, solder material 21, and a suitable cleaner to remove any residual solder paste material 22. Other methods may be contemplated within the disclosure using the specific materials and steps described above, such as where the machine applies a mixture of the solder material 21 and the solder paste material 22 to the pad 14C and then places the assembly in a furnace to melt the solder material 21. The component 14 may then be subjected to a cleaning bath to remove excess solder paste material 22, with the composition of the cleaning bath depending on the configuration of the component 14.

[0038] Advantages of the methods 100 and 200 disclosed above include the retention and consistency of the application of the shape memory properties of the SMA element 12. By eliminating the crimping of end connections or interference fit to the SMA element 12 in favor of directly soldering the SMA element 12 to the component 14, the number of applications suitable for the integration of SMA elements 12 can be increased. The methods 100 and 200 can also result in assemblies with improved heat transfer properties. Furthermore, the time and expense associated with crimping the end connections to the faces of the SMA element 12 are eliminated. Solid end attachments are enabled, for example, by allowing the SMA element 12 to be attached directly to a control circuit board or other component 14.These and other advantages will be apparent to one of ordinary skill in the art in light of this disclosure.

[0039] While the best modes for carrying out the disclosure have been described in detail, those skilled in the art described herein will recognize various alternative designs and embodiments in which the invention can be practiced within the scope of the following claims.

Claims

[1] A method of brazing a shape memory alloy (SMA) element onto a component, the method comprising: tinning an end face of the SMA element with a solder material and a predetermined solder paste material, wherein the solder paste material contains concentrated phosphoric acid or tin fluoride and the solder material has a melting temperature not exceeding 260°C (500°F), wherein the predetermined solder paste material contains the concentrated phosphoric acid with a mass of at least 80% phosphoric acid; positioning the tinned face of the SMA element relative to a surface of the component; the direct soldering of the tinned face of the SMA element to the surface of the component using the solder material; and Regulating the amount of heat penetrating into the depth of the SMA element while tinning and directly soldering the SMA element, thereby protecting the shape memory capabilities of the SMA element. [2] The method of claim 1, further comprising: detecting an oxide layer on the SMA element and using a leaded solder material in response to the detected oxide layer. [3] A method according to claim 2, wherein the leaded solder material contains at least 70% elemental lead mass. [4] The method of claim 1, further comprising: determining the absence of an oxide layer on the SMA element, and using a lead-free solder material in response to the determined absence of the oxide layer. [5] The method of claim 1, wherein the predetermined solder paste material contains the tin fluoride. [6] The method of claim 1, further comprising: immersing the SMA element in an acid bath prior to tinning the face of the SMA element for a calibrated time sufficient to remove an oxide layer from the SMA element. [7] The method of claim 6, wherein immersing the SMA element in an acid bath comprises immersing the SMA element in a mixture of hydrofluoric acid and nitric acid. [8] The method of claim 1, wherein the solder material contains elemental tin and elemental silver. [9] The method of claim 1, wherein the SMA element is an SMA wire made of nickel titanium. [10] A method of brazing a nickel-titanium (NiTi) shape memory alloy (SMA) wire to a component, the method comprising: immersing the NiTi SMA wire in an acid bath for a calibrated time sufficient to remove an oxide layer from the NiTi SMA wire, resulting in a clean SMA wire; tinning one end face of the clean NiTi SMA wire with a non-leaded solder material and a solder paste material, wherein the solder paste material contains concentrated phosphoric acid or tin fluoride and wherein the predetermined solder paste material contains the concentrated phosphoric acid with a mass of at least 80% phosphoric acid; positioning the tinned end face of the clean NiTi SMA wire against a surface of the component; directly soldering the tinned end face of the clean NiTi SMA wire to the surface of the component using the solder material, wherein the solder material has a melting temperature not exceeding 260 °C (500 °F); and Regulating the amount of heat penetrating into the depth of the NiTi SMA wire while tinning and directly soldering the NiTi SMA wire, thereby protecting the shape memory capabilities of the NiTi SMA wire.

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