Method for soldering shape memory alloys

A method of soldering a shape memory alloy (SMA) element to a component includes positioning a tinned end of the SMA element with respect to a surface of the component, and then directly soldering the tinned end to the surface using solder material having a low liquidus temperature of 500° F. or less when an oxide layer is not present on the SMA element. The end may be soldered using lead-based solder material at a higher temperature when an oxide layer is present. The end may be tinned with flux material containing phosphoric acid or tin fluoride prior to soldering the SMA element. The SMA element may be submersed in an acid bath to remove the oxide layer. The solder material may contain tin and silver, antimony, or zinc, or other materials sufficient for achieving the low liquidus temperature. Heat penetrating the SMA element is controlled to protect shape memory abilities.

TECHNICAL FIELD

The present disclosure relates to a method for soldering shape memory alloys.

BACKGROUND

Shape memory alloys (SMA) are a class of materials exhibiting pseudo-elasticity and shape memory. Deformation of an SMA element such as an SMA wire is temporary and reversible by application of an external stimulus such as heat or an electrical signal. Shape memory capabilities of an SMA element are due in large part to a temperature- and stress-dependent solid-state change of phase that occurs due to a cooperative atomic rearrangement.

Certain mechatronic applications use SMA elements to carry and transmit a load and/or a displacement, for example SMA wire-based control actuators. However, solder materials of the types conventionally used to join conductive wires in electronic devices do not bond well to SMA materials such as nickel-titanium. Therefore, current practices for joining SMA elements to a component include crimping a metal end attachment onto the distal ends of the SMA element and then fastening the crimped end attachment to a surface of the component. However, the crimping of an SMA element has certain performance limitations, including potential slippage or fatigue over time at or adjacent to the crimped end attachments.

SUMMARY

A method of soldering a shape memory alloy (SMA) element to a component is disclosed herein. The method, including specific steps for protecting the critical shape memory properties of the SMA element, enables direct soldering an end of a SMA element, e.g., an SMA wire, to a component. For instance, the SMA element may be soldered directly to a contact pad of a surface mount or through-hole of a printed circuit board assembly, or directly to other SMA elements. The present approach is intended to help address the manufacturing problem in which conventional solder and flux combinations do not bond well to the materials of construction of typical SMA elements, e.g., nickel-titanium (NiTi). The method disclosed herein, which enables direct soldering of the SMA element without the use of conventional end crimps or other intervening 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.

In a particular embodiment, a method of soldering an SMA element to a component includes tinning an end of the SMA element with a predetermined flux material and solder material, positioning the tinned end of the SMA element with respect to a surface of the component, and directly soldering the tinned end of the SMA element to the surface of the component. When the SMA element does not have an oxide layer, the solder material has a liquidus temperature that does not exceed 500° F. Higher temperatures may be used when an oxide layer is present, and the solder material may be leaded in those instances. The solder material in an example embodiment may be tin-based, although other materials may be used within the intended inventive scope, including lower percentage mixtures of tin and lead, or mixtures of indium with lead, silver, or tin, with various other example material combinations set forth below. The method includes controlling an amount of heat penetrating into a depth of the SMA element while tinning and directly soldering the SMA element to thereby protect shape memory abilities of the SMA element.

The method may include soldering the tinned end of the SMA element to the surface of the component using a lead-free solder material when the oxide layer is not present.

The method may optionally include submersing the SMA element in an acid bath for a calibrated duration sufficient for producing a clean SMA element. The bath may 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.

In various non-limiting example embodiments, the solder material may contains at least 3.5% elemental silver by weight, e.g., KAPP ZAPP 3.5, or 5% elemental antimony by weight, or 9% to 15% zinc by weight. Other materials may be used to achieve the required liquidus temperature.

The SMA element may be constructed of nickel titanium and may be configured as an SMA wire in some embodiments.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, an example soldering station10is shown schematically inFIG. 1. As described below with reference toFIG. 3, however, the disclosure is not limited to manual processes. The soldering station10ofFIG. 1is configured for soldering a shape memory alloy (SMA) element12to a surface of a component14. For instance, the soldering station10may be used to solder the SMA element12to a contact pad14C, e.g., an electrical contact or actuator surface of an example printed circuit board assembly as shown. Useful applications of SMA materials extend well beyond the realm of electronics, as is well known in the art, and therefore the printed circuit board assembly is merely illustrative. The component14may be optionally embodied as a control board, with the SMA element12selectively connected to apply a force or a load to the control board, again without being limited to such an application.

The soldering station10ofFIG. 1includes a controller16in communication with a soldering iron18having a thermally-controlled soldering pencil19. A clip-on style heat sink11may be used to secure the SMA element12, as well as to act as a heat sink as set forth below. A fume hood15may also be used to help remove fumes that may be generated by the soldering process in the vicinity of the operator. Also arranged at the soldering station10are required materials and equipment as set forth below, which are required for successfully soldering the SMA element12according to a method100shown inFIG. 2. The required materials include solder material21and the flux material22of the specific compositions described below. Conventional tip-tinning material24may be used to help clean oxides and dirt from the soldering pencil19prior to the soldering operation.

As noted above, conventional soldering materials are largely or wholly ineffective when applied to SMA materials, particularly nickel-titanium SMA wires. Therefore, the specific material compositions, welding temperatures, and techniques disclosed herein are intended to enable the direct soldering of SMA elements12to the component14. As used herein, the terms “direct” and “directly” require the absence of intervening structure between the SMA element12and the surface to which the SMA wire12is to be soldered. By way of example, conventional approaches include crimping a metallic end piece to an end of an SMA wire, then soldering or fastening the metallic end piece to a surface. Such a process is considered to be indirect, as the material of the SMA wire itself is not soldered to the surface.

SMA wires and other SMA elements12, when manufactured, have the ability to shrink at a calibrated activation temperature, e.g., by up to 6% in length, thus allowing the SMA element12to be used as an actuator. If the SMA element12is overheated during the soldering process, which could occur using conventional soldering techniques, the shape memory abilities of the SMA element12could be degraded or lost altogether. Therefore, all soldering steps of the method100described below require application of controlled amounts of heat to a localized area of the SMA element12, in part by controlling the amount of heat at the tip19T. This allows the flux material22to sufficiently wet the SMA element12so that the solder material21can securely bond to the SMA element12. Heat is thereafter removed as quickly as possible to prevent heat penetration into the depth of the SMA element12. In other words, heat is retained locally so the SMA element12does not lose its shape memory abilities. In general terms, with specific examples disclosed below, the method100uses heat sinks such as conductive clip-on style heat sinks11shown inFIG. 1, low liquidus temperature solder material21, and specific flux materials22to minimize the effect on shape memory properties.

The controller16ofFIG. 1includes an input device13operable for setting a desired soldering temperature. For example, a temperature dial as shown may be rotated to a desired temperature setting, which in the scope of the present method100may range from about 375° F. to about 700° F., with soldering temperatures of about 450° F. to about 550° F. being generally suitable, with higher temperatures possibly being used when an oxide layer is present on an outer surface of the SMA element12as explained below. Other embodiments of the temperature controller16may be digital in design and operation. The setting of a desired soldering temperature via the input device13results in the resistive heating of a tip19T of the soldering pencil19to the desired temperature.

Within the scope of the method100ofFIG. 2and the automated method200ofFIG. 3, the flux material22may be an active or acidic compound having a low liquidus temperature. As is known in the art, the term “liquidus temperature” refers to the temperature above which a given material is in a completely liquid state. Within the scope of the present disclosure, the term “low liquidus temperature” refers to a temperature of no more than about 500° F. in a particular embodiment. The flux material22is used to help remove light layers of surface oxides from the SMA element12. The use of certain chemicals in the flux material22, such as tin fluoride (SnF2), may help remove oxide layers. Another example mixture of the flux material22is a sufficiently concentrated form of phosphoric acid, e.g., a mixture containing at least 80% phosphoric acid. The SMA elements12are tinned with the solder material21and the flux material22in ends or other areas of the SMA element12that will eventually be directly soldered to the component14.

The solder material21used in the methods100and200may be a suitable non-leaded solder material whenever oxide layers are not present on the surfaces of the SMA element12, or are present at such diminished levels as to not unduly interfere with direct bonding to the component14. A leaded and flux cored solder material21, e.g., containing elemental tin and fluoride, may be selectively used when oxide layers are present at sufficiently high levels relative to a calibrated threshold. When a leaded flux cored form of the solder material21is used, a higher relative soldering temperature may be selected via the temperature input device13, e.g., about 600° F. to 700° F. To minimize the need for leaded solder, the methods100and200could include the removal of oxide layers from the SMA element12in a separate preparation phase, such as via abrasion or the use of an electrochemical bath, and/or by the use of the flux material22having tin and fluoride.

With respect to example embodiments of the solder material21, a tin-based mixture may be used. For instance, a mixture of at least 85% elemental tin (Sn) is suitable in some embodiments. Within the scope 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 a suitable material such as elemental zinc (Zn), silver (Ag), or antimony (Sb). Within the stated ranges, effective example mixtures may include a mixture of 95% Sn and 5% Sb, i.e., Sn95Sb5, Sn96.5Ag3.5, Sn91Zn9, and Sn85Zn15. In other example embodiments, zinc chloride or zinc fluoride may be used, particularly when surface oxides are present. Those of ordinary skill in the art will appreciate that various other solder materials21may be envisioned having a threshold low liquidus temperature of no more than 500° F. within the intended inventive scope, including but not limited to Sn 95.5Cu4Ag0.5, Sn90Zn7Cu3,Pb70Sn30 to Pb55Sn45, Sn50Pb50, Sn50Pb48.5Cu1.5, Sn60Pb40 to Sn95Pb5, Sn60Pb38Cu2, Sn60Pb39Cu1, Sn63Pb37P0.0015-0.04, Sn62Pb37Cu1, Pb80Sn18Ag2, Sn43Pb43Bi14, Sn46Pb46Bi8, Bi52Pb32Sn16, Bi46Sn34Pb20, Sn62Pb36Ag2, Sn62.5Pb3Ag2.5, 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.

The soldering temperature may be selected depending on the nature of the SMA element12and the presence or absence of layers of surface oxides. Generally, the soldering temperature will be in the range of between about 450° F. and 700° F., i.e., sufficiently higher than the low liquidus temperature of the soldering material21. Higher temperatures within the example range may be used for oxide-coated SMA elements12, with a lower temperature being more desirable for SMA elements12lacking an oxide layer, as well as for minimally impacting the shape-memory properties of the SMA element12. Because soldering temperature will exceed the melting point of the solder material21, pure tin will generally be soldered at temperatures above 450° F. Adding 3.5% elemental silver, for instance, will tend to reduce this temperature to about 430° F.

Referring toFIG. 2, the method100in an example embodiment begins with step S102, wherein the SMA element12is first cut to a desired length and then evaluated for the presence of oxide surface layer or film. For instance, the SMA element12may be viewed under a high-power microscope or subjected to other testing or inspection techniques that can reveal oxide layers on the outer surfaces of the SMA element12. The method100then proceeds to step S104.

At step S104, the levels of any oxides detected at step S102may be compared to a calibrated threshold. The method100proceeds to step S107when detected oxide levels are below the calibrated threshold, and to step S105or step S106when the detected oxide levels exceed the calibrated threshold. Specifically, step S105may be executed in a process in which the oxide layers are separately removed as part of the method100before direct soldering, while step S106may be executed when direct soldering of oxide-containing SMA elements12is desired.

At optional step S105, the layers of oxides may be gently removed from the surface of the SMA element12, such as by chemical or acid etching, via gentle abrasion, or other suitable processing steps. For instance, the SMA element12may be submersed in an acid oxide-removal bath for a calibrated duration suitable for removing the oxide layers, followed by a thorough rinsing of the SMA element12with a bath of clean liquid water or other suitable cleaning solution. In a possible embodiment, an oxide-coated SMA element12may 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 S105may also or alternatively include manually abrading the surface of the SMA element12with fine-grit sandpaper or other abrasive material to gently remove the oxide film without abrading the underlying surface of the SMA element12. The method100proceeds to step S107after the oxide layers have been removed below the level of the threshold applied at step S104.

Step S106includes setting the soldering temperature of the controller16ofFIG. 1to a threshold temperature suitable for soldering oxide-coated SMA element12. The soldering temperature may exceed 600° F. when, as noted below, flux cored solder wire or a leaded solder material21is used. Examples of suitable flux cored solder wire include a mixture of about 70% to 80% lead (Pb), 10% to 20% tin (Sn), and 1% to 5% silver (Ag), ALU-SOL 45D, or other suitable lead-based mixtures. Depending on the configuration of the controller16and the temperature input device13, step S106may require rotating a dial or selecting a digital setting via a keypad. At each step of the method100, the amount of heat penetrating into a depth of the SMA element12is carefully controlled, i.e., via the clip-on style heat sinks11and the use of the specific materials and oxide removal processes disclosed herein, especially while tinning and directly soldering the SMA element. This is done to protect shape memory abilities of the SMA element. The method100then proceeds to step S108.

Step S107includes setting the temperature of the controller16to a threshold temperature level suitable for soldering relatively clean, oxide-free SMA elements12. Execution of step S107is predicated on a decision at step S104that detected oxide levels are sufficiently low for proceeding with direct soldering with a lead-free solder material21, for instance KAPP ZAPP 3.5, or removal of any oxide layers at step S105to reach such sufficiently low oxide levels. Soldering temperatures may be about 450° F. to 550° F. in step S107, with the temperature set as set forth in step S106above. The method100then proceeds to step S109.

At step S108, solder material21may be applied to the SMA element12. A leaded flux-core version of the solder material21may be used for this purpose, for instance containing about 70% elemental lead by weight. The method100then proceeds to step S110.

At step S109, an end of the SMA element12is tinned with the flux material22, which is a cored flux material22in this instance, and solder material21such that the SMA element12is coated with sufficiently coated with solder material21. The tip19T of the soldering pencil19may be cleaned as necessary with the tip-tinning material24. As is known in the art, tinning is the process of applying flux material22and solder material21to an end of the SMA element12so as to ensure a bond of sufficient integrity is formed between the solder material22and the SMA element12prior to attaching the SMA element12to the component14. The method100then proceeds to step S111.

Step S110includes cleaning the tip19T with the tip-tinning material24, which in this instance should be lead-free. An example of a suitable lead-free tip-tinning material24includes a mixture of tin, ammonium phosphate, and diammonium phosphate. The method100then proceeds to step S111.

Step S111includes dipping an end of the SMA element12that was previously tinned into the solder material21, with the composition of the solder material21depending on whether or not oxide layers were present at step S102and not removed at step S105. That is, when oxides are detected, a leaded, flux-cored version of the solder material21may be used to help remove the oxide and tin the SMA element12. When oxides are not detected, the solder material21may be lead-free.

Step S111may also include holding the tinned end with the clip-on style heat sinks11. For instance, alligator clips may act as a wire holder for the SMA element12, but also serves as a suitable heat sink, further protecting the shape memory effect of the SMA element12. The tip19T of the soldering pencil19is coated with soldering material21of the types noted above so that a small pool of molten solder is present on the tip19T. The solder pool on the tip19T is then touched to the SMA element12where flux material21coats the SMA element12, running the SMA element12through the molten pool sufficiently to tin the SMA element12. The method100then proceeds to step S113.

Step S113includes tinning, with the tip-tinning material24ofFIG. 1, any contact surfaces of the component14to which the SMA element12will be soldered. By way of example, step S113may include tinning a contact pad14C of the component14ashown inFIG. 1. The method100then proceeds to step S115.

At step S115, the soldering pencil19is held against the contact pad14C of the component14while a small bead of solder material21is applied to the contact pad14C. The clean SMA element12is moved into the molten pool while the solder material21remains molten. The soldering pencil19is removed once the SMA element12is properly positioned. The SMA element12is held in place until the solder material21eventually cools, usually only a few seconds. The heat sink11can be released. The method100repeats with each solder joint that is formed.

Other embodiments of the method100ofFIG. 2may be envisioned by one of ordinary skill in the art. For instance, the method200ofFIG. 3may include an automated or semi-automated wave soldering process in which the SMA element12is first subjected to a series of baths at step202. In an example embodiment, a first bath of acid, in a step analogous to step S105ofFIG. 2, may be used to remove the oxide layers, while a second bath of clean water removes any acid residue. Additional baths may be used to apply flux material22to coat the end of the SMA element12and dip the coated end of the SMA element12into the solder material21.

At step204, the tinned end of the SMA element12may be placed in contact with a previously-tinned contact pad14C of the component14and heated via a heat gun, oven, or other heating source to melt the solder material21. At step206, the SMA element12and tinned contact pad14C could be pressed together in a clamp or press and allowed to cool. Alternatively, the SMA element12may be placed in a pick-and-place machine of the type known in the art and temporarily adhered to the contact pad14C.

At step208, the SMA element12and component14can be sent through another series of baths of flux material22, solder material21, and an appropriate cleaner to remove any residual flux material22. Other processes may be envisioned within the scope of the disclosure using the specific materials and steps outlined above, e.g., having the machine apply a mixture of the solder material21and flux material22to the contact pad14C, then send the assembly through an oven to melt the solder material21. The component14can thereafter be sent through a cleaning bath to remove excess flux material22, with the composition of the cleaning bath depending on the configuration of the component14.

Advantages of the methods100and200disclosed above include the maintenance and consistency of application of the shape memory properties of the SMA element12. By eliminating crimping of end connections or crimps to the SMA element12in favor of directly soldering the SMA element12to the component14, the number of applications suitable for inclusion of SMA elements12may be increased. The methods100and200may also lead to assemblies having improved heat transfer characteristics. Moreover, the time and expense associated with crimping end connections to the ends of the SMA element12are eliminated. Stronger end attachments are made possible, for instance by allowing the SMA element12to attach directly to a control circuit board or other component14. These and other benefits will be apparent to one of ordinary skill in the art in view of this disclosure.