Abstract:
The present invention relates to a soldering method and the like comprising a structure for making it possible to solder microsize objects to each other. The soldering method is a method realizing the soldering by using a fiber laser apparatus capable of minutely adjusting the spot size of outputted laser light, and prepares the fiber laser apparatus and a spatial optical system before soldering the objects. The fiber laser apparatus includes an amplification optical fiber having a single core structure and outputting amplified single-mode light, and a seed light source supplying seed light to the amplification optical fiber. The spatial optical system includes a collimator collimating the outputted laser light from the fiber laser apparatus, and a condenser lens converging the outputted laser light transmitted through the collimator to solder which is set. Light having a pulse width of not shorter than a microsecond or continuous light outputted as outputted laser light from the fiber laser apparatus is applied to the solder set between objects to be soldered through the spatial optical system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method and apparatus for performing soldering by irradiating solder or an object to be soldered with laser light. 
         [0003]    2. Related Background Art 
         [0004]    A technique which performs soldering by irradiating solder set between objects with laser light is disclosed in Japanese Patent Application Laid-Open No. HEI 6-77638 (Patent Document 1), for example. The soldering technique, disclosed in Patent Document 1, guides laser light outputted from a laser light source with an optical fiber, irradiates solder set between objects with the laser light outputted from the leading end of the optical fiber, and thereby performs soldering. 
       SUMMARY OF THE INVENTION 
       [0005]    The inventors have studied the prior art described above in detail, and as a result, have found problems as follows. 
         [0006]    Namely, the prior art performing soldering by laser light irradiation cannot sufficiently reduce the spot size of laser light when converging the laser light to a soldering location. It is therefore difficult for the prior art to solder microsize electronic devices and the like arranged in a length of 0.1 mm or less, for example. In particular, needs for soldering techniques in minute areas have recently been increasing as electronic devices have become smaller. 
         [0007]    In order to overcome the above-mentioned problems, it is an object of the present invention to provide a soldering method and apparatus of enabling the soldering in minute areas. 
         [0008]    A soldering method according to the present invention is a method which realizes the soldering by using a fiber laser apparatus capable of minutely adjusting the spot size of outputted laser light, and performs the soldering between objects by using the fiber laser apparatus and a spatial optical system. In particular, the soldering method prepares a fiber laser apparatus, prepares a spatial optical system, controls a seed light source included in the fiber laser apparatus so as to yield desirable outputted laser light, and irradiates solder set between objects with thus obtained outputted laser light. 
         [0009]    The fiber laser apparatus to be prepared includes an optical fiber which has a single core structure and which outputs single-mode light, and a seed light source for supplying seed light to the optical fiber. In the specification, the wording “single core structure” includes the structure that only one or more core regions are concentrically arranged like a dual core, but does not include a multi-core structure such that a plurality of core regions are dotted within a central region of an optical fiber. The spatial optical system prepared includes a collimator collimating the outputted laser light from the fiber laser apparatus, and a condenser lens converging the outputted laser light transmitted through the collimator. The seed light source included in the fiber laser apparatus is controlled such that light having a pulse width of not shorter than a microsecond or continuous light is outputted as the outputted laser light from the fiber laser apparatus. By way of the spatial optical system, the outputted laser light from the fiber laser apparatus, which is obtained by controlling the seed light source as mentioned above, is applied to the solder set between the objects. 
         [0010]    Preferably, in the soldering method according to the present invention, the objects to be soldered are heated before irradiating the solder set between the objects with laser light. In particular, the objects are initially heated by irradiation with the outputted laser light from the fiber laser apparatus converged by the spatial optical system. Thereafter (after the objects are heated), the outputted laser light from the fiber laser apparatus, which is converged by the spatial optical system, is applied to the solder set between the objects, whereby the objects can be soldered more efficiently to each other. Namely, peripheral areas of the soldering part can be prevented from being heated unnecessarily. 
         [0011]    The soldering method according to the present invention may comprise the step of removing an unnecessary solder part after soldering the objects. Namely, after soldering the objects, an unnecessary solder part generated at the time of soldering the objects is irradiated with light having a pulse width of a nanosecond or less outputted from one selected from the fiber laser apparatus and another fiber laser apparatus irradiates by way of the spatial optical system, whereby the unnecessary solder part can be removed. 
         [0012]    In the soldering method according to the present invention, the spatial optical system is adjusted so as to converge the outputted laser light from the fiber laser apparatus such that the spot size of the outputted laser light, applied to the solder from the fiber laser apparatus, falls within the range of 1 μm to 100 μm. 
         [0013]    In the soldering method according to the present invention, it is preferable that the optical fiber includes a Yb-doped optical fiber, whereas the fiber laser apparatus includes a wavelength conversion device for converting the wavelength of the output light from the optical fiber. In this case, the light whose wavelength is converted to 532 nm by the wavelength conversion device irradiates the solder by way of the spatial optical system. 
         [0014]    In the soldering method according to the present invention, the seed light source preferably includes a semiconductor laser, whereas the fiber laser apparatus has an oscillation adjustment mechanism adjusting an oscillation condition of the semiconductor laser as a MOPA-type laser apparatus. 
         [0015]    The soldering method according to the present invention may comprise a step of protecting a soldering part in the objects. Namely, before soldering the objects, one of the objects is bonded to a surface of a plastic sheet with an adhesive. Thereafter, in the state where the one object bonded to the plastic sheet is soldered with the other object, the objects are soldered to each other. As another protecting means, respective soldering parts in the objects are covered with a plastic sheet after soldering the objects. Thereafter, as the outputted laser light from the fiber laser apparatus, light having a pulse width of not shorter than a microsecond or continuous light irradiates the plastic sheet by way of the spatial optical system, thereby forming a plastic protective film in the soldering parts. 
         [0016]    A laser soldering apparatus according to the present invention irradiates solder set between objects with laser light, thereby soldering the objects. In particular, the laser soldering apparatus comprises a fiber laser apparatus and a spatial optical system, whereas the fiber laser apparatus outputs light having a pulse width of not shorter than a microsecond or continuous light as output light. 
         [0017]    The fiber laser apparatus is one outputting single-mode light as outputted laser light, and includes an optical fiber, a seed light source, and an oscillation adjustment mechanism. The optical fiber includes an amplification optical fiber having a single core structure and outputting amplified single-mode light, for example. The seed light source supplies seed light to the optical fiber. The oscillation adjustment mechanism enables both continuous and pulsed oscillations in the optical fiber. 
         [0018]    On the other hand, the spatial optical system includes a collimator collimating the outputted laser light from the fiber laser apparatus, and a condenser lens converging the outputted laser light having transmitted through the collimator. 
         [0019]    In the laser soldering apparatus according to the present invention, the oscillation adjustment mechanism preferably comprises a structure for enabling an oscillation of a nanosecond pulse in the optical fiber. Also, it is preferable that the fiber laser apparatus has a pulse width adjustment mechanism for adjusting the pulse width of the outputted laser light in order to regulate an oscillation condition in the optical fiber. 
         [0020]    The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
         [0021]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1A  is a view for explaining an initial step of the soldering method according to the present invention while showing the structure of an embodiment of the laser soldering apparatus according to the present invention, whereas  FIG. 1B  is a plan view specifically showing an arrangement of objects to be soldered; 
           [0023]      FIG. 2  is a view for explaining an intermediate step of the soldering method according to the present invention while showing the structure of an embodiment of the laser soldering apparatus according to the present invention; 
           [0024]      FIG. 3  is a view for explaining the final step of the soldering method according to the present invention while showing the structure of an embodiment of the laser soldering apparatus according to the present invention; 
           [0025]      FIG. 4  is a view showing the structure of a fiber laser apparatus employed in the laser soldering apparatus according to the present invention; 
           [0026]      FIG. 5  is a view showing another structure of a fiber laser apparatus employed in the laser soldering apparatus according to the present invention; and 
           [0027]      FIG. 6  is a graph showing the wavelength dependency of absorption ratio of Sn. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    In the following, embodiments of the soldering method and laser soldering according to the present invention will be explained in detail with reference to  FIGS. 1A ,  1 B, and  2  to  6 . In the explanation of the drawings, constituents identical to each other will be referred to with numerals identical to each other without repeating their overlapping descriptions. 
         [0029]      FIGS. 1A ,  2 , and  3  are views for sequentially explaining the steps of the soldering method according to the present invention while showing the structure of an embodiment of the laser soldering apparatus according to the present invention.  FIG. 4  is a view showing the structure of a fiber laser apparatus employable in the laser soldering apparatus according to the present invention.  FIGS. 1A ,  2 , and  3  show not only a laser soldering apparatus  1 , but also a substrate  91  and coaxial cable center conductors  92  which are objects to be soldered, and solder  93  and a plastic  94 . 
         [0030]    The laser soldering apparatus  1  comprises a fiber laser apparatus  10 , a guide optical fiber  20 , and a spatial optical system  30 . The spatial optical system  30  includes a collimator  21 , a beam expander  31 , and a condenser lens  32 . As shown in  FIG. 4 , the fiber laser apparatus  10  comprises an optical amplifier  11 , a seed light source  12 , and an oscillation adjustment mechanism  13 . The optical amplifier  11  includes an amplification optical fiber  14 , a pumping light source  15 , and an optical coupler  16 . 
         [0031]    The amplification optical fiber  14  is an optical device having a single core structure and outputting amplified light as single-mode light, an example of which is a Yb-doped optical fiber amplifying light having a wavelength of 1064 nm. The pumping light source  15  is an optical device outputting pumping light to be supplied to the amplification optical fiber  14 , and includes a semiconductor laser device, for example. The seed light source  12  is an optical device outputting seed light to be amplified in the amplification optical fiber  14 , and includes a semiconductor laser device, for example. The oscillation adjustment mechanism  13  drives the seed light source  12 , so as to enable both continuous and pulsed oscillations, and adjusts the pulse width in the case of pulsed oscillation (functions as a pulse width adjustment mechanism). 
         [0032]    The pumping light outputted from the pumping light source  15  is supplied to the amplification optical fiber  14  through the optical coupler  16 . The supplied pumping light pumps elemental Yb contained in the amplification optical fiber  14 . The seed light source  12  driven by the oscillation adjustment mechanism  13  outputs seed light. The seed light is fed into the amplification optical fiber  14  through the optical coupler  16 , and is amplified in the amplification optical fiber  14 . Namely, the fiber laser apparatus  10  has a MOPA (Master Oscillator Power Amplifier) structure. The light amplified in the amplification optical fiber  14  is outputted from the fiber laser apparatus  10  as outputted laser light. 
         [0033]    The outputted laser light from the fiber laser apparatus  10  is fed into the guide optical fiber  20  from one end thereof and propagates through the guide optical fiber  20 . The outputted laser light having propagated through the guide optical fiber  20  is collimated (outputted as parallel light into the space) by the collimator  21  provided at the other end of the guide optical fiber  20 . The parallel light outputted from the collimator  21  is expanded by the beam expander  31  in terms of the luminous flux diameter, and then is converged by the condenser lens  32 . Thus converged outputted laser light irradiates the solder  93  set between the objects (substrate  91  and coaxial cable center conductors  92 ) to be soldered. 
         [0034]      FIG. 1B  is a view showing a state of arrangements of copper patterns  91   a  provided on a substrate  91  and the coaxial cable center conductors  92  arranged with intervals P, and a spot S of the outputted laser light. Specifically, the example shown in  FIG. 1B  illustrates a state in which the width of each copper line pattern  91   a  (the width of the electrode pad part) formed on the substrate  91  is 100 μm, the diameter of each coaxial cable center conductor  92  is 60 μm, and cream solder is applied as the solder  93  between the copper patterns on the substrate  91  and the coaxial cable center conductors  92 . The laser soldering apparatus  1  scans the spot S over the substrate  91  such that the solder  93  is irradiated with the outputted laser light, so as to solder the coaxial cable center conductors  92  to the copper patterns  91   a  of the substrate  91 , respectively. 
         [0035]    At the time of soldering, the single-mode light (outputted laser light) outputted from the fiber laser apparatus  10  is light having a pulse width of a microsecond or greater or continuous light, and the outputted laser light from the spatial optical system  30  irradiates the objects (substrate  91  and coaxial cable center conductors  92 ) to be soldered or solder  93 . Since the fiber laser apparatus  10  outputs single-mode light or the luminous flux diameter of the light outputted from the fiber laser apparatus  10  is expanded by the spatial optical system  30  before the light is converged, the spot diameter of the light converged by the spatial optical system  30  can become smaller. 
         [0036]    Suppose a case where light having a wavelength λ of 1064 nm outputted from the fiber laser apparatus  10  expands its luminous flux diameter D to 10 mm with the beam expander  31  and then is converged by the condenser lens  32  having a focal length f of 100 mm. Let a be the beam quality factor (M 2 ) of light outputted from the guide optical fiber  20 . Here, the minimal spot diameter d of the light converged by the condenser lens  32  is obtained by the expression of d=1.27·f·λ·a/D. In general, the beam quality factor a of light outputted from an optical fiber is said to be 1. 
         [0037]    Therefore, the minimal spot diameter d of the light converged by the condenser lens  32  is about 13.5 μm. Thus, the fiber laser apparatus  10  can converge laser light to minute areas and consequently perform microsize soldering, whereby the coaxial cable center conductor  92  having a diameter of 60 μm can be soldered to the copper pattern  91   a  having a width of 100 μm formed on the substrate  91 . 
         [0038]    In general, the spot diameter D of the light incident on the condenser lens  32  is adjusted such that the spot diameter d of the light converged by the condenser lens  32  becomes 1 μm to 100 μm. When the spot diameter d of the light converged by the condenser lens  32  is less than 1 μm, the optical system is not easy to adjust, whereby the soldering operation becomes troublesome. When the spot diameter d of the light converged by the condenser lens  32  exceeds 100 μm, on the other hand, unnecessary solder parts increase. When the spot diameter d of the light converged by the condenser lens  32  falls within the range of 1 μm to 100 μm, the soldering operation becomes easy while unnecessary solder parts are less. 
         [0039]    When a converging point of light having a pulse width of not shorter than a microsecond or continuous light is positioned at the objects (substrate  91  and coaxial cable center conductors  92 ) to be soldered or solder  93 , the objects (substrate  91  and coaxial cable center conductors  92 ) to be soldered or solder  93  can be heated without dissipating the solder  93 . In this case, the substrate  91  and coaxial cable center conductors  92  can be soldered to each other in a short time (see  FIGS. 1A and 1B ). 
         [0040]    Before soldering, the objects (substrate  91  and coaxial cable center conductors  92 ) to be soldered may be irradiated with the outputted laser light from the spatial optical system  30 . This preheats the objects to be soldered, and improves attachment of solder  93  when the solder  93  is irradiated with the outputted laser light from the spatial optical system  30  (see  FIGS. 1A and 1B ). 
         [0041]    An unnecessary solder part  93   a  may occur at the time of soldering. It will be preferred in this case if light having a pulse width of a nanosecond or less outputted as outputted laser light from the fiber laser apparatus  10  (or another fiber laser apparatus) irradiates the unnecessary solder part  93   a  through the spatial optical system  30 . This can favorably remove the unnecessary solder part  93   a  (see  FIG. 2 ). 
         [0042]    Here, it will be preferred if the pulse width of the outputted laser light irradiating the unnecessary solder part  93   a  is a nanosecond or less. When the irradiation power of irradiating outputted laser light per unit time is made greater, the unnecessary solder part  93   a  is rapidly heated without a lapse of time in which heat generated by light absorption is conducted. Such ablation can easily remove the unnecessary solder part  93   a.    
         [0043]    Light having a pulse width of a nanosecond or less can be outputted as the outputted laser light, in the case that the modulation period of a driving signal supplied to a semiconductor laser device acting as the seed light source  12  is adjusted. Light having a pulse width of a nanosecond or less can also be outputted, in the case that a pulse compressor which compresses the pulse width is provided. 
         [0044]    It will also be preferred when the coaxial cable center conductors  92  are bonded to the surface of a plastic sheet with an adhesive, and are soldered to the substrate  91  in the state where the coaxial cable center conductors  92  bonded to the plastic sheet are in contact with the substrate  91 . Alternatively, after soldering the coaxial cable center conductors  92  and the substrate  91  to each other, the soldering parts of the coaxial cable center conductors  92  and substrate  91  may be covered with a plastic sheet  94 , and the outputted laser light (light having a pulse width of not shorter than a microsecond or continuous light) from the fiber laser apparatus  10  may irradiate the plastic sheet  94  from the upper side through the spatial optical system  30 . In this case, the plastic sheet  94  covering the soldering parts forms a protective film (see  FIG. 3 ). Namely, the soldering parts in the coaxial cable center conductors  92  and substrate  91  are covered with the plastic protective film. As the plastic sheet  94 , polyacetal, polycarbonate, or polyethylene terephthalate is used favorably, for example. 
         [0045]      FIG. 5  is a view showing another structure of the fiber laser apparatus  10  employable in the laser soldering apparatus according to the present invention. The fiber laser apparatus  10 A shown in  FIG. 5  is employed in place of the fiber laser apparatus  10  ( FIG. 4 ) included in the laser soldering apparatus  1  shown in  FIGS. 1A ,  2 , and  3 . The fiber laser apparatus  10 A shown in  FIG. 5  differs from the fiber laser apparatus  10  shown in  FIG. 4  in that it further comprises a wavelength conversion device  17 . 
         [0046]    The wavelength conversion device  17  is an optical device which inputs light having a wavelength of 1064 nm from a Yb-doped optical fiber acting as the amplification optical fiber  14  and generates light with a wavelength of 532 nm having an optical frequency which is twice that of the former light. As such a wavelength conversion device  17 , a nonlinear optical crystal such as KTP, for example, is favorably used. The light having the wavelength of 532 nm outputted from the wavelength conversion device  17  is converged on the objects (substrate  91  and coaxial cable center conductors  92 ) to be soldered or solder  93  through the guide optical fiber  20  and spatial optical system  30 . 
         [0047]    Thus irradiating the objects (substrate  91  and coaxial cable center conductors  92 ) to be soldered or solder  93  with the light having the wavelength of 532 nm enables soldering of further smaller areas. In general, the light absorption ratio of metals is greater at the wavelength of 532 nm than at the wavelength of 1064 nm. For example, as  FIG. 6  shows the wavelength dependency of absorption ratio of Sn, the light absorption ratio of Sn at the wavelength of 532 nm is several times that at the wavelength of 1064 nm. Therefore, soldering can be performed more efficiently when the light at the wavelength of 532 nm is utilized. 
         [0048]    The soldering method and laser soldering apparatus according to the present invention enables soldering of objects having a size further smaller than before. 
         [0049]    From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.