Abstract:
This invention discloses a solderability testing apparatus which comprises a sample parts holding means having a sample parts holding member for holding a sample; an external force detection means for supporting such sample parts holding means; a solder paste container for containing a solder paste which is internally added with a flux; and a heating means for heating the solder paste; wherein such apparatus has a flux wetting preventive layer at least on the surface of a sample holding portion of the sample parts holding member.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention claims priority to priority document no. 2001-051611 filed in Japan on Feb. 27, 2001, and incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solderability testing apparatus and a solderability testing method. 
     2. Description of the Related Art 
     To achieve a desirable soldering, enough metallic bonding should be formed between a metal composing a base material (e.g. copper foil (or land portion) on printed circuit boards, and electrode portion of surface mounted devices) and solder (which is generally made of an alloy of tin and lead. It is thus necessary to ensure wetting of the surface of the base material with the solder (more specifically, tin). The wetting solder (tin) diffuses into the base material and forms therein an alloy layer formed with such base material through metallic bond, which is a final form of the soldering. So that testing the wetting balance between the base material and the solder can provide a quantitative evaluation of solderability. 
     Known apparatuses for evaluating wetting balances of solder, flux, solder alloy, solder paste (also referred to as cream solder) and so forth provided to lead portions of lead parts, lead and electrode portions of surface mounted parts, or land portions on printed circuit boards include an apparatus disclosed in Japanese Laid-Open Patent Publication No. H7-72064, and an apparatus specified based thereon by Standards of Electronic Industries Association of Japan (EIAJ) ET-7404, “Method for Testing Solderability of Surface Mounted Parts Using Solder Paste (Equilibrium Method)”. The solderability testing apparatuses disclosed in these documents are suitable for solderability testing based on the equilibrium method. 
     As schematically shown in FIG. 1, the solderability testing apparatus specifically comprises a sample parts holding means  20 , an external force detection means  10  for supporting such sample parts holding means  20 , a solder paste container  30 , and a heating means  40 . The external force detection means  10  has a load cell (high-sensitivity load sensor). The solder paste container  30  contains a solder paste  31  which is internally added with a flux. The sample parts holding means  20  comprises a sample parts holding member  23  for holding a sample (or standard test piece)  50 , an expansion sliding portion  21  for supporting such sample parts holding member  23 , and an electromagnetic clutch  22  for locking such sliding portion  21 . The sliding portion  21  is suspended at the upper end thereof from the external force detection means  10 . 
     The solder paste container  30  is supported by a holder  32 , and such holder  32  can ascend or descend, together with the solder paste container  30 , with the aid of a stepping motor  33 . The heating means  40  has a solder bath  41  which serves as a heat source. Solder  42  contained in the solder bath  41  is heated by a heater  43  to be brought into a molten state. The temperature of the solder  42  is monitored with a temperature sensor (e.g., thermocouple), not shown, and results of the measurement are fed back to control the heater  43 . This allows the molten solder  42  in the solder bath  41  to be kept at a predetermined temperature. By dipping the solder paste container  30  in the solder bath  41  containing the molten solder  42  then successfully heats the solder paste  31  contained in such solder paste container  30  to thereby keep the molten state thereof at a predetermined temperature. The solder bath  41  can be ascend or descend with the aid of the stepping motor  44  provided thereunder. 
     FIG. 6A is a partial schematic view of the sample parts holding member  23  in a state of holding a sample  50  (e.g., surface mounted parts). The sample parts holding member  23  of the conventional solderability testing apparatus has been made of all sort of metals which can form structural member (except for those having a melting point of 500° C. or lower, or those possibly act as a solder poison such as zinc and aluminum), which can be typified by steel and stainless steel material. 
     In the solderability testing, the sample  50  is held by the sample parts holding member  23 , the stepping motor  33  is activated so as to raise the holder  32  together with the solder paste container  30  containing the solder paste  31 , and the lower end of the sample  50  goes into the solder paste  31  and finally reaches the bottom plane of the solder paste container  30 . Thereafter the solder paste container  30  pushes the sample  50  upward while being raised by the ascending holder  32 . Thus the sample parts holding member  23  moves upward within the sliding portion  21  of the sample parts holding means  20  as much as the length of ascending path of the sample  50 . After the ascending of the holder  32  together with the solder paste container  30  comes to the end, the sliding portion  21  is locked by means of the electromagnetic clutch  22 . The lower end of the sample  50  is now in contact with the bottom plane (upper bottom) of the solder paste container  30 . The stepping motor  33  is then activated to descend the holder  32  together with the solder paste container  30 . Thus the sample  50  is held so that the lower end thereof is dipped in the solder paste  31  to a predetermined depth so as to keep a predetermined gap between such lower end and the upper bottom of the solder paste container  30 . 
     Since the external force detection means  10  composing the load cell is applied with a load which is ascribable to the weights of the sample parts holding means  20  and the sample  50 , so that such load is canceled as a tare so as to attain a load-zero status. 
     Then the stepping motor  44  is activated to raise the solder bath  41 . This allows the solder paste  31  contained in the solder paste container  30  to be quickly heated to a temperature of the molten solder  42  and brought into a molten state. In the melting process of such solder paste  31 , acting forces exerted on the sample  50 , which are typified by buoyancy attributable to the solder paste  31  and surface tension of the molten solder paste  31 , are detected by the load cell which composes the external force detection means  10 , and then output as electric signals. 
     In the process of soldering, acting forces effecting between the molten solder paste  31  and the sample  50  are considered as two ways; that are acting force f 1  ascribable to the surface tension of the solder paste  31 ; and buoyancy f 2  from the molten solder paste  31 , while ignoring the weight of adhered solder. The acting force f 1  and buoyancy f 2  can be expressed by the following equations (1) and (2), respectively, where force (tension) directed downward is defined as positive force. 
     
       
         f 1 =γl cos Θ)  (1)  
       
     
     
       
         f 2 =−ρvg  (2)  
       
     
     , where, meanings of γ, Θ, l, ρ, v and g are respectively as follows: 
     γ=boundary tension between molten solder paste and flux; 
     Θ=contact angle of molten solder paste with sample; 
     l=outer peripheral length of a sample measured at contact plane with molten solder paste; 
     ρ=density of molten solder paste; 
     v=volume of displaced molten solder paste; and 
     g=gravitational acceleration. 
     When heating of the solder paste  31  starts, the surface of the sample  50  starts to be wet with the flux preliminarily mixed into the solder paste  31 , where the flux is responsible for removing oxide film or foreign matters from the surface of the sample  50  to thereby clean such surface of the sample  50 . 
     Then the solder paste  31  starts to melt, buoyancy ascribable to such molten solder paste  31  starts to effect, and wetting with such solder paste  31  also starts when the temperature of the sample reaches a predetermined level. The force F exerted on the sample  50  herein is expressed as an equation below. 
     
       
           F=f   1   +f   2   =γl  cos Θ−ρ vg    
       
     
     Time-course of such acting force (expressed by a wetting curve), time-course of the heating process, and wetting status of the sample  50  are shown in FIG.  5 . Heating of the sample  50  immersed into the solder paste  31  starts at a point A in FIG.  5 . Status of the sample  50  and solder paste  31  is expressed with a reference alphabet (a) in FIG.  5 . 
     After complicated processes between points A and B, which include run of the solder paste  31 , evaporation of the solvent and wetting of the flux (where the acting force ascribable to surface tension of the flux becomes maximum at the point B), the solder paste  31  starts to melt at such point B. Upon beginning of the melting of the solder paste  31 , the buoyancy attributable thereto begins to effect, which is detected as a repulsive force against the sample  50  (a downward force in FIG.  5 ). Such repulsive force becomes maximum at a point C, and the entire solder paste  31  completes the melting. Also the contact angle between the molten solder paste  31  and the sample  50  reaches maximum as schematically indicated by reference alphabet (b) in FIG. 5 (&gt;90°). By this point of time, a condition for wetting of the surface of the sample  50  with the molten solder paste  31  is already satisfied, so that the wetting starts. The acting force (referred to as tension hereinafter) ascribable to the surface tension of the molten solder paste  31  increases at the point C and thereafter, and suction force for the sample  50  (upward force in FIG. 5) is begun to be detected. The acting force F becomes zero at a point D, where the buoyancy and tension are kept in balance with each other. 
     The contact angle attained at this point of time is 90° as specifically indicated by reference alphabet (c) in FIG.  5 . The contact angle then decreases below 90° at the point D and thereafter, where the molten solder paste  31  creeps up the non-dipped portion of the sample  50  to thereby form a meniscus as specifically indicated by reference alphabet (d) in FIG.  5 . 
     The time period from the start of the heating (the point A) to the point D is referred to as wetting time t w  (zero-cross time). The point D can be obtained approximately as an intersection of the zero line and wetting curve shown in FIG.  5 . It is to be noted now that load difference between the zero line and the initial line indicating the zero-load status before the testing is started represents the weight of the solder paste bonding (remaining) on the sample  50  at the time point when the testing is completed. The shorter the wetting time t w  is, the better the wetting balance between the solder paste  31  and sample  50  becomes, which indicates better solderability. 
     When the heating of the solder paste  31  begins, wetting of the surface of the sample  50  with the flux mixed into such solder paste  31  begins. More specifically, the flux creeps upward on the surface of the sample  50 , instantaneously invades into the micro-gap between the sample  50  and sample holding portion of the sample parts holding member  23 , and may even instantaneously reaches the top portion of the sample parts holding member  23 . Such status is schematically illustrated in FIG. 6B, where the portion having the flux crept thereon is shown with hatching for easy understanding. This is attributable to the surface tension of the flux. Once such creeping-up of the flux such that reaching the top portion of the sample parts holding member occurs, the load cell detects surface tension and weight of the flux excessively applied thereto and also detects acting force caused by abrupt movement of the flux, which adversely affect detection result of the acting force (disturbance of the wetting curve) to a considerable degree. Or acting force ascribable to the surface tension of the flux can heavily affect the acting force detected by the load cell. The load cell which composes the external force detection means  10  is designed to detect extremely small force (in the order of several mN(Newton) to ten and several mN). So that such invasion and creeping-up of the flux into or onto the unnecessary portions will prevent the load cell composing the external force detection means  10  from precisely detecting the acting force exerted on the sample  50 . Such wetting of the flux will even be more abrupt and rapid as compared with that of the solder paste, so that it becomes difficult to determine wetting time t w . A wetting curve for an exemplary case with such event is shown in FIG.  7 . 
     The wetting t w  can generally be estimated by personal-computer-assisted analysis of detected output of the external force detection means  10  (more specifically, load cell). However for the case shown in FIG. 7, the personal-computer-assisted analysis will determine a point B as a wetting time t w  (zero-cross time), while an actual one should be determined as a point A, which erroneously shortens the observed wetting time t w ′ than the actual wetting time t w . Wetting curves obtained for the case with such phenomenon lacks reproducibility and consistency as typically shown in FIG. 8, which makes it extremely difficult to resolve such problem in the analysis of the wetting time on the software basis. FIG. 8 shows six wetting curves, and two out of six are expressed in an overlapped manner. 
     SUMMARY OF THE INVENTION 
     It is therefore an aspect of the present invention to provide a solderability testing apparatus and a solderability testing method, both of which can completely prevent the unnecessary flux wetting such that the flux invades upwardly into the gap between the sample and sample parts holding member, or even creeps up to the top portion of the sample parts holding member, and can ensure precise measurement of the wetting time particularly for the case that small-sized SMDs (Surface Mounted Devices) such as those of 0603 type or 1005 type are tested. 
     A solderability testing apparatus according to first and second aspects of the present invention, and a solderability testing method according to first to third aspects of the present invention are essentially based on the Standards of Electronic Industries Association of Japan (EIAJ) ET-7404, “Method for Testing Solderability of Surface Mounted Parts Using Solder Paste (Equilibrium Method)”. 
     The solderability testing apparatus according to a first aspect of the present invention for attaining the foregoing aspect is such that comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; wherein such apparatus has a flux wetting preventive layer at least on the surface of a sample holding portion of the sample parts holding member. 
     A solderability testing method according to a first aspect of the present invention for attaining the foregoing object is such that using a solderability testing apparatus comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; such apparatus having a flux wetting preventive layer at least on the surface of a sample holding portion of the sample parts holding member; wherein such method comprises a step of heating and melting the solder paste using a heating means while keeping a part of a sample, which is held by a sample parts holding member, being dipped therein, and measuring time-dependent changes in the acting force effected between the molten solder paste and the sample using the external force detection means. 
     In the solderability testing apparatus or solderability testing method according to the first aspect of the present invention, the flux wetting preventive layer may be provided at least on the surface of a sample holding portion of the sample parts holding member, or may be provided on the most or entire surface of the sample parts holding member. More specifically, the flux wetting preventive layer may typically be formed on the surface of the sample holding portion of the sample parts holding member and the portion above thereof (portion onto which the flux can creep up). 
     The solderability testing apparatus according to a second aspect of the present invention for attaining the foregoing object is such that comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; wherein a sample holding portion of the sample parts holding member is made of a material having a poor wetting balance in respect of the flux. 
     A solderability testing method according to a second aspect of the present invention for attaining the foregoing object is such that using a solderability testing apparatus comprises: (A) a sample parts holding means having a sample parts holding member for holding a sample; (B) an external force detection means for supporting such sample parts holding means; (C) a solder paste container for containing a solder paste which is internally added with a flux; and (D) a heating means for heating the solder paste; such sample parts holding member having a sample holding portion which is made of a material having a poor wetting balance in respect of the flux, wherein such method comprises a step of heating and melting the solder paste using a heating means while keeping a part of a sample, which is held by a sample parts holding member, being dipped therein, and measuring time-dependent changes in the acting force effected between the molten solder paste and the sample using the external force detection means. 
     In the solderability testing apparatus or solderability testing method according to the second aspect of the present invention, at least the sample holding portion of the sample parts holding member may be made of a material having a poor wetting balance in respect of the flux, while it is also allowable that the entire portion of the sample parts holding member is made of a material having a poor wetting balance in respect of the flux. It is also preferable to form the sample holding portion of the sample parts holding member with a material having a poor heat conductivity. 
     In the solderability testing apparatus or solderability testing method according to the first aspect of the present invention, the material composing the flux wetting preventive layer preferably has a contact angle È to flux of larger than 90°, and such material is preferably selected from polymer material, cermet and ceramic. The polymer material herein preferably has excellent heat resistance, wear-proof property and water repellency, where preferable examples of which include various engineering plastics such as fluorocarbon resin or derivatives thereof (e.g., polytetrafluoroethylene) and polyoxymethylene (POM) resins. The cermet or ceramic herein preferably has excellent water repellency. The cermet refers to a composite material obtained by sintering ceramic and metal powder, or a composite material composed of ceramic, metal and so forth, and examples of which include those in which metal such as iron (Fe), nickel (Ni), cobalt (Co), chromium (Cr), molybdenum (Mo) or the like is combined with silicon (Si), boron (B), various carbides (TiC, ZrC, B 4 C, WC, SiC, etc.), oxides (Al 2 O 3 , ZrO 2 , ThO 2 , etc.) or nitrides (W—N, Mo—N, TaN, B—N, etc.). The ceramic can be exemplified by so-called new ceramics such as alumina, mullite, magnesia, forsterite, zirconia, titania, yttria or the like; vitreous materials such as borosilicate glass, potassium borosilicate glass or the like; quartz glass and phosphosilicate glass. 
     Possible methods for forming the flux wetting preventive layer depend on materials composing such layer, where examples thereof include such that dipping the sample holding portion of the sample parts holding member (or entire portion thereof if necessary) into solution of a material composing such flux wetting preventive layer, which is followed by drying; such that coating solution of a material composing such flux wetting preventive layer on the portion the flux wetting preventive layer is to be formed, which is followed by drying; such that spraying solution of a material composing the flux wetting preventive layer to a portion the flux wetting preventive layer is to be formed, which is followed by sintering or flame coating if necessary; such that coating (depositing) the material; and such that forming film of a material composing the flux wetting preventive layer on the portion the flux wetting preventive layer is to be formed by physical vapor deposition (PVD) process such as sputtering or vapor deposition, or chemical vapor deposition (CVD) process. It is also allowable to employ direct processing or forming of plastic materials. 
     In the solderability testing apparatus or solderability testing method according to the second aspect of the present invention, the material composing at least sample holding portion (or the entire portion if necessary) of the sample parts holding member preferably has a contact angle E to flux of larger than 90°, and such material is preferably selected from various engineering plastics such as polymer materials having excellent heat resistance, processability, wear-proof property and strength (e.g., fluorocarbon resin or derivatives thereof typified by polytetrafluoroethylene, and polyoxymethylene resins), and from the foregoing cermet or ceramic. 
     In the solderability testing apparatus according to the first and second aspects of the present invention, and in the solderability testing method according to the first and second aspects of the present invention (all of which may simply be referred to as “the present invention” hereinafter), the sample parts holding means preferably comprises an expandable sliding portion for supporting the sample parts holding member and an electromagnetic clutch for locking such sliding portion. The sliding portion is preferably suspended at the upper end thereof from the external force detection means; or the sample parts holding member, which is made of a flexible (springy) material, preferably has on the outer periphery thereof a sleeve covering thereof, and is suspended at the upper end of such sleeve or at the sample parts holding member per se from the external force detection means, although the present invention is by no means limited to these constitutions. 
     In the present invention, the external force detection means can be composed, for example, of a high-sensitivity load sensor such as load cell, or an electronic balance. The solder paste container can specifically be composed of testing jig plate I or II specified in EIAJ ET-7404. The heating means preferably has a heating bath containing liquid metal (e.g., solder bath containing solder, heating bath containing fusible alloy such as Wood&#39;s metal), which can ensure a larger contact surface area than a heating plate can. 
     In the solderability testing method according to the first and second aspects of the present invention (generally referred to as “the solderability testing method of the present invention” hereinafter), the acting force effected between the molten solder paste and the sample is specifically understood as a synthetic force of buoyancy exerted on the sample from the solder in the molten solder paste and the surface tension (tension). 
     The present invention is applicable to evaluation of solderability of the samples, or solder wetting balance listed below: 
     (1) lead portion of lead parts; 
     (2) electrode portion (terminal portion) of surface mounted parts; 
     (3) land portion provided on printed circuit boards; 
     (4) flux; 
     (5) solder alloy (including lead-free solder such as Sn—Cu and Sn—Cu—Ag solders); 
     (6) solder paste (also referred to as cream solder); 
     (7) raw materials before being processed into part terminal leads or lead frames, such as wire material (Cu/CP wire), hoop material (to be processed into lead frame, terminal and so forth, and exemplified by 426 alloy, 42 alloy, etc.), steel sheet (tin-plated or zinc-plated steel plate such as tin plate or galvanized steel sheet); 
     (8) materials obtained by subjecting the materials described above in (7) to various plating processes (evaluation of solder wetting balance of plating chemicals); 
     (9) evaluation of solderability of films formed using surface treatment apparatuses such as PVD and CVD apparatuses on the materials described above in (7); 
     (10) performance test of solderability in apparatuses and methods of surface treatment such as PVD and CVD; and 
     (11) surface treatment materials (BTA, imidazole) other than flux in relation to the solderability. 
     For the evaluation of (1), (2) or (3), lead part, surface mounted part, printed wiring board, or land portion on such printed wiring board is assumed as the sample. In such cases, it is preferable to use for example a standard solder paste specified by EIAJ ET-7404, or a standard (reference) paste preliminarily defined by the user. When items (1) to (3), and (7) to (10) are to be evaluated, the evaluation preferably follows the solderability testing method according to the first or second aspect of the present invention. Conditions for such testing are preferably in compliance with the recommended testing conditions specified by EIAJ ET-7404, while not being limited thereto. 
     On the other hand, for the evaluation of item (4), it is preferable to use a copper oxide wire (0.6 mm in diameter) or a reference material defined by the user (e.g., electrodes subjected to lead-free plating with Sn—Cu, Sn, Sn—Ag or the like) as a standard sample, and to prepare the solder paste using a component solder powder specified by EIAJ ET-7404. For the evaluation of items (5), (6) and (11), it is preferable to use a copper oxide wire (0.6 mm in diameter) as a standard sample. When items (4), (5), (6) and (11) are to be evaluated, the evaluation preferably follows the solderability testing method according to the first or second aspect of the present invention. 
     In the solderability testing methods according to the first and second aspects of the present invention, temperature elevation profile attained in the process in which the solder paste is heated using a heating means to be brought into a molten state (elevation profile of sample temperature) preferably follows the rapid heating profile specified by EIAJ ET-7404, while being not limited thereto. 
     In the present invention, at least the surface of the sample holding portion of the sample parts holding member has formed thereon the flux wetting preventive layer, or at least the sample holding portion of the sample parts holding member is made of a material having a poor wetting balance, which is advantageous for surely preventing the flux from invading into the gap between the sample and sample holding portion of the sample parts holding member, and eventually from creeping up to the sample parts holding member. This ensures precise measurement of the wetting time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description of the presently preferred exemplary embodiment of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic drawing of the solderability testing apparatus; 
     FIG. 2 is a partial schematic view of the sample parts holding member of the solderability testing apparatus according to the present invention; 
     FIG. 3 is a graph showing exemplary wetting curves of a terminal of a 1005-type part subjected to lead-free surface treatment (Sn—Cu plating) according to Embodiment 1 of the invention; 
     FIG. 4 is a graph showing exemplary wetting curves of the lead-free solder paste (Sn—3.5Ag—0.5Cu) according to Embodiment 1 of the invention; 
     FIG. 5 is a drawing for explaining time-dependent changes in the acting force applied by the molten solder paste onto the sample (wetting curve), time-dependent changes in the heating process, and wetting statuses of a sample; 
     FIG. 6A is a schematic drawing of the sample parts holding member in a state of holding a sample, and 
     FIG. 6B is a schematic drawing of the sample parts holding member in a state the flux creeps up to the upper portion thereof; 
     FIG. 7 is a graph showing a wetting curve (of the foregoing part) involving disturbance due to the flux creepage up to the sample parts holding member (corresponded to the state shown in FIG.  6 B); and 
     FIG. 8 is a graph showing exemplary wetting curves (of the foregoing lead-free solder paste) involving disturbance due to the flux creepage up to the sample parts holding member (corresponded to the state shown in FIG.  6 B). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described referring to embodiments of the present invention (simply referred to as embodiment, hereinafter). 
     (Embodiment 1) 
     The embodiment 1 relates to the solderability testing apparatus and solderability testing method according to the first aspect of the present invention. Constitution of the solderability testing apparatus of the embodiment 1 can be almost similar to that of the conventional solderability testing apparatus previously explained referring to FIG. 1, so that detailed description thereof will be omitted. 
     A partial schematic view of the sample parts holding member  23  of the solderability testing apparatus of the embodiment 1 is shown in FIG.  2 . The sample parts holding member  23  is made of a steel material or stainless steel, and has the flux wetting preventive layer  24  comprising a flux creepage preventive agent at least on the surface of the sample holding portion  23 A thereof. Now the drawing shows the flux wetting preventive layer  24  with hatching for easy understanding. Specifically, the flux wetting preventive layer  24  was formed by spraying SFCOAT SIF-200 AEROSOL (product of Seimi Chemical Co., Ltd., major component: fluorocarbon resin, available as an isopropanol solution) onto the surface of the sample holding portion  23 A and around thereof of the sample parts holding member  23 , and then drying at 200° C. using an industrial drier. Such spraying and baking finish were repeated three times to thereby form the flux wetting preventive layer  24  on the sample holding portion  23 A and around thereof of the sample parts holding member  23 . 
     Thus fabricated sample parts holding member  23  was attached to the solderability testing apparatus shown in FIG.  1 . The sample  50  employed herein was a chip-type tantalum capacitor having 42-alloy terminals with Sn-plated surface in a size of 6.2 mm×5.8 mm×1.2 mm (product of Nichicon Corporation), or an 1005-type chip resistor having terminals with solder plating on Ag—Pd baked (thick-film) electrodes (product of Taiyo Electric Co., Ltd.). The solder paste for the testing employed herein was a standard solder paste specified in the EIAJ ET-7404 (product of Tarutin Kester Co., Ltd.). The testing employed the testing conditions specified in the EIAJ ET-7404 (angle of dipping: horizontal, dipping direction:  1 A); rapid seating profile specified in EIAJ ET-7404 as the temperature elevation profile (elevation profile of sample temperature) attained when the solder paste  31  is heated to a molten state using the heating means  40 ; testing jig plate II specified in the EIAJ ET-7404 as the solder paste container  30 ; and the depth of dipping of 0.1 mm to 0.2 mm (depending on part size). Specific procedures of the solderability testing are the same with those explained above in the Description of the Related Art. 
     Thus obtained wetting curves are shown in FIGS. 3 and 4. FIGS. 3 and 4 show wetting curves observed at 6 points and 4 points, respectively. Observation of the sample holding portion  23 A of the sample parts holding member  23  after the testing showed almost no flux creepage onto such sample holding portion  23 A. It was also found that the wetting curves were obtained in a highly consistent manner, which ensured precise measurement of the wetting time. 
     (Embodiment 2) 
     The embodiment 2 also relates to the solderability testing apparatus and solderability testing method according to the first aspect of the present invention. While the sample parts holding member  23  in the embodiment 2 is also made of a material same as that for the embodiment 1, the entire surface of which has formed thereon the flux wetting preventive layer  24  comprising a fluorocarbon resin derivative. More specifically, the entire portion of the sample parts holding member  23  was dipped in an isopropanol solution or perfluorocarbon solution of a fluorocarbon resin derivative (content of the fluorocarbon resin derivative of 0.2 to 1%), and the solution was then dried using a hair drier to thereby form the flux wetting preventive layer  24 . Dipping only the sample holding portion  23 A (and around thereof) of the sample parts holding member  23  into such solution and then drying thereof can form the flux wetting preventive layer only on the surface of the sample holding portion  23 A and around thereof of the sample parts holding member  23 . It is also allowable to coat using a brush or the like such solution onto the entire surface of the sample parts holding member  23  or onto the sample holding portion  23 A (and around thereof) of the sample parts holding member  23 , in place of dipping into such solution, and then dry such solution. Such dipping into or coating of such solution may be repeated twice or more times. 
     Thus obtained sample parts holding member  23  was attached to the solderability testing apparatus shown in FIG. 1, and then subjected to the solderability testing similarly to the embodiment 1. Observation of the sample holding portion  23 A of the sample parts holding member  23  after the testing showed almost no flux creepage onto such sample holding portion  23 A. It was also found that the wetting curves were obtained in a highly consistent manner, which ensured precise measurement of the wetting time. 
     (Embodiment 3) 
     The embodiment 3 also relates to the solderability testing apparatus and solderability testing method according to the first aspect of the present invention. While the sample parts holding member  23  in the embodiment 3 is also made of a material same as that for the embodiment 1, the member has formed thereon the flux wetting preventive layer  24  comprising a fluorocarbon resin derivative. More specifically, a film of the flux wetting preventive layer  24  was formed on the sample holding portion  23 A and around thereof of the sample parts holding member  23  by sputtering using a polytetrafluoroethylene target. 
     Thus obtained sample parts holding member  23  was attached to the solderability testing apparatus shown in FIG. 1, and then subjected to the solderability testing similarly to the embodiment 1. Observation of the sample holding portion  23 A of the sample parts holding member  23  after the testing showed almost no flux creepage onto such sample holding portion  23 A. It was also found that the wetting curves were obtained in a highly consistent manner, which ensured precise measurement of the wetting time. 
     (Embodiment 4) 
     The embodiment 4 also relates to the solderability testing apparatus and solderability testing method according to the first aspect of the present invention. While the sample parts holding member  23  in the embodiment 4 is also made of a material same as that for Embodiment 1, the member has formed thereon the flux wetting preventive layer  24  comprising a ceramic. More specifically, alumina was applied by ceramic coating and then baked to thereby form the flux wetting preventive layer  24  onto the sample holding portion  23 A and around thereof of the sample parts holding member  23 . 
     Thus obtained sample parts holding member  23  was attached to the solderability testing apparatus shown in FIG. 1, and then subjected to the solderability testing similarly to Embodiment 1. Observation of the sample holding portion  23 A of the sample parts holding member  23  after the testing showed almost no flux creepage onto such sample holding portion  23 A. It was also found that the wetting curves were obtained in a highly consistent manner, which ensured precise measurement of the wetting time. 
     (Embodiment 5) 
     The embodiment 5 also relates to the solderability testing apparatus and solderability testing method according to the first aspect of the present invention. While the sample parts holding member  23  in Embodiment 5 is also made of a material same as that for Embodiment 1, the member has formed thereon the flux wetting preventive layer  24  comprising a cermet. More specifically, the flux wetting preventive layer  24  comprising cermet was formed on the sample holding portion  23 A and around thereof of the sample parts holding member  23  by sputtering using SiC and TaN targets. 
     Thus obtained sample parts holding member  23  was attached to the solderability testing apparatus shown in FIG. 1, and then subjected to the solderability testing similarly to the embodiment 1. Observation of the sample holding portion  23 A of the sample parts holding member  23  after the testing showed almost no flux creepage onto such sample holding portion  23 A. It was also found that the wetting curves were obtained in a highly consistent manner, which ensured precise measurement of the wetting time. 
     In such embodiments 1 to 5, composing the flux wetting preventive layer  24  with a material having a low heat conductivity can prevent heat supplied from the heating means  40  to the sample  50  from conducting (dissipating) toward the sample parts holding member  23 , which successfully raises accuracy in the temperature control of the solder paste  31 , and reduces a ratio of temperature elevation time included in the wetting time (delay in the temperature elevation), which ensures more precise measurement of the wetting time. Heat conductivity of a material composing such flux wetting preventive layer  24  is typically {fraction (1/100)} or below of that for metal (Ni), that is 0.2 to 1.0 W·m −1 ·K −1  (300K). 
     (Embodiment 6) 
     The embodiment 6 relates to the solderability testing apparatus and solderability testing method according to the second aspect of the present invention. Constitution of the solderability testing apparatus of the embodiment 6, except for the constitution of the sample parts holding member  23 , can be almost similar to that of the conventional solderability testing apparatus previously explained referring to FIG. 1, so that detailed description thereof will be omitted. 
     In the embodiment 6, the sample parts holding member  23  similar to that shown in FIG. 2 was obtained by machining a block of fluorocarbon resin (more specifically, polytetrafluorotehylene). 
     Thus obtained sample parts holding member  23  was attached to the solderability testing apparatus shown in FIG. 1, and then subjected to the solderability testing similarly to the embodiment 1. Observation of the sample holding portion  23 A of the sample parts holding member  23  after the testing showed almost no flux creepage onto such sample holding portion  23 A. It was also found that the wetting curves were obtained in a consistent manner, which ensured precise measurement of the wetting time. It was also confirmed that composing the sample parts holding member  23  with such fluorocarbon resin prevent heat supplied from the heating means  40  to the sample  50  from conducting (dissipating) toward the sample parts holding member  23 , which successfully raises accuracy in the temperature control of the solder paste  31 , and reduces a ratio of temperature elevation time included in the wetting time (delay in the temperature elevation), which ensures more precise measurement of the wetting time. In general, heat conductivity of a material composing at least the sample holding portion of the sample parts holding member is typically {fraction (1/100)} or below of that for metal (Ni), that is 0.2 to 1.0 W·m −1 ·K −1  (300K). 
     It is now also allowable to fabricate the sample holding portion  23 A (a portion expressed by the hatching in FIG. 2, for example) of the sample parts holding member  23  with a fluorocarbon resin (more specifically, polytetrafluoroethylene), to fabricate other portions of the sample parts holding member  23  with other material (steel material or stainless steel), and to assemble these portions to thereby obtain the sample parts holding member  23 . 
     Although the invention has been described referring to Preferred Embodiments, the present invention is by no means limited thereto. It is therefore to be understood that any constitutions of the solderability testing apparatus and testing conditions for the solderability testing method described in the Preferred Embodiments can properly be modified.