JP2012038710A - Flexible flat cable, flexible printed board, method of manufacturing flexible flat cable, and method of manufacturing flexible printed board - Google Patents

Abstract

A flexible flat cable, a flexible printed circuit board, a flexible flat cable manufacturing method, and a flexible printed circuit board manufacturing method capable of suppressing the occurrence of whiskers are provided.
A flexible flat cable according to the present invention includes a conductor having a conductive base material made of copper or a copper alloy, and a conductive layer provided on the surface of the conductive base material. A first insulating layer 20 provided above and a second insulating layer 22 provided below the conductor 10, and the conductive layer 102 is a copper-tin intermetallic compound layer 61 formed on the conductive substrate 100. And a tin-bismuth solid solution formed on the copper-tin intermetallic compound layer 61 and composed of tin and bismuth, and a residual tin layer 62 containing at least tin and unavoidable impurities.
[Selection] Figure 1

Description

  The present invention relates to a flexible flat cable, a flexible printed circuit board, a flexible flat cable manufacturing method, and a flexible printed circuit board manufacturing method. In particular, the present invention relates to a flexible flat cable having a conductive layer containing a tin alloy, a flexible printed board, a method for producing a flexible flat cable, and a method for producing a flexible printed board.

  Many signal lines are used in recent electric appliances. And when transmitting a signal to a moving part in an electric device, the flexible flat cable (FFC) or flexible printed circuit board (FPC) which has the said part and the part which transmits a signal to the said part are flexible. ). A plating layer of tin, silver, gold, nickel or the like is provided on the surface of the FFC and FPC wiring materials in order to prevent the wiring materials from being oxidized and corroded. Here, since tin is soft, it is easily deformed when pressure is applied. Therefore, when tin plating is performed on the wiring layer of the male and female terminals, the tin plating is deformed by the fitting between the male terminal and the female terminal, and the contact area between the male terminal and the female terminal increases. Since contact resistance can be reduced by this, tin plating is widely performed on the surface of the wiring material. Here, when a plating layer containing tin is provided on the surface of the wiring, a needle-like crystal called a whisker is precipitated from the plating layer, and the wiring having the plating layer and another wiring are short-circuited There is.

  Therefore, conventionally, after tin plating with a thickness of less than 0.2 μm to less than 1.0 μm is applied to the electrical connection portion of the electrical conductor component, the ratio of the tin-plated tin to the electrical conductor is set to 50% or more by heat treatment. A method for manufacturing an electrical conductor component is known (see, for example, Patent Document 1).

  In the method of manufacturing an electrical conductor component described in Patent Document 1, since the heat treatment is performed at a temperature equal to or higher than the melting point of tin to form a tin alloy layer, the amount of the tin layer that becomes a whisker generation source can be reduced. It is possible to reduce the generation of whiskers without using lead.

JP 2006-127939 A

  However, since the method of manufacturing an electric conductor component described in Patent Document 1 performs heat treatment at a temperature equal to or higher than the melting point of tin, tin having large crystal grains may be generated due to recrystallization of tin. In some cases, the stress generated in the electric conductor cannot be sufficiently relaxed, and it is difficult to suppress the generation of whiskers.

  Therefore, the objective of this invention is providing the manufacturing method of the flexible flat cable which can suppress generation | occurrence | production of a whisker, a flexible printed circuit board, a flexible flat cable, and the manufacturing method of a flexible printed circuit board.

  In order to achieve the above object, the present invention provides a conductor having a conductive base made of copper or a copper alloy, a conductive layer provided on the surface of the conductive base, and a first provided above the conductor. An insulating layer; and a second insulating layer provided below the conductor, wherein the conductive layer includes a copper-tin intermetallic compound layer formed on the conductive substrate, and the copper-tin intermetallic compound. Provided is a flexible flat cable formed on a layer and comprising a tin-bismuth solid solution composed of tin and bismuth, and a residual tin layer containing at least tin and inevitable impurities.

  Moreover, as for the said flexible flat cable, it is preferable that a tin-bismuth solid solution contains 3 mass% or less bismuth.

  Moreover, it is preferable that the said flexible flat cable further contains the bismuth crystal which the remaining tin layer precipitated, without being able to dissolve completely.

  Further, in the above-mentioned flexible flat cable, the tin-bismuth solid solution contained in the conductive layer has a lattice constant of the tin-bismuth solid solution measured by using an X-ray diffractometer of 0.584 nm to 0.585 nm in the a-axis. And preferably in the range of 0.3185 nm to 0.32 nm on the c-axis.

In the flexible flat cable, the tin-bismuth solid solution contained in the conductive layer has a lattice volume of the tin-bismuth solid solution measured by using an X-ray diffractometer of 0.1085 nm ³ or more and 0.109 nm ³ or less. It is preferable to be within the range.

  Moreover, this invention is provided between the 1st insulating film and the 2nd insulating film, and the said 1st insulating film and the said 2nd insulating film in order to achieve the said objective, Copper or a copper alloy And a wiring circuit having a conductive layer provided on a surface of the conductive base material, the conductive layer being formed on the conductive base material, a copper-tin intermetallic compound Provided is a flexible printed circuit board comprising: a layer, a tin-bismuth solid solution composed of tin and bismuth formed on the copper-tin intermetallic compound layer, and a residual tin layer containing at least tin and inevitable impurities .

  In the flexible printed board, the tin-bismuth solid solution preferably contains 3% by mass or less of bismuth.

  Moreover, it is preferable that the said flexible printed circuit board further contains the bismuth crystal which the remaining tin layer precipitated, without being able to dissolve completely.

  In the flexible printed circuit board, the tin-bismuth solid solution contained in the conductive layer has a lattice constant of 0.584 nm or more and 0.585 nm or less in the a-axis when the tin-bismuth solid solution is measured using an X-ray diffractometer. And preferably in the range of 0.3185 nm to 0.32 nm on the c-axis.

In the flexible printed circuit board, the tin-bismuth solid solution contained in the conductive layer has a lattice volume of the tin-bismuth solid solution measured using an X-ray diffractometer of 0.1085 nm ³ or more and 0.109 nm ³ or less. It is preferable to be within the range.

  In order to achieve the above object, the present invention provides a base material preparation step for preparing a conductive base material made of copper or a copper alloy, and tin-bismuth alloy plating is applied to the surface of the conductive base material to form a tin- A plating step for forming a substrate with a bismuth alloy plating layer, a heat treatment step for applying a heat treatment at a temperature equal to or higher than the melting point of the tin-bismuth alloy plating layer to the substrate with a tin-bismuth alloy plating layer, and after the heat treatment step, A cooling step of cooling the substrate with the tin-bismuth alloy plating layer at a cooling rate of 200 ° C./sec or more to form a substrate with a conductive layer; and above the substrate with the conductive layer after the cooling step. An insulating layer forming step of providing a second insulating layer below the first insulating layer, wherein the conductive layer includes a copper-tin intermetallic compound layer formed on the conductive substrate, and the copper-tin Formed on intermetallic compound layer Tin consisting of tin and bismuth which - and bismuth solid solution, and tin, method of manufacturing a flexible flat cable consisting of a inevitable impurities including at least the remaining tin layer.

  Moreover, as for the manufacturing method of the said flexible flat cable, it is preferable that tin-bismuth alloy plating contains 3 mass% or more and 10 mass% or less bismuth in pure tin.

  Moreover, as for the manufacturing method of the said flexible flat cable, it is preferable that a conductive layer contains the said tin- bismuth solid solution containing 3 mass% or less bismuth.

  In order to achieve the above object, the present invention provides a base material forming step of forming a conductive base material made of copper or a copper alloy on the first insulating film, and tin-bismuth on the surface of the conductive base material. A plating step for performing alloy plating, a heat treatment step for performing a heat treatment at a temperature equal to or higher than a melting point of the tin-bismuth alloy plating, and cooling the tin-bismuth alloy plating at a cooling rate of 200 ° C./sec or more after the heat treatment step. A cooling step of forming a conductive layer, and a coating step of coating the conductive layer with a second insulating film after the cooling step, wherein the conductive layer is a copper- A flexible film comprising a tin intermetallic compound layer, a tin-bismuth solid solution composed of tin and bismuth formed on the copper-tin intermetallic compound layer, and a residual tin layer containing at least tin and unavoidable impurities. Method for manufacturing a Blu printed substrate is provided.

  Moreover, as for the manufacturing method of the said flexible printed circuit board, it is preferable that tin-bismuth alloy plating contains 3 mass% or more and 10 mass% or less bismuth in pure tin.

  Moreover, it is preferable that the manufacturing method of the said flexible printed circuit board contains the said tin- bismuth solid solution in which a conductive layer contains 3 mass% or less bismuth.

  According to the flexible flat cable, the flexible printed circuit board, the flexible flat cable manufacturing method, and the flexible printed circuit board manufacturing method according to the present invention, the flexible flat cable, the flexible printed circuit board, and the flexible flat cable manufacturing method capable of suppressing the occurrence of whiskers. And the manufacturing method of a flexible printed circuit board can be provided.

(A) is a top view of the edge part of the flexible flat cable which concerns on the 1st Embodiment of this invention, (b) is sectional drawing in the AA of (a). It is a schematic diagram of the crystal lattice of pure tin metal. It is a schematic diagram of the state which inserted the flexible flat cable which concerns on the 1st Embodiment of this invention in the connector. It is a schematic diagram of the state in which the connector pin of the connector is contacting the surface of the conductor of the flexible flat cable which concerns on the 1st Embodiment of this invention. It is a schematic diagram of the partial cross section of the flexible printed circuit board concerning the 2nd Embodiment of this invention. It is sectional drawing after performing plating on a conductive substrate, and is a sectional view after performing heat treatment and quenching treatment on this conductive substrate with a plating layer.

[First Embodiment]
Fig.1 (a) shows the outline | summary from the upper surface of the edge part of the flexible flat cable which concerns on the 1st Embodiment of this invention, (b) is the flexible flat in the AA line of Fig.1 (a). An outline of the cross section of the cable is shown. FIG. 2 is a schematic diagram of a crystal lattice of pure tin metal.

(Outline of the structure of the flexible flat cable 1)
The flexible flat cable (FFC) 1 according to the first embodiment has a conductive layer 102 provided on the surface, and a plurality of conductive substrates 100 that are rectangular conductors having a rectangular or elliptical cross section. The first insulating layer 20 and the second insulating layer 22, which are insulating films, are arranged with a predetermined gap between them. Specifically, as shown in FIGS. 1A and 1B, the flexible flat cable 1 includes a conductive base material 100 made of copper, a copper alloy, or the like, and a conductive material provided to cover the surface of the conductive base material 100. A conductor 10 having a layer 102, a first insulating layer 20 provided above the conductor 10, and a second insulating layer 22 provided below the conductor 10 are provided. And in the edge part of the flexible flat cable 1, a part of 1st insulating layer 20 is predetermined | prescribed from the edge part of the flexible flat cable 1 so that a part of surface of the conductive layer 102 of the conductor 10 may be exposed outside. Only the distance has been removed.

(Conductor 10)
The conductor 10 includes a conductive substrate 100 made of a metal material such as copper or a copper alloy, and a conductive layer 102 provided in contact with the surface of the conductive substrate 100. A plurality of conductors 10 are arranged in parallel between the first insulating layer 20 and the second insulating layer 22, for example. The dimension of the conductive substrate 100 is determined according to the dimension of the flexible flat cable 1 determined by the application of the flexible flat cable 1. As an example, when the plurality of conductors 10 are provided between the first insulating layer 20 and the second insulating layer 22 at intervals of 0.5 mm, the conductive substrate 100 has a width of 0.3 mm and a thickness of It is about 35 μm. In this case, a conductive layer 102 having a thickness of about 30 times the thickness of the conductive substrate 100 (that is, a thickness of about 1 μm) is provided on the surface of the conductive substrate 100.

  Here, the conductive layer 102 will be described in detail with reference to FIGS. FIG. 6A shows a cross-sectional structure of a lead-free tin-bismuth alloy plating layer 60 in which a predetermined amount of bismuth is added to pure tin so as to contact the surface of the conductive substrate 100. In b), the tin-bismuth alloy plating layer 60 is further subjected to a heat treatment / cooling treatment (a treatment method in which the tin-bismuth alloy plating layer is heated at a temperature equal to or higher than the melting point of the tin alloy plating layer and once melted and then cooled / solidified). The conductive layer 102 formed of the copper-tin intermetallic compound layer 61 made of tin and copper formed on the conductive substrate 100 side and the residual tin layer 62 containing bismuth formed on this layer is formed. This is a cross-sectional structure of the fish.

  When a heat treatment is applied to the tin-bismuth alloy plating layer 60 formed by adding bismuth to pure tin containing inevitable impurities, a tin-bismuth solid solution composed of tin and bismuth is formed in the remaining tin layer 62. The bismuth concentration contained in the tin-bismuth solid solution is a maximum of about 3% by mass.

  Further, when the amount of bismuth added to the tin-bismuth alloy plating layer 60 is increased and heat treatment is performed, a tin-bismuth solid solution composed of tin and bismuth is formed in the residual tin layer 62 and bismuth crystals are formed. Also precipitate. Specifically, the remaining tin layer 62 in the conductive layer 102 includes a tin-bismuth solid solution composed of tin and bismuth, and the remainder is formed of bismuth crystals that are precipitated without being completely dissolved in tin and inevitable impurities.

  Here, the amount of bismuth atoms added to the tin-bismuth alloy plating layer 60 is the amount of bismuth atoms in the tin-bismuth solid solution formed in the residual tin layer 62 and the amount of bismuth crystals deposited without being completely dissolved. It is approximately equal to the sum of the amount of atoms.

  It should be noted that whether the bismuth crystals are precipitated without being completely dissolved depends on the initial bismuth concentration added to the tin-bismuth alloy plating layer 60 and the heat treatment time (for example, the longer the heat treatment time, the longer the remaining tin layer 62). The film thickness is reduced, and the bismuth concentration in the residual tin layer 62 is increased and is likely to be precipitated.) Depending on the conditions, it may not be precipitated.

  The tin-bismuth alloy plating layer 60 reduces the amount of tin atoms contained in the residual tin layer 62 on the surface of the conductive substrate 100, and the residual tin layer 62 becomes conductive when heat treatment is performed. Thickness of 2 μm or less, preferably 0.3 μm or more and 1.5 μm or less, more preferably for the purpose of not disappearing from the conductive substrate 100 (not only the copper-tin intermetallic compound layer 61) Is formed with a thickness of 0.5 μm or more and 1.5 μm or less.

The tin-bismuth solid solution contained in the residual tin layer 62 has a lattice constant of 0.584 nm or more and 0.585 nm or less in the a-axis, as measured by an X-ray diffractometer. It is defined in the range of 0.3185 nm or more and 0.32 nm or less on the axis. The tin-bismuth solid solution contained in the residual tin layer 62 has a lattice volume of the tin-bismuth solid solution measured using an X-ray diffractometer in the range of 0.1085 nm ³ or more and 0.109 nm ³ or less. Is done. As described above, the concentration of bismuth that can be dissolved in tin is up to about 3% by mass, and within this concentration range, the lattice constant of the tin-bismuth solid solution changes (the lattice constant increases as the bismuth concentration increases). Furthermore, even if the amount of bismuth added is increased to precipitate bismuth crystals, the lattice constant of the tin-bismuth solid solution does not change.

The lattice constant of the tin-bismuth solid solution can be calculated by calculation using an X-ray diffraction pattern measured when the conductive layer 102 which is a plating layer is irradiated with X-rays. The crystal lattice of pure tin metal is a rectangular parallelepiped, and tin atoms 3 are arranged as shown in FIG. The long side of the crystal lattice is the a-axis 30 and the short side is the c-axis 32. In pure tin metal, the a-axis 30 is 0.5831 nm, and the c-axis 32 is 0.3182 nm. From these values, the volume of the crystal lattice can be calculated, and is 0.10819 nm ³ for pure tin metal. Similarly, the lattice volume of the tin-bismuth solid solution can also be calculated from the lattice constant measured and calculated by the X-ray diffractometer.

(First insulating layer 20 and second insulating layer 22)
Each of the first insulating layer 20 and the second insulating layer 22 is formed of an insulating polymer material, for example, polyethylene terephthalate (PET). Each of the first insulating layer 20 and the second insulating layer 22 is provided with a flame retardant adhesive layer on one surface, and is attached to the surface of the conductor 10 via the adhesive layer. The adhesive layer is formed, for example, with flame retardant polyester as a main component.

(Fitting of flexible flat cable 1 to connector 5)
FIG. 3 shows an outline of a state in which the flexible flat cable according to the first embodiment of the present invention is inserted into the connector, and FIG. 4 shows the conductor of the flexible flat cable according to the first embodiment of the present invention. The outline of the state where the connector pin of the connector is in contact with the surface is shown.

  As shown in FIG. 3, the connector 5 into which the flexible flat cable 1 is inserted includes a plurality of connector pins 50 that are electrically connected to the plurality of conductors 10 included in the flexible flat cable 1. The connector pin 50 is a metal terminal formed from a metal material such as copper. When the flexible flat cable 1 is inserted into the connector 5, each of the plurality of connector pins 50 comes into contact with each of the surfaces of the plurality of conductors 10, and the flexible flat cable 1 is fitted into the connector 5.

  By this fitting, the tip 50a of the connector pin 50 contacts the surface of the conductor 10 as shown in FIG. 4 and the tip 50a applies pressure to the conductor 10. Thereby, the pressure from the connector pin 50 is applied to the conductor 10 having the conductive layer 102 containing tin on the surface, and stress is generated in the conductor 10. Here, when the conductive layer 102 does not contain the tin-bismuth solid solution according to the present embodiment, this stress causes the tin atoms to move inside the conductive layer 102 and cause recrystallization or diffusion. Sometimes. In this case, it is considered that the whisker is generated and grows through the surface of the conductor 10 to the outside due to recrystallization or diffusion phenomenon of tin atoms. Alternatively, the copper contained in the conductive base material 100 diffuses into the conductive layer 102 to generate an intermetallic compound of copper and tin, thereby generating a stress in the conductive layer 102. It is considered that whiskers are generated and grow by being pushed out of the conductor 10.

  However, since the conductor 10 included in the flexible flat cable 1 according to the present embodiment has at least a tin-bismuth solid solution and the remaining tin layer 62 containing tin and unavoidable impurities on the surface, The movement of tin atoms or the diffusion of copper into the conductive layer 102 is suppressed. Therefore, the generation and growth of tin whiskers on the surface of the conductor 10 is suppressed.

(Method for manufacturing flexible flat cable 1)
The flexible flat cable 1 is manufactured through the following steps. First, the electroconductive base material 100 which consists of copper or a copper alloy is prepared (base material preparation process). The dimensions of the conductive substrate 100, thermal characteristics such as thermal conductivity, electrical characteristics such as electrical conductivity, and mechanical characteristics such as tensile strength are characteristics required for the flexible flat cable 1 to be manufactured. It is decided according to. Next, a tin-bismuth alloy plating layer 60 as a plating layer is formed on the surface of the conductive substrate 100 by a plating method to form a substrate with a tin-bismuth alloy plating layer (plating step). The conductive layer 102 as a plating layer is formed of the same material as that described in FIG.

  Subsequently, heat treatment at a temperature equal to or higher than the melting point of the tin-bismuth alloy plating layer 60 is performed on the substrate with the tin-bismuth alloy plating layer (heat treatment step). In the heat treatment step, a heat treatment for confirming the melting of the tin-bismuth alloy plating layer 60 as the plating layer is performed. Then, the substrate with the tin-bismuth alloy plating layer is cooled at a cooling rate of 200 ° C./sec or more immediately after the heat treatment step or continuously with the heat treatment step (cooling step). Thereby, the copper-tin intermetallic compound layer 61 made of tin and copper formed on the conductive substrate 100 side, and bismuth added to tin on this layer keep the shape of the crystal lattice of tin. A conductive layer composed of a tin-bismuth solid solution dissolved in tin and a residual tin layer 62 that always includes tin and unavoidable impurities, and may contain bismuth crystals that are not completely dissolved depending on conditions. Formed on the conductive substrate 100. Furthermore, while providing the 1st insulating layer 20 above the base material with a conductive layer which passed through the cooling process, the 2nd insulating layer 22 is provided below (insulating layer formation process). Thereby, the flexible flat cable 1 which concerns on this Embodiment as shown in FIG. 1 is obtained.

(Effects of the first embodiment)
In the flexible flat cable 1 according to the present embodiment, the outermost surface of the conductive substrate 100 is covered with the residual tin layer 62 containing the tin-bismuth solid solution, so that lead is not added to the conductive layer 102. The whisker length is such that the occurrence and growth of tin whiskers can be suppressed, and even if the whisker is generated, a failure (that is, a failure in which one conductor 10 and another conductor 10 are short-circuited) does not occur. It can be suppressed. Thereby, the flexible flat cable 1 corresponding to a RoHS command and REACH regulation can be provided. For example, since at least the conductive layer 102 made of a tin alloy containing a tin-bismuth solid solution is formed in a portion to which pressure is applied when the flexible flat cable 1 is fitted to the connector 5, the pressure is applied to the conductor 10 to form the conductor 10. Even when stress is generated inside, the generation and growth of tin whiskers can be suppressed.

  Moreover, the flexible flat cable 1 according to the present embodiment forms the conductive layer 102 by plating the surface of the conductive substrate 100 with a tin alloy containing bismuth and then performing heat treatment and quenching treatment. Compared with the case where a method such as plating or base plating is employed, an increase in cost due to an increase in the manufacturing process can be suppressed.

  In addition, the flexible flat cable 1 according to the present embodiment has a thickness of tin-bismuth in consideration of the fact that the plating layer, that is, the conductive layer 102 flows and disappears on the surface of the conductive substrate 100 by the heat treatment in the manufacturing process. Since the alloy plating layer 60 is provided, a phenomenon in which the conductive substrate 100 is exposed on the surface of the conductor 10 (for example, a phenomenon called “red skin” when the conductive substrate 100 is made of copper or a copper alloy) can be suppressed. .

[Second Embodiment]
FIG. 5 shows an outline of a partial cross section of a flexible printed board according to the second embodiment of the present invention.

  In the flexible printed circuit board 7 which concerns on 2nd Embodiment, the member to which the code | symbol same as the member which comprises the flexible flat cable 1 which concerns on 1st Embodiment is attached | subjected concerns on 1st Embodiment. Since it has substantially the same configuration and function as the members, detailed description will be omitted except for differences.

  The flexible printed circuit board 7 according to the second embodiment includes a wiring circuit 12, a first insulating film 24 that is attached to one surface of the wiring circuit 12, and a first that is attached to the other surface of the wiring circuit 12. 2 insulating films 26. That is, the wiring circuit 12 is provided between the first insulating film 24 and the second insulating film 26. Moreover, the wiring circuit 12 has the electroconductive base material 100 and the electroconductive layer 102 which covers the surface of an electroconductive base material similarly to the conductor 10 of 1st Embodiment.

  Moreover, the flexible printed circuit board 7 is manufactured as follows, for example. First, the electroconductive base material 100 which consists of copper or a copper alloy is affixed on the 1st insulating film 24 (base material formation process). Note that the conductive substrate 100 can also be formed by depositing a conductive material on the first insulating film 24.

  Next, a mask pattern for a desired circuit pattern is formed on the conductive substrate 100 provided on the first insulating film 24 (mask process). Then, by performing an etching process on the conductive substrate 100 using the formed mask pattern as a mask, a base of a circuit pattern having a desired shape is formed (etching process). And the tin-bismuth alloy plating layer 60 is formed on the said foundation | substrate using a plating method (plating process).

Subsequently, heat treatment at a temperature equal to or higher than the melting point of plating is performed in the same manner as in the first embodiment (heat treatment step). After the heat treatment step, the plating is cooled at a cooling rate of 200 ° C./sec or more as in the first embodiment (cooling step). As a result, a copper-tin intermetallic compound layer 61 composed of tin and copper is formed on the conductive base material 100, and the bismuth added to tin on this layer is tin while maintaining the shape of the crystal lattice of tin. A wiring circuit 12 is formed in which a conductive layer 102 formed of a tin-bismuth solid solution dissolved therein and a residual tin layer 62 including bismuth precipitated without being completely dissolved is formed. Furthermore, the flexible printed circuit board 7 according to the second embodiment is manufactured by covering the wiring circuit 12 with the second insulating film 26.
Whether the bismuth crystal is precipitated without being completely dissolved depends on conditions such as the initial bismuth addition amount of the tin-bismuth alloy plating layer 60 and heat treatment as in the above-described flexible flat cable.

(Knowledge obtained by the inventor)
The flexible flat cable 1 according to the first embodiment and the flexible printed circuit board 7 according to the second embodiment will be described as supplements based on the following knowledge obtained by the present inventors. .

  That is, the present inventor has obtained the knowledge that generation of tin whiskers can be suppressed by adding bismuth to the material constituting the conductive layer 102. The reason why the generation of tin whiskers can be suppressed by adding bismuth to the material constituting the conductive layer 102 is that tin and bismuth are in a state where bismuth is dissolved in the crystal lattice while maintaining the crystal lattice of tin. This is probably because the diffusion of tin in the conductive layer 102 is suppressed due to the formation of the solid solution in the conductive layer 102.

Further, in the step of forming the conductive layer 102, that is, the cooling step after the heat treatment step, by cooling the substrate with the tin-bismuth alloy plating layer at a cooling rate of 200 ° C./sec or more, the conductive substrate 100 It is suppressed that copper is diffused into the tin-bismuth alloy plating layer 60 to form an intermetallic compound such as Cu ₆ Sn ₅ (the formation of the copper-tin intermetallic compound layer 61 proceeds during the cooling step). Can be suppressed). Furthermore, since the cooling step suppresses coarsening of particles such as tin in the conductive layer 102, it is considered that the generation of stress is reduced in the conductive layer 102 which is a plating layer.

  Based on these findings, the present inventor has obtained knowledge that the generation and growth of whiskers can be suppressed in the conductive layer 102 according to the first and second embodiments as described above.

A conductor with tin-bismuth alloy plating for a flexible flat cable was produced as the conductor according to Example 1. Specifically, first, an electrolytic plating solution of tin-bismuth alloy (a plating solution in which the concentration of bismuth added to tin is 3% by mass) was prepared. Next, a tin-bismuth alloy plating layer 60 having a bismuth addition concentration of 3 mass% was formed on the surface of the conductive substrate 100 by electroplating. The temperature of the electrolytic plating solution was maintained at 40 ° C. so that the thickness of the tin-bismuth alloy plating layer 60 became 1 μm, and the current density was set to 100 mA / cm ² and plating treatment was performed for a predetermined time. . After the tin-bismuth alloy plating layer 60 is formed on the surface of the conductive substrate 100, the conductive substrate 100 on which the tin-bismuth alloy plating layer is formed is placed on a plate whose temperature is 280 ° C. Heated for 10 seconds. And after confirming that the surface of the tin-bismuth alloy plating layer 60 was melted, the conductive base material 100 on which the tin-bismuth alloy plating layer 60 was formed was introduced into water at 25 ° C. It was taken out from. Thus, the surface of the conductive substrate 100 is composed of a copper-tin intermetallic compound layer 61 composed of tin and copper, and a residual tin layer 62 containing a tin-bismuth solid solution formed on this layer. A conductor according to Example 1 in which the conductive layer 102 was formed was obtained.

  A conductor with a tin-bismuth alloy plating for a flexible flat cable was produced as the conductor according to Example 2. Specifically, first, a molten metal plating tank was prepared in which a plating solution of tin-bismuth alloy (a plating solution in which the concentration of bismuth added to tin was 3% by mass) was maintained at 250 ° C. Next, the conductive base material 100 made of copper, the surface of which was previously cleaned, was put into a molten metal plating tank, and hot dip plating was performed. By adjusting the immersion time of the conductive substrate 100 in the molten metal plating tank, the thickness of the plating layer formed on the surface of the conductive substrate 100 was adjusted to 1 μm. After hot dipping, the conductive substrate 100 on which the tin-bismuth alloy plating layer 60 having a bismuth addition concentration of 3 mass% was formed was placed on a plate at a temperature of 280 ° C. and heated for 10 seconds. . Then, after confirming that the surface of the plating layer was melted, the conductive base material 100 on which the plating layer was formed was put into water at 25 ° C., and taken out from water after 5 seconds. Thereby, the copper-tin intermetallic compound layer 61 made of tin and copper on the surface of the conductive substrate 100, the tin-bismuth solid solution formed on this layer, tin, and inevitable impurities are included. A conductor according to Example 2 in which the conductive layer 102 composed of the remaining tin layer 62 was formed was obtained.

(About heat treatment temperature)
It was confirmed as follows that a temperature equal to or higher than the melting point of the tin-bismuth solid solution was suitable as the temperature of the heat treatment step.

  First, a sample was prepared by applying a tin-bismuth alloy plating layer 60 to which 3% by mass of bismuth was added to the surface of a copper test piece. Next, under the same conditions as described in Example 1 and Example 2, the sample was placed on a plate heated to 280 ° C., which is higher than the melting point of the tin-bismuth solid solution, and heated for 10 seconds. And after confirming that the surface of the plating layer was melted, the sample was put into water at 25 ° C., and the sample was taken out from water after 5 seconds. As a result, a sample according to Example 3 was obtained.

  On the other hand, the sample which concerns on the comparative example which gave the tin-bismuth alloy plating layer 60 to which 3 mass% bismuth was added to the surface of the copper test piece was prepared as a comparative example. And about the sample which concerns on a comparative example, it heat-processed for 5 minutes at each temperature of 150 degreeC, 180 degreeC, and 210 degreeC which are the temperature below melting | fusing point of a tin-bismuth solid solution. And after heat processing, the sample which concerns on a comparative example was created by throwing in 25 degreeC water and cooling, respectively. A sample heated to 150 ° C. is a sample according to Comparative Example 1, a sample heated to 180 ° C. is a sample according to Comparative Example 2, and a sample heated to 210 ° C. is a sample according to Comparative Example 3.

  Each of the sample according to Example 3 and the samples according to Comparative Examples 1 to 3 was fitted into a 100-pin connector used for a flexible flat cable and left at room temperature for 2 weeks. Thereafter, each sample was removed from the connector, and the number and maximum length of whiskers generated around the fitting marks were measured with an electron microscope. The results are shown in Table 1. The whisker generation rate is obtained from a calculation formula of (number of pins in which whiskers are generated) / (total number of pins).

  Referring to Table 1, the whisker generation rate can be reduced to less than 25% by heat treatment at a temperature equal to or higher than the melting point of the tin-bismuth alloy plating layer employed in Example 3, and the longest whisker length should be 11 μm. It was shown that On the other hand, referring to the results of Comparative Examples 1 to 3, in the heat treatment at a temperature lower than the melting point of the tin-bismuth alloy plating layer, the occurrence rate of whiskers is 30 even when the heating is performed for a long time as compared with Example 3. % And the longest whisker length was also 12 μm or more. Therefore, it was shown that the heat treatment temperature is preferably a temperature equal to or higher than the melting point of the tin-bismuth alloy plating layer.

(Regarding bismuth addition concentration, tin-bismuth lattice constant, and lattice volume)
The optimum conditions of bismuth addition concentration, tin-bismuth lattice constant, and lattice volume were confirmed as follows.

  First, a sample was prepared by applying a tin-bismuth alloy plating layer 60 in which 3% by mass of bismuth was added to pure tin on the surface of a copper test piece. And the heat processing and the cooling process were performed to the said sample similarly to Examples 1-3, and the sample concerning Example 4 was obtained. On the other hand, as the sample according to the comparative example, samples each provided with a tin-bismuth alloy plating layer 60 having bismuth addition concentrations of 1 mass%, 5 mass%, and 10 mass% were prepared. And the heat processing and the cooling process were performed to these samples similarly to Examples 1-3, and the sample which concerns on Comparative Examples 4-6 was obtained. The sample according to Comparative Example 4 has a bismuth addition concentration of 1% by mass, the sample according to Comparative Example 5 has a bismuth addition concentration of 5% by mass, and the sample according to Comparative Example 6 has a bismuth addition concentration of 10% by mass. %.

  Each of the sample according to Example 4 and the samples according to Comparative Examples 4 to 6 was fitted into a 100-pin connector used for a flexible flat cable and left at room temperature for 2 weeks. Thereafter, each sample was removed from the connector, and the number and maximum length of whiskers generated around the fitting marks were measured with an electron microscope. Moreover, the sample which concerns on Example 4 and Comparative Examples 4-6 was measured using the X-ray-diffraction apparatus, and the lattice constant and the lattice volume were computed from the diffraction pattern. The results are shown in Table 2.

  Referring to Table 2, the lattice constant value and the lattice volume value of the remaining tin layer 62 change according to the change in the addition concentration of bismuth to the tin-bismuth alloy plating layer 60, and the addition concentration is 5% by mass. Above, it became a substantially constant value. In addition, the number of whiskers generated and the longest whisker length decreased with increasing concentration of addition. When the addition concentration of bismuth was 5% by mass or more, it was confirmed from the analysis of the X-ray diffraction pattern of the remaining tin layer 62 that bismuth crystals were precipitated. When bismuth crystals are precipitated, whisker generation can be suppressed, but the plating layer tends to be hard and brittle. In a flexible flat cable and a flexible printed board provided with a conductive layer containing bismuth crystal, handling becomes difficult if the plating layer becomes too hard and brittle. Therefore, it was shown that the addition concentration of bismuth in pure tin is preferably 3% by mass or more and 10% by mass or less.

(Cooling rate)
The optimum conditions for the cooling rate of the cooling process were confirmed as follows.

  First, a sample was prepared by applying a tin-bismuth alloy plating layer 60 having a bismuth addition concentration of 3 mass% on the surface of a copper test piece. And the sample which concerns on Example 5 was obtained by performing the heat processing for 280 degreeC for 10 second to the said sample, and cooling with the cooling rate of 200 degreeC / sec or more. On the other hand, as a sample according to the comparative example, the cooling rate was adjusted to 50 ° C./sec and 2 ° C./sec, and the cooling was performed, whereby Comparative Example 7 (cooling rate: 50 ° C./sec) and Comparative Example 8 (cooling rate: 2 C./sec) was obtained.

  Each of the sample according to Example 5 and the samples according to Comparative Examples 7 to 8 was fitted into a 100-pin connector used for a flexible flat cable and left at room temperature for 2 weeks. Thereafter, each sample was removed from the connector, and the number and maximum length of whiskers generated around the fitting marks were measured with an electron microscope. The results are shown in Table 3.

  Referring to Table 3, in the samples according to Comparative Examples 7 to 8 cooled at a cooling rate slower than the cooling rate of Example 5, the longest whisker length was 13 μm or more. Moreover, although the incidence rate of whiskers was 20% or less in Example 5, it was 25% or more in Comparative Examples 7-8. Therefore, it was shown that the cooling rate in the cooling step is preferably 200 ° C./sec or more.

  While the embodiments and examples of the present invention have been described above, the embodiments and examples described above do not limit the invention according to the claims. It should be noted that not all combinations of features described in the embodiments and examples are necessarily essential to the means for solving the problems of the invention.

DESCRIPTION OF SYMBOLS 1 Flexible flat cable 3 Tin atom 5 Connector 7 Flexible printed circuit board 10 Conductor 12 Wiring circuit 20 1st insulating layer 22 2nd insulating layer 24 1st insulating film 26 2nd insulating film 30 a axis 32 c axis 50 Connector pin 50a Tip 60 Tin-bismuth alloy plating layer 61 Copper-tin intermetallic compound layer 62 Remaining tin layer 100 Conductive substrate 102 Conductive layer

Claims (16)

A conductor having a conductive base made of copper or a copper alloy, and a conductive layer provided on the surface of the conductive base;
A first insulating layer provided above the conductor;
A second insulating layer provided below the conductor,
The conductive layer includes a copper-tin intermetallic compound layer formed on the conductive substrate, a tin-bismuth solid solution formed on the copper-tin intermetallic compound layer and made of tin and bismuth, and tin And a flexible flat cable comprising a residual tin layer containing at least inevitable impurities.   The flexible flat cable according to claim 1, wherein the tin-bismuth solid solution contains 3% by mass or less of bismuth.   3. The flexible flat cable according to claim 1, wherein the remaining tin layer further includes bismuth crystals that are precipitated without being completely dissolved.   In the tin-bismuth solid solution contained in the conductive layer, the lattice constant of the tin-bismuth solid solution measured using an X-ray diffractometer is 0.584 nm or more and 0.585 nm or less in the a axis, and in the c axis. The flexible flat cable according to any one of claims 1 to 3, which is in a range of 0.3185 nm to 0.32 nm. The tin-bismuth solid solution contained in the conductive layer has a lattice volume of the tin-bismuth solid solution measured using an X-ray diffractometer in a range of 0.1085 nm ^{3 to} 0.109 nm ^3. The flexible flat cable in any one of 1-4. A first insulating film and a second insulating film;
A wiring circuit provided between the first insulating film and the second insulating film and having a conductive base material made of copper or a copper alloy, and a conductive layer provided on a surface of the conductive base material; With
The conductive layer includes a copper-tin intermetallic compound layer formed on the conductive substrate, a tin-bismuth solid solution composed of tin and bismuth formed on the copper-tin intermetallic compound layer, and tin And a flexible printed circuit board comprising a residual tin layer containing at least inevitable impurities.   The flexible printed circuit board according to claim 6, wherein the tin-bismuth solid solution contains 3% by mass or less of bismuth.   The flexible printed circuit board according to claim 6, wherein the residual tin layer further includes bismuth crystals that are precipitated without being completely dissolved.   In the tin-bismuth solid solution contained in the conductive layer, the lattice constant of the tin-bismuth solid solution measured using an X-ray diffractometer is 0.584 nm or more and 0.585 nm or less in the a axis, and in the c axis. The flexible printed circuit board according to claim 8, which is in a range of 0.3185 nm to 0.32 nm. The tin-bismuth solid solution contained in the conductive layer has a lattice volume of the tin-bismuth solid solution measured using an X-ray diffractometer in a range of 0.1085 nm ^{3 to} 0.109 nm ^3. The flexible printed circuit board according to 8 or 9. A base material preparation step of preparing a conductive base material made of copper or a copper alloy;
A plating step of forming a substrate with a tin-bismuth alloy plating layer by performing tin-bismuth alloy plating on the surface of the conductive substrate;
A heat treatment step of applying a heat treatment at a temperature equal to or higher than the melting point of the tin-bismuth alloy plating layer to the substrate with the tin-bismuth alloy plating layer;
After the heat treatment step, a cooling step of cooling the substrate with the tin-bismuth alloy plating layer at a cooling rate of 200 ° C./sec or more to form a substrate with a conductive layer;
An insulating layer forming step in which a first insulating layer is provided above the substrate with a conductive layer that has undergone the cooling step, and a second insulating layer is provided below;
The conductive layer includes a copper-tin intermetallic compound layer formed on the conductive substrate, a tin-bismuth solid solution composed of tin and bismuth formed on the copper-tin intermetallic compound layer, and tin And a method for producing a flexible flat cable comprising a residual tin layer containing at least inevitable impurities.   The method for producing a flexible flat cable according to claim 11, wherein the tin-bismuth alloy plating contains 3% by mass or more and 10% by mass or less of bismuth in pure tin.   The method for producing a flexible flat cable according to claim 11 or 12, wherein the conductive layer includes the tin-bismuth solid solution containing 3% by mass or less of bismuth. A base material forming step of forming a conductive base material made of copper or a copper alloy on the first insulating film;
A plating step of performing tin-bismuth alloy plating on the surface of the conductive substrate;
A heat treatment step of performing a heat treatment at a temperature equal to or higher than the melting point of the tin-bismuth alloy plating;
A cooling step of cooling the tin-bismuth alloy plating at a cooling rate of 200 ° C./sec or more to form a conductive layer after the heat treatment step;
A coating step of coating the conductive layer with a second insulating film after the cooling step;
The conductive layer includes a copper-tin intermetallic compound layer formed on the conductive substrate, a tin-bismuth solid solution composed of tin and bismuth formed on the copper-tin intermetallic compound layer, and tin And a method for producing a flexible printed circuit board comprising a residual tin layer containing at least inevitable impurities.   The said tin-bismuth alloy plating is a manufacturing method of the flexible printed circuit board of Claim 14 containing 3 mass% or more and 10 mass% or less bismuth in pure tin.   The method for producing a flexible printed board according to claim 14 or 15, wherein the conductive layer includes the tin-bismuth solid solution containing 3% by mass or less of bismuth.

Images