JP4986026B2 - Metal surface bondability / connectivity evaluation method and evaluation apparatus - Google Patents

Description

  This invention is a method for evaluating the bondability / connectivity of metal surfaces to evaluate the solid-phase bondability and electrical connectability of two metals of the same or different types, and is used directly in the implementation of this method. It is related with the evaluation apparatus which performs.

  A bonding method is known in which bump electrodes or metal wires are ultrasonically bonded to electrodes of a printed wiring board. In this method, two different metals such as bumps and metal wires are stacked on the electrode surface, and ultrasonic vibration parallel to the joint surface is applied in a state where a pressure (load) perpendicular to the joint surface is applied. It is what is joined.

  Generally, there are adsorbates and oxide films on the surfaces of the metals to be joined, and dirt adheres during storage and component mounting processes. Also, the metal surface is not smooth when viewed microscopically. For example, some printed circuit board electrodes have a copper foil surface plated with nickel or the like and then gold plated. In this case, the underlying nickel diffuses into the surface gold plating, and oxide or Form hydroxide.

  In ultrasonic bonding, two metals collide with each other by ultrasonic vibration to destroy the adsorbate and oxide film on the surface, and the contact surface is mechanically cleaned and smoothed to bond (solid phase bonding). Promoted. However, in actual bonding, even when the bonding conditions on the device side (ultrasonic output, applied pressure, application time, etc.) are constant, the roughness of the metal surface and the degree of contamination can be caused by deformation of the bonding surface, bonding strength, etc. This greatly affects the bonding state.

JP-A-9-293744

Conventionally, as a method for evaluating the quality of the bonding state in this solid phase bonding, the method of Patent Document 1 (particularly, paragraphs 0042 to 0043) is known. In this method, two metals are actually bonded (solid phase bonding) by wire bonding , and the tensile strength test of one metal is used. A method using a shear strength test of the joint is also known.

  In addition, in the case where electrical connection of this contact portion (connection portion) is performed in a state where two metals are in contact with each other like a connector, a conventional method for evaluating a joint portion (method using a tensile strength test or a shear strength test). Cannot be used. For this reason, it has been necessary to measure the actual contact electric resistance using a special measuring instrument capable of measuring a minute resistance by actually bringing two metals into contact with each other.

However, the electrical resistance (junction resistance, connection resistance) of the two metal solid-phase junctions and the contact part of the two metals itself varies depending on the joining / connecting conditions such as the contact area. It is necessary to make a comparison using resistance values measured under a certain condition in which a constant load is applied to the vertical direction. For this reason, even if the combination of two metals was the same, there was a problem that the quality of electrical connectivity could not be evaluated between metals with different contact areas and conditions. In other words, the method of actually measuring the resistance value cannot be compared with measurement results with different measurement conditions or measurement environments, and thus represents the contact resistance (resistivity) specific to each metal surface without depending on the contact area. It cannot be an evaluation value (evaluation coefficient) that can be used as an index.

The present invention has been made in view of such circumstances, and the bonding property in the case where two metals are solid-phase bonded and the electric connectivity in the case where the two metals are brought into contact with each other to make an electrical connection are obtained. It is a first object of the present invention to provide a metal surface bondability / connectivity evaluation method that enables evaluation without depending on measurement conditions such as contact area and measurement environment.

  A second object of the present invention is to provide a metal surface bondability / connectivity evaluation apparatus which is directly used in the implementation of this method.

According to the present invention, a first object is a metal surface bondability / connectivity evaluation method for evaluating the bondability of solid phase bonding of two metals and the electrical connectability of contact portions of two metals, comprising: a ) The two metals are brought into contact with each other, and the change in the measured resistance R including the electric resistance of the contact portion of both the metals with respect to the change in the compressive load F perpendicular to the contact surface of these metals is measured; b) the obtained measured resistance The evaluation coefficient ρ _f is obtained by applying R and the load F to the following expression; expression: R = ρ _f / F + C (where ρ _f , C is a constant) c) The smaller the evaluation coefficient ρ _f is, the better the connectivity and connection It is achieved by the method for evaluating the bondability / connectivity of a metal surface comprising the steps a), b) and c) described above.

Similarly, the second object is a metal surface bondability / connectivity evaluation apparatus for evaluating the bondability of solid phase bonding of two metals and the electrical connectivity of contact portions of two metals, A pressurizer for applying a compressive load F perpendicular to the contact surfaces of the two metals, a pressure detection unit for measuring the load F by the pressurizer, a load control unit for controlling the load F by the pressurizer, Using the power source for supplying the current I to the contact portion, the resistance measurement portion for obtaining the actual resistance R including the electric resistance of the contact portions of the two metals, the load F and the actual resistance R, the equation [R = ρ _f / F + C] (where [rho _f, C is constant) comprise an evaluation coefficient calculating unit for obtaining an assessment factor [rho _f from an evaluation unit for bonding properties and connectivity as determined assessment value [rho _f is less evaluates to good, This is achieved by a metal surface bonding / connectivity evaluation apparatus characterized by the following.

As will be described later, the inventor of the present invention applies a load applied to two metal contact portions, that is, a load (pressure force) F in a direction perpendicular to the contact surface, and an electric resistance of a series connection circuit including these contact portions ( It was discovered that the relationship of the formula [R = ρ _f / F + C] (where ρ _f , C is a constant) holds between the measured resistance) R and the present invention was made based on this discovery. is there. In other words, ρ _f can be regarded as an almost constant constant, based on knowing that this is an index (evaluation coefficient) indicating the inherent plastic deformation and surface cleanliness of the metal surface, that is, the ease of adhesion. .

Therefore, it is possible to determine whether the joining property and the electrical connectivity at the contact portion between the two metals are good or bad based on the magnitude of the evaluation coefficient ρ _f . That is, according to equation (7) described later, this evaluation coefficient ρ _f is proportional to the surface yield stress σ _y , so the smaller the evaluation coefficient ρ _f , the smaller the yield stress σ _y, and the greater the plastic deformability. It can be determined that the bondability and the electrical connectivity are improved. Further, when there are a lot of oxides and dirt on the surface, the contact resistivity is large and the evaluation coefficient ρ _f is also large.

According to the method of the first aspect of the invention, for this reason, the evaluation coefficient ρ _f is obtained and based on the magnitude, that is, the smaller the obtained evaluation coefficient ρ _f is, the better the connectivity and connectivity are. by evaluation, it is possible to determine the quality of the electrical connection of the contact portion of the bondability and two metal in the case of solid phase bonding two metal. For this reason, it is not necessary to actually join and connect two metals to be joined and connected, and the metal surface can be evaluated without being affected by the measurement conditions and measurement environment.

Further, according to the apparatus of the eighth aspect of the invention, an apparatus for evaluating a metal surface can be obtained which is directly used for carrying out the invention of the first aspect.

principle

  Next, the principle of the present invention will be described. In FIG. 1, reference numeral 1 is a printed wiring board, 2 is an electrode made of a copper foil circuit pattern formed on the surface of the printed wiring board 1, and 3 is a flexible printed wiring board. To do. Therefore, here, a description will be given assuming that a large number of bumps (projection electrodes) 4 are formed. The bump 4 is pressed and connected to the electrode 2 from above. The electrode 2 and the bump 4 are the same or different metals in the present invention. The flexible printed wiring board 3 is pressed against the electrode 2 by a load (pressure) F from above the bump 4 by a pressing member 6 fixed to the pressing device 5. This load F is variable. In many cases, a plating film such as gold plating is formed on the surface of the electrode 2 or the bump 4. In this case, the metal surface is considered to be this plating film. The bump 4 is obtained by, for example, applying a nickel base plating to a copper core and performing gold plating thereon.

Found resistance R between the electrode 2 and the bumps 4, the resistance or contact resistance R _C of the contact surface between the electrode 2 and the bumps 4, the sum of the wiring resistance R _P. Therefore, the following equation (1) is established.
R = R _C + R _P (1)

The contact resistance R _C is the resistance R _S which depends on the physical (macroscopic) the contact area S of the contact surface, the sum R _S + R _f and does not depend on the contact area S resistance R _f (resistance area-independent) Become. That is, R _C = R _S + R _f (2)

Here, the area-dependent resistance R _S defines a two-dimensional contact resistivity ρ _C (Ω · cm ² ).
R _S = ρ _C / S
Therefore, the contact resistance R _C is R _C = ρ _C / S + R _f (3)
It becomes. On the other hand, if the contact interface is in a plastically deformed state, the average pressure (F / S) of the contact interface is constant and can be expressed by the _yield pressure σ _yield .
σ _yield = F / S (4)

Here, the yield pressure σ _yield is expressed as follows using the yield stress σ _y and the constant φ determined by the aspect ratio and the friction coefficient according to the “slip deformation theory of plane strain” known in the field of plastic deformation. It is known that
σ _yield = φ · σ _y (5)

From the equations (1) to (5), the actually measured resistance R is
R = ρ _C · φ · σ _y · (1 / F) + R _f + R _P (6)
Here, the following adhesion parameter ρ _f (Ω · N) is defined.
ρ _f = ρ _C · φ · σ _y (7)
Therefore, the measured resistance R is
R = ρ _f · (1 / F) + C (8)
However, C is a constant and C = (R _f + R _P ).

Here, the adhesion parameter ρ _f includes a two-dimensional contact resistivity ρ _C indicating the cleanliness of the metal surface, a constant φ indicating the film thickness and surface shape (aspect ratio), and a yield stress σ _y corresponding to the hardness. It is determined by. Therefore, it can be considered as an index indicating the inherent plastic deformability of the metal surface, that is, the ease of adhesion. Therefore, in the present invention, this adhesion parameter ρ _f is used as an evaluation coefficient. The constant φ changes in the process of compressive deformation by applying a load, but it can be considered that the contact between two metals is almost constant during the application of the load.

The above equation (8) is as shown in FIGS. That is, as shown in FIG. 2B, the actually measured resistance R is a straight line with respect to the reciprocal of the load F (1 / F), the slope of which is the adhesion parameter ρ _f , and the intercept of the resistance R with the coordinate axis is (R _f + R _P )

According to the present invention, from the equation (8), the measured resistance R becomes a linear function (straight line) with respect to the reciprocal of the load F (1 / F) within the plastic deformation range of the metal surface, and the slope ρ _{f of} this straight line is the plastic deformation of the metal surface. It is based on what is considered to show ease, that is, easy adhesion.

  Next, the principle of the present invention will be described further qualitatively. In the electrical conduction between metals, if a very small current such as a tunnel effect is ignored, a force (load F) is applied to the metal contact interface on the surface where an insulating or high resistance oxide or dirt exists. The tip of the minute unevenness on the surface is plastically deformed, and the internal metal new surface is exposed by this deformation. Metal contact is performed in a microscopic region by the contact between the new metal surfaces, and this serves as an electrical conduction path to develop electrical conduction. Therefore, as the load F increases, the cross-sectional area of the conduction path increases and the current increases. In addition, since this conduction path is a metal bonded (solid phase bonded) portion, the mechanical bonding strength (connection strength) also increases.

  That is, in order to evaluate the quality / characteristics of the bondability or connectivity of the metal surface (plated film), it is only necessary to evaluate the ease of increase of the bond area (microscopic bond area). As this evaluation method, mechanical strength measurement (tensile strength test and shear strength test) can be considered as described above, but these methods are very difficult to measure because they are microscopic. And has the above-mentioned problems. Therefore, in the present invention, instead of mechanical strength measurement, the ease of increasing the bonding area is evaluated from the electrical resistance value.

Although this principle is as described above, it can also be explained as follows. On the plastic deformation surface, the average pressure (yield pressure σ _yield ) at the interface, which is the ratio (F / s) between the applied load F and the true contact area (cross-sectional area of the electrical conduction path) s, is constant. This yield pressure σ _yield is given by a relational expression (σ _yield = φ · σ _y ) with the _yield stress σ _y which is a physical property value indicating the hardness of the surface (plated film). Here, the constant φ depends on the surface (film thickness of the plating film) and the uneven shape (aspect ratio).

Further, by defining a two-dimensional contact resistivity ρ _C and representing the resistance value (contact resistance) R _C of the contact surface as a ratio (ρ _C / S) of this ρ _C and the macroscopic contact area S, it is true The relationship (s / S) between the contact area s and the macroscopic contact area S can be related to the magnitude of the contact resistivity ρ _C (∝ (S / s) · F). That is, it is considered that the increase in the true contact area s due to the increase in the load F corresponds to the contact resistivity ρ _C , and the adhesion parameter ρ _f shown in the equation (7) is a parameter indicating the ease of the increase in the contact area s. Is to introduce.

In the present invention, as described above, it is evaluated that the smaller the evaluation coefficient ρ _f is, the better the bondability / connectivity is . One of the two metals and the electrode of the printed wiring board may be a flexible printed wiring board that is this a solid phase joining the other (claim 2). Also it may be evaluated both connectivity and the other as the electrode of one of the printed wiring board as the press contact terminal (claim 3).

Gold plating is performed on one surface of the metal to be bonded / connected, and the bondability / connectivity of this gold-plated layer (film) can be evaluated ( claim 4 ). The actual resistance R may be measured continuously while increasing the load F ( Claim 5 ), or the load F and the actual resistance R may be measured intermittently.

The evaluation coefficient ρ _f can be determined from the slope of the straight line when the actual measurement value is written in the coordinate system of 1 / F and R ( claim 6 ). Further, a plurality of evaluation coefficients ρ _f can be obtained from the equation (8) for different values of F and R, and the average value of the obtained evaluation coefficients can be used as the final evaluation coefficient ρ _f ( claim 7 ). In this case, the calculation of the average value may be determined using various mathematical methods, and for example, the least square method may be used. The power source used in the apparatus of claim 8 can be a DC power source that supplies a direct current I to the contact portion ( claim 9 ).

  FIG. 3 is a conceptual diagram of an apparatus for evaluating a metal surface used directly for carrying out the method of the present invention. In this figure, 10 is a printed wiring board, 12 is a circuit pattern made of copper foil formed on the surface thereof, and part of it is an electrode 14. A plating film such as gold plating is formed on the surface of the electrode 14. The surface of the circuit pattern 12 is covered with an insulating material 16.

  Reference numeral 18 denotes a flexible printed wiring board in which a base film 20, a circuit pattern 22 made of copper foil, and a coverlay 24 are laminated. A part of the circuit pattern 22 is exposed at the cutout portion of the base film 20, and this exposed portion becomes an electrode connected to the electrode 14 of the printed wiring board 10. A large number of bumps (projection electrodes) 26 are formed on the electrodes of the flexible printed wiring board 18. The bumps 26 are pressed from above onto the electrodes 14 on the printed wiring board 10 side, and the fine irregularities of the bumps 26 are crushed (plastically deformed), so that they are solid-phase bonded to the electrodes 14. The electrode 14 and the bump 26 are two kinds of metals of the present invention.

Reference numeral 28 denotes a clip connected to the circuit pattern 12 of the printed wiring board 10, and reference numeral 30 denotes a clip connected to the circuit pattern 22 of the flexible printed wiring board 18. These clips 28 and 30 sandwich a portion where the insulating material 16 and the cover lay 24 are partially cut out to expose a part of the circuit patterns 14 and 22 with a constant load. For this reason, the contact resistance between the clips 28 and 30 and the circuit patterns 14 and 22 is constant, and this resistance value is included in the wiring resistance R _P shown in the equation (1).

Reference numeral 32 denotes a pressure member, the tip of which is flat and presses the contact portion (joint portion) between the bump 26 of the flexible printed wiring board 18 and the electrode 14 of the printed wiring board 10 from above. The macroscopic contact area S of the tip surface of the pressure member 32 is constant. The pressurizing member 32 is pressed against the joint by pressure (load) F by a pressurizer 36 through a pressure detecting element 34 as a pressure detecting unit. The pressure detection element 34 is formed by, for example, a load cell (piezoelectric element). The pressurizer 36 can change the applied pressure (load) F continuously or discontinuously. The contact resistance R _C between the bump 26 and the electrode 14 changes (decreases) as the pressure (load) F changes (increases).

  Reference numeral 38 denotes a DC power supply, which applies a constant voltage V between the two clips 28 and 30. Reference numeral 40 denotes a voltmeter for measuring the voltage V between the clips 28 and 30, and 42 is an ammeter for measuring the current I between the clips 28 and 30. Reference numeral 44 denotes a control device including a microcomputer, which includes a resistance measurement unit 46, a load control unit 48, an evaluation coefficient calculation unit 50, and a control unit 52. The resistance measurement unit 46 obtains an actual resistance R based on the voltage V and current I measured by the voltmeter 40 and the ammeter 42. That is, it is obtained by R = V / I.

Here the measured resistance R and the contact resistance R _C between the bump 26 and the electrode 14 described above, the sum of the wiring resistance R _P. That is, R = (R _C + R _P ). The contact resistance R _C is a resistance that depends on the macroscopic contact area S of the contact portion (joint portion). The wiring resistance R _P is a constant value (fixed value).

The load control unit 48 controls the pressurizer 36 so that the applied pressure F detected by the pressure detection element 34 becomes the applied pressure F commanded by the control unit 52. The evaluation coefficient calculation unit 50 determines the adhesion parameter ρ _f based on the measured resistance R = (R _C + R _P ) that is the output of the resistance measurement unit 46 and the applied pressure F detected by the pressure detection element 34. Is calculated.

The calculation unit 50 calculates the above formula (8) to obtain ρ _f . For example, the actually measured resistance R can be obtained while gradually increasing the pressure F from 0, and can be obtained from the slope of the graph of FIG. In addition, the measured resistance R with respect to a plurality of applied pressures F is obtained, and the evaluation coefficient ρ _f can be determined based on the obtained plurality of coefficients ρ _f using an appropriate mathematical means such as an average value or a least square method.

The obtained evaluation coefficient ρ _f is input to the evaluation unit 54 and finally the metal surface is evaluated (evaluation of the bonding property between the bump 26 and the electrode 14). That determines that the higher the evaluation coefficient [rho _f is greater bondability and connection is poor, better bonding properties and connection resistance as evaluation factor [rho _f is small.

The reason for this will be described qualitatively as follows using equation (7). That is, the evaluation coefficient ρ _f is proportional to the variable σ _{y because} it is a product of ρ _C , φ considered as a constant and σ _y considered as a variable. This variable σ _y is the yield stress of the joint surface and indicates the softness of the surface. Accordingly, the ease of joining the joining surfaces increases as the yield stress σ _y decreases. From this, it can be determined that the smaller the evaluation coefficient ρ _f (that is, the smaller the yield stress σ _y ), the better the bondability and connectivity.

  FIG. 4 is a diagram showing an example of actual measurement. In this actual measurement example, a flexible printed wiring board 3 is joined to a printed wiring board 1 as shown in FIG. 1, and the electrode 2 is a copper foil. Further, the bump 4 is formed by bonding a copper core (ball) onto a circuit pattern made of the copper foil of the flexible printed wiring board 3 by imprinting (pressing with a mold), and performing nickel base plating and gold plating on the surface. Is. A thermosetting insulating resin is sandwiched between the electrode 2 and the bump 4 to pressurize and bond them.

FIG. 4A shows the relationship between the load F (unit: N) and the measured resistance R (unit: Ω) using the thickness of the gold plating as a parameter. FIG. 4B shows the relationship between the reciprocal 1 / F (N ⁻¹ ) of the load F and the contact resistance R _S. Here, the measured resistance R cannot be simply compared between lots because the wiring resistance R _P changes greatly if the production lot differs. Therefore, select a load range that can be regarded as plastic deformation of the plating film, subtract the intercept of this approximate line from the total resistance value R, extract only the load-dependent contact resistance R _S , and rearrange the relationship with the load. Thus, a straight line whose inclination is an adhesion parameter is obtained as shown in FIG.

In FIG. 4B, lot A and lot B have different terminal patterns. When measured with two types of lots A and B, it can be seen that the adhesion parameter ρ _f hardly changes between the lots A and B and varies depending on the plating thickness. That is, the adhesion parameter ρ _f is small for thick gold with good bondability (large plating thickness), whereas ρ _f is large for thin gold (small plating thickness). From this, it can be seen that the adhesion parameter ρ _f is an effective parameter as a quality index regarding the adhesion (bondability) of the plating thickness. That is, it can be seen that the reliability of the method of the present invention is high.

The evaluation unit 54 may compare the evaluation coefficient ρ _f obtained by measurement with a predetermined constant value, or may display it in several stages based on a predetermined standard. . The evaluation unit 54 may simply display the obtained evaluation coefficient ρ _f , and in this case, the operator may evaluate from this display.

Principle explanatory diagram of the present invention The figure which similarly shows the change of the resistance R for the principle explanation, and the load F The figure which shows Example 1 of this invention The figure which shows the example of measurement of this invention

Explanation of symbols

10 Printed wiring board 14 Electrode (one metal to be joined)
26 Bump (the other metal to be joined)
34 Pressure detector (load cell)
36 Pressurizer 44 Control Device 46 Resistance Measurement Unit 48 Load Control Unit 50 Evaluation Coefficient Calculation Unit 54 Evaluation Unit

Claims (9)

A method for evaluating the bondability / connectivity of a metal surface for evaluating the bondability of solid phase bonding of two metals and the electrical connectivity of a contact portion of two metals ,
a) contacting the two metals and measuring changes in the measured resistance R including the electric resistances of the contact portions of the two metals with respect to changes in the compressive load F perpendicular to the contact surfaces of these metals;
b) Obtain the evaluation coefficient ρ _f by applying the measured resistance R and the load F obtained to the following equation;
Formula: R = ρ _f / F + C (where ρ _f is a constant)
c) Evaluate that the smaller the evaluation coefficient ρ _f is, the better the bondability and connectivity are;
A metal surface bondability / connectivity evaluation method comprising the steps a), b), and c).   One of the two metals is an electrode of a printed wiring board, and the other is a flexible printed wiring board bonded to the electrode by solid phase bonding. Evaluation methods.   2. The method for evaluating metal surface bondability / connectivity according to claim 1, wherein one of the two metals is an electrode of a printed wiring board, and the other is a press contact terminal connected to the electrode by press contact. .   4. The method for evaluating the bondability / connectivity of a metal surface according to claim 2 or 3, wherein one surface of the metal to be bonded / connected is subjected to gold plating, and the bondability and electrical connectability of the gold plating layer are evaluated.   The method for evaluating the bondability / connectivity of a metal surface according to claim 1, wherein in step a) of claim 1, the measured resistance R is continuously measured while the load F is continuously increased. In step b) of claim 1, the reciprocal 1 / F of load F and resistance R are taken as two coordinate axes of an orthogonal coordinate system, and the slope of the obtained straight line is used as the evaluation coefficient ρ _f. And connectivity evaluation method. In step a) of claim 1, a plurality of different measured resistances R _n against a load F _n measured, step b) final evaluation coefficient average value of a plurality of evaluation factors [rho _fn for these measurements at [rho _f The metal surface bondability / connectivity evaluation method according to claim 1. A metal surface bondability / connectivity evaluation apparatus for evaluating the bondability of two metal solid-phase bonds and the electrical connection of two metal contacts,
A pressurizer for applying a compressive load F perpendicular to the contact surface of the two metals;
A pressure detector for measuring the load F by the pressurizer;
A load control unit for controlling the load F by the pressurizer;
A power source for supplying a current I to the contact portion of the two metals;
A resistance measuring unit for obtaining an actual resistance R including the electric resistance of the contact part of the two metals;
An evaluation coefficient calculation unit for obtaining an evaluation coefficient ρ _f from the formula [R = ρ _f / F + C] (where ρ _f is a constant) using the load F and the measured resistance R;
An evaluation unit for evaluating that the smaller the obtained evaluation coefficient ρ _f is, the better the connectivity and connectivity are;
A metal surface bondability / connectivity evaluation apparatus comprising:   The metal surface bonding / connectivity evaluation apparatus according to claim 8, wherein the power source is a DC power source that supplies a DC current I to the contact portion.

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