SU1763874A1 - Method of coulometric measurements of metal cover thickness of objects - Google Patents

Abstract

The invention relates to a measurement technique, in particular, to methods for controlling the thickness of metal coatings of objects with a small cross-sectional area, and can be used to control electroplating on wire and strip current leads. The item to be controlled is placed as an anode in an electrolytic cell and an arbitrary part of it is immersed in an electrolyte to determine the thickness of this type of coating, which additionally contains a complexing agent, which gives soluble complexes with metal ions of the coating. Then, in the potential-dynamic mode, the peak value of the anodic limiting current in terms of complexation, proportional to the surface of the immersed area, is measured. Thereafter, the coating residue is anodically dissolved, the amount of electricity passed in the dissolution process is measured, the amount of electricity consumed in measuring the surface is added to it, and the thickness of the coating is determined on the basis of the total amount of electricity according to Farad’s law. The method allows measurements to be made on miniature items of arbitrary shape, as well as on old coatings, 3 sludge. sl C

Description

The invention relates to a measurement technique, in particular, to methods for controlling the thickness of metal coatings (electroplating and chemical), and can be primarily used in the electrical and electronics industry to control the thickness of coatings on the current leads of electrical and radio elements, microchips, miniature parts electrical contact devices and electronic devices.

All known methods of coulometric measurement of the thickness of metallic coatings are based on the use of Farad's law:

6 Q3 / S F pj (1) where d is the thickness of the coatings, cm;

Q - the amount of electricity, C;

E - electrochemical gram-equivalent metal, g / mol;

S is the surface of the dissolved area, cm2;

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VI

four

F is the Farade number equal to 96490 C / mol;

p is the density of the coating metal, g / cm.

As can be seen from formula (1), in order to determine the thickness of the coating, along with the amount of electricity and the electrochemical characteristics of the metal, it is necessary to know the surface area S of the coating to be dissolved. In the majority of well-known methods of measuring the thickness of metal platings, the size of the working surface is set in a constructive (mechanical) way using pressure sensors (see, for example, 1). Being pressed to a controlled surface, such a sensor provides a constant area of dissolution defined by the diameter of the hole in the pressure pad.

Obviously, such a method and thickness measuring devices based on it are well applicable only to flat parts or parts with a sufficiently large radius of curvature. Meanwhile, in a number of industries, primarily in the electronic and radio engineering industry, a very common and important technical task is the cross-sectional area (for example, with a diameter of 0.3 to 1.2 mm). The use of clamping sensors with actually achievable diameters of the solvent coating area (1.5–4 mm) is simply impossible in this case.

Two approaches are known for solving the problem of coulometric determination of the thickness of metallic coatings on such miniature parts. The first of these 2 is reduced to fixing the depth of immersion of such a part in the electrolyte using a special device. For example, to determine the thickness of a tin coating deposited on a thin copper wire, a strictly defined length of wire (3 cm) is immersed in the electrolyte, anodically dissolved by a predetermined current, the amount of electricity is measured and the thickness of the coating is calculated using formula (1).

It can be seen that, as in the previously described methods, the working surface area of the dissolved portion of the coating is determined only once in advance during calibration — by calculating from a geometric shape (in this case, as the lateral surface of a cylinder whose height is equal to the depth of the submerged part L) and further, its constancy is provided in a constructive way. The disadvantage of this method, besides the constructive complexity, is that it is applicable provided that not only the shape and size

controlled detail, but also the depth of its immersion in the electrolyte.

Another option for coulometric measurement of the thickness of electroplated coatings on parts with a small radius of curvature, which is closest to the claimed technical essence and the achieved result, is method 3, which allows an arbitrary depth of immersion of the part into the electrolyte and based on measuring the surface of the submerged part of the part before each thickness determination. To do this, a piece of correct shape to be controlled with a small radius of curvature is coaxially placed in a cylindrical cell, which is the cathode, and an arbitrary part of L is immersed in a solution intended for coulometric determination of the thickness of this coating. Then, in the same solution and with the same system geometry, the ohmic resistance of the ROM cell is measured, which is determined by the ohmic resistance of the thin layer of the solution adjacent to the anode, and therefore is inversely proportional to the immersed surface size S.

Having determined the surface S and the optimum current density of the anodic solution i of this type of coating, the optimum current of the anodic dissolution for this particular part and the depth of its immersion in the solution (I i, S) is determined, the amount of electricity passed from the beginning to the end of dissolution is measured , and on its basis the thickness of the coating is determined by formula (1).

The experience of practical use of this method has shown that it gives good results when controlling the thickness of fresh metal coatings on small parts of regular shape (round, square, rectangular in cross section, and the dimensions of the cross sections do not change with the depth). When monitoring the coatings after long storage, the known 3 method gives a significant error and often becomes practically inapplicable. This is due to the appearance of oxide layers on the surface of the coating, representing additional resistance for alternating current and thereby breaking the inversely proportional relationship between ROM and S,

This interrelation between ROM and S is not maintained for fine details of irregular shape. Such parts (for example, cylindrical or ribbon current leads having holes, bevels, bends, thickenings, etc.) are very common in the production of electrocontact equipment and electronic devices. Obviously, with coaxial immersion of such parts into a cylindrical cell, the current distribution over the depth of the submerged part of the part will not be uniform. As a result, regardless of the immersion depth, the S k / RoM communication will not function properly. and therefore, it becomes unreasonable and the application of a known method.

The purpose of the claimed invention is to expand the field of application of the method, namely, to ensure the possibility of controlling the thickness of old metal coatings, as well as coatings on small parts of irregular shape. As a result, the nomenclature of controllable objects can be significantly increased.

The goal is achieved by immersing an arbitrary part of the object under test into an electrolyte solution, determining the surface area of the submerged area and simultaneously conducting anodic dissolution of the coating on it, measuring the amount of electricity passed from the beginning to the end of the dissolution. The surface areas of the submerged area determine the thickness of the coating according to Farad’s law. The difference lies in the fact that the complexing agent forming soluble complexes with the metal ions of the coating is additionally introduced into the electrolyte for coulometric measurement of the thickness of the coating, and to determine the surface area of the submerged area, the anode current limit measured by the depletion of the electrode layer along the complexing agent is measured and when calculating the thickness of the coating according to Farad’s law, the total amount of electricity, including its part, of surface area,

Figure 1 is a diagram of the process of coulometric determination of the thickness of metal coatings on miniature parts of irregular shape; Fig. 2 shows an example of the implementation of the method in controlling the thickness of silver coatings; on fig.Z is an example of the implementation of the method for controlling the thickness of gold coatings.

The claimed method is carried out as follows (see Fig. 1). The part 1 with a small radius of curvature (including irregularly shaped) to be inspected is placed in a cell 2, which is simultaneously a cathode and an arbitrary part of part I is immersed in electrolyte 3, intended for coulometric determination of the thickness of this type of coating. The electrolyte composition additionally introduces the complex formation gel for the metal ions of the coating at a known concentration. Then,

do not change the geometry of the system, anodically polarize a part of the part and measure the peak value of the anodic current n caused by the merging of the anode layer along the complexing agent, and determine the surface

0 submerged part parts

5 k.p./Sk, (2)

where k is a constant depending on the potential sweep rate and the kinematic parameters of the discharge and diffusion

5 ions;

Ck concentration of complexation.

After registration, the anodic potential sweep is stopped and broken.

0 chain of polarization. Simultaneously with the determination of the surface of the immersed area, the amount of electricity consumed by the SR is measured. Then, sign S and the optimum current density of the anodic dissolution

5 of this type of coating i, establish the required anodic dissolution current of the immersed part of the object (I i.S). Thereafter, the amount of electricity passed from the beginning to the end of the dissolution is measured.

0 of the coating Q2, it is summed up with the amount of electricity spent on the determination of the surface, and on the basis of this value of the surface of the submerged area the thickness of the coating is determined by formula (1).

5 A comparative analysis of the inventive method and the prototype shows that its differences are as follows:

1. The electrolyte for coulometric determination of the thickness of the metal coating further contains a known concentration of the complexing agent for dissolving metal ions.

2. The determination of the surface area of the submerged part of the part is not carried out.

5 by measuring the ohmic resistance of the cell at a high frequency of the alternating current, and by measuring the limiting anode current, which in the conditions of the implementation of the inventive method is proportional to the concentration of the complexing agent and the surface area. The coaxial arrangement of the part in the cell and its correct shape is not required, and sensitivity to the resistance of the surface layers is also excluded.

3. Simultaneously with the determination of the surface, the amount of electricity expended on this is measured.

4. When calculating the thickness of the coating according to Farad’s law, the total amount of coating dissolution electricity is taken into account, incl. and its dol, spent on the definition of S.

PRI me p 1.

As an example of the implementation of the method, consider the case of measuring the thickness of a gold coating on a nickel substrate on a current lead wire with a diameter of 0.5 mm. The wire lead is connected to the anode terminal and placed in an electrochemical cell, the body of which is simultaneously the cathode. In this case, hydrochloric acid solution can be used as an electrolyte, since It is known that SG ions are strong complexing agents for gold ions. Having immersed an arbitrary part of a gold-plated electrical lead into the electrolyte, the anode current limit for SG ions is measured with a suitable device (for example, a PA-2 polarograph). As can be seen from FIG. 3a, the peak value of In at a fixed potential sweep rate of 0.1 V / s is really proportional to the SG concentration and the immersion depth L (FIG. 3b). After measuring n, the further anodic potential sweep ceases. Knowing the concentration of chloride and the constant K, (from measurements with a known surface S), the surface of the submerged part S is determined by equation (2).

Then, using the surface value found (S 0.079 cm2 at L 5 mm) and the optimum anodic dissolution rate Au in a solution containing 1 g / l of HCI (1 mA / cm), the required anodic current 1a is established, which is then kept constant. The process of anodic etching of the coating is carried out until the potential of the anode sharply changes from the potential of gold to the potential of nickel. The amount of electricity spent in pickling is measured by Ch IA. The amount of electricity Q1 spent on measuring the surface is added to it. The latter can be found either by integrating the l (t) curve with potentiometric recording of the anodic limiting (peak) current, or by the simplified formula Qi ln.tn/2, the graphical justification of which is shown in Fig.1. Here, n is the peak value of the limiting anode current, tn is the time required for sweeping anodic polarization from zero to the peak potential at the selected fixed potential sweep rate V of 0.1 V / s. Know the total amount of electricity.

spent on the dissolution of the coating, and taking into account the size of the surface, formula (1) determines the thickness of the gold layer.

For example, p2.

Determination of the thickness of a silver coating on a copper base. The electrolyte used was a solution containing 20 g / l of H2SCM + 10 g / l of thioule resin, giving strong complexes with silver ions. As can be seen from FIG. 2, in this case, clear anodic peaks are also obtained, and at constant concentrations of thiourea and potential sweep rates, the peak current is proportional to the immersion depth L, i.e. can be used to measure the surface. Further operations in determining the thickness of the coating are carried out in the same manner as in Example 1.

Thus, the proposed method significantly expands the class of controlled parts:

firstly, due to the possibility of controlling the thickness of coatings on miniature parts of a regular shape if there are poorly conducting oxide layers on their surface (after thermal cycling, long-term storage, etc.);

secondly, it opens up the possibility of controlling the thickness of coatings on small parts of irregular shape that are inaccessible to known methods.

Claims (1)

Claims A method for coulometric measurement of the thickness of metal coatings of objects with a small cross-sectional area, which means that an arbitrary area of an object is immersed in a cell with an electrolyte solution, the surface area of the submerged section is determined and anodic dissolution of the submerged area is simultaneously performed, the amount of electricity From the beginning to the end of dissolution and taking into account the measured value and the value of the surface area of the immersed area, the thickness of the met In order to expand the field of application, the complexing agent for the metal ions of the coating is introduced into the electrolyte, the surface area of the submerged area is determined by the measured limiting value of the anodic dissolution current by the complexing agent, and when measuring the amount of electricity it is taken into account, spent on determining surface area. ro - s o t 80 - O l - 91 - o z with. 1: -, - - h: -.- t-; s T .. .ЈGCWT-.A; Mr. G iccvij3G4:  Jf v 1 .- I - 1-1- t -J - J  t 7Ј.: c-ir: -; - - - X „/ TF -: T-w / (Ј - ioggesh: e li since g -th-- ed , - J W8E9AI  - № And 60 -0 30 -0.10, B FIG. 3 In, 60 20 0 2 8 L, MV