SU791475A1 - Apparatus for monitoring solderability at low-temperature soldering - Google Patents

Description

This invention relates to the field of soldering the leads of electronic devices to printed circuit boards. It can also be used when soldering other types of products in the instrument and mechanical engineering. A device is known for controlling the capacity during low-temperature soldering 1. It contains a bath with solder, a mechanism for immersing a sample in solder, a clamp with a sample fixed at the end of the barrel, the other end of which has the ability to pivotally rotate around a horizontal axis. In the case of an ECM, the sample is introduced into the bath with solder tangentially to its surface. A disadvantage of this device is the imperfection of the operation of controlling the capacity, which is carried out visually by calculating the relative area of areas not wetted by solder and comparing with the standard. This prevents the wetting time and a number of other quantitative characteristics of the power to be controlled. In addition, the device is not very productive; The results are controlled by the subjective factor, and the results are not documented by the device (they are not recorded in the form of graphical or digital information). All this ultimately leads to a decrease in the quality of soldering, since it complicates the optimum conditions of the solder. The arrangement is known (for controlling the capacitance 2, which contains a bath with solder, a sample immersion mechanism, a bath with a flux. The drive ensures that the sample is alternately immersed in flux and solder with intermediate drying of the flux. Immersion of the sample is vertical. The disadvantage of this device is the same This is a visual estimation of the capacity, which is carried out with a tenfold increase in the number of unsightly-ohl sections, which makes it difficult to choose opt: the general conditions and the quality of the n-ayik are reduced. methods of processing the results of measuring the percentage of unwetted area. This requires the accumulation of an appropriate number of measurement results, which reduces the performance and control efficiency. The closest to the technical essence and the achieved result is a device for monitoring the capacity (The menisograph contains a solder bath with a vertical back-up mechanism translational movement, suspension with clip for sample crease and force transducer, electrically connected with recording device 3. xMenisk af allows register ka diagram tape recorder and then measure the important characteristics. The drawback of the device is the limited amount of control information obtained, since it does not allow controlling such parameters as adhesion work, cohesion work and work spent on maintenance that are important for assessing the capacity. It also does not allow to estimate the difference in wetting angles when the sample is immersed in the solder (the contact angle) and removed from the solder (flow angle). This difference also characterizes the capacitance. These disadvantages make it difficult to choose the most appropriate materials (solder fluxes, coatings) and optimal soldering regimes, which leads to a decrease in the quality of soldering. The aim of the invention is to increase the number of parameters for estimating occupancy by controlling the work of adhesion and cohesion and the work expended on maintenance. The goal is achieved by the fact that the reciprocating movement mechanism is equipped with a bath movement sensor, and the self-recording device is made in the form of a two-piece recording recorder potentiometer, one input of which is connected to the movement sensor and the other with a force sensor. Figure 1 shows the block diagram of the device; figure 2 - constructive implementation of the main mechanisms; Fig. 3 shows typical diagrams obtained by controlling the capacity of the proposed device (a) and the known type of Menisograph (b); 4, examples of the definition of the work of adhesion, cohesion and maintenance. The device consists of a solder bath with a heating table 1, those of the skating unit 2, a mechanism 3 for vertical reciprocating movement of the bath, suspensions with a clip for securing sample 4, a force sensor 5, a bath movement sensor 6, a two-coordinate writing potentiometer 7, a replaceable bath 8 with solder (see FIG. 2) installed on the heating table 9 with the possibility of vertical reciprocating movement in two directions: an adjusting 10, with which the table is connected; Paruy 11 screw – nut c from by the calculating vernier, and along the guides 12 of the bath stroke. The guides 12 are mounted on the device body 13. The guides 10 are connected by means of a hinge 14 to a reciprocating mechanism, which includes a device for converting rotary motion into reciprocating in the form of a crank mechanism 15 (or a screw-nut pair not shown in FIG. 2). rotation of the screw) gearbox 16 with an electric motor 17 direct current, mounted on the housing 13 of the device. The sensor b of the bath movement is arranged as follows. Moving guides 10 are connected via a rack-toothed or friction gear 18 — kinematically connected to the slider axis of the variable resistor 19. This resistor is included in a bridge circuit with a similar resistor 20. The bridge circuit is powered from a low-voltage stabilized DC source, and the useful signal from the bridge diagonal arrives at one of the inputs of the two-coordinate recording recorder potentiometer 7 of the PDS-02i type. Sample 21 is suspended above the bath 8 using the clamp 22 on the free end of the auxiliary measuring beam 23 , Which is part of force sensor 5 (see figure 1). In the middle part of the beam 23, one end is fixedly fixed to a thin steel filament 24, the other end of which is fixed at the end of the measuring tube of a mechatronic converter (mechatronic) 25. A cciM mechatronic is a BMXCS type electron tube with mechanically controlled SMM anodes. It is mounted in the housing 26, mounted on a cantilever plate 27, which at its other end is attached to the housing 13 of the device. Above the housing 26 of the mechanotryn 25, also a bracket 28 with pressure adjustment is attached to the base of the housing 13. screw 29 interacting with the mechanotron housing 26. The attachment point of the filament 24 to the auxiliary measuring beam 23 divides the latter into two arms — 30 and 31. The shoulder 30 is rigid; the shoulder 31 is a flat spring with a rigidity less than that of the mechanotron 25 (by rigidity is meant the force per unit deflection at a given point , in particular in the place of attachment of the thread). The spring arm 31 engages the console in the split liner 32 with a screw 33. It is possible to change the position of the insert when adjusting the length of the arm 31 by moving it left or right in the sleeve 34, which

is attached to the device body 13. The measuring beam arm 30 is provided with adjustable stops 35, which limit the movement of the end of the beam 12 beyond the permissible limits under the effect of random forces.

The device works as follows.

The bath 8 with the solder being tested is installed on the heated table 9. After the solder has been melted and exhausted and at a predetermined temperature, which is established and maintained by the unit 2, the bath movement drive motor 17 is switched on and transferred to the lowest position. Wire 21 or wire (flat) leads of transistors of diodes, microcircuits and other electronic devices to be tested for capacitance are used as sample 21. You can also use single leads of these devices, in the form of wire segments, plates. (foils, etc. Sample 21 is fastened to clamp 22 so that the serviceable leads are positioned vertically above the bath and their ends are not touched; solder mirrors. To eliminate such a touch, the bath is manually lowered by screw pair 11 along mounting guides 10. After installing and fixing the sample, the bath is raised again with the help of a screw pair 11 almost until the end of the mirror sample touches the solder mirror and mark this position as the extreme upper point of the bath rise on the vernier screw pair 11. Then turn on the electric motor 17 and transfer well to the lowest position. Then, with a screw pair 11 with a vernier, the bath is lifted in the direction of 10 by an amount corresponding to the first predetermined depth of the sample in the solder. If you now turn on the bath drive motor 17 again, then the extreme upper point of the bath will be higher than in the initial position by the value established on the conius. Obviously, the sample will be immersed in the solder at the same depth. The immersion depth is chosen in the range of 0.5-1.5 mm. Sample Flux Before Dipping Solder

In the process of testing the sample capacity, the bath rises from the lower position to the upper point and then lowers with the help of a displacement drive from the electric motor 17. At the same time, the sample is gradually introduced into the solder by its end to the depth, and then lowering the bath is removed from her. This working movement of the bath is transmitted using a rack and a friction gear. 18 to the axis of the slider of the wire linear resistor 19 of the bridge circuit of the sensor b. Get 1 S1 electric to ignite output

The bridge circuit of the sensor is proportional to the height of the bath or the depth (h) of the sample immersion. This signal is fed to the input of a horizontal scan of the two-coordinate self-recording potentiometer; as a result, during one reciprocating movement of the bath, the feather of the recording device performs horizontal reciprocating movement. If speed

If the lift and the amplitude of the bath move are constant, then obviously the pen moves at a constant speed on the screen, and a horizontal time scale of the bath cannot be applied to the horizontal scale of the sweep.

The weight of the specimen with the clamp in the initial state, when the end of the specimen does not touch the solder mirror, is balanced by the deflection of the spring arm

0 31 of the measuring beam 23. This deflection is transmitted by means of a steel thread 24 to the measuring rod of the mechanotron 25. The deviation of the rod of the mechanotron from the horizontal position is converted into an electrical signal directly proportional to the magnitude of this deviation in the sensor 5 (see Fig. 1), which is further arrives at the input of a vertical screwdriver of a two-coordinate self-recording potentiometer 7. Sensor 5 uses a commercially available power supply unit for diode mehronotrons. Next, set5 the zero position of the mechatronic sensor. For this purpose, the screw 29 is rotated upward into the casing of the mechanotron 26. At the same time, by bending the plate 27, the casing of the mechanotron 25 is deflected and its bar is measured from the horizontal position. Thereby, the tension of the string 24: j is adjusted to the zero point of the mecharotron rod to the zero position (middle position). Electro signal at

5 this at the output of the sensor 5 is missing.

Thus, when moving the bath up until the design of the z. Needle touches the solder, the pen of the recorder draws a horizontal line. Starting from the moment when the end face of the specimen of the mirror 36 solders (see FIGS. On the measuring beam 23, the resulting force acts

55 from the oppositely directed buoyant force, proportional to the weight of the solder in the volume displaced by the sample, and the pulling force (in the presence of wetting). Wetting may occur with some delay, therefore at first there will sometimes be a push force. This produces a negative meniscus 37, and the recorder records

jj5 reduction of the resulting effort. acting on the sample 38. In the presence of subsequent wetting, the time taken to restore the resulting force to zero is measured. 39 This time is called the wetting time and is one of the parameters for estimating the density. The shorter the time, the more intense the sample wetting, and the lower the Jjy4me. In the presence of wetting, surface tension forces are applied at point 40 at the interface of the three phases. In accordance with the well-known rule, the equilibrium condition can be expressed by the relation. , The value of a% -f Hbfj-f ° is the name of adhesive tension. The product and the perimeter of the wetting determines the magnitude of the engaging force. Adhesive tension causes the solder to rise near the submerged part of the sample — a positive meniscus. Essentially, the solder weight in the meniscus volume balances the adhesive force. As follows from the above equality, the q value is the larger, the smaller the wetting angle 9. The magnitude and sign of the resultant force, acting on beam 23, depend not only on the wetting angle, but also on the ratio of the volume of the submerged part of the sample to the perimeter of it being wetted. At small values of this ratio, which is typical of thin wires, foil, as well as with sufficiently intensive wetting (for example, gilded leads), a decrease in the resultant force is observed. Because of the smallness of the buoyancy force, it is neglected, and the resultant force is equalized to the pulling adhesive force. The maximum magnitude of the engaging force 41, divided by the wetting perimeter, serves as the second quantity of the convicts as a characteristic of the estimation of capacity, the larger it is, the better the capacity is. When the specimen is immersed, the wetting angle is reduced at the initial stage 42 to the magnitude of the boundary angle of leakage. and then remain constant until the end of the dive. The magnitude of the pulling force, respectively, in the initial part increases, and then remains constant (the most typical case is considered, the curve may have a different appearance). Point 43 corresponds to the maximum immersion depth. The PL under the curve of variation of the pull-in force from the immersion depth h is the work of the adhesion force during the immersion, or the work of adhesion of the solder in the conditions of the immersion (see Fig. 4). The greater the amount of work adhesion released during immersion. the better it is. The theoretical relationship of work with adhesion to paema is known, but none of the known devices for estimating paema is able to determine this value. In the Menisograph, the change in the resultant force acting on the sample is recorded by a single-channel self-recording instrument depending on the time (see Fig.). This device allows only two parameters of the estimation of capacity to be determined: the wetting time and the amount of retracting adhesion force. However, at close values of the magnitude of the drag force, but at different intensities of its growth or with a complex shape of the pull-in curve, comparison and selection of optimal soldering conditions are best carried out by the magnitude of the adhesion work. This improves the accuracy of the estimation of the cost. Data for the evaluation of consumption on the prior device is obtained at the extraction stage of the sample from the solder. First, with the start of extraction of the surface-pg, the wetting angle is reduced, since the marginal angles of flow are always less than the marginal corners of the leakage. This leads to an increase in adhesive tension. (© from the marginal flow), and therefore the inverse of the branch of the recorded curve 44 lies more than the right. Secondly, the nature of the measured adhesive force changes. Since the sample is depleted, when it is removed from the sample, an additional effort is expended on the formation of two new solder interface surfaces with a rounding medium (flux, gas). The specific work expended on this is 2 () and is the work of cohesion (bonding of solder particles) under the conditions of immersion soldering K. This work is determined by the area H) 1 under the return stroke (see Fig. 4). The work of cohesion U (in the presence of maintenance) reflects the magnitude of the surface tension of the solder, in particular, the degree of influence of the flux on it. The more intensively the flux reduces the tension of the solder, the lower the WK. This characteristic is also not measured by known devices. The difference between the work of cohesion and adhesion. WQ is the work spent on maintenance (see Fig. 4). The work of serving W & SA. characterizes the capacity: less than; this rz.bota, i.e. the more work done by the adhesion of Wa. and the less expenditure of work when it comes to gin, the better is the capacity. The resulting diagram is a wet hysteresis loop. It is formed due to the difference in the marginal angles of flow and churn. This difference is quantitatively quantitative} 1o characterize the contingent

the difference between the engaging force on the forward and reverse branches (section 45 in FIG. 4). The smaller this difference, the smaller the wetting angle when immersed and the less it differs from the contact angle when the sample is removed. In the ideal case, the difference becomes equal to zero, and the forward and reverse branches in the considered horizontal section 45 1 coincide with each other.

Thus, the device allows controlling the capacity according to five parameters: wetting time, retracting adhesive force, adhesion work and cohesion and service. Each of the listed parameters refines the estimate of the capacity and complements one friend. The use of the device. Provides the opportunity to more reasonably choose materials for soldering, it is more accurate to set optimal modes and permissible limits of their deviation.

Claims (3)

1.C.J. Thwaites and B.F. Mutter, MetaCu-urgischa aspekte des weichto- 0 tens. Metalt, v. 25, No. 6, 1971, p. 626. 2.M.Schafer Feingeratetechnik,, jg. 23, 3, 1974, p. 115-118. 3.D. Hockay, The Menixograph: 5 a method of so derabi itij measurement. Circuits manufacturing, V. 13-7, 1973, p. 52-56 (prototype).