JP6143097B2 - Manufacturing method of electronic parts - Google Patents

Description

  The present invention relates to a method for manufacturing an electronic component, and more particularly to a method for manufacturing an electronic component including a step of applying a resin to a substrate.

  For example, when manufacturing an electronic component in which a component mounted on a substrate is coated with a resin, as shown in a schematic diagram of FIG. 7, a dispenser nozzle is mounted on a mounting surface 101f of a collective substrate 101 on which the component 21 is mounted. The resin 7 discharged from 51 is applied (see, for example, Patent Document 1).

JP 2004-014871 A

  While the resin is applied to a plurality of aggregate substrates, the amount of resin discharged may change due to changes in the viscosity of the resin contained in the syringe part of the dispenser, and the amount of resin applied to the aggregate substrate (application amount) may vary. is there. In that case, for example, the height of the resin mold part changes and the module height varies. When grinding the surface of the mold resin to eliminate module height variation, if the resin height differs due to variation in the amount of resin applied, even if the resin surface is ground in the same grinding time, There arises a problem that the resin height varies.

  In view of such circumstances, the present invention intends to provide a method for manufacturing an electronic component capable of suppressing variations in the amount of resin applied.

  In order to solve the above problems, the present invention provides a method of manufacturing an electronic component configured as follows.

The method for manufacturing an electronic component includes a first step of preparing a substrate and a second step of applying a resin to the substrate. The second step includes (a) applying the resin to the substrate on at least one of the substrates, and applying a predetermined number of predetermined times of two or more times , and (b) the last step A measurement step of measuring the weight of the resin applied in the immediately preceding application step, which is the application step immediately before the application step, and (c) the weight of the resin measured in the measurement step, and the immediately preceding application step. Based on the time when the resin is applied to the substrate, the immediately preceding application speed, which is the weight per unit time of the resin applied to the substrate in the immediately preceding application step, is calculated, Based on the target application amount that is the weight of the resin to be applied to the substrate and the final application amount that is the difference between the weight of the resin applied to the substrate immediately before the last application step, In the coating step And a final application time calculation step of calculating a final application time that is a time for applying the resin to (d) the final application time calculated in the final application time calculation step in the final application step. The resin is applied to the substrate.

  According to the above method, even if the weight per unit time of the resin applied to the substrate changes with the passage of time, the weight of the resin applied to the substrate after the last application step matches the target application amount. Thus, the application amount of the resin applied in the last application step can be adjusted by the calculated final application time. Therefore, variation in the amount of resin applied can be suppressed.

  Preferably, in the measurement step, the substrate is placed on a weighing scale before and after the immediately preceding application step, and the weight of the resin applied to the substrate in the immediately preceding application step is measured. In the applying step, the resin is applied to the substrate while the substrate is lifted and the substrate is separated from the weighing scale.

  In this case, the substrate is lifted when the resin is applied, and the substrate is lowered when the weight is measured. When measuring the weight, the coated resin applied to the substrate is in a state of being surely separated from the nozzle or the like for applying the resin, and the weight of the coated resin can be accurately measured. Variation in the amount can be suppressed with high accuracy.

  In addition, the resin can be applied and the weight can be measured by raising and lowering the substrate. Since it is only necessary to raise and lower the substrate, the moving distance of the substrate can be shortened, and the process time can be shortened by measuring the weight and applying the resin in one apparatus.

  Preferably, in the coating step, the substrate is placed on a stage equipped with a heating device, and the resin is applied to the substrate in a state where the substrate is heated using the heating device.

  In this case, since the resin can be applied while heating the substrate to a constant temperature, it is possible to suppress a change in the viscosity of the applied resin between a plurality of application steps, and to further suppress variations in the resin application amount.

  Preferably, in the first step, the board having components mounted on a plurality of parts divided into individual pieces is prepared. In the second step, the resin is applied to the substrate so that the component is covered with the resin.

  In this case, for example, a mold resin can be applied to a collective substrate for forming a module (composite electronic component), and variations in the amount of the mold resin applied can be suppressed. Therefore, variation in module height can be reduced, and grinding variation can be reduced when the surface of the mold resin is ground.

  Preferably, the second step includes the coating step twice for one substrate.

  In this case, it is possible to suppress variations in the amount of resin applied while simplifying the process.

  Preferably, the second step includes at least three application steps for one substrate, and (a) the last application step is performed on the one substrate. A current measurement step of measuring the weight of the resin applied in the current application step, which is the application step prior to the immediately preceding application step, and (b) the weight of the resin measured in the current measurement step; Based on the time when the resin was applied to the substrate in the current application step, the current application weight, which is the weight per unit time of the resin applied to the substrate in the current application step, was calculated, and the calculated Based on the current application weight and the weight of the resin to be applied to the substrate in the next application process, which is the application process subsequent to the current application process, the time for applying the resin to the substrate in the next application process. Next time painting Anda next coating time calculating step of calculating a time, in (c) the next coating step, during the next coating the next coating time calculated by the time calculation step, applying the resin to the substrate.

  In this case, by calculating the application time and increasing the number of times of application to a single substrate, it is possible to increase the adjustment accuracy of the resin application amount and further suppress variations in the resin application amount. . In particular, when the amount of resin applied to a single substrate is large, or when the change in the coating weight per unit time of the resin applied to the substrate is large, the effect of suppressing variation in the amount of resin applied is high.

  Preferably, the second step includes (a) a plurality of times of the applying step, the measuring step, and the final applying time calculating step with respect to the first substrate, and the last applying step. In the step, the resin is applied to the substrate during the final application time calculated in the final application time calculation step, and (b) the second and subsequent sheets up to a predetermined number of the substrates, Applying the resin for the same time as the application step of the first substrate, corresponding to the application step of the first substrate, and applying the resin to the first substrate; The second and subsequent coating steps of the same number are included.

  In this case, for the second and subsequent substrates up to a predetermined number of substrates, calculation of the application time is omitted, and the resin is applied under the same conditions as the first substrate, thereby suppressing variations in the amount of resin applied. However, the resin can be efficiently applied to the substrate. In particular, the effect is high when the application weight per unit time of the resin applied to the substrate has a small change with time.

  According to the present invention, it is possible to suppress variations in the amount of resin applied.

It is explanatory drawing of the manufacturing method of an electronic component. (Example 1) It is sectional drawing of an electronic component. (Example 2) It is explanatory drawing of the manufacturing method of an electronic component. Example 3 It is explanatory drawing of the manufacturing method of an electronic component. Example 3 (A) It is explanatory drawing of the manufacturing method of an electronic component, (b) It is a top view of a board | substrate. Example 4 It is a flowchart of the manufacturing method of an electronic component. (Example 5) It is the schematic which shows the manufacturing method of an electronic component. (Conventional example 1)

  Embodiments of the present invention will be described below with reference to FIGS.

  <Example 1> The manufacturing method of the electronic component of Example 1 is demonstrated, referring FIG.

  The manufacturing method of the electronic component of Example 1 includes a first step of preparing a substrate, a second step of applying an uncured resin to the substrate, and a third step of curing the applied resin. .

  FIG. 1 is an explanatory view schematically showing a method of manufacturing an electronic component. As shown in FIG. 1, in the second step of applying a resin to the substrate, a liquid resin 20 discharged from the nozzle 4 of the dispenser 2 is applied to the substrate 10. By measuring the weight of the substrate 10 before and after the application of the resin 20 with the electronic balance 30, the weight of the resin 20 applied to the substrate 10 can be determined.

  In the dispenser 2, the resin is stored in the syringe unit 6, and the resin stored in the syringe unit 6 is discharged from the nozzle 4 by applying pressure to the piston (not shown) of the syringe unit 6 using an air cylinder or the like. Discharge.

  The viscosity of the resin stored in the syringe unit 6 changes due to a temperature change or the like. When the viscosity of the resin stored in the syringe unit 6 changes, the amount of the resin 20 discharged from the nozzle 4 per unit time changes. Therefore, if the time for applying the resin 20 to the single substrate 10 is the same, while the resin 20 is being applied to the plurality of substrates 10, the resin 20 for each substrate 10 is increased over time. Application amount varies.

  However, the viscosity change of the resin 20 discharged from the nozzle 4 is gentle. Therefore, while the resin 20 is applied to one substrate 10, the amount of the resin 20 discharged from the nozzle 4 per unit time, that is, the application amount per unit time is the same or substantially the same. Moreover, the time (application time) for applying the resin 20 by pressurizing the piston of the syringe part is proportional to the weight (application amount) of the applied resin 20. Therefore, the coating process is divided into two times for one substrate, and if the coating amount per unit time and the remaining coating amount are known after the first coating process, the optimum in the second coating process. By calculating the appropriate application time and applying the resin 20 with the calculated application time, it is possible to perform accurate weight control application with respect to the target application weight.

  The detailed procedure is as follows.

(1) Weight measurement before application First, the weight (A (g)) of the substrate before applying the resin is measured.

(2) First application step Next, pressure is applied to the piston of the syringe part of the dispenser, and the resin discharged from the nozzle of the dispenser is applied to the substrate. At this time, the amount of resin applied is set to be smaller than the amount of resin (target amount) to be applied to one aggregate substrate. For example, it should be about half of the target amount.

  The time for performing the first application (first application time) is defined as the time during which pressure is applied to the piston of the syringe part of the dispenser, for example. First application time: T1 (seconds) may be a predetermined time or an actually measured time. That is, when the predetermined time is reached, the pressure applied to the piston of the syringe unit may be stopped or the time during which the pressure is applied to the piston of the syringe unit may be measured.

(3) Weight measurement after application Next, the weight B1 (g) of the substrate on which the resin is applied by the first application is measured. Then, the weight of the resin applied by the first application (first application amount): P1 (g) is calculated from the difference in the weight of the substrate before and after the first application process: B1-A. That is, P1 is calculated by the following equation (1).
P1 = B1-A (1)

  When measuring the weight using an electronic balance, in the weight measurement before the first coating step, the display of the measurement value of the electronic balance is reset with the collective substrate placed on the weighing pan (measured value). The first application amount: P1 (g) may be obtained from the measurement value displayed in the weight measurement after the first application process.

(4) Calculation of the first coating speed Next, from the first coating amount: P1 (g) and the first coating time: T1 (seconds), the following formula (2) is used to calculate the first coating speed: The weight per unit time of the resin applied in the application step (first application speed): R1 (g / second) is calculated.
R1 = P1 / T1
= (B1-A) / T1 (2)

(5) Calculation of Second Application Time Next, the target application amount Me (g), which is the weight of the resin to be applied in the second application process, that is, the weight of the resin applied to one substrate, and 1 From the difference between the first coating amount: P1 (g) and the first coating speed: R1 (g / second), the second coating time: T2 (second) is calculated using the following equation (3). calculate.
T2 = (Me−P1) / R1 (3)

(6) Second application step Next, during the calculated second application time: T2 (seconds), pressure is applied to the piston of the syringe part, and the resin discharged from the nozzle of the syringe part is applied to the substrate.

  When the resin is applied to the substrate by the above procedures (1) to (6), the weight of the resin applied to the substrate after the second application process can be brought close to the target application weight: Me with high accuracy.

  For example, even if the amount of resin discharged from the nozzle varies due to a characteristic change (viscosity change) of the resin itself discharged from the nozzle of the syringe part or a change in the discharge state of the dispenser (change in discharge pressure, etc.) Since the second coating time: T2 is appropriately corrected according to the first coating speed: R1, the coating amount accuracy can be maintained.

  That is, even if the weight per unit time of the resin applied to the substrate changes with the passage of time, the weight of the resin applied to the substrate after the second application step matches the target application amount. The amount of resin applied in the second application step can be adjusted by the second application time. Thereby, the dispersion | variation in the application quantity of resin can be suppressed.

  The coating process for one substrate can be performed three times or more. However, if the coating process is performed twice as in the first embodiment, variation in the amount of resin applied can be suppressed while simplifying the process. .

  In general, even if the weight per unit time of the resin applied to the substrate changes with time, the unit time of the resin applied to the single substrate in the application step immediately before the last application step The weight per unit time (immediately before application speed) and the weight per unit time of the resin applied in the last application step are the same or substantially the same.

  Therefore, assuming that the weight per unit time of the resin applied in the last application process is the same as the previous application speed, the difference between the target application amount and the weight of the resin applied after the previous application process. Based on the weight of the resin to be applied in the final application process (final application weight) obtained from the above, an appropriate application time (final application time) in the final application process is calculated. In the last application step, if the resin is applied to the substrate for the calculated final application time, is the weight of the resin applied to the substrate in the last application step equal to the weight of the resin to be applied in the last application step? It becomes almost the same. As a result, the weight of the resin applied to the substrate after the final application process matches or substantially matches the target application amount.

  In other words, even if the weight per unit time of the resin applied to the substrate changes with the passage of time, the final weight is such that the weight of the resin applied to the substrate after the last application step matches the target application amount. The application amount of the resin applied in the application step can be adjusted by the calculated final application time. Therefore, variation in the amount of resin applied can be suppressed.

<Modification 1> When the coating process for one substrate is performed in three steps, the time for applying the resin in the third coating process: T3 (seconds) is a unit in which the resin is applied in the second coating process. Amount per time: R2 (g / sec), target amount of resin applied to the substrate: Me (g), weight of resin applied to the substrate in the first application step: P1 (g), 2 From the weight of the resin applied to the substrate after the second application step: P2 (g), calculation is performed using the following equation (4).
T3 = (Me−P1−P2) / R2 (4)

  By increasing the number of times of application by calculating the application time on a single substrate, it is possible to increase the adjustment accuracy of the resin application amount and further suppress variations in the resin application amount. In particular, when the amount of resin applied to a single substrate is large, or when the change in the coating weight per unit time of the resin applied to the substrate is large, the effect of suppressing variation in the amount of resin applied is high.

Note that the application time of the second application process: T2 (seconds) may be determined as appropriate. For example, the following is performed so that the weight of the resin after the second application process becomes 2/3 of the target application amount. (5) is used for calculation.
T2 = {(2/3) Me-P1} / R1 (5)

  Example 2 In Example 2, resin 20 is applied so as to cover component 12 as shown in the cross-sectional view of FIG.

  First, as a first step, a frame-shaped dam that surrounds the component 12 by applying a highly viscous dam agent to the substrate 10 (collective substrate) on which the component 12 is mounted on a plurality of parts divided into individual pieces. 22 is formed. The dam agent may be a resin, or a member previously molded into a ring shape may be bonded onto the substrate.

  Next, as a second step, an uncured resin 20 is applied to the substrate 10 prepared in the first step. The resin 20 is applied to the inside of the dam 22.

  The second step is the same as in Example 1, and the step of applying the resin 20 is divided into two times, and the optimal second application time is calculated based on the first application result to obtain 2 A second coating process is performed.

  Next, as a third step, the resin 20 is cured. For example, when the applied resin 20 is a thermosetting resin, it is heated above the curing start temperature, and when it is an ultraviolet curable resin, it is irradiated with ultraviolet rays.

  Next, as a fourth step, the cured resin is ground as necessary, and then the substrate 10 is divided into individual pieces. For example, the substrate 10 is cut together with the cured resin 20 using a dicing apparatus, and is divided into individual pieces of modules (composite parts).

  In the second embodiment, as in the first embodiment, it is possible to suppress variation in the amount of resin applied. Therefore, variation in module height can be reduced, and grinding variation can be reduced when the surface of the mold resin is ground.

  Example 3 FIGS. 3 and 4 are explanatory diagrams of Example 3. FIG. As shown in FIGS. 3 and 4, in the third embodiment, the steps of the first and second embodiments are performed by using an automatic machine including a stage 44 on which the substrate 10 is placed and a lifting mechanism (not shown) that lifts and lowers the stage. To do.

  On the weighing pan 32 of the electronic balance 30, a jig 40 on which a plurality of support columns 42 are erected is placed. The stage 44 is formed with a through hole 44p through which the support column 42 of the jig 40 is inserted, and moves up and down without contacting the support column 42.

  When the stage 44 is raised as indicated by an arrow 46 in FIG. 3, the support column 42 is retracted from the upper surface 44 a of the stage 44, and the substrate 10 is supported by the upper surface 44 a of the stage 44. In this state, the resin 20 is applied to the substrate 10.

  As shown by an arrow 48 in FIG. 4, when the stage 44 is lowered, the support column 42 protrudes from the upper surface 44 a of the stage 44, and the substrate 10 is supported by the support column 42. In this state, the weight of the substrate 10 and the jig 40 is measured with the electronic balance 30.

  The substrate 10 is lifted when the resin 20 is applied, and the substrate 10 is lowered when the weight is measured. When measuring the weight, the applied resin 20x applied to the substrate 10 is separated from the nozzle 4 for applying the resin 20, and the weight of the applied resin 20x can be accurately measured. Can be suppressed with high accuracy.

  Also, by moving the substrate 10 up and down by the stage 44, it is possible to apply resin and measure the weight. Since it is only necessary to raise and lower the substrate 10, the movement distance of the substrate can be shortened, and the process time can be shortened by measuring the weight and applying the resin in one apparatus.

  A heater may be embedded in the stage 44. In this case, since the resin 20 can be applied while heating the substrate 10 to a constant temperature, it is possible to suppress a change in viscosity of the applied resin 20x between a plurality of application processes, and to further suppress variations in the amount of resin applied. it can.

  That is, when the viscosity of the coated resin changes between coating processes, the resistance state of the coated resin with respect to the resin discharged from the nozzle changes, and the amount of resin discharged from the nozzle changes. Changes in the amount of resin discharged from the nozzles contribute to variations in the amount of resin applied. If the resin 20 is applied while a heater is embedded in the stage 44 and the substrate 10 is heated to a certain temperature, the change in the viscosity of the applied resin 20x can be suppressed between the application processes. Can be further suppressed.

  In addition, the resin 20 may be applied to the upper surface 44a of the stage 44 in a state where the substrate 10 is adsorbed and fixed by vacuum suction.

  <Example 4> In Example 4, an application | coating process is performed using an automatic machine and a robot.

  Fig.5 (a) is a perspective view which shows an application | coating process typically. FIG. 5B is a plan view of the substrate 10 on which the resin 20 is applied. As shown in FIGS. 5 (a) and 5 (b), the automatic machine 50 moves along the guide portions 52 and 54 in two different directions (preferably two directions perpendicular to each other) indicated by arrows 52x and 54y. 56 moves. By attaching the dispenser 2 to the moving part 56 and discharging the resin 20 from the nozzle 4 of the dispenser 2, the moving part 56 is moved according to a predetermined route, thereby, as indicated by arrows 58 and 59 on the substrate 10, as shown in FIG. Resin 20 is drawn and applied.

  Instead of the automatic machine 50, the dispenser 2 may be attached to a robot capable of three-dimensional movement, and the resin 20 may be drawn and applied on the substrate 10.

  When the automatic machine 50 is used in the coating process of the first embodiment, in the first coating process, the nozzle 4 moves along a preset route at a preset moving speed, and performs drawing coating. In the second application step, the resin is applied for the calculated second application time: T2. At this time, with the calculated second application time: T2, the movement speed of the moving unit 56, that is, the drawing speed is controlled so as to complete the movement of the predetermined path, that is, the drawing, and the resin 20 is applied. To do.

  The same control is performed when the number of coatings is increased and the third coating is applied at time T3 as in Modification 1. That is, the moving speed of the moving unit 56, that is, the drawing speed is controlled so that the movement of the predetermined path of the third application, that is, the drawing is completed at time T3.

  In addition, you may change a drawing pattern, ie, the movement path | route of the moving part 56, with the frequency | count of an application | coating process.

  In Example 4, the weight of the resin applied to one substrate can be made constant while keeping the drawing pattern constant. Since the resin can be uniformly applied in a predetermined pattern, it is possible to suppress variation in the amount of resin applied to each piece, particularly when the pieces are divided from the aggregate substrate.

  <Example 5> The manufacturing method of the electronic component of Example 5 is demonstrated, referring FIG.

  If the change over time in the weight per unit time (application speed) of the resin applied to the substrate is small, the second and subsequent substrates may be applied even if the resin is applied under the same conditions as the first substrate. Variation in the amount of coating can be suppressed. Therefore, in Example 5, the same process as in Example 1 is performed for the first substrate, but for the second and subsequent aggregate substrates, weight measurement before and after coating other than the coating process and the second coating time are performed. Calculation is omitted.

  That is, as shown in the flowchart of FIG. 6, for the first substrate (# 10, Y at # 20), after measuring the weight of the substrate before coating: A (# 22), the coating time: T1 The first application is performed (# 24). Next, the weight of the substrate after application: B1 was measured (# 28), and the second application time: T2 was calculated (# 28) using the above-described formulas (1) to (3). Application time: Second application is performed at T2 (# 30).

  For the second substrate (N at # 32 and # 20), the same first application time as the first substrate: the first application is performed at T1 (# 50), and then the first substrate Second application time: The second application is performed at T2 (# 52).

  Similarly, when the number of substrates reaches a predetermined number: N (N in # 54 and # 26), coating is performed under the same conditions as the first substrate (# 50 to # 56).

  When the number of substrates reaches a predetermined number: N (Y in # 26), the process returns to the beginning of the above series of procedures (# 10), and thereafter the same is repeated.

  As described above, by applying the resin under the same condition as that of the first substrate for the second and subsequent substrates up to a predetermined number of sheets: N, variation in the amount of resin applied is suppressed. The resin can be efficiently applied to the substrate. In particular, the effect is high when the application weight per unit time of the resin applied to the substrate has a small change with time.

  <Summary> The application process for applying the resin to a single substrate is performed in a plurality of times, and at least the last application process is performed with an optimal application time, thereby suppressing variations in the amount of resin applied. Can do.

  The present invention is not limited to the above embodiment, and can be implemented with various modifications.

DESCRIPTION OF SYMBOLS 2 Dispenser 4 Nozzle 6 Syringe part 10 Board | substrate 12 Parts 20, 20x Resin 22 Dam 30 Electronic balance 32 Weighing pan 40 Jig 42 Support pillar 44 Stage 44a Upper surface 44p Through-hole 50 Automatic machine 52, 54 Guide part 56 Moving part

Claims (7)

A first step of preparing a substrate;
A second step of applying a resin to the substrate;
With
In the second step, for at least one of the substrates,
Applying the resin to the substrate, a predetermined number of application steps of two or more predetermined times ;
A measuring step for measuring the weight of the resin applied in the immediately preceding application step, which is the application step immediately before the last application step;
Based on the weight of the resin measured in the measurement step and the time during which the resin is applied to the substrate in the immediately preceding application step, per unit time of the resin applied to the substrate in the immediately preceding application step Is applied to the substrate immediately before the last application step and the target application amount that is the weight of the resin to be applied to the substrate. A final application time calculation step of calculating a final application time, which is a time for applying the resin to the substrate in the final application step, based on a final application amount that is a difference from the weight of the resin being,
Including
In the final application step, the resin is applied to the substrate during the final application time calculated in the final application time calculation step. In the measurement step, the substrate is placed on a weighing scale before and after the immediately preceding application step, and the weight of the resin applied to the substrate in the immediately preceding application step is measured.
2. The method of manufacturing an electronic component according to claim 1, wherein, in the application step, the resin is applied to the substrate in a state where the substrate is lifted and separated from the scale.   2. The application step, wherein the substrate is placed on a stage equipped with a heating device, and the resin is applied to the substrate in a state where the substrate is heated using the heating device. Or the manufacturing method of the electronic component of 2. In the first step, preparing the substrate on which components are mounted in a plurality of parts divided into pieces,
4. The manufacturing of an electronic component according to claim 1, wherein in the second step, the resin is applied to the substrate so that the component is covered with the resin. 5. Method.   4. The method of manufacturing an electronic component according to claim 1, wherein the second step includes two coating steps for one substrate. 5. In the second step, for one substrate,
Including at least three application steps,
Furthermore, for the one substrate,
A current measurement step of measuring the weight of the resin applied in the current application step, which is the application step prior to the application step immediately before the last application step;
Based on the weight of the resin measured in the current measurement step and the time during which the resin is applied to the substrate in the current application step, the unit time of the resin applied to the substrate in the current application step Based on the calculated current application weight and the weight of the resin to be applied to the substrate in the next application process, which is the next application process after the current application process, A next application time calculating step of calculating a next application time which is a time for applying the resin to the substrate in the next application step;
Including
4. The electron according to claim 1, wherein in the next application step, the resin is applied to the substrate during the next application time calculated in the next application time calculation step. 5. A manufacturing method for parts. The second step includes
For the first substrate,
A plurality of the application steps;
The measuring step;
The final application time calculating step;
Including
In the last application step, during the final application time calculated in the final application time calculation step, the resin is applied to the substrate,
From the second board onwards, up to a predetermined number of boards,
The application step of the first substrate corresponding to the application step of the first substrate and applying the resin for the same time as the application step of the first substrate. The manufacturing process of the electronic component according to any one of claims 1 to 6, further comprising a coating process of the second and subsequent sheets the same number of times.

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