A temperature control method during the welding process of laser soldering equipment

Abstract

The present invention relates to the field of welding control, and discloses a temperature control method for the welding process of laser soft soldering equipment. Under the premise of an accurate mathematical model, the model is discretized, the discretization formula is listed and Z-transformed to obtain the controlled object of each cycle; in each cycle, the PID controller and the controlled object are connected in series, the closed-loop transfer function is obtained, the pole configuration is performed, and the controller parameters are calculated; the error is calculated for the first heating cycle, and the error is input to the PID controller of the next cycle for operation, and the result value is the laser power; and so on, to ensure that the error each time is within a controllable range. After experiments, the results confirmed that the self-correcting PID control method proposed in this article has significantly improved the control accuracy and adjustment time. Compared with the fixed parameter PD control algorithm, the present invention improves the response speed and control accuracy of the system, and the power control amount is smoother and consumes less energy.

Description

Technical Field

[0001] The present invention relates to the field of welding control, in particular to a temperature control method during the welding process of laser soft soldering equipment. Background Art

[0002] As integrated circuit products develop towards miniaturization, the pin spacing and pad size of surface mount components are getting smaller and smaller.

[0003] As structures become increasingly complex, the solderable areas of surface mount components are becoming increasingly smaller. Searching for solutions, many researchers have discovered that laser soldering technology can effectively address this challenge. The laser soldering process consists of four stages: In the first stage, the laser heats the solder paste from room temperature until it begins to melt; in the second stage, the laser continues to heat the solder paste until it completely melts and becomes liquid tin; in the third stage, the liquid tin is heated to a certain temperature; and in the fourth stage, heating is stopped and the solder is cooled to room temperature, forming the solder joint. Temperature control during these four stages is critical; the actual temperature must be consistent with the target temperature to produce a high-quality solder joint. Therefore, temperature control during the soldering process is crucial, and the error between the actual and target temperatures should be minimized or eliminated.

[0004] Temperature control is a complex technical issue, and there are many different methods for temperature control. The traditional temperature control method is positional PI control, which is based on the system's mathematical model. However, this method is not particularly effective. The variance of the error value during the entire control process is large, and the output power is also quite oscillatory. Using a self-correcting PID controller significantly improves the welding effect, reduces the error, and makes the output power curve smoother, thereby increasing the system's response speed and control accuracy. However, the temperature control method currently used by most machines is still the traditional PI control method, and therefore this method has not yet been widely promoted and used. To this end, we propose a temperature control method for the welding process of laser soft soldering equipment. Summary of the Invention

[0005] (1) Technical problems solved

[0006] To address the shortcomings of existing technologies, the present invention provides a temperature control method for laser soldering equipment during welding. Unlike traditional PID control methods, this method eliminates the need for manual adjustment of PID parameters. Instead, the controller automatically calculates appropriate PID parameters using a pole placement method. Because certain system parameters vary with temperature, a discretization method is employed, recalculating the PID parameters throughout each control cycle. This differs from the fixed parameters of traditional PID controllers.

[0007] (2) Technical solution

[0008] To achieve the above-mentioned purpose, the present invention provides the following technical solution: a temperature control method during the welding process of a laser soldering device, comprising the following steps:

[0009] Step 1: Based on an accurate mathematical model, discretize the model, list the discretization formula and perform Z-transformation to obtain the controlled object for each cycle;

[0010] Step 2: In each cycle, connect the PID controller and the controlled object in series, find the closed-loop transfer function, perform pole configuration, set the characteristic polynomial to the expected characteristic polynomial, and calculate the PID controller parameters;

[0011] Step 3: Calculate the error of the first heating cycle and input the error into the PID controller of the next cycle for operation. The result value is the laser power.

[0012] Step 4: Repeat this process to ensure that the error is within a controllable range.

[0013] Preferably, the PID controller is a self-correcting PID controller, which is designed using a pole placement method based on an existing basic mathematical model and adopts incremental PID control:

[0014]

[0015] Therefore, ΔP(k)=P(k)-P(k-1)=g0e(k)+g1e(k-1)+g2e(k-2);

[0016] Where g0 = k p +k i +k d ,g1=-k p -2k d ,g2=k d

[0017] Perform Z transform and get:

[0018]

[0019] In short:

[0020] Where F = 1-z -1 ,G=g0+g1z -1 +g2z -2 , P(k) is the laser power value calculated in the kth cycle, T r (k) is the target temperature of the kth cycle, and T(k) is the output temperature of the kth cycle.

[0021] Preferably, the pole placement method is as follows: after determining the desired closed-loop characteristic polynomial, since the closed-loop characteristic polynomial depends on the PID controller and the controlled object, and since the controlled object is known, the specific parameter values ​​of the PID controller can be deduced in reverse;

[0022] The accused are:

[0023] A(z -1 )T(k)=z -1 B(z -1 )P(k)+r(k);

[0024] Where A=1-az -1 , B = b, a and b are parameters related to the system, both are known parameters, r(k) is the disturbance value, and the derivation process of the closed-loop transfer function is as follows:

[0025]

[0026] Preferably, the self-correcting PID controller uses a pole placement method to make the closed-loop characteristic polynomial of the entire control system equal to the desired characteristic polynomial, and solve the equation. By solving the equation, the parameters of the PID controller can be obtained;

[0027] The expected characteristic polynomial is:

[0028] Am(z -1 )=1-a1z -1 +a2z -2 +...+a n z -n .

[0029] Preferably, the self-tuning PID controller recalculates the PID parameters every cycle. Since system parameters a and b vary with temperature, and the solder paste temperature fluctuates over time after heating begins, the parameters of the controlled object also change over time. Therefore, compared to conventional PID controllers that use fixed P, I, and D parameters, self-tuning PID is more suitable for systems that vary over time.

[0030] (3) Beneficial effects

[0031] Compared with the prior art, the present invention provides a temperature control method for laser soldering equipment during welding, which has the following beneficial effects:

[0032] 1. The temperature control method during the welding process of the laser soft soldering equipment introduces the idea of ​​adaptive control on the basis of the conventional PID controller, automatically adjusts the controller parameters, and can adapt to changes in the controlled process parameters. Compared with the existing conventional PID control method, this algorithm has smaller errors and better control effects, enabling the laser welding equipment to obtain better control performance.

[0033] 2. The temperature control method during the welding process of the laser soft soldering equipment is smoother than the control signal of conventional PID control. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is the temperature control box flow chart;

[0035] Figure 2 This is a schematic diagram of the laser soldering system principle;

[0036] Figure 3 To find the flow chart of PID controller parameters;

[0037] Figure 4 The temperature curve of solder paste when the system is open-loop and constant power is given during soldering;

[0038] Figure 5 For a given target temperature curve, the system closed-loop control, using conventional PID controller and self-correcting PID controller, the effect comparison chart;

[0039] Figure 6 Adding noise to the sensor transmission signal and using the self-correcting PID control algorithm to achieve the control effect diagram;

[0040] Figure 7 In order to adjust the system parameters within a certain range, the control effect diagram of the self-correcting PID control algorithm is adopted. DETAILED DESCRIPTION

[0041] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0042] See also Figure 1-7A temperature control method for the welding process of a laser soft soldering device includes the following steps: placing the board to be soldered on the pan / tilt platform of the device, observing through the device's camera, finding the pad to be soldered, setting the coordinate value and the amount of solder paste, and then applying solder paste to the pad. Next, the solder paste is heated, using a laser as a heat source to radiate the lead and pad, transferring heat to the substrate through the solder. When the temperature reaches the soldering temperature, the solder wets and spreads to form a solder joint. During heating, since the laser energy output by the device is not completely absorbed by the solder paste and the solder paste composition is relatively complex, closed-loop control must be adopted during heating. The solder paste temperature is measured using an infrared sensor on the device, and the measured value is transmitted to the controller as a feedback signal. The controller can then use a PID control algorithm to calculate the appropriate power value based on the error between the target value and the feedback value. The laser soft soldering device uses the calculated power value to heat the solder paste in order to reduce or eliminate the error between the target temperature and the actual temperature.

[0043] The following briefly describes the derivation process of the mathematical model of laser soldering: According to the principle of conservation of energy, a mathematical model of the welding process can be established. The energy emitted by the laser generator consists of two parts: the energy absorbed by the solder paste and the energy emitted.

[0044] Based on the direction of the absorbed energy, the absorbed energy is also divided into two parts: the energy absorbed by the actual solder paste and converted into internal energy and the energy transferred to the substrate through the solder paste.

[0045] The mathematical model is established as shown in formula (1):

[0046] MC(T-T0)+HA(T-T0)t=Pt*α (1)

[0047] Differentiating both sides of the equation, we get:

[0048]

[0049] Where M is the mass of solder paste, A is the pad area, C is the specific heat capacity of solder paste, H is the heat dissipation coefficient, P is the laser power, T is the current temperature value, T0 is the ambient temperature, and α is the absorption rate of solder paste to laser.

[0050] Since the soldering equipment uses a digital controller and is programmed using software, the model must be discretized and approximated. The discretization result is:

[0051] T(k)=[a k *P(k)+b k *T(k-1)+c k *T0] / d k (3)

[0052] in, T(k-1) and T(k) represent the temperature values ​​of the k-1th cycle and the kth cycle, respectively. T0 is the ambient temperature, and P(k) is the laser power of the kth cycle.

[0053] After discretization, the controlled object within each cycle is different. We only need to ensure temperature control within each cycle. For the first cycle, we can set an appropriate laser power based on the pad size. Starting from the second cycle, we use a self-correcting PID control method to calculate the laser power based on the deviation from the previous cycle, thereby controlling the temperature. This process continues in this manner, ensuring temperature control within each cycle.

[0054] The self-correcting PID control method is implemented as follows:

[0055] Perform z-transform on equation (3):

[0056] A(z -1 )T(k)=z -1 B(z -1 )P(k) (4)

[0057] Where: A(z -1 )=1-az -1 ,B(z -1 )=b,

[0058] The incremental PID is expressed as:

[0059]

[0060] ΔP(k)=P(k)-P(k-1)=g0e(k)+g1e(k-1)+g2e(k-2) (6)

[0061] Where g0 = k p +k i +k d ,g1=-k p -2k d ,g2=k d

[0062] Perform z-transform on equation (6):

[0063]

[0064] Abbreviated as

[0065] Among them, g0, g1, g2 are the parameters to be determined

[0066] By combining equations (4) and (8) into a system of equations and transforming them, the system transfer function can be obtained as follows:

[0067]

[0068] Characteristic polynomial

[0069] A(z -1 )F(z -1 )+z -1 B(z -1 )G(z -1 )(10)

[0070] Contains parameters g0, g1, g2 to be determined

[0071] This system is a second-order system. Considering the adjustment time and overshoot, the controller sets the damping ratio ξ to 0.707, and the natural frequency ω n Take 1, the corresponding closed-loop pole is -0.707±j0.707. Therefore, the expected characteristic polynomial of the second-order system is

[0072] 1-1.3205z -1 +0.4966z -2 (11)

[0073] By making equations (10) and (11) equal and solving the equations, we can obtain the values ​​of g0, g1, and g2. Substituting the parameter values ​​into equation (6), we can calculate the laser power.

[0074] The temperature control method during the welding process of the laser soft soldering equipment of the present invention introduces the idea of ​​adaptive control on the basis of conventional PID. It is not only simple to implement and has small error, but also when the parameters of the controlled object change with temperature, the self-correcting PID controller can recalculate the appropriate PID parameters. It is very suitable for widespread use in the welding process of laser soft soldering equipment.

[0075] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A temperature control method during the welding process of laser soldering equipment, characterized in that: The following steps are involved: Step 1: Based on an accurate mathematical model, discretize the model, list the discretization formula and perform Z-transformation to obtain the controlled object for each cycle; Step 2: In each cycle, connect the PID controller and the controlled object in series, find the closed-loop transfer function, perform pole configuration, set the characteristic polynomial to the expected characteristic polynomial, and calculate the PID controller parameters; Step 3: Calculate the error of the first heating cycle and input the error into the PID controller of the next cycle for operation. The result value is the laser power. Step 4: Repeat this process to ensure that the error is within a controllable range. The PID controller is a self-correcting PID controller, which is designed based on the existing basic mathematical model using the pole placement method and adopts incremental PID control: Therefore ΔP(k)=P(k)-P(k-1)=g0e(k)+g1e(k-1)+g2e(k-2); Where g0 = k p +k i +k d ,g1=-k p -2k d ,g2=k d Perform Z transform and get: In short: Where F = 1-z -1 ,G=g0+g1z -1 +g2z -2 , P(k) is the laser power value calculated in the kth cycle, T r (k) is the target temperature of the kth cycle, and T(k) is the output temperature of the kth cycle; The pole placement method is as follows: after determining the desired closed-loop characteristic polynomial, since the closed-loop characteristic polynomial depends on the PID controller and the controlled object, and since the controlled object is known, the specific parameters of the PID can be deduced in reverse; The accused are: A(z -1 )T(k)=z -1 B(z -1 )P(k); Among them, A(z -1 )=1-az -1 ,B(z -1 )=b, M is the mass of solder paste, A is the pad area, C is the specific heat capacity of solder paste, H is the heat dissipation coefficient, α is the absorptivity of solder paste to laser, and Δt is the time interval between adjacent cycles; The derivation process of the closed-loop transfer function is as follows:

2. The temperature control method during the laser soldering process according to claim 1, characterized in that: The self-correcting PID controller uses the pole configuration method to make the closed-loop characteristic polynomial of the entire control system the expected characteristic polynomial. By solving the equation, the parameters of the PID controller can be obtained. The expected characteristic polynomial is: Am(z -1 )=1-a1z -1 +a2z -2 +...+a n z -n 。

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