Today's electronics industry revolves around the high-speed communication and supercomputing of the Internet. The requirements for electronic products are getting higher and higher. A large number of micro-precision electronic products are constantly testing the response speed and R & D level of major enterprises in the market. In general, this is undoubtedly an industrial upgrade of the electronics industry.
Due to the changing market situation, both the foundry and its outstanding OEM manufacturers or the R & D equipment development companies that are more in line with the development of the times have challenged the new process and new processes. Traditional OEMs The factory needs a new process with higher quality and more efficiency to solve the problems faced by the enterprise. R & D enterprises need to respond quickly to new market demands to maintain their own technological leadership. The development of the technology industry has also brought more opportunities to companies with strength and readiness.
Friends who are familiar with Songde Technology know that Songde Technology focuses on the new laser soldering process research and is committed to the research and development of electronic products laser non-contact welding products for many years. He has spared no effort in scientific research and published many technical articles. Part of the scientific research results have been announced from time to time. Newcomers can check some articles published by Sundell Lab through the Internet.
There are not many gossips. Today, the theme of the Sundell laboratory is: the influence of laser optical shapes on laser soldering of electronic products.
We all know that the laser pumped light is transmitted through a fiber coupling and transmitted to a plane, which is generally circular. Most optical combination lenses shape the laser to the required size for some applications, such as marking , Cutting, welding and other fields.
For laser soldering, many times also use the optical form of the department to solder electronic products, but there are different laser application forms than those mentioned above.
Single-point laser soldering is a better choice for irregular solder joints, because the arrangement of irregular solder joints mainly depends on the mobility of the equipment.
Regarding the regularly arranged solder joints, although single-spot soldering is also a good choice, if the output and quality requirements are relatively high, if there is an array of light, it will undoubtedly double the efficiency.
For example, when the solder joints of the product appear in pairs, some changes are made to the optical system combination lens. The light emitted from the optical fiber is shaped by the columns of the lens, and a beam of light is divided into two separate beams. The fine adjustment system adjusts the distance between the lenses to adjust the distance between the two light spots.
Establishment of initial conditions
At the beginning of optical design, the initial conditions are established and the initial structure of the optical lens is designed. The material of the lens group is selected from high-quality optical glass, the material is anti-reflection treatment, the light transmittance is 99%, the refractive index = 1.51630, dispersion = 0.00806 Abbe number = 64.06 . Laser source is 940 semiconductor laser
The foundation of the lens 3D model is the design of the optical surface in the lens. There are generally two types of partial differential equations and trial and error. Partial differential equations are based on Snell's law and conservation of energy to establish a polynomial to solve the surface shape of reflection and refraction. The coordinates of discrete points are calculated by programming and then imported into the software to form. The trial and error method is to directly create the corresponding surface in the software and then pass The simulation ray traces the result, and modifies the surface according to the deviation between the result and the actual requirements. This article uses the second method to build the model directly in Solid Works software.
Zemax simulation can be used to obtain a 2d planar illumination distribution of one of the light spots as shown below.
The optical morphology is analyzed in the laboratory under the two beam spot analyzers produced:
Actually, there is a slight difference between the obtained light spot and the theoretical light spot, but such a difference is within the range allowed by the experimental design, so we define this optical form as meeting the experimental requirements.
Select a two-pin wire harness for dual-spot welding, so that the two laser beams are irradiated onto two pads, and the solder covers two solder joints. After a series of parameter settings, observe the effect of laser soldering, as shown in Figure 1, Figure 2, Figure 3, you can see the state of the molten tin:
Because the final spot of the two beam spots is not at the center of the red cross, there is no need to worry about the pcb board at the center of the cross being burned. In addition, the machine experimented by Songde Technology has a temperature control system. The protection of the board is more comprehensive, so we can directly observe the welding effect of the two welding points by this center of gravity. From the above experiments, it can be seen that the dual-spot design is more efficient for regularly occurring solder joints than single-spot soldering.
In order to fully understand the optical shape distribution of a single light spot, the optical edge response spectrum of the FFT LINE / EDGE SPREAD in the theoretical calculation is retrieved, as shown in the figure below.
By analogy, through the optical design, the above method can also be used to design an optical system with 3-4 light spots. If you encounter more array solder joints, you can use the grating array light welding method to pass a beam of light through a special grating optical element. The device is divided into several light spots with equal spacing. For the method of simultaneous welding of multiple solder joints, the principle is shown below:
Special grating devices can be combined to form a multi-light spot array welding, as shown in the ABCDEF ... focus point above.
Design and application of rectangular light spot
Through the above two-spot design method, a rectangular spot design can also be performed.
A micro-array structure is established on the light-emitting surface of the lens. As can be seen from the above dual-dot diagram, the light output from a single lens group is a circular light spot. In order to change the shape of the light spot, a rectangular array structure must be added to the surface to obtain a rectangular light spot. Compressed; different micro-structures can be obtained here to obtain different shapes of initial light spots.
In fact, the initial spot obtained can not meet our experimental requirements, nor can it meet the actual production requirements. Therefore, it is necessary to combine other lenses for optical shaping multiple times, as close as possible to the ideal optical form we need.
After several experiments, a rectangular light spot (ideal light spot) that meets the requirements can be basically obtained. Figure 5.
At this time, the rectangular light spot shape will have a more uniform heating and filling effect on the rectangular solder joint. For example, if a square gold-plated pad is heated with a circular light spot, the periphery of the pad is easy to fall off, because the unevenness of heat may cause the gold-plated pad to fall off entirely.
The rectangular light spot can be uniformly heated around the rectangle by adjusting the length and width of the rectangle, and the temperature control is more handy.
As shown in Figure 7, the welding of slender large rectangular pads and wire harness products. If circular light spots are used, the wires and pads cannot be uniformly heated, resulting in high local temperatures. Either the product is burned or the tin is not completely melted and cannot be covered. The entire pad. The optical form used in Figure 6 is a rectangular light spot form. By adjusting the rectangular aspect ratio, the optical form that satisfies the soldering can be obtained. As can be seen in the screenshot of the soldering process, the entire pad is heated at the same time as the end of the solder bar. Therefore, there is no azimuth warping caused by the deformation and distortion of the tin bar. The entire process of tin melting is completed in a short time, and the entire pad is smoothly covered. The crystallization process of the solder joint is very full and round.
Use a round light spot, as shown in Figure 8. Soldering with the same parameters slows the melting of the tin. The molten tin does not flow to a lower temperature because the pad heat is uneven, and then the tin wire is warped directly. Product analysis Yellowing phenomenon, welding quality NG.
There is also an optical form of the rectangular spot. The slenderness ratio is higher, almost a linear stripe spot. Generally, the higher the slenderness ratio, the more complicated the lens system is required. When arranged in a long row, both circular and rectangular spots are less efficient. In this case, you need to use an in-line light spot.
The fractal light spot design is a partial differential equation that is based on Snell's law and conservation of energy to establish a polynomial to solve the surface profile of reflection and refraction. The coordinates of discrete points are calculated by programming, and then imported into the software for forming calculations.
The light emitted by the laser beam described above is transmitted to the end surface of a certain plane through the optical fiber, which is generally circular. At this time, although the laser beam appears circular on this plane, its own beam is in a divergent state. In order to obtain a slender light spot, we first need to uniformly shape the emission state of the light spot. A combination of multiple lens groups and non-conventional lenses is used to obtain an elongated light spot shape as shown in Figure 8:
For the slender light spot, you can observe the distribution of the zemax dot map to determine the density of the falling points of the light. Increasing the density of the light can roughly see that the light beam distribution of the light spot in the shape of the shape is relatively average, as shown below:
The line / edge spread of the strip light collected by software is shown below
The edge light collection curve shows relatively even light distribution density at the edge, so it can be judged that this optical form also meets the expected design.
When laser welding soft board dense wiring, if one point welding is used to remove tin one by one, the efficiency will undoubtedly be very low, and the solder resist strips of dense solder joints are very narrow, and even a little accident will lead to tin and burn out. . The contact processing HOTBAR can complete this kind of welding at one time, but HOTbar also faces the problems of tin, burns, and blistering. Because the hot source of the HOTbar is a thermal head, the temperature of the thermal head is relatively high, cold plates conducted to normal temperature are prone to warp, white bubbles, tin slag and other uncontrollable factors, and the hotbar is prone to tin oxidation and other characteristics. This also limits this process. Important factor. The slender light spot uses the principle of hot pressing Hotbar to shape the laser shape into an slender one. Because the laser can directly heat the pad without the need for heat conduction from the heating medium (thermal head), the temperature step change can be used by the upper position. The machine directly sets the entire process of preheating and welding. The product in the figure below is taken as an example to test how a dozen light spots are welded in one go.
An automatic solder paste soldering operation is performed on the product by using the integrated testing platform of Song Deer Technology's spot laser laser soldering system. (Full automatic spot soldering equipment)
The display result under CCD imaging after the automatic tinning is completed is shown below
Because it is an experimental environment, we did not let the machine directly perform the automatic cycle, and returned the machine to the coaxial CCD of the laser welding head to observe the point tin situation. After confirming that the point tin consistency was good, we performed laser welding. The parameters do some column settings.
Taking into account the shape of the slender light spot, the distance between the solder joints is 0.1mm, and the welding point is too small, it is necessary to increase the camera magnification for the human eye to observe. When the magnification is changed, the conditions for the slender light temperature are also set After making a series of restrictions, after confirming that they are correct, soldering is started.
First observe the temperature curve of the laser heating, as shown below:
The temperature of the laser heating pad during the heating stage, pre-heating stage and soldering stage is tightly controlled within the set range without any abnormality, so we exclude the various effects caused by the unstable temperature, and then pay attention to The change in the concentration of force into the product welding is shown below:
Judging from the effect of the experimental finished product, it is undoubtedly a relatively successful experiment. The slender light spot can not be more suitable for welding of this kind of product.
In fact, it can complete the intensive solder joint welding in a short time in a short time, thanks to the excellent optical design ability and excellent laser control ability of Songde Technology. We know that lasers generally belong to destructive processing, such as three major applications: marking, cutting, welding, all use the high energy and high density of laser to impact the product itself; and the use of lasers for micro-precision welding of electronic products is Never use this destructive processing method to complete. The application of laser in the soldering industry is mainly completed by the principle of light and heat. The short heating time of the laser and the characteristics of non-contact heating require strong research and development capabilities. Optical design and structural design, laser control, electrical control, The host computer control is indispensable. It is precisely because the Songde team keeps close to the pain points of the market, anxious customers, and has the customer's needs as their own responsibility. It keeps forging ahead and continuously investing in R & D and design. Accumulate a lot of good reputation here. This young team of Songde Technology will keep abreast of the development of the industry and continue to develop a new generation of soldering products, which will contribute a little force to the laser industry and the industrial upgrade of the electronics industry.