Although, in smt production, we call patch glue, solder paste, and stencil auxiliary materials, but their importance cannot be ignored. Among them, stencil is the first link of the whole process, and its quality directly affects printing quality. According to statistics, in the SMT process, more than 60% of SMT defects caused by printing, of which only 35% are caused by poor templates, and 60% of assembly defects and 87% of reflow soldering defects are also caused by poor templates . Therefore, the template plays a vital role in the quality and production efficiency of SMT. Choosing a high-quality template can improve the quality of the SMT process.

The factors that affect the quality of the steel mesh are mainly reflected in three aspects. The first is the quality of the material, that is, the quality, hardness and elasticity of the steel sheet itself. Secondly, the design of the formwork includes the selection of the thickness of the steel sheet, the opening size and shape of the hole. Among them, the thickness and opening size determine the amount and accuracy of solder paste coating, which is a very important part of the entire production process, and the shape of the opening has an impact on the quality of tin applied. The third is the manufacturing method of the template, including dimensional accuracy, trimming straightness, roughness and shape of the opening wall. Dimensional accuracy is the basic requirement for use, and the roughness and shape of the opening wall determine the quality of tin application. Among them, the first two factors are the most important factors affecting the quality of the formwork due to the variability in the selection of the steel sheet and the completion of the design. However, the manufacturing method of the formwork is variable.

At present, there are three methods for manufacturing templates, namely chemical etching, laser cutting, and electroforming. The three methods have their own advantages and disadvantages. Through the comparison of the production process and the quality of the template, usually, the laser template is used, which has the following advantages,

Good quality: non-contact processing, no stress and no deformation, even tension distribution after stretching.

By adjusting the laser focus position, the opening is automatically formed into a tapered shape, which is conducive to the demolding of solder paste.

The cut edges are smooth, comparable to electroformed templates.

Fast speed: less production process, easy operation, fast production speed and short delivery date.

Low cost: less process, so less consumables, high template reuse rate, up to more than 300,000 times.

Realize machine automation control, easy operation and save human resources.

High precision: use design documents directly, without photography steps, and eliminate the factors of misalignment.

The hole has small dimensional accuracy and high positional accuracy, which is very suitable for high-density design.

Strong functions: the only process that realizes the rework of existing templates, such as hole increase, hole repair, and hole expansion.

No pollution: There is no chemical liquid in the production process, no pollution to the environment, and no harm to the health of operators.

Based on the importance of the template to the SMT process and the advantages of the laser template, we will make an in-depth and detailed discussion on the cutting quality of the SMT laser template, which is positive and important for the improvement of the process and the overcoming of some problems in practical applications. meaning.

1. SMT laser steel mesh cutting

After the laser is focused and irradiated on the material, the light energy is converted into heat energy, which makes the temperature of the material to be cut rise rapidly, and then melts or vaporizes it. At the same time, the air flow coaxial with the beam is ejected from the nozzle, blowing the melted or vaporized material away from the bottom of the incision. With the relative movement of the laser and the material to be cut, a kerf is formed on the material to achieve the purpose of cutting. If the blown gas reacts exothermicly with the material being cut, this reaction will provide the additional heat source required for cutting. The air flow also has the effect of cooling the cut surface, reducing the heat-affected zone and keeping the focusing lens free from contamination.

From the perspective of cutting precision, laser cutting can be roughly divided into high-power cutting and precision cutting. Laser precision cutting is mainly used in precision machinery and electronics industries. The focus of the application is thin plates less than 0.5mm, generally with complex structures, and the size is less than 200μm. SMT laser template is one of its typical applications. The commonly used SMT template is made of stainless steel, and the auxiliary gas is usually industrial oxygen or compressed air.

2. Cutting quality analysis

The analysis of cutting quality should start with the cutting process and equipment, that is, the laser cutting machine. For a long time, laser precision cutting has been monopolized by foreign countries, and the country relies on imports. Until 2006, Shenzhen Musen Technology Co., Ltd. developed the first domestic machine with independent The high-precision laser template cutting machine with intellectual property rights broke the international monopoly and passed the national verification. The performance has reached the same international level and has been put into production and sales. It has epoch-making significance in the laser cutting and SMT industries. Musen’s StencilCut series laser stencil cutting machine is taken as an example to analyze the quality of SMT laser stencil cutting.

Laser cutting machines can be roughly divided into three parts: laser, mechanical electronic control and software. The following will discuss their influence on cutting quality from these three aspects in turn.

2.1 Influence of laser parameters on cutting quality

In cutting, the “knife” is the most critical link. Therefore, the laser parameters are the key factors in the cutting process, including spot diameter, laser power, repetition frequency, focus position, etc., which will be analyzed one by one below.

1) Influence of spot diameter

The precision of laser cutting has a lot to do with the beam mode and spot diameter after focusing. In the cutting, the laser adopts the laser in the fundamental mode mode, and the light intensity of the fundamental mode beam in any section is distributed according to the Gaussian function, which is called the Gaussian beam. The range determined by the point where the light intensity falls to 1/e2 of the central value is the spot radius, and 86.5% of the beam energy is contained within this circle. The diameter of the spot has the greatest influence on the width of the slit. The smaller the diameter of the spot, the smaller the slit, and the higher the cutting accuracy.

Two important optical mirror groups in the optical path are the beam expander and the focus mirror. The purpose of the beam expander is to reduce the divergence angle of the laser and obtain a beam as close to parallel light as possible. The focusing mirror is used to reduce the spot size, increase the energy density of the beam, and improve the precision and efficiency of processing. Assume that the beam waist radius of the laser is ω0, the beam quality factor is M2, the laser wavelength is λ, the focal length of the focusing mirror is f, and the collimation magnification of the beam expander is A. According to the laser principle, when the beam waist of the object Gaussian beam is within the lens When the object space focal plane is on, the image space Gaussian beam waist is also on the image space focal plane.

The wavelength λ and beam quality M2 of the laser are determined by the laser, and a smaller spot size can be obtained by reducing the laser wavelength and selecting a high-quality laser, that is, a smaller M2. In terms of the mirror group, the focal length of the focusing lens can be reduced and the expansion lens can be increased. The beam mirror multiple is used to achieve a smaller spot size, but while reducing the spot diameter, the depth of focus will be shortened, and the thickness of the plate that can be cut will also be reduced. Therefore, a reasonable focal length and beam expander should be selected according to the actual situation.

The relative position of the focal point and the steel sheet has a great influence on the width of the kerf and the shape of the trimming edge. After focusing, the focal point is generally located on the surface of the steel sheet. In this way, a certain taper will automatically appear on the trimming edge during cutting, which is conducive to solder paste demoulding.

In addition, there will be strong reflection when the laser is focused on the steel sheet, and the reflected light will return to the laser along the original optical path. When the reflected light reaches a certain intensity, the laser cannot work stably or even damage the laser. Therefore, the reflected light must be suppressed. Adding an optical isolator at the exit of the laser can solve this problem.

2) Effect of laser power and laser repetition rate

Energy E is the product of power P and time t. When the cutting speed is constant, that is, the laser irradiation time is constant. As the laser output power increases, the energy obtained by the material per unit time increases, and the temperature of the material increases, resulting in a heat-affected zone Widen, the deformation increases, and the width of the kerf also increases.

When the laser power is constant, the longer the irradiation time, the more energy the steel sheet will get, and the heat will be conducted to the non-processing area, and the heat capacity of the steel sheet itself is small, so the temperature of the steel sheet will rise rapidly and cause thermal deformation . Therefore, the difference between laser precision cutting and traditional high-power cutting is the pulse working mode. The advantage of pulse is that the energy required for metal melting is brought in in a very short time, the overall heating of the part is low, and there will be no overheating and undesired melting during continuous laser processing.

The laser works in pulse mode, which uses high energy density to instantly melt and gasify materials, and punch a series of continuous holes on the steel sheet to obtain continuous slits to realize continuous cutting of the steel sheet. In this process, the weight of adjacent laser spots, that is, the weight ratio of the spot, is a key parameter, which refers to the percentage of the adjacent spot weight area to the spot area, which can be obtained from a simple geometric relationship (in the cutting process, the The spot deformation on the steel sheet is small and can be considered as a circle), which is related to the laser repetition frequency, pulse width and cutting speed. It has a great influence on the smoothness of the cut edge and the width of the slit. The higher the weight ratio, the smoother the cut edge, the better the quality.

When other parameters remain unchanged, the higher the repetition frequency, the shorter the time for the single pulse to interact with the material, the smaller the thermal effect, and the smaller the slit width. At the same time, the higher the repetition frequency, the higher the repetition rate of the spot and the better the trimming effect. Therefore, increasing the repetition rate of the laser can improve the cutting quality. Early foreign template cutting machines used YAG lasers, while StencilCut used fiber lasers. The main reason is that fiber lasers have many advantages. First, the cutting quality is high: the laser repetition rate is high, so the cutting edge continuity is good, and the cutting side wall Smooth; second, low cost of use: no need to replace lamp tubes, deionized water and filter elements, which can save the cost of consumables; third, good product performance: low power can save electricity costs and long service life; fourth, it is convenient to use : Lightweight, easy to assemble, simple optical path correction.

2.2 Influence of cutting speed

The cutting speed determines the production efficiency. On the premise of ensuring the cutting quality, it is a problem that cannot be ignored for the development of modern enterprises to increase the productivity as much as possible and reduce the processing cost.

When other parameters remain unchanged, the change of cutting speed means the time change of the interaction between laser and material, that is, the change of laser energy density. The faster the cutting speed, the smaller the laser energy density. When the cutting speed is low, the laser energy density is too high, so that the material around the slit is also melted or gasified, resulting in more slag and rough slits, and poor cutting quality. As the speed increases, when the laser energy density reaches a suitable range, the material will be completely melted or gasified, and the material will be removed under the action of the auxiliary gas to form a smooth and uniform kerf; the speed increases to At a limit value, the energy obtained by the material is not enough to completely melt or gasify, and the material cannot be completely cut; in addition, when the repetition frequency is constant, the cutting speed increases to a certain level and the slit will change from a straight state to a Discontinuous small holes, therefore, there is a critical speed, and when it is greater than this critical value, cutting becomes punching.

2.3 Effect of auxiliary gas

The use of auxiliary gas in laser cutting is to remove the molten material in the incision, so that the cutting process can continue smoothly, and at the same time, protect the lens from damage. Provides extra energy to accelerate cutting.

1) Influence of gas type

When cutting iron and its alloys, O2 is usually used as an auxiliary gas. Iron and oxygen can undergo violent oxidation reactions to provide additional heat for cutting. Therefore, compared with inert gases or nitrogen, the use of oxygen can effectively increase the cutting speed. SMT stencils are usually made of stainless steel sheets, and oxygen cutting can achieve very good results.

StencilCut series cutting machines use industrial oxygen as the auxiliary gas, which is fully responsive, while foreign equipment uses compressed air, only one-fifth of which is oxygen. In contrast, the advantages of using industrial oxygen are: less gas consumption, smaller air pressure, lower laser power and faster response speed; under the same laser power, industrial oxygen can reach a maximum The cutting depth; in the case of the same plate thickness, oxygen cutting can obtain a higher cutting speed.

2) Influence of gas pressure, nozzle structure and nozzle position

The basic requirement of laser cutting for auxiliary gas is that the gas flow into the incision is large and the speed is high, so that there is enough momentum to eject the molten material, and there is enough gas to have a sufficient exothermic reaction with the material. Gas pressure and gas flow are important parameters, the higher the oxygen pressure and the higher the flow rate, the faster the combustion chemistry and material removal will be. At the same time, the reaction product at the outlet of the slit can also be rapidly cooled. In nearby non-cutting areas, the gas acts as a coolant, reducing the heat-affected zone. However, the pressure of the gas is not as high as possible. When the gas pressure is too low, the molten material at the incision cannot be completely removed, which will form burrs and reduce the cutting speed; as the gas pressure increases, the gas flow increases and the slag discharge capacity improves. , a smoother edge can be obtained; however, when the pressure is too high, not only the gas consumption will be increased, but also the air flow will be disturbed, and a vortex will be formed on the surface of the workpiece, which will reduce the slag removal effect and increase the slit width slightly. . Therefore, only by choosing an appropriate gas pressure can a more ideal cutting quality be obtained.

During the laser cutting process, the laser beam must pass through the nozzle to generate a gas flow field. The shape of the nozzle and the position of the nozzle have an important influence on the flow rate of the gas and the distribution of the flow field. The refractive index of the gas is related to the density. When the air pressure is too high, a shock wave will be generated in the flow field. The density of the gas at the shock wave will change suddenly, and the laser will be refracted between the air flow interfaces of different layers, resulting in a focal point. Changes that affect cutting speed and trimming quality.

2.4 Influence of Mechanism and Electronic Control Part

In addition to a good “knife”, high-precision laser cutting also needs a high-precision platform. The dimensional accuracy of laser cutting mainly depends on the mechanical accuracy and control accuracy of the cutting equipment. When using pulsed laser and using high-precision cutting equipment and control technology, the dimensional accuracy can reach micron level. For a long time, China has been unable to master the technology of precise positioning, and laser precision cutting has been monopolized by foreign countries.

The stability of the machine and the mechanical precision of the platform are the basic requirements to ensure the accurate opening position and opening size of the formwork. There are strict requirements when designing and installing the mechanism part of StencilCut. The control part adopts an infinite loop control system, that is, the position The accuracy of the servo system for detection and feedback mainly depends on the accuracy of the measurement components and the precision of the digital-to-analog converter. The minimum scale of the measurement component, that is, the optical ruler, is 0.5 μm. After assembly, it undergoes laser interference correction and uses a laser interferometer to measure the platform movement. After the distance is compared with the actual given distance, the movement error is compensated by the motion controller so that the movement distance reaches the set target. In this way, the positioning accuracy of the whole machine can reach 3 μm, and the repeatability can reach ±1 μm.

2.5 Influence of software part

In addition to the four basic requirements of high stability, complete functions, easy operation and friendly interface, the operating software should also have the ability to deal with some special problems in laser cutting, so as to make up for the defects and deficiencies in the process of design and file transfer .

1) Graphics conversion file

During the printing process, the 90° corner part of the SMT laser stencil will retain solder paste during use, which will affect the subsequent use and cleaning of the stencil, and will also cause insufficient solder and sharpening of the soldering effect. , collapse etc. For this problem, when designing the opening of the SMT template, it will be improved if the right angle is replaced with a circular arc lead angle. However, due to the difference in software and the uneven operation level of the operator, during the process of converting the CNC file, The arc chamfer part will become an irregular curve composed of short straight lines or short arcs, which greatly reduces the production efficiency and quality of SMT templates. Therefore, companies are working hard to find this problem solutions, but the previous solutions are more cumbersome and not perfect. After years of hard work, Musen Technology has independently developedThe StencilCut software solves this problem, and the implementation is simple, the operation is convenient, and the effect is good. When converting files, the arcs in the source files can be transferred perfectly.

2) Path optimization

Due to differences in design software and drawing personnel, the cutting path of the exported drawing file will show a lot of randomness during cutting, which will undoubtedly increase the cutting time and reduce production efficiency. Therefore, the path must be optimized before cutting. Currently , there are many types of path optimization algorithms, and there are many researchers, but the final effect depends on two aspects: one is the optimization rate of the path; the other is the length of the optimization time. The package software of StencilCut integrates the function of path optimization, with fast calculation speed and high path optimization rate, which can reach more than 60%, that is to say, at the same cutting speed, only 40% of the original time is needed.

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