Although we refer to SMT adhesive, solder paste, and steel mesh as auxiliary materials in SMT production, their importance cannot be ignored. Template is the first step of the entire process, and its quality directly affects printing quality. According to statistics, in the SMT process, more than 60% of SMT defects are caused by printing, of which 35% are caused only by poor templates. In addition, 60% of assembly defects and 87% of reflow welding defect defects are also caused by poor templates. Therefore, templates play a crucial role in the quality and production efficiency of SMT, and selecting high-quality templates can improve the quality of SMT processes.

The factors that affect the quality of the template are mainly reflected in three aspects. Firstly, the material quality, namely the quality, hardness, and elasticity of the steel sheet itself. Secondly, the design of the template includes the selection of steel sheet thickness, the size and shape of the hole opening. The thickness and opening size determine the amount and accuracy of solder paste application, which is a very important part of the entire production process. The shape of the opening has an impact on the quality of the applied tin. The third is the manufacturing method of the template, including dimensional accuracy, straightness of the cutting edge, roughness and shape of the open hole wall. Dimensional accuracy is a basic requirement for use, and the roughness and shape of the open hole wall determine the quality of tin application. The first two factors have stability after selecting the steel sheet and completing the design, but the manufacturing method of the template has variability, which is the biggest factor affecting the quality of the template.

At present, there are three manufacturing methods for steel mesh, namely chemical etching, laser cutting, and electroforming. Each of the three methods has its advantages and disadvantages. By comparing production processes, template quality, and other aspects, laser templates are usually used, which have the following advantages,

Good quality: non-contact processing, no stress or deformation, and uniform tension distribution after stretching the mesh.

By adjusting the laser focus position, the opening automatically forms a cone, which is beneficial for solder paste demolding.

Smooth cut edges, comparable to electroforming templates.

Fast speed: fewer production processes, simple operation, fast production speed, and short delivery date.

Low cost: fewer processes, therefore less consumables, high template reuse rate, up to 300000 times or more.

Implement machine automation control, easy to operate, and save human resources.

High precision: Directly using design documents without photography steps, eliminating factors of misalignment.

The size accuracy of the hole is small, and the position accuracy is high, making it very suitable for high-density design.

Strong functionality: The only process that enables rework of existing templates, such as adding holes, patching holes, and expanding holes.

No pollution: The production process has no chemical solution, no pollution to the environment, and is harmless to the health of operators.

Based on the importance of templates for SMT processes and the advantages of laser templates, we will conduct a thorough and detailed discussion on the cutting quality of SMT laser templates. This has positive and important significance for improving the process and overcoming some problems in practical applications.

1. SMT laser template cutting

After being focused, the laser irradiates the material, converting light energy into heat energy, causing the temperature of the cut material to rapidly rise, and then melting or vaporizing it. At the same time, the airflow coaxial with the beam of light is sprayed out of the nozzle, blowing the melted or vaporized material away from the bottom of the incision. With the relative motion of the laser and the material being cut, a cutting seam is formed on the cutting material to achieve the purpose of cutting. If the blown gas and the material to be cut produce exothermic reaction, this reaction will provide additional heat source required for cutting. The air flow also has the functions of cooling the cut surface, reducing the Heat-affected zone and ensuring that the focus lens is not polluted.

From the perspective of cutting precision, laser cutting can be roughly divided into high-power cutting and precision cutting. Laser precision cutting is mainly applied in precision machinery and electronic industries, with a focus on thin plates less than 0.5mm, which generally have complex structures and sizes less than 200 μ m. The SMT laser template is one of its typical applications. The commonly used SMT template material is 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, namely laser cutting machines. For a long time, laser precision cutting has been monopolized by foreign countries and relies on imports domestically. It was not until 2006 that Shenzhen Musen Technology Co., Ltd. developed the first high-precision laser template cutting machine with independent intellectual property rights in China, which broke the international monopoly and passed national verification. Its performance has reached the same level as the international market, It has been put into production, use, and external sales, and has epoch-making significance in the laser cutting and SMT industry. Therefore, we take Mumori’s StencilCut series laser template cutting machines as an example to analyze the quality of SMT laser template cutting.

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

2.1 The influence of laser parameters on cutting quality

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

1) The influence of spot diameter

The precision of laser cutting is closely related to the beam mode and the diameter of the focused spot. In cutting, the laser adopts the fundamental mode laser, and the intensity of the fundamental mode beam in any section is distributed according to the Gaussian function, which is called Gaussian beam. The range determined by the point where the light intensity drops to 1/e2 of the center value is the spot radius, which contains 86.5% of the beam energy within this circle. The diameter of the light spot has the greatest impact on the width of the cutting seam. The smaller the diameter of the light spot, the smaller the cutting seam, and the higher the cutting accuracy.

The two important optical mirror groups in the optical path are beam expanders and focusing mirrors. The beam expander is used to reduce the laser divergence angle and obtain a beam as close as possible to parallel light. Focusing mirror is used to reduce the size of the light spot, increase the energy density of the beam, and improve the precision and efficiency of processing. Let the waist radius of the laser be ω 0, beam quality factor M2, laser wavelength λ， The focal length of the focusing mirror is f, and the collimation power of the beam expander is A. According to the laser principle, when the waist of the object Gaussian beam is on the object focal plane of the lens, the waist of the image Gaussian beam is also on the image focal plane.

The wavelength of the laser λ The beam quality M2 is determined by the laser, and a smaller spot size can be obtained by reducing the laser wavelength and selecting a high-quality laser, i.e. a smaller M2. In terms of the mirror group, a smaller spot size can be achieved by reducing the focal length of the focusing lens and increasing the magnification of the beam expander. However, while reducing the spot diameter, the focal depth will be shortened, and the thickness of the plate that can be cut will also be reduced. Therefore, it is necessary to choose a reasonable focal length and beam expander based on the actual situation.

The relative position of the focal point and the steel sheet has a significant impact on the width of the cutting seam and the shape of the cutting edge. After focusing, the focal point is generally located on the surface of the steel sheet, so that a certain taper will automatically appear on the cutting edge during cutting, which is conducive to the demolding of the solder paste.

In addition, when the laser is focused on the steel plate, there will be strong reflections, which will return to the laser along the original optical path. When the reflected light reaches a certain intensity, it can cause the laser to fail to work stably or even damage the laser. Therefore, it is necessary to suppress the reflected light, and adding a light isolator at the laser outlet can solve this problem.

2) The Influence of Laser Power and Laser Repetition Frequency

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. With the increase of laser output power, the energy obtained by the material in unit time increases, and the material temperature increases, resulting in the widening of the Heat-affected zone, the increase of deformation, and the increase of the slit width.

When the laser power is constant, the longer the irradiation time, the more energy the steel sheet obtains, and the heat will be transmitted to the non processing area. Moreover, the steel sheet itself has a small thermal capacity, which will cause a rapid increase in the temperature of the steel sheet and cause thermal deformation. Therefore, the difference between laser precision cutting and traditional high-power cutting lies in the use of pulse working mode. The advantage of pulse is that the energy required for metal melting is brought in in an extremely short time, and the overall heating of the parts is relatively low, avoiding overheating and unwanted melting phenomena during continuous laser processing.

Laser works in a pulsed manner, utilizing high energy density to instantly melt and vaporize materials, and drilling a series of continuous holes on steel sheets to obtain continuous slits, achieving continuous cutting of steel sheets. In this process, the weight of adjacent laser spots, i.e. the weight rate of the spot, is a key parameter. It refers to the percentage of the weight area of adjacent spots in the spot area, which can be obtained from a simple geometric relationship (the deformation of the spot hit on the steel sheet during the cutting process is small and can be considered as still circular). It is related to the laser repetition frequency, pulse width, and cutting speed. It has a significant impact on the smoothness of the cut edge and the width of the cut seam. The higher the weight ratio, the smoother the cut edge and the better the quality.

When other parameters remain unchanged, the higher the repetition frequency, the shorter the time for a single pulse to interact with the material, resulting in a smaller thermal effect and a smaller slit width. The higher the repetition frequency, the higher the repetition rate of the light spot and the better the trimming effect. Therefore, increasing the repetition frequency of the laser can improve the cutting quality. Early foreign template cutting machines all used YAG lasers, while StencilCut used fiber lasers. The main reason is that fiber lasers have many advantages. Firstly, the cutting quality is high: the laser repetition rate is high, so the cutting continuity is good and the cutting side wall is smooth; Secondly, the use cost is low: there is no need to replace the lamp tube, deionized water, and filter element, which can save consumables costs; Thirdly, the product has good performance: low power can save power costs and long service life; Fourthly, it is convenient to use: lightweight, easy to assemble, and easy to calibrate the optical path.

2.2 Impact of cutting speed

Cutting speed determines production efficiency. While ensuring cutting quality, maximizing productivity and reducing processing costs is an issue that cannot be ignored for the development of modern enterprises.

When other parameters remain unchanged, the change in cutting speed implies a temporal change in the interaction between the laser and the material, i.e. a change in 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, causing the material around the cutting seam to be melted or vaporized, resulting in more slag and rough cutting seam, resulting in poor cutting quality. As the speed increases, when reaching a suitable range, the laser energy density is large enough, and the material will completely melt or gasify. Under the action of auxiliary gas, the material can be removed to form a smooth and uniform cutting seam; When the speed increases to a limit value, the energy obtained by the material is not sufficient to completely melt or gasify it, and the material cannot be completely cut; In addition, when the repetition frequency is constant, increasing the cutting speed to a certain extent will cause the cutting seam to change from a straight state to a discontinuous small hole. Therefore, there is a critical speed, and when it exceeds this critical value, cutting will become punching.

2.3 Effects of auxiliary gases

The auxiliary gas used for laser cutting is to eliminate the molten material in the incision, so that the cutting process can be smoothly and continuously carried out, and at the same time, protect the lens from damage. In addition, if the auxiliary gas and the material to be cut have exothermic reaction, it can also provide additional energy for cutting and accelerate the cutting process.

1) The impact of gas types

When cutting iron and its alloys, O2 is usually used as an auxiliary gas. Iron and oxygen can undergo a violent oxidation reaction, providing additional heat for cutting. Therefore, compared to inert gas or nitrogen, the use of oxygen can effectively improve the cutting speed. SMT templates usually use stainless steel sheets, and using oxygen cutting can achieve very good results.

The StencilCut series cutting machines use industrial oxygen as the auxiliary gas, with sufficient reaction. However, foreign equipment uses compressed air, of which only one-fifth is oxygen. In contrast, the advantages of using industrial oxygen include: less gas consumption, lower gas pressure, lower laser power, and faster reaction speed; Under the same laser power, industrial oxygen can achieve a larger cutting depth; Under the same plate thickness, oxygen cutting can achieve higher cutting speed.

2) The 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 sufficient exothermic reaction with the material. Gas pressure and gas flow rate are important parameters. The higher the oxygen pressure, the higher the flow rate, and the faster the combustion chemical reaction and material removal speed. At the same time, it can also quickly cool the reaction products at the slit outlet. In the nearby non cutting area, the gas acts as the coolant to narrow the Heat-affected zone. But the higher the gas pressure, the better. 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 rate increases, the slag removal ability improves, and smoother cutting edges can be obtained; But when the pressure is too high, it not only increases the gas consumption, but also causes turbulence in the airflow, forming eddies on the surface of the workpiece, reducing the slag removal effect, and slightly increasing the width of the cutting seam. Therefore, selecting the appropriate gas pressure can achieve ideal cutting quality.

In the process of laser cutting, the laser beam needs to pass through the nozzle to generate a gas flow field. The shape and position of the nozzle have a significant impact on the gas flow rate and distribution of the flow field. The refractive index of a gas is related to its density. When the pressure is too high, a shock wave will be generated in the flow field. At the shock wave, the density of the gas will undergo a sudden change, and the laser will refract between the airflow interfaces of different layers, resulting in a change in the focus position and affecting the cutting speed and edge quality.

2.4 Impact of Mechanism and Electronic Control

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

The stability of the machine and the mechanical accuracy of the platform are the basic requirements for ensuring the accurate opening position and size of the template. There are strict requirements when designing and installing the mechanism part of StencilCut. The control part adopts a dead cycle control system, which is a servo system equipped with position detection feedback. Its accuracy mainly depends on the accuracy of the measurement component and the accuracy of the digital to analog converter. The minimum scale of the measurement component, namely the optical ruler, is 0.5 μ m. After assembly, after laser interference correction, use the laser interferometer to measure the moving distance of the platform. After comparing with the actual given distance, use the motion controller to compensate the moving error, so that the moving distance can reach the set goal. This can achieve a positioning accuracy of 3% for the entire machine μ m. Repetitive accuracy of ± 1 μ m。

2.5 Impact of Software Part

In addition to the four basic requirements of high stability, complete functionality, easy operation, and user-friendly interface, the operating software should also have the ability to handle some special problems in laser cutting, in order to compensate for the defects and deficiencies in the design and transfer process.

1) Convert graphics to files

During the printing process, the 90 ° corner part of the SMT template may cause retention of solder paste during use, which affects the subsequent use and cleaning of the template. At the same time, it can also cause insufficient solder, pinching, and collapse of the soldering effect. Regarding this issue, when designing the opening of the SMT template, changing the right angle to an arc guide angle will improve it. However, due to differences in software and varying levels of operation by operators, the arc guide angle will become an irregular curve composed of short straight lines or short arcs during the conversion of CNC documents. This greatly reduces the production efficiency and quality of the SMT template. Therefore, Companies are striving to find a solution to this problem, but previous solutions have been cumbersome and incomplete. wood