In the process flow of SMT, one of the important steps is to print the solder paste on the PCB pads accurately, with accurate opening position and opening size, accurate opening taper size, smooth sidewall, no burr, material Uniform thickness, no stress, uniform distribution of template tension, etc.
With the development of SMT towards fine-pitch components, the pitch of SMD package pins is getting smaller and smaller, and the trend of shrinking package size poses a serious challenge to solder paste printing. This has higher quality requirements for the precision of SMT printing templates. In the following, we will analyze and discuss the factors affecting the cutting quality of SMT templates from various aspects, promote and improve the quality of laser templates, so that this process technology can be fully applied, so that the quality and production efficiency of SMT can be more improved.
Brief description of SMT process flow
SMT is Surface Mount Technology (Surface Mount Technology), which is a new assembly technology developed relative to the traditional THT through-hole mounting technology. Due to the different types of assembly processes, the specific SMT process flow is also different. At present, the SMT process flow is usually carried out in the following steps:
Production Preparation→Stencil Making→Screen Printing Solder Paste/Dispensing→Mounting→Reflow Soldering→Inspection Test→Rework/Packaging
Among them, screen printing is a process of printing solder on a substrate using a stencil (stencil). In the SMT process, it is to print the solder paste on the connection pads of the circuit board through the SMT stencil, which is the first and most important part of SMT assembly. necessary process
The role of templates
A type of flat die that is required to apply solder paste prior to screen printing/dispensing, namely SMT or SMD solder paste stencils. SMT laser stencil technology is the key first step in the SMT manufacturing process. The application of this technology produces accurate screen printing solder paste stencils, so that solder paste leakage can be accurately realized
The principle of laser cutting process
Laser processing technology uses the energy released when a laser beam hits the surface of a steel plate to melt and vaporize stainless steel
SMT laser cutting template machine generally consists of three parts: laser head, mobile positioning system and software. Using raw electronic data, the device is directly driven by a computer, and the laser beam is concentrated in a small area through lenses and mirrors. The high concentration of energy enables rapid localized heating to vaporize the stainless steel. The processed sheet stainless steel material is stretched on the fixture of the worktable, and the mobile positioning system drives the worktable or the laser head, so that the material to be cut moves at a high speed under the cutting head. The laser head is composed of a light source part and a cutting head. The light source part generates a focused laser beam with a very short wavelength. The laser beam passes through the cutting head and is vertically focused on the surface of the material to be cut, heating, melting and evaporating the material to be cut to form a slit. Closed kerfs form pad openings
Because the laser's energy is so concentrated, only a small amount of heat is transferred to the rest of the steel, resulting in little or no deformation. We can use the laser to accurately cut complex-shaped blanks without further processing. When cutting stainless steel, because the focus is on the bottom of the steel, it can produce an inverted trapezoidal opening that is consistent with SMT solder paste leakage
Advantages of Laser Stencils
At present, there are three manufacturing methods for templates, namely chemical etching, laser cutting, and electroforming. The three methods have their own advantages and disadvantages. By comparing the production process and template quality, the currently used laser template has the following advantages:
low cost
The laser cutting process is few, the consumables are few, and the template reuse rate is high, and its usage rate can reach more than 300,000 times. Laser cutting can realize automatic control of the machine, easy to operate and save human resources
high speed
Laser cutting is easy to operate, fast in production and short in delivery time
Good quality
Laser cutting adopts non-contact processing, no stress and no deformation, and the tension is evenly distributed after stretching the net. By adjusting the laser focus position, the opening is automatically formed into a cone, which is conducive to the release of the solder paste. Smooth cut edges, comparable to electroforming templates
High precision
Laser cutting uses design documents directly, with no photography steps, eliminating misplacement factors. High accuracy of hole placement after laser cutting, ideal for high-density designs
no pollution
There is no chemical liquid in the production process of laser cutting, no pollution to the environment, and no harm to the health of operators
Quality Control of Laser Templates
Cut Quality Analysis
According to statistics, in the SMT process, SMT defects caused by printing exceed 60%, of which 35% are caused by poor stencils. In addition, 60% of assembly defects and 87% of reflow soldering defects are also caused by poor stencils. . Therefore, the template plays a crucial role in the quality and production efficiency of SMT, and high-quality templates can significantly improve the quality of the SMT process
Main Factors Affecting Template Quality
template design
The design of the formwork includes the choice of the thickness of the steel plate, the size of the opening of the hole and the shape of the opening. The thickness and opening size determine the coating amount and accuracy of the solder paste, which is a very important part of the entire production process. The shape of the opening has an impact on the quality of the tin.
quality of material
The material quality factor is relatively stable. Generally, imported 304 stainless steel is selected, and its hardness, elasticity and other indicators can meet the requirements. This is a relatively stable factor
Template making
The production of the template includes the dimensional accuracy, the straightness of the cutting edge, the roughness and shape of the opening hole wall. Dimensional accuracy is the basic requirement for use, and the roughness and shape of the opening hole wall determine the quality of tinning
Parameters of the laser
Laser cutting machine can be roughly divided into three parts: laser, mechanism electronic control and software. In cutting, "knife" is the most critical link. Therefore, laser parameters are the key factors in the cutting process, including spot diameter, laser Power, repetition frequency, focus position, etc., the above parameters must be debugged and verified many times to find the best control point of each parameter, and finally get the most satisfactory opening quality
Laser power and repetition rate
The energy E is the product of the power P and the 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 material temperature increases, resulting in the heat affected zone. Wider, the deformation increases, and the slit width also increases
The laser works in a pulsed mode. The principle is to use high energy density to melt and vaporize the material in an instant, punch a series of continuous holes on the steel sheet to obtain continuous slits, and realize continuous cutting of the steel sheet. In this process, the degree of repetition of adjacent laser spots, that is, the repetition accuracy of the spot, is a key parameter, which refers to the percentage of the heavy area of the adjacent laser spots in the spot area, which can be obtained from a simple geometric relationship (during the cutting process, the The deformation of the light spot on the steel sheet is small and can be considered to be still circular), which is related to the laser repetition frequency, pulse width and cutting speed. It has a great influence on the smoothness of the cutting edge and the slit width. The higher the repeatability, the smoother the cutting edge and the better the quality.
cutting speed
Cutting speed determines production efficiency. On the premise of ensuring cutting quality, it is a problem that cannot be ignored for the development of modern enterprises to improve productivity and reduce processing costs as much as possible.
When other parameters remain unchanged, the change of cutting speed means the time change of the interaction between the laser and the material, that is, the change of the 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 large, so that the material around the slit is also melted or vaporized, resulting in more slag and rough slits and poor cutting quality. With the increase of speed, when reaching a suitable range, the laser energy density is large enough, the material will be completely melted or vaporized, and the material will be removed under the action of auxiliary gas, and a smooth and uniform slit can be formed; the speed increases to At a limit value, the energy obtained by the material is not enough to make it completely melt or vaporize, and the material cannot be completely cut; in addition, when the repetition frequency is constant, the cutting speed increases to a certain extent, so that the cutting seam will change from a straight state to a flat state. Discontinuous small holes, therefore, there is a critical speed, above this critical value, cutting will become punching
Design of opening size
The software part is used for data reception, aperture design, processing and control and driving of the laser head and movement system. Among them, the part of opening design and processing has the ability to deal with some special problems in laser cutting, and makes up for the defects and deficiencies in the design and transfer process. For example: a circuit board has 0402chip, 0.5 mm pich QFP IC, and large pads larger than 0805chip, even with a side length of more than 2.0 mm, and the board thickness is fixed, in order to optimize the amount of solder paste on all pads , it is necessary to find a balance point in the opening size
auxiliary gas
The auxiliary gas used in laser cutting is to remove the molten material in the incision, so that the cutting process can be carried out smoothly and continuously, and at the same time, protect the lens from damage. Provides extra energy to speed up cutting
gas pressure
The basic requirements for auxiliary gas in laser cutting are 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, the higher the flow rate, the faster the combustion chemical reaction and material removal
At the same time, the reaction product at the outlet of the slit can also be rapidly cooled. In the nearby non-cut area, the gas acts as a coolant, shrinking the heat-affected zone. However, the higher the gas pressure is, the better. When the gas pressure is too low, the molten material at the incision will not be exhausted, which will cause burrs and reduce the cutting speed. , a smoother cutting edge can be obtained; but when the pressure is too high, it will not only increase the gas consumption, but also make the air flow turbulent, form eddy currents on the surface of the workpiece, reduce the slag removal effect, and the slit width will also slightly increase . Therefore, only by selecting the appropriate gas pressure can the ideal cutting quality be obtained.
Extended application of laser stencil
In addition to making the entire template of silk-screen solder paste, the laser can also make multiple small templates, that is, to generate multiple PCB silk-screen paste small templates on a single steel sheet material, which saves manpower, material resources, and is convenient for storage, etc. advantage
At the same time, when the product needs to be repaired, a "rework mini-template" can be specially designed to rework or refurbish individual components. For example, a template for a certain component can be made, and the required template size can be determined according to its position on the printed board for subsequent maintenance work.
