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Figure 2: The LPKF TechPaper explores the relationships between process and quali-ty parameters for challenging stencil applications.
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Figure 3: Fiducial and text engraving with insufficient (top), proper (middle), and excessive (bottom) power.
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Figure 4: Cutting a stencil with different heights: On the right side of the image, the welded overlays are coloured yellow and the stepped removals red.
Stencils and cutting elements have not been immune from the constantly increasing demand for quality. An application team of LPKF laser specialists has been focusing closely on quality features of solder paste stencils, micro-cutting elements, and filter applications – typical applications for LPKF stencil laser systems. In test runs and a series of measurements, the experts have carefully scrutinized individual quality aspects, summarising the findings in the LPKF TechPaper “Quality Considerations for Stencil Applications.”
When processing solder paste stencils, a laser system using cutting gases is employed for precision processing of metal foils. These foils – up to a maximum size of 60 x 160 centimeters – are used as stencils for applying solder paste. Laser cutting systems produce apertures of any shape with a precision and speed that would have been unthinkable only a few years ago. However, the demands are growing due to the increasing miniaturisation and compression of components. If the dependency of the process and quality parameters is known, then the modern StencilLasers of today will already be able to meet the demands of upcoming production processes.
An ideal result for cases where cutting, text and fiducial engraving, stepped stencils, cutting elements, and punching are used depends on many factors. Parameter changes do not always affect processing results in a linear fashion. When there is exact knowledge of the relationships, the cutting result can be precisely adapted to the required quality specifications.
Quality Optimisation During the Cutting Process
For a precise description of quality aspects when cutting the stencils, the application team has taken into consideration the parameters of taper, roughness, burrs, ridges, the heat-affected zone, etc. for laser processing. Each parameter depends on different framework conditions.
A taper is a conically shaped extension that is the result of the difference between the cutting diameter on the laser entry and exit sides. With a positive taper, the laser entry side has a larger diameter than that of the exit side. This helps the solder paste to be released and makes it easier for it to adhere to the board when the stencil is lifted.
The focal position affects the diameter of the laser beam and therefore has a direct effect on various parameters. A defocused laser beam generates an unusually large taper and also increases the roughness of the cut edge. The focal position of the laser determines the heat-affected zone in the edge area of a cut as well. The diffusing warmth caused by processing with the laser beam reduces as the cutting gas pressure grows, and increases in proportion to the material thickness.
When the laser parameters are adjusted, the cutting gas type hardly affects the heat-affected zone; in contrast, the amount of energy fed through the laser beam does have an effect. This means that low energy results in a high melt viscosity, which is therefore able to remain in the cutting gap for a long time and can emit warmth into the edge areas. Excessive energy causes the melt to be expelled quickly, with the edge areas absorbing a large portion of the beam energy.
Another quality criterion is the melt emitted on the laser exit side, the ridge. The ridge can be affected by the cutting gas pressure and minimized by adjusting the laser energy. The ridge is thus reduced the higher the cutting gas pressure is, since the melt does not adhere as readily at the exit point. The ridge can be reduced when the laser power is lower, but this results in greater roughness of the cut edge.
A burr cannot be avoided during fusion cutting with microsecond pulses. Scattered burr protrusions several microns in height form that are loosely linked to the edge of the cut. The burr usually occurs at the end of an aperture and is produced when the material bends before the end of the cutting line. The burr is affected by the parameters of aperture shape, aperture size, and the pierce position, length, and angle. However, it is possible to optimize the height of the burr for individual apertures.
The LPKF TechPaper investigates the individual aspects of the quality parameters and shows their connection to process management parameters.
Engraving and Material Removal
In addition to cutting apertures, modern laser systems are also well-suited for other applications. However, there are some important differences in the quality aspects here.
The engraving of text and fiducials needs to be as visible and create as little thermal distortion through remelting as possible. The thermal distortion increases at medium power the thinner the material is, with the contrast varying. Pulse duration and power need to be adapted to the materials. For example, fiducials are filled with less line spacing than text, and the contour has to enable scanning to be as precise as possible.
Stepped stencils help when dosing the solder paste quantity. A step-down stencil has thinner material to reduce the solder paste deposit for fine components; when a step-up stencil is used, components subjected to mechanical stress, such as plug connections, are given a greater volume of solder paste.
During stepped removal, both the processing time and the roughness of the step surface also affect the heat-affected zone and the expulsion. There is less roughness once greater step depths are produced through a higher number of passes. The higher the power, the more the heat-affected zone increases – and thus the thermal distortion and roughness as well. The expulsion is a kind of ridge that forms around the step. The expulsion is made up of small molten balls that are blown out of the step by the cutting gas. During ablation, more material can be deposited when there is higher laser power.
Welding Techniques in Focus
Step-up stencils are produced in the laser process by a weld buildup of sheets in the reinforcement area. First, the reinforcements are attached using spot welding. Then the StencilLaser cuts the apertures for the entire stencil.
In the step-up procedure, the shear strength is an important aspect, since the spot welds are subjected to shear force from the doctor blade when the solder paste is applied. The maximum force generally increases with the welding depth, while the shear strength decreases slightly. Air pockets in the spot welds reduce the shear strength. It is even possible to positively affect the number of splashes and the expulsion height.
CW (continuous wave) welding is used for a continuous weld seam. The fine welding process not only makes uniform depth possible, but also an evenly raised seam. Here as well, the welding depth and the melted material volume are decisive. The aspect ratio, i.e. the ratio of the processed depth of the weld seam to the lateral extension on the surface, can be adjusted through power and speed.
During fine welding, a distinction is made between penetration welding and heat conduction welding. While material is melted on through heat conduction during heat conduction welding, higher-temperature melt during penetration welding produces a vapor that exerts pressure on the melt and achieves a greater welding depth through displacement. At high speeds, a vapor-filled capillary (keyhole) is formed here. The farther the working point extends into the penetration welding area, the higher the aspect ratio becomes.
In contrast, punching produces fine holes in the metal foils. Through defocusing, the hole diameter can be varied and set precisely.
Punching on the fly mode is even faster. The laser continuously emits pulses, each of which produces a hole. The speed determines the process time. At the extreme end, hundreds of thousands of holes per minute are generated. If the speed is set too high, an elongated hole could result. Material deformations may lead to defocusing and interrupt the punch process. The maximum material thickness is restricted, since the thermal distortion is also dependent on the material thickness and increases with the melt volume. In order to avoid overlapping heat-affected zones due to closely bunched bores, the maximum speed that can be set needs to be oriented to the cooling time and the size of the hole pattern taken into consideration.
In percussion drilling, however, each aperture or cutting line is pierced using multiple pulses. They first produce a hole in the material before starting the cutting process, resulting in splashes on the material surface. They should be as small and as few as possible, since they can form deposits and plug the inside walls of the nozzle. A larger hole diameter reduces nozzle plugging; at the same time, the quantity of melted material increases, which needs to be moved out of the hole.
With cutting elements, the outer contour needs to be taken into account in addition to the cutouts in the individual parts, whereby special cutting and cutting-fast procedures ensure exact outer contours. If tabs are needed to hold the parts in place, it must be easy to break them out of the surrounding material. The tab width varies according to material type, material thickness, and sheet tension. The thicker the material, the smaller the required tab size, whereby the tab width must be increased the greater the sheet tension is.
Quality when processing the micromaterial of metal foils is a complex field. The investigations in the LPKF TechPaper “Quality Considerations for Stencil Applications” examine the relationships between process parameters and quality criteria and form a sound basis for optimum cutting results.
LPKF Laser & Electronics AG