The printing process is a process that follows fluid dynamics. In principle, it is a very simple process.

The patch stencil is a key factor, which must be paid great attention to. Many printing problems can be avoided if basic design and manufacturing rules are strictly followed. We have countless instances where companies have spent a lot of money on printing presses and inspection equipment and still have serious printing problems. There are many design rules that people consider standard practice. However, many people are unaware that these ground rules exist.

Jiali Chuang Stencil Department studies and recommends that the frame supports and protects the patch stencil, typically made of aluminum, and the tube is welded by tungsten inert gas, or by casting. The size of the frame usually depends on the method used. Casting is fine for smaller frames, but when frames get bigger it’s more efficient to use TIG welded tubes. The patch stencil is mounted on the frame by glue and polyester or stainless steel mesh, which pulls the patch stencil taut to maintain tension and avoid warping or twisting.

The main frame is designed to fix the frameless patch steel mesh. The concept arose out of high-mix operations, using a large number of patch stencils, whose storage became an issue. It also eliminates frame costs. The user installs the patch steel mesh into the frame and tightens it through the tensioning mechanism on the frame. Over time, I believe this method will have difficulty maintaining proper patch stencil tension. Unless the patch stencil is handled with care, the risk of damage is high. SMD stencils are manufactured by three methods. Chemical etching and laser cutting, while electroforming adds material chemically (additional method).

Chemical etching, or etching of stencils, is the original and most common manufacturing method. A photoimageable resist is laminated to both sides of the foil, and a two-sided phototool containing the aperture image is carefully positioned with the foil. The etch resist is developed, exposing the areas to be removed. Photosensitive tools incorporate an etch factor that compensates for lateral etching. The metal foil is then placed in a chemical etch chamber to create openings by removing exposed material. This method is acceptable for components with pin pitches of 0.65 mm or greater.

Laser cutting, also known as laser stencil, involves fewer steps than chemical etching. A programmable laser machine is used to cut the opening, which naturally produces tapered or trapezoidal hole walls (chemical etching can also produce this effect, if desired). In some cases, this will improve solder paste release. Typically, the openings on the plate side are larger than the openings on the scraper side. Typically laser cutting is used for close-pitch components, but can also be used for full boards. Hybrid technology patch stencils use a combination of chemical etching and laser cutting. Chemical etching is used for larger holes, while laser cutting is used for denser holes.

Electroformed stencils also use a photoimageable resist, which is placed on the cathode metal core. The resist is thicker than the desired patch stencil thickness. When the resist is developed, resist pillars are formed where openings are desired. Nickel is electroplated on the cathode metal core until the desired thickness of the patch stencil is achieved. After electroplating, the resist posts are removed and the patch stencil is removed from the metal core. This method is primarily used in applications where paste release is an issue and very good accuracy and precision are required. Two additional processes, polishing and nickel plating, are used to further improve the surface finish and eliminate surface irregularities. This improves solder paste release and therefore improves die attach stencil performance. I’m a big fan of polish.

The volume of the material (solder paste and glue) is mainly controlled by the size of the opening and the thickness of the metal foil. Material release is affected by various factors. As far as patch stencils are concerned, the most critical factors involving patch stencils include aspect ratio, area ratio, surface finish and geometry of hole walls. The aspect ratio is the width of the opening divided by the thickness of the patch stencil (W/T). The area ratio is the area of the pad divided by the area of the sidewall of the opening. Tests have shown that the aspect ratio should be greater than 1.5 and the area ratio should be greater than 0.66 to ensure sufficient material release. My own experiments have confirmed these recommendations several times. Slightly reducing all openings prevents solder paste from printing on the solder mask and creating solder balls. Jia Lichuang Stencil Division provides detailed recommendations by component type. Typically, a reduction of 0.1mm in each direction is sufficient to prevent solder balling due to solder paste overprinting.

Insert-mounted components can be reflow soldered using a solder paste-through-hole printing process. This method works best when the pins are round and the hole diameter is 0.15~0.20mm larger than the pin diameter. Square pins are more difficult than round pins, and thick pins are difficult because it requires a very high volume of solder paste. More detailed information can be found in the specification of laser stencil perforation. A stepped patch stencil is used to vary the amount of solder paste. Step-down stencils are typically used in fine-pitch applications, resulting in reduced die stencil thickness on fine-pitch pads. Step-by-step mounting stencils are rare, but it can increase the volume of solder paste in localized areas (for example, when solder paste vias are printed for insert-mounted components).

Printing defects can be divided into six categories: Positioning Alignment. This involves the alignment of the die stencil to the area where the material is attached – either the pads (solder paste) or the span between pads (glue). The maximum allowable positioning error should be 15% of the length or width of the pad for solder paste applications and 15% of the length or width of the opening for adhesive applications. collapse. This is a material related defect, either due to the viscosity of the glue or solder paste being too low, or due to overheating exposure. The amount of slump should be limited to 15% of the length or width of the pad for solder paste and 15% of the length or width of the opening for adhesive. thickness. The final print thickness should not vary by more than ±20% of the desired print thickness. Less material may result in insufficient solder or open circuits, and missing components in the case of glue.

Too much material may cause the solder joints to be too full or solder bridges, which may pollute the solder joints or open the circuit in the case of the glue. Hollow out. This is the result of too much pressure on the scraper, too soft a scraper blade, or too large an opening. This defect may cause insufficient tin in the solder dot, or insufficient glue in the glue dot to secure the component. The amount of hollowing out should be limited to a maximum variation of no more than 20% from highest to lowest. dome. Usually the result of improper squeegee blade height adjustment or insufficient squeegee pressure, it increases the amount of material that can cause solder bridging, contamination, or open solder joints. This variation should be limited to 20% of the print thickness. slope. This can happen due to excessive blade pressure. It is more common in solder paste and may cause insufficient solder.

The variation should not exceed 20% from the highest point to the lowest point. Solder paste printing has evolved to a period of compromise as it becomes increasingly difficult to meet the needs of each pin shape. This dilemma can be controlled by good patch stencil design and manufacturing technology. Work closely with your SMT stencil supplier to follow the guidelines in the Laser Stencil Aperture Specification.

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