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

The stencil, a key element, deserves considerable attention. Many printing problems can be avoided if basic design and manufacturing rules are strictly followed. I have noticed numerous instances where companies have spent large sums of money on printing presses and inspection equipment and continue to have serious printing problems. There are many design rules that people consider standard practice. Unfortunately, many people are unaware that these ground rules exist.

Later this year, IPC will issue IPC-7525 – a set of stencil design guidelines created by a panel of industry experts. This document will remove some of the “witchcraft” surrounding formwork design and fabrication, which will be of great benefit to our industry. The frame supports and protects the formwork, typically made of aluminum, by welding the tubes with tungsten inert gas (TIG, 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 formwork is attached to the frame by means of glue and polyester or stainless steel mesh, which pulls the formwork taut, maintaining tension and avoiding warping or twisting.

Main frame, designed to hold frameless formwork. The concept arose out of high-mix operations, using a large number of templates, whose storage became an issue. It also eliminates frame costs. The user installs the template into the frame and pulls it tightly through the tensioning mechanism on the frame. Over time, I believe this method will have difficulty maintaining proper stencil tension. Unless the formwork is handled with care, the risk of damage is high. The templates were fabricated by three methods. Chemical etching and laser cutting remove material (subtractive method), while electroforming adds material chemically (additive method).

Chemical etching 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 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 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.

Electroforming also uses a photoimageable resist that is placed on the cathode metal core. The resist is thicker than the desired 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 stencil thickness is achieved. After plating, the resist posts are removed and the 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 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 templates are concerned, the most critical factors related to templates include aspect ratio, area ratio, surface finish and geometry of cell walls. The aspect ratio is the width of the opening divided by the thickness of the 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. IPC-7525 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 paste-in-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 IPC-7525. A stepped stencil is used to vary the amount of solder paste. Step-down stencils are typically used in fine-pitch applications, resulting in reduced stencil thickness on fine-pitch pads. Step-up stencils are rare, but can increase paste volume in localized areas (for example, when paste vias are printed for insert-mounted components).

Printing defects can be divided into six categories: Registration. This involves the alignment of the 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. Slump (slump). 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 (scoop). 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 formwork design and manufacturing techniques. Work closely with your stencil supplier to follow the guidelines in IPC-7525.

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