This article shows that when ordering a stencil for a printing process, there is a clear empirical curve. When the familiarity with its technology helps produce the desired result, the template becomes a constant in an otherwise variable assembly operation.

"Good templates get good print results and then automated help to make the results repeatable."
The procurement of templates is not only the first step in the assembly process, it is also the most important step. The main function of the template is to help the deposition of solder paste. The goal is to transfer the exact amount of material to the exact position on the bare PCB. The less solder paste is blocked on the template, the more it deposits on the board. Therefore, when something goes wrong during the printing process, the first reaction is to blame the template. However, it should be remembered that there are more important parameters than the template that can affect its performance. These variables include the press, the particle size and viscosity of the paste, the type of blade, material, hardness, speed, and pressure, the separation of the template from the PCB (sealing effect), the flatness of the solder mask, and the planarity of the component.

Template manufacturing technology

The three main techniques for template fabrication are chemical etch, laser cutting, and electroforming. Each has unique advantages and disadvantages. Chemical etching and laser cutting are substractive processes. Electroforming is an incremental process. Therefore, the comparison of certain parameters, such as price, may be a comparison between apples and oranges. However, the main consideration should be performance that is commensurate with cost and turnaround time.

In general, chem-etched stencils and other techniques are equally effective when used in applications where the tightest spacing is 0.025" or more. Conversely, laser cutting and electroforming should be considered when dealing with spacings below 0.020". Template. Although the latter type of template is also good for spacings above 0.025", it may be difficult to say about its price and cycle time.

Chemical etching template

Chemical etching templates are the main type of template world. They have the lowest cost and fastest turnover. Chemically etched stainless steel stencils are produced by applying an anti-corrosion agent on the metal foil, positioning the sensitization tool with a pin to expose the pattern on both sides of the metal foil, and then etching the metal foil from both sides simultaneously using a double-sided process. Since the process is double-sided, the corrosive passes through the holes or openings created by the metal, not only from the top and bottom but also horizontally. The inherent characteristic of this technology is the formation of a blade or hourglass shape (Figure 1). At a pitch below 0.020", this shape creates an opportunity to block solder paste. This defect can be reduced by an enhancement process called electropolishing.

Electropolishing is an electrolysis back-end process that “polises” the pore walls, resulting in reduced surface friction, good paste release, and reduced voids. It also greatly reduces the underside of the template. Electropolishing is achieved by attaching a metal foil to the electrode and immersing it in an acid bath. The current causes the corrosive agent to first attack the rougher surface of the hole and the effect on the hole wall is greater than the effect on the top and bottom surfaces of the metal foil, resulting in a "polishing" effect (Figure 2). Then, the metal foil is removed before the etchant acts on the top and bottom surfaces. In this way, the surface of the hole wall is polished so that the solder paste will be effectively scraped (rather than pushed) by the scraper on the surface of the template and fill the hole.

Another technique for improved solder paste release with pitches below 0.020" is trapezoidal section apertures (TSAs).

Trapezoidal section hole (TSA) is a hole (Figure 3) whose contact surface (or bottom surface) of the template is larger than the scraper surface (or top surface) by a size of 0.001 to 0.002". The trapezoidal section hole can be completed in two ways: by selecting Specially modified components, ie, the dimensions of the contact surface of the double sided development tool are made larger than the size of the scraper surface, or the template of all trapezoidal section holes, which can be generated by changing the pressure setting of the top and bottom surfaces of the etchant spray. After polishing, the hole wall geometry allows solder paste release of 0.020" or less. In addition, the resulting paste deposit is a trapezoidal "brick" shape that promotes stable placement of components and fewer solder bridges.

A stepdown, or dual-level template, can be easily produced by chemical etching techniques. The process reduces the amount of tin of the selected component by forming a hole in the down step. For example, in the same design, most 0.050" to 0.025" pitch components (usually requiring a 0.007" thick template) and several 0.020" pitch QFPs (quad flat packs), in order to reduce the QFP solder paste volume, this A 0.007" thick template creates a 0.005" thick downward step area. The downward step should always be on the stencil side of the stencil because the contact surface of the stencil must be horizontal throughout the plate (figure 4). Nonetheless, it is recommended to provide a spacing of at least 0.100" between the QFP and the surrounding elements to allow the scraper to dispense solder paste completely on both levels of the stencil.

Chemically etched templates are also best for generating half-etched fiducial and subtitle names. The fiducials used in the press vision system alignment can be half etched and then filled with black resin to provide a contrast that is easily recognizable by the vision system and a smooth metallic background. Caption blocks containing part numbers, production dates, and other related information can also be half-etched on the template for marking purposes. Both processes are accomplished by developing only half of the double-sided.

Chemical etching limitations. In addition to the defects of the blade edge, the chemically etched template has another limitation: the aspect ratio. Simply stated, this ratio limits the smallest hole opening that can be etched according to the metal thickness at hand. Typically, for a chemically etched template, the aspect ratio is defined as 1.5:1. Therefore, for a 0.006" thick template, the smallest hole opening would be 0.009" (0.006"x1.5=0.009"). In contrast, for electroformed and laser-cut stencils, the aspect ratio is 1: 1, ie, a 0.006" opening can be created on a 0.006" stencil by any process.

Electroforming

Electroforming, an incremental rather than degressive process, produces a nickel metal template with unique sealing features that reduce the need for solder bridging and the cleaning of the underside of the template. The process provides near-perfect positioning, without geometric limitations, smooth trapped walls with inherent trapezoidal shape and low surface tension, improving solder paste release.

Photoresist is developed on a substrate (or core mold) on which an opening is to be formed, and then the template is electroplated around the photoresist atom by layer and layer by layer. As can be seen in Figure 5, the nickel atoms are deflected by the photoresist, creating a trapezoidal structure. Then, when the template is removed from the substrate, the top surface becomes a contact surface, resulting in a sealing effect. A continuous nickel thickness in the 0.001 to 0.012" range can be selected. This process is ideally suited for ultra-fine-pitch requirements (0.008 to 0.016") or other applications. It can achieve a 1:1 aspect ratio.

As for the disadvantages, because it involves a photosensitive tool (although on one side) there may be a malposition. If the plating process is not uniform, the sealing effect will be lost. Also, the seal "block" may be removed if the cleaning process is too hard.

Laser cut template

Produced directly from the customer's original Gerber data, the laser-cut stainless steel stencil features no photographic steps. Therefore, the opportunity for malposition is eliminated. Template making has good position accuracy and reproducibility. The Gerber file is transmitted (and directly driven) to the laser machine with the necessary modifications. Less physical interference means less chance of error. Although there are major problems with metal slag produced by a laser beam (evaporated molten metal), current laser cutters produce slag that is rarely easily removed.

There is also the problem that a "scallop-like" appearance occurs around the hole, causing the hole wall to be rough. Although this increases surface friction, the roughness is in the vertical plane. However, recent laser machines have an internal vision system that allows metal foil to be cut without borders. This is significant because templates can be fabricated by first chemically etching standard pitch elements and then laser cutting fine-pitch elements. This "hybrid" or combined template yields the advantages of both technologies, reducing costs and faster turnaround. In addition, the entire template can be electropolished to provide a smooth hole wall and good paste release. The main disadvantage of the laser cutting process is that the machine cuts each hole individually. Naturally, the more holes, the longer the time spent, the higher the cost of the template. However, if the design allows, costs can be reduced by using a hybrid template process. In accordance with the focus of the laser beam, trapezoidal holes are automatically generated. The opening of the hole is actually cut from the contact surface of the stencil; the template is then turned over with the squeegee facing up.

Laser technology is the only process that allows existing templates to be reworked, such as enhancing holes, enlarging existing holes, or enhancing fiducials.

Other progress

In addition to laser cutting and electroforming, the most important advancement in template fabrication is the transfer of electronic data. As recently as 1995, most of the pictures provided to the template maker were film positives, matching the pattern on light copper one by one. The modification of the opening of the element involves repeated photography techniques and manual operations. The process also depends on the quality of the film prints provided. Finally, repeating the picture in steps is a heavy task.

Today, electronic file transfer via modems and e-mails is the most common method of providing graphical data on the fly. Selective decoration, step-and-repeat patterns, and geometric transformations can be easily and accurately completed. Also, because the mailing of the film is eliminated, the turnaround time can be reduced by almost one day.

With the transfer of Gerber files, the pad geometries can be changed from squares and rectangles to “home plate,” “plaid,” “zip,” and other shapes (Figure 6) as a way to reduce solder paste volume. . By modifying the geometry to adjust the amount of solder paste, in combination with selecting the correct metal plate thickness, the need for a stepdown plate can also be eliminated. A single-thickness template, after proper design, is always better than a two-stage tool from a process perspective.

Adhesive Stencil

Electronic files also make it easy for computer-aided design (CAD) operators to determine a pad-shaped centroid point. With this ability, the solder paste layer in the design document can be converted into round and oval shapes. Show the size of the components (Figure 7). Therefore, a template can be made to "print" instead of dispensing. Printing is faster than Epoxy, giving this device to other jobs.

Rework template

A more recent innovation occurred in the area of ​​rework. There are now "small" templates that are specifically designed to rework or renovate individual components. Templates for individual components can be purchased, such as standard QFPs and ball grid arrays (BGAs). Of course there are corresponding scrapers, or small scrapers.

price comparison

The price of chemically etched templates is driven by the size of the frame. Although metal foil is the focus of the template making process, the frame is the single, most expensive fixed cost. Its size is largely determined by the type of press. However, most presses can accommodate more than one frame size. (Frame size is an industry standard). Most template suppliers maintain a standard framework for inventory, ranging in size from 5x5" to 29x29". Because the cost of the empty foil is not as much as the frame, the metal thickness has no effect on the price. And because all the holes are etched at the same time, the number is also irrelevant.

The price of electroformed formwork is mainly driven by the thickness of the metal. Plating to the desired thickness is a major consideration: Thicker templates are less costly than thinner ones.

Laser cut template prices are based on the number of holes designed.

The laser cuts one hole at a time, that is, the more holes, the higher the cost. Also add the required frame size. A hybrid template that uses a laser to cut fine pitch and chemically etch standard spacing elements may be a cost effective method when many openings are required. However, for less than 2500 holes

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