The production process of steel mesh includes chemical etching, laser cutting, and electroformed.
1. Chemical etches process: data file PCB→film production→exposure→development→etching→steel cleaning→sheet.
Features: one-time molding, faster speed, low price.
Disadvantages: easy to form an hourglass shape (not enough etching) or larger opening size (over etching); objective factors (experience, medicament, film) have a large impact, many production links, large cumulative errors, not suitable for fine pitch steel mesh production method; The production process is polluted, which is not conducive to environmental protection.
2. Laser cutting process: Film making PCB→acquisition of coordinates→data file→data processing→laser cutting→grinding→netting.
Features: high precision of data production, the small influence of objective factors; trapezoidal opening is good for demolding; can be used for precision cutting; the price is moderate. Disadvantages: cutting one by one, the production speed is slow.
3. Electroform process: coating photosensitive film on the substrate → exposure → developing → electroforming nickel → forming → steel sheet cleaning → netting. Features: smooth hole walls. Advantages: especially suitable for ultra-fine pitch steel mesh production method.
Disadvantages: the process is difficult to control, the production process is polluted, which is not conducive to environmental protection; the production cycle is long and the price is too high.
The opening design of the stencil should consider the mold release of the solder paste, which is determined by three factors: ①The width to thickness ratio/area ratio of the opening. ②The geometric shape of the sidewall of the opening. ③The smoothness of the hole wall. Among the three factors, the latter two are determined by the manufacturing technology of the steel mesh, and we consider the first one more. Because the laser steel mesh is very cost-effective, here we focus on the opening design of the laser steel mesh. First of all, we understand the aspect ratio and area ratio: Width-thickness ratio: the ratio of the opening width to the thickness of the steel mesh. Area ratio: The ratio of the opening area to the cross-sectional area of the hole wall, as shown in the following figure: Generally speaking, to obtain a good demolding effect, the width-to-thickness ratio should be greater than 1.5 and the area ratio should be greater than 0.66. When to consider the aspect ratio and when to consider the area ratio? Generally, if the length of the opening does not reach 5 times the width, the area ratio should be considered to predict the release of the solder paste. In other cases, the aspect ratio should be considered.
Of course, when designing the opening of the steel mesh, one should not blindly pursue the aspect ratio or area ratio while ignoring other process issues, such as continuous tin and multiple tins. In addition, for chip components above 0603 (1608), we should think more about how to prevent tin beading.
The above mainly talks about the opening design of the solder paste process stencil, let’s briefly introduce the opening design of the glue process stencil (SMT template): Due to its characteristics, the experience value of the opening design is very important. The openings of the rubber-printed steel mesh are generally open in long strips or round holes; when positioning non-MARK points, two positioning holes should be opened.
The thickness of the printed rubber stencil is generally selected from 0.15 mm to 0.2mm. Tips for designing the opening of the stencil (SMT template):
1. Fine-pitch IC/QFP, in order to prevent stress concentration, it is better to have rounded corners at both ends. The same is true for BGA and 0402, 0201 pieces with square holes.
2. The anti-tin bead opening method of chip components is best to choose the concave opening method, which can effectively prevent component tombstones.
3. When designing the steel mesh, the width of the opening should at least ensure that the 4 largest solder balls can pass smoothly.
4. Post-processing Etching and electroforming steel mesh generally do not do the post-processing. The steel mesh post-processing mentioned here is mainly for laser steel mesh. Because metal slag will adhere to the walls and openings after laser cutting, surface polishing is generally required; of course, polishing is not only to remove slag (burrs) but also to roughen the surface of the steel sheet. Increase the surface friction to facilitate the rolling of the solder paste and achieve a good soldering effect. If necessary, you can also choose “Electropolishing” to completely remove the slag (burr) and improve the hole wall.
5. Cleaning SMT steel mesh should be cleaned before, during, and after use (usually cleaning with SMT steel mesh cleaning machine): Wipe before use; wipe the bottom of the steel mesh regularly during use, In order to keep the stencil demolding smoothly; clean the stencil in time after use, so that the same good demolding effect can be obtained next time. The stencil cleaning methods generally include wiping and ultrasonic cleaning: Wipe Wipe the stencil with a lint-free cloth (or special stencil wipe paper) pre-soaked with detergent to remove the solidified solder paste or glue. It is characterized by convenience, no time limit, and low cost; the disadvantage is that it can incompletely clean the steel mesh, especially the densely spaced steel mesh. In addition, some printers have an automatic wipe function, which can be set to wipe the bottom of the stencil automatically after a few times of printing. This process also uses a special steel mesh to wipe the paper, and the machine will spray cleaning agent on the paper before the action. Ultrasonic cleaning There are two main types of ultrasonic cleaning: immersion and spray. Some manufacturers use a semi-automatic ultrasonic cleaning machine to clean the steel mesh. The choice of cleaning agent The ideal stencil cleaning agent must be practical, effective, and safe for people and the environment. At the same time, it must be able to well remove the solder paste (glue) on the stencil. There is a special stencil cleaning agent, but it may wash out the stencil, so you should be cautious when using it. If there is no special requirement, alcohol or deionized water can be used instead of a special cleaner for steel mesh.
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