The production process of steel mesh includes chemical etching, laser cutting, and electroforming.
1. Chemical etching 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, small influence of objective factors; trapezoidal opening is good for demolding; can be used for precision cutting; moderate price. Disadvantages: cutting one by one, 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: difficult to control the process, polluted production process, not conducive to environmental protection; long production cycle, high price.
The opening design of the stencil should consider the mold release of the solder paste, determined by three factors: ① The width to thickness ratio/area ratio of the opening. ② The geometric shape of the sidewall of the opening. ③ The smoothness of the hole wall. Among the three factors, the latter two are determined by the manufacturing technology of the steel mesh, and the focus is on the first one. Laser steel mesh is cost-effective; focus is on the opening design of the laser steel mesh. Understanding the aspect ratio and area ratio is essential: 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. To achieve 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. For openings less than 5 times the width, consider the area ratio to predict paste release; otherwise, consider the aspect ratio.
Designing the opening of the steel mesh should not solely focus on aspect or area ratios, but also consider process issues like continuous tin and multiple tins. For chip components above 0603 (1608), preventing tin beading is crucial.
The above mainly discusses the opening design of solder paste process stencils; briefly introducing the glue process stencil (SMT template) opening design: Experience is vital for the rubber-printed steel mesh where openings are typically in long strips or round holes; two positioning holes should be opened when positioning non-MARK points. The stencil thickness is generally 0.15 mm to 0.2mm. Tips for stencil opening design:
1. Use rounded corners for fine-pitch IC/QFP and BGA or square holes for 0402, 0201 pieces to prevent stress concentration.
2. Choose the concave opening method for chip components to prevent tombstoning.
3. Ensure openings allow the largest solder balls to pass smoothly.
4. Post-processing for laser steel mesh involves surface polishing to remove slag and roughen the surface for better soldering.
5. Cleaning SMT steel mesh before, during, and after use is essential for smooth demolding. Wiping with a lint-free cloth soaked in detergent is common; ultrasonic cleaning is also effective. Alcohol or deionized water can substitute a special cleaner.
1. Chemical etching 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, small influence of objective factors; trapezoidal opening is good for demolding; can be used for precision cutting; moderate price. Disadvantages: cutting one by one, 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: difficult to control the process, polluted production process, not conducive to environmental protection; long production cycle, high price.
The opening design of the stencil should consider the mold release of the solder paste, determined by three factors: ① The width to thickness ratio/area ratio of the opening. ② The geometric shape of the sidewall of the opening. ③ The smoothness of the hole wall. Among the three factors, the latter two are determined by the manufacturing technology of the steel mesh, and the focus is on the first one. Laser steel mesh is cost-effective; focus is on the opening design of the laser steel mesh. Understanding the aspect ratio and area ratio is essential: 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. To achieve 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. For openings less than 5 times the width, consider the area ratio to predict paste release; otherwise, consider the aspect ratio.
Designing the opening of the steel mesh should not solely focus on aspect or area ratios, but also consider process issues like continuous tin and multiple tins. For chip components above 0603 (1608), preventing tin beading is crucial.
The above mainly discusses the opening design of solder paste process stencils; briefly introducing the glue process stencil (SMT template) opening design: Experience is vital for the rubber-printed steel mesh where openings are typically in long strips or round holes; two positioning holes should be opened when positioning non-MARK points. The stencil thickness is generally 0.15 mm to 0.2mm. Tips for stencil opening design:
1. Use rounded corners for fine-pitch IC/QFP and BGA or square holes for 0402, 0201 pieces to prevent stress concentration.
2. Choose the concave opening method for chip components to prevent tombstoning.
3. Ensure openings allow the largest solder balls to pass smoothly.
4. Post-processing for laser steel mesh involves surface polishing to remove slag and roughen the surface for better soldering.
5. Cleaning SMT steel mesh before, during, and after use is essential for smooth demolding. Wiping with a lint-free cloth soaked in detergent is common; ultrasonic cleaning is also effective. Alcohol or deionized water can substitute a special cleaner.
