The key factors affecting PCB stencil quality include stencil material, thickness, opening type, opening size, and stencil wall flatness. For this reason, special attention must be paid to the PCB stencil throughout its design, processing, inspection, and production processes.
Common types of PCB stencils include SMT stencil, etching stencil, and laser stencil. MaxiPCB is a professional stencil manufacturer integrating related services.
1. What is PCB Stencil?
A PCB stencil is essentially a thin steel sheet, with a fixed size to match the solder paste printer. Its common thickness ranges from 0.08mm, 0.10mm, 0.12mm, 0.15mm to 0.18mm.
The early stencil was made of wire mesh, so it was also called a mask. It initially used nylon polyester mesh, then switched to wire mesh and copper wire mesh for better durability, and now uses stainless steel wire mesh. However, regardless of the wire mesh material, it had the drawbacks of poor shape stability and low precision. With the rapid development of SMT technology, the requirements for stencils have become increasingly high, so stainless steel stencils (now commonly referred to as SMT stencils) have gradually replaced wire mesh stencils.
The main function of a PCB stencil is to assist in solder paste deposition, transferring an accurate amount of solder paste to the exact position on the bare PCB. A standard PCB stencil consists of a mesh frame, wire mesh, and steel sheet. The steel sheet is equipped with multiple holes corresponding to the positions on the PCB that require printing. During use, the PCB is placed under the stencil, and the solder paste is leaked onto the PCB through the fixed-position holes on the stencil.
2. Types of PCB Stencils
2.1 SMT Stencil
SMT stencil is a traditional prototype stencil, mainly used in PCB SMT assembly. There are many mature SMT stencil suppliers in the market, and many customers prefer to customize laser-cut stencils from nearby suppliers for convenience.
2.2 Etching Stencil
Etching stencil is a type of stencil made by an etching process. MaxiPCB uses 301 steel sheets to produce etching stencils, which are suitable for PCB printing with angular positions and spacing ≥ 0.4mm. It can be used with sheet copying and film, and can also adopt CAD/CAM and exposure modes simultaneously. The price of etching stencils is not calculated based on the number of components, and scaling can be performed according to different component positions. Compared with laser stencils, etching stencils have a shorter production cycle and lower cost.
Process flow of chemical etching stencil: Data file & PCB → Film making → Exposure → Development → Etching → Steel sheet cleaning → Wire mesh mounting.
Advantages: One-time forming, fast production speed, and low cost.
Disadvantages: Prone to forming an hourglass shape (insufficient etching) or excessive opening size (over-etching); greatly affected by objective factors (operator experience, chemicals, film), with more production links and large cumulative errors, making it unsuitable for fine-pitch stencil production; the preparation process is environmentally polluting.
2.3 Laser Stencil
Laser stencil is made by laser cutting technology. In recent years, laser stencil technology has developed rapidly, with continuous updates of stencil materials. There are more and more laser-cut stencil suppliers, making it more convenient for customers to customize laser stencils.
Laser stencil is currently the most commonly used type in the SMT industry. Its characteristics include: direct production using data files, reducing production error links; extremely high opening position accuracy (overall error < 4μm); the stencil opening has a special geometry that facilitates solder paste printing.
To make a laser stencil, PCB and data files are required. The PCB data must be correctly edited without distortion, damage, or breakage. The acceptable data file formats for MaxiPCB include GERBER, HPGL, JOB, PCB, GWK, CWK, PWK, DXF, PDF, PAD2000, POWER PCB, GCCAM4, PROTEL, AUTOCADR14 (2000), CLIENT98, CAW350W, and V2001. The data must include the SMT solder paste layer and character layer data to check the front/back side of the data, component categories, etc.
Process flow of laser cutting stencil: Film making & PCB preparation → Coordinate acquisition → Data file input → Data processing → Laser cutting → Grinding → Mesh mounting.
Advantages: High data production accuracy, little impact from objective factors; trapezoidal opening is conducive to demoulding; capable of precision cutting; moderate price.
Disadvantages: Laser cutting is performed one by one, resulting in a relatively slow production speed.
