Manufacturers engaged in PCBA labor and materials boast a comprehensive production line equipped with various specialized machines. Let me share some insights on the effective use of scrapers in the PCBA processing and production process.
1. The Angle of the Scraper
In the PCBA processing and production process, the angle of the squeegee significantly influences the vertical force applied to the solder paste. A smaller angle results in a greater vertical component force. By adjusting the squeegee angle, the pressure exerted can be modified. If the angle exceeds 80°, the solder paste will only advance without rolling, leading to minimal vertical force. Consequently, the solder paste may not effectively fill the openings of the stencil window. The optimal squeegee angle should be set between 45° and 60°, as this promotes favorable rolling characteristics for the solder paste.
2. The Speed of the Scraper
The squeegee operates at a high speed, which increases the pressure of the solder paste. Considering the actual scenario of the solder paste being pressed into the opening, the duration for this pressing action is reduced. If the squeegee moves too quickly, the solder paste may slide across the printing stencil without rolling. This is particularly noticeable when printing fine-pitch QFP patterns, as the solder paste pattern on the pad perpendicular to the squeegee tends to be fuller compared to the opposite side. To ensure uniformity in solder paste distribution during fine-pitch QFP printing, the printing machine features a function that rotates the squeegee at a 45° angle. The maximum printing speed should guarantee that the solder paste on the FQFP pads is applied uniformly and adequately in both vertical and horizontal directions. Typically, controlling the squeegee speed at 20 to 40 mm/s yields the best board brushing effect.
3. The Pressure of the Scraper
As the solder paste rolls, it applies a positive pressure that affects the vertical balance of the squeegee mechanism, commonly referred to as printing pressure. Insufficient printing pressure can lead to incomplete scraping of the solder paste, while excessive pressure may cause leakage behind the stencil and scratches on the steel plate’s surface. Generally, the squeegee pressure is set between 5 to 12 N/(25 mm). The ideal squeegee pressure should be just enough to effectively scrape the solder paste off the surface of the steel plate.
4. Scraper Width
If the squeegee is too wide relative to the PCB, it will require greater pressure and more solder paste, resulting in waste. Typically, the width of the squeegee should be the length of the PCB in the printing direction plus approximately 50 mm, ensuring that the squeegee head makes contact with the metal stencil.
5. Printing Gap
It is generally advisable to maintain a zero distance between the PCB and the stencil (earlier practices suggested a gap of 0 to 0.5 mm, but zero distance is preferred for FQFP). Some printing machines may require the PCB surface to be slightly elevated compared to the stencil. After adjustments, the stencil should be minimally supported from underneath, but the height of this support should not be excessive to avoid damaging the stencil. From the squeegee’s operational perspective, it should glide smoothly over the stencil, effectively scraping away excess solder paste without leaving scratches on the stencil surface.
6. Separation Speed
After the solder paste is printed, the instantaneous speed at which the steel plate detaches from the PCB is a crucial parameter affecting print quality. The capability to adjust this speed reflects the printing machine’s quality, especially in precision printing. Early printing machines separated at a constant speed, while advanced models incorporate a brief pause when the steel plate lifts away from the solder paste pattern to ensure optimal print quality.
7. Shape of the Squeegee and Materials
The material and shape of the squeegee head have long been important topics in solder paste printing. There are various shapes and materials available, categorized mainly into two types: polyurethane hard rubber and metal squeegees. Currently, with the widespread use of stainless steel stencils, metal scrapers—constructed from high-hardness alloys—are commonly employed. These metal squeegees exhibit excellent fatigue resistance, wear resistance, and bending resistance.
