Stencil Printing in Contract PCB Assembly
The stencil printing process serves two primary functions in contract pcb assembly. First, it allows for the precise placement of a material such as solder paste, flux or encapsulant onto a substrate. This helps ensure that components are positioned correctly and will stay in place during subsequent reflow. Second, it allows the printed deposit to properly adhere to the pads on the board during reflow. This helps to reduce the occurrence of solder bridging and ‘bleeding’ during reflow, which can compromise the quality and reliability of the finished product.
Stencils are created using a computer-aided design (CAD) program to specify the locations of apertures that align with the places where SMT components will be placed on the board. These apertures are then cut from a sheet of stainless steel or other material, usually by etching, laser cutting or electroformed foil. Stencil material must be selected to meet specific performance characteristics such as durability or flexibility. Standard stencils streamline manufacturing by providing cost-effective solutions for commonly used component footprints. However, these standard sizes must be carefully specified to provide the best possible print quality and to avoid product defects that may result in rework or waste.
Solder paste is a thixotropic material that requires energy in the form of print head motion to change its viscosity and flow evenly into stencil apertures. There is a general ‘5 ball rule’ that indicates that ideally, five solder particles should span the width of the smallest aperture. During the design phase, QualiEco Circuits Pty Ltd engineers carefully consider these factors when selecting a stencil’s material and aperture size.
The Role of Stencil Printing in Contract PCB Assembly
As the squeegee blade travels across the stencil during the print cycle, solder paste fills each aperture and then releases to the pads on the board. This is a critical step because the deposits of properly sized and shaped paste are essential to ensuring accurate and repeatable component placement. In addition, the ‘aspect ratio’ of the apertures must be correctly specified to minimize product defect rate.
The aspect ratio is the relationship between a stencil’s aperture height and its wall thickness. The higher the aperture wall thickness, the less the squeegee can apply to the surface of the pad. The squeegee’s pressure must be sufficient to overcome the force of the nozzle’s retraction and the adhesive forces of the solder paste and the pad to ensure that enough paste is deposited on the pad for proper electrical connections. The squeegee must also be able to separate easily from the stencil after each print, without leaving behind any residue. This is achieved through a clean, well-designed and maintained stencil.
In addition, it’s important to double-check the manufacturer’s specific requirements regarding lead times. Typically, lead times vary according to the complexity of your order and the manufacturer’s current workload. Be sure to communicate your desired timeline with each manufacturer during the quoting process to avoid any delays in production.