Solder stencils are precision tools, and it is of paramount importance in the SMT (surface mount technology) process. It helps apply solder depots on PCBs. The Surface Mount Device (SMD) components are placed on placed within these depots. The PCB engineer then solders it with a reflow- or vapor phase process. The application of solder paste needs to be even and precise. A solder stencil is helpful for effective and repeatable solder paste deposition.
Solder stencil helps in the creation of depots defined in shape and size. Gerber files are typically used in the solder stencil fabrication process. It would help if you designed the stencil as per the industry standards or IPC standards. When creating SMT stencils, you need to address four primary types of components. It includes the pitch components, chip components, ball grid arrays (BGAs), and bottom-terminated components, including LEDs. Considering these components during each solder stencil design review helps to produce good print results during PCB assemblies.
To ensure the optimum results for the production of your solder stencil, you need to consider certain design principles carefully. It will enable your stencil to facilitate an easier, faster, and accurate deposition of solder paste. The solder stencil consists of a laser cut in the stainless steel foil. This cut matches with the places on the circuit board where you need to mount various devices.
Through these cuts, the engineer applies the solder paste using a metal squeegee blade. Solder stencils help to eliminate the tedious, error-prone process of hand-soldering the prototype circuit boards.
Here are the essential features you need to consider while designing the solder stencil.
Table of Contents
1. Stencil Type
You need to assess your project needs to identify the solder stencil type pertinent to your project. The stencil technologies used in the industry are laser-cut, electroformed, chemically etched plastic, and hybrid. Hybrid stencils combine chemically etched and laser-cut to form the apertures on the stencil. You can also customize the material for constructing the solder stencil so that it can adapt to the harsh working conditions of your PCB fabrication and assembly environment.
Generally, SMT stencils used during prototyping are made of stainless steel. Therefore, it helps to reduce mistakes and meet the soldering requirements quickly. Mylar and Kapton SMT stencils are another popular variant, and it assures optimal performance during hand soldering.
2. Support The Desired Solder Paste Application Process
While designing the solder stencil, take care of the solder paste application process you intend to deploy. You can either choose a framed stencil or a frameless one. Framed stencil are laser-cut solder paste stencils mounted in a stencil frame, useful for high-volume screen printing.
It provides better locational positional and dimensional accuracy. Frameless SMT Stencils are low-cost laser cut stencils and are not glued to the frame. It is helpful for prototyping PCB assembly or short runs.
3. Aperture Design
The factors which influence the aperture design of solder stencil are aspect/area ratio, board finish, component type, pad footprint, solder paste type, solder mask opening, board finish, and user process requirement. Regulation of aperture design using the right metrics, including area ratio and aperture design, is essential to deposit the desired volume of solder paste with uniformity and precision.
4. Aperture Size Vs. Board Pad Size
You need to reduce the aperture size in comparison to the board pad size. Based on your original pad design, you can modify the solder stencil aperture. Reducing the area of aperture size helps to enhance printing, reflow, or SMD stencil cleaning. It minimizes the risk of any misalignment of the aperture with the board pad. It also reduces the probability of printing solder paste off the pad, which can cause solder balls or solder bridging.
This would be best to achieve better positional accuracy for the stencil apertures to align them with the PCB pad. You can ‘fiducial marks’ or registration marks to both PCB and stencil for better alignment. It enables the printer to align the board with the stencil automatically.
Area ratio and aspect ratio helps to calculate the thickness of stencil foil. The area ratio is the area of the aperture opening divided by the size of the side walls. The aspect ratio is the aperture width divided by stencil thickness. An aspect ratio of 1.5 or greater is generally acceptable for solder paste transfer.
Moreover, the stencil should also satisfy most fine-pitch QFPs and BGAs while considering the smallest chip size. Precision in solder stencil thickness helps to release the desired amount of solder paste.
6. Multilevel or Stepped Stencils
Some PCBs require a thin stencil for acceptable pitch devices and a larger stencil for the remaining parts. Hence, it would be best to design a multilevel or ‘stepped’ stencil for such circuit boards. To create step-up areas, you need to add materials to increase the volume and height of solder paste in the designated areas. You can remove the materials from the stencils to create step-down regions to minimize the volume and size of solder paste.
Solder Stencils support a fast, mass solder paste deposition process. These design features will help you to create high-precision SMD-Stencils to deposit the solder paste in the designated places so that components appear in the right places on the printed circuit board.
In addition, achieving an accurate design for the solder stencil helps avoid errors in the PCB fabrication and assembly process.