Design Guidelines for Industry-Standard Solder Stencil

Solder paste printing is one of the important processes in PCB assembly. The designing of a stencil directly affects the solder paste application. In a previous article, we discussed stencils and their types in detail. Step-up/step-down and nano-coated stencils are examples of recent advances that have expanded the potential of JLCPCB. In electronics hardware design, design rules are just as crucial as the necessary tools. This essay examines key design principles for producing useful and production-ready content today.

What Are Industry-Standard Solder Stencils?

Solder paste stencil is applied using industry-standard stencils, which are commonly laser-cut stainless steel foils with precise perforations that line up with PCB pads. These foils are typically 100µm to 120µm thick. Usually utilized for SMT assembly lines, these stencils are made to integrate easily with pick-and-place machines and automated stencil printers.

They are designed to meet common SMT requirements without the need for specialized features like thickness stepping, and they adhere to standards set by organizations such as the IPC, SMTA, and IEC. Click here to view the whole standard IPC guidelines for PCB stencils.

Considerations for Solder Stencil Designing:

The key design features of an effective solder paste stencil are as follows:-

• Stencil thickness
• Aperture design
• Stencil material and mounting
• Stencil alignment
• PCB design specific

7 Key Design Guidelines for Standard SMT Stencils:

1. Stencil Thickness Selection

The amount of solder paste put down depends on the thickness of the pattern. Give most of your attention to the board's smallest pitch component.  In a thick stencil due to a phenomenon named surface tension, solder paste may stick to the inner walls of the aperture. According to a general guideline, the smallest aperture should preferably have at least five solder particles.

2. Aperture Design Guidelines

To prevent solder flaws like bridging and solder beads, this is crucial. For accurate deposition the apertures are made to be a bit smaller than the pads. However, for certain parts, an aperture with a specific design is required to deposit less paste on the inner edge of the part, which helps to prevent the formation of "mid-chip" solder balls. Here are a few instances:

3. Aperture Reduction (Pad-to-Aperture Ratio)

Apertures are frequently made slightly smaller than the actual pad dimensions in order to prevent bridging and extra solder.

• For most components, the typical reduction is between 5 and 7%.
• For dependable printability, IPC-7525 suggests a stencil-to-pad area ratio >0.66.

Formula: Area Ratio = (Aperture Area) / (Aperture Wall Height × Aperture Perimeter)

Keep this ratio above 0.66 to ensure clean paste release.

4. Fiducial Marks

When printing solder paste onto pads, it is very important that the pattern is lined up correctly with the PCB.  This is accomplished by applying fiducial marks, which are registration marks, to the PCB and stencil. As you can see from our most recent comprehensive piece on fiducial marks, adding them to the stencil will guarantee proper alignment.

5. Stencil Frame and Border

Stencils can be frameless (for rapid prototyping or manual printing) or framed (glued into aluminum frames).

• Typical frame dimensions are 23 x 23 inches.
• Keep the distance between the aperture area and the frame edge at least 10 mm.
• Mark the stencil's position, thickness, and part number clearly.

6. Stencil Material

The stencil's capacity to remove solder paste from the apertures will depend on the kind of material it is made of.  Although stainless steel is usually used, nickel, which is about 50% more expensive than stainless steel, can be used for PCB designs that include fine pitch components.

Chemical etching was used to create the initial generation of stencils, which worked well for larger apertures but became more difficult to remove solder from as component sizes shrank.  This led to the development of laser-cut stencils, which created a much finer, trapezoidal-shaped opening that improved paste release.  The possibility of electropolishing the aperture walls and, more subsequently, the technique of electro-forming the stencil were developed as component sizes decreased.

7. Window Effect design:

There are instances where the PCB design features big copper pads underneath the component. Devices that use power electronics typically experience this.  In this case, a window effect is present to ensure a good electrical link and to let the device's heat escape.  The device may lift and the outer leads may not solder if solder paste is placed over the entire pad.  A "window effect" in the design can be used to decrease the solder volume, which is shared in the image above.

Conclusion:

For the majority of PCB assemblies, these fundamental design guidelines will yield good print results. I think it's critical that the PCB manufacturer comprehends the design guidelines provided by their stencil supplier. An essential step in the assembly process is printing. There will be "knock on" repercussions throughout the procedure if mistakes are made at this point. It is crucial to utilize a stencil that has been designed with all the necessary considerations in order to provide good, repeatable printing results. Before experimenting with more complex methods like step stencils, electroformed versions, or nano-coated foils, PCB designers and assembly engineers can establish a strong foundation by becoming proficient in these standard processes.