A Beginners Guide to Step-Up and Step-Down Stencils

As everyone knows, companies are moving down to 2nm, and electronics are at their peak right now. The component sizes of a PCB likewise drastically shrink during a ten-year period. From 1206, BGAs have shrunk to 0402, 0201, and even smaller. Due to the small size of the components and the requirement for high throughput, defects such as tombstoning, bridging, or poor connections may occur during the soldering process. In this case, two specialized stencil technologies - step-up and step-down stencils are helpful.

Fine pitch components with a pitch of 0.4mm are often adjacent to devices that need much higher paste heights. This amounts to a significant amount of SMT's overall production. The capacity to print devices with a 0.5mm pitch while simultaneously printing smaller devices, like 01005 or CSP, with a 0.3mm pitch and noticeably lower paste heights is an emerging factor in electrical production. Printing multiple paste heights inside a single stencil in a single printing operation is possible with a step stencil. Let's look into it, and in this tutorial, I'll describe what they are, when to utilize them, and how they impact the caliber of your assembly.

What is a Stencil?

A stencil is a thin piece of steel that has been cut with perfectly matching holes to match the pads on the PCB. To assemble SMT components on a PCB, solder paste needs to be applied to the board's pads. The most effective and accurate way to do this is via a stencil. Check out this paper about stencils and nanocoating that we recently discussed.

At high production lines, an operator places the stencil on the PCB, squeezes solder paste through the holes in the stencil, and deposits it on the board's pads. When the operator removes the stencil from the board, some solder paste remains on the pads. The board covered with solder paste is then fed into a pick-and-place machine, which positions the appropriate SMT components on the paste. The filled boards are then soldered in reflow furnaces.

Problems with Traditional Stencils:

The thickness of the stencil and the size of the pad's aperture or opening determine how much solder paste the stencil applies to the pad. The quantity of solder paste on a pad is essential for proper soldering of a component. If the volume is small, the component might not be soldered correctly. In the event that the volume is substantial, excess solder could cause bridging with adjacent pads. as a combination of fine-pitch components and large connectors or heat pads on the same board. This creates conflicting volume needs for solder paste that standard stencils cannot handle.

For SMT components like 0402 chip components and QFPs with 0.5 mm pitch or smaller, incredibly small stencil apertures are needed. Using such tiny perforations on a 0.2 mm thick stencil is a challenge for paste release. When the operator lifts the stencil, these apertures function best with stencils that are 0.127 mm thick.

What are Step Stencils?

A step stencil essentially controls how much solder paste is applied to the PCB for both large and small SMT components. For big SMT components, a thicker stencil and more solder paste are needed for optimal soldering. However, since small SMT components require less solder, a thin stencil offers a better option. Therefore, for a board that has both tiny and large SMT components, a step stencil with thick sections for large components and thin parts for small components is required.

（This stencil is manufactured by JLCPCB）

Because electronic equipment typically has smaller and thinner form factors, engineers must use microscopic Surface Mount Technology (SMT) components on Printed Circuit Boards (PCBs). SMT components of big, medium, and fine pitches are frequently combined to form the board. Nevertheless, the PCB surface can be coated using a customized volume of solder paste thanks to the step stencil's change in thickness.

Types of Step Stencils:

These modifications are often made by chemical etching, laser welding, or electro-polishing during the stencil manufacturing process. There are two types of step stencils: step-up and step-down.

Step-Up Stencil

PCBs with numerous small components and a number of large SMT components need step-up stencils. They are thicker in some places to allow for more solder paste deposition. The basic stencil is made of a steel sheet that is 0.1 mm thick; for the larger SMT components, the thickness increases to 0.15 mm.

Step-Down Stencil

PCBs with a number of large components and small SMT components need step-down stencils. They are thinner in certain areas so that less solder paste is applied. The base stencil is made of steel sheet that is 0.15 mm thick; for the small SMT components, the thickness decreases to 0.1 mm.

Step stencils can be used with or without frames. Frameless step stencils are more practical for prototype assembly, whereas framed step stencils are used by assemblers for production runs. The cost of frameless step stencils is lower than that of frame stencils.

Manufacturing Process of Step Stencils:

Step stencils have been available on the market for more than a decade and are made using a variety of production techniques. The first technique to create step stencils was wet chemical etching, which was followed by laser cutting/welding, milling, and electroforming.

Even though chemical etching is becoming less popular because to its limited capacity for miniaturization, laser cutting and electroforming are currently the most widely employed methods. The two technologies differ greatly in terms of material, production method, and finishing process. Stainless steel is mostly used in the laser cutting process, while nickel is the primary material used in the electroforming process.  Creating step-up or step-down areas requires sophisticated techniques:

• Laser Welded Foils : Additional foil sections are welded onto the base stencil.

• Chemical Etching : Selective material is removed to produce either additions (steps-up) or depressions (step-down).

• Nano-coating compatibility : Many manufacturers provide nano-coated versions to increase paste release even more and cut cleaning requirements.

Key in this procedure are excellent CAD design and tight cooperation with your stencil supplier.

Stencil Fabrication Techniques:

Manufacturers use one of three technologies for fabricating step stencils. They use:

• Photo-Chemical Etching Technology
• Micro-Machining Technology
• Welding Technology Using Laser Devices

1. Photo-Chemical Etching Technology:

Photo-chemical etching technology is the primary method for producing step stencils. The fabricator creates resist using a photographic process on areas of the stencil that are unlikely to be etched. After that, they etch an etchant onto the stencil surface to remove material until the desired thickness is achieved.

2. Micro-Machining Technology :

The fabricator attaches a chilled vacuum plate to a simple stencil sheet on a milling machine. They use the CNC-based milling machine to remove minuscule quantities of material at a time until they reach the desired thickness. After placing the stepped foil on a frame, they used a laser beam to cut the perforations.

3. Welding Technology Using Laser Devices :

The fabricator uses two sheets : The basic thin sheet and a thicker sheet for laser-welding. Using CNC machinery, they cut the step's region from the thicker, simpler thin sheets. After inserting the thicker foil cutout black into the thin sheet's cutout region, they laser-weld the joint. Using laser beams to cut perforations, the fabricators.

Benefits of Using Step Stencils:

Conclusion:

In conclusion, step-up and step-down stencils are becoming increasingly crucial in PCB assembly, despite their appearance as sophisticated production tools. Simply put, when component sizes decrease and board complexity increases, regulated solder paste deposition becomes necessary. Complex PCBs require varying amounts of solder paste for a number of SMT components. Using varying foil thicknesses on the same stencil makes it easier to deposit enough solder on coarse pitch areas while carefully controlling the solder paste deposit in fine pitch locations.

By understanding when and how to use these specialized stencils, even novices may significantly increase the reliability and quality of their assemblies. Before you acquire your next stencil, consider whether a step stencil can solve common issues on your board. Consult your manufacturer, look into your options, and don't be afraid to move up or down.