In surface mount technology (SMT) assembly, the quality of solder joints determines the reliability of the printed circuit board assembly (PCBA). One important factor in SMT assembly is the solder paste stencil.
Whether producing a single prototype or thousands of mass production units, a well-designed SMT stencil ensures precise solder paste deposition, stable solder joints, and long-term product performance.
At PCBONLINE, we treat every SMT project with the IPC-A-Class 2/3 standards using automatic solder paste printing and pick-and-place. For each SMT PCBA project, we custom-design and manufacture solder paste stencils for both prototype and high-volume PCB assembly.
Solder Paste Stencils Matter to Soldering Quality
During SMT assembly, solder paste is applied onto PCB pads before components are placed and soldered in a reflow oven. A solder paste stencil is a thin sheet with laser-cut apertures to control the amount and location of solder paste.
If the stencil design is poor, soldering defects can occur.
- Insufficient solder results in weak or open joints.
- Excessive soldering can result in bridging, short circuits, and solder beads.
- Misaligned solder results in tilted components or unreliable connections.
At PCBONLINE, we never compromise on quality and never apply solder paste manually or using simplified stencils.
For every SMT order, regardless of whether it is a prototype build or a mass production batch, we design a professional SMT stencil. This ensures that prototype boards and final mass production boards share the same soldering quality, making the prototype a true representation of the final product.
Types of Solder Paste Stencils We Provide
To meet different PCB SMT assembly requirements, PCBONLINE can design and provide two solder paste stencil types.
Stainless steel stencils
Made from high-quality stainless steel, our solder paste stencils are laser-cut for precision and long-lasting durability. They can be framed or frameless, and are suitable for prototype and mass production needs.
- The most commonly used option.
- Provides high durability, precise aperture definition, and repeatable solder paste deposition.
- Suitable for most prototype and mass PCB assemblies.
Step stencils (step-up and step-down)
A step stencil is a specialized solder paste stencil that features areas of different thicknesses. It is used for PCBs that require different solder paste thicknesses in different regions, including:
Step-down stencils: Areas are thinned to reduce solder paste volume. These are ideal for fine-pitch ICs, QFNs, and BGAs, preventing solder bridging and voids.
Step-up stencils: Areas are thickened to increase solder paste volume. These are suitable for large components such as inductors, capacitors, connectors, and shielding cases, ensuring enough solder for a strong joint.
PCBONLINE's Solder Paste Stencil Design Tips
The key to solder paste stencil design is designing the stencil apertures properly. Our engineering team has rich experience in solder paste stencil design. We don't just copy pad shapes but also adjust aperture designs to prevent solder defects and form long-term solder joints.
Here are some proven stencil design techniques used at PCBONLINE:
Fine-pitch ICs and QFPs – To prevent stress concentration, we design the two ends of the aperture with rounded corners.
BGAs and ultra-small passives (0402, 0201) – Square-shaped apertures are used for consistent solder ball formation.
QFN ground pads – When applying a cross-shaped ("+") aperture, we ensure at least 0.5 mm clearance from the PCB pad edge to avoid solder beads and guarantee effective grounding.
USB, Mini USB, and CF/SD connectors – For connector grounding apertures, we extend the length in a 1:1.2 ratio to increase solder volume. To reduce stress and prevent solder beads, we use an inward concave opening method.
General ball-grid aperture rule – The aperture width must allow at least four solder balls to pass through freely, ensuring reliable reflow results.
Example: rectangular corner pads on ICs
For ICs with rectangular corner pads, our engineers extend the stencil slot beyond the pad outline. This ensures that during reflow, the solder paste has enough volume to climb up the vertical edges of the pad, forming a solid, complete solder joint.
Without this adjustment, solder paste would only wet the pad bottom, leaving the sides partially unsoldered and reducing joint strength.
The above engineering adjustments demonstrate PCBONLINE's solder paste professionalism. We take care of all details to bring the best soldering reliability.
PCBONLINE Provides SMT Stencils for Prototype PCB Assembly
PCBONLINE provides PCB assembly from 1 piece to 100,000 pcs. Some people may assume that solder paste stencils are unnecessary for prototype PCBs since the quantity is low. However, we insist on professional and automatic SMT assembly, so we provide SMT stencils to all SMT projects.
Benefits of using solder paste stencil for prototype PCB assembly:
- Consistency with mass production – The prototype behaves the same as the production unit, making testing and validation accurate.
- Fewer defects – Proper solder paste deposition reduces rework, saving time and cost.
- Faster transition to mass production – No need to redesign or adjust the stencil later; the same setup can scale directly.
At PCBONLINE, "no shortcuts in prototypes" guarantees that you can rely on our PCB assembly quality consistency.
Using a Solder Paste Stencil in SMT Assembly
For each of our automatic SMT lines, we place a solder paste stencil in the solder paste printing machine. Our SMT production process includes:
- Automatic solder paste printing with precision stencil alignment.
- 3D SPI inspection for paste volume and uniformity.
- High-speed Yamaha YSM20R pick-and-place machines for small-footprint surface mount components.
- Yamaha YSM10 flexible placement machine for large-footprint SMD parts.
- Pre-reflow 3D AOI inspection to verify correct placement and polarity.
- 10-zone lead-free reflow oven with optimized thermal profiles.
- Post-reflow 3D AOI inspection for solder joint quality.
Our fully automated SMT process using the carefully engineered solder paste stencil ensures reliable SMT assembly for both prototypes and mass production.
Partner with PCBONLINE for SMT PCB Assembly
If you are looking for a trusted partner from prototype PCBA to batch production, parter with PCBONLINE with guaranteed reliability. We provide SMT assembly, PTH assembly, PCB fabrication, component sourcing, fixtures, enclosures, box-build assembly, value-added services, testing, and shipping the finished devices you can sell.
PCBONLINE is an OEM PCBA source factory manufacturer founded in 2005. It has two large advanced PCB manufacturing bases independently fabricating all types of PCBs, one EMS (electronic manufacturing service) PCB assembly factory with two component warehouses, stable supply chains, and an R&D team. Besides, it keeps long-term cooperation with the top 3 mold/enclosure manufacturers in China to provide the molds, jigs, and enclosures for full box-build solutions.
Each SMT line at PCBONLINE is equipped with three advanced pick-and-place machines, supporting up to 376 SMD placements per PCB, enabling us to handle all types of SMT assembly, including high-density and large-format PCBs.
Our SPI and dual AOI systems perform 3D surface scanning, offering thorough and accurate inspections for superior quality control throughout the process.
We conduct first-article inspections both before prototype assembly and before mass production to ensure the highest PCBA quality and prevent costly issues.
By BOM component sourcing and in-stock warehousing, as well as PCB optimization advice, we reduce the circuit board assembly costs without quality sacrifice.
Our experienced R&D and engineering teams provide one-on-one support, solving technical and non-technical challenges while offering optimization suggestions to ensure your project runs smoothly from start to finish.
High-quality PCBA manufacturing certified with ISO 9001:2015, ISO 14001:2015, IATF 16949:2016, RoHS, REACH, UL, and IPC-A-610 Class 2/3.
When your project enters the bulky production stage, PCBONLINE refunds the fees of prototyping, including its value-added services.
Whether you need 5 prototype circuit board assemblies or 10,000 production units, PCBONLINE ensures that every PCBA meets the highest reliability standards. To get a quote from the SMT PCBA manufacturer PCBONLINE, contact info@pcbonline.com.
Conclusion
Solder paste stencils are important for PCB assembly from prototype to mass production, as they matter to solder joint consistency, reliability, and product performance. To ensure the success of your PCBA projects, send the quote request from the table on the right to work with PCBONLINE.
PCB assembly at PCBONLINE.pdf
