PCB Assembly\nExpress works closely with Stencils Unlimited to provide optimized SMT stencil\ndesigns. Each stencil is carefully tailored to your PCB layout, component mix,\nand assembly requirements. They:<\/p>\n\n\n\n
- \n
- Review
stencil design as part of DFM analysis<\/li>\n\n\n\n - Optimize
thickness based on component mix<\/li>\n\n\n\n - Apply
aperture modifications where needed<\/li>\n\n\n\n - Ensure
compatibility with assembly process<\/li>\n<\/ul>\n\n\n\nComponent Misalignment (Shifting and Skewing)<\/strong><\/strong><\/h4>\n\n\n\n
This occurs when\nan SMT component is not properly positioned on the printed circuit board after\nplacement or reflow in the PCB assembly process. It may shift (move off its\npads) or skew (rotate out of alignment), resulting in poor solder joints and\npotential electrical issues. A properly designed stencil helps reduce this\nmisalignment by providing uniform solder paste deposition, creating balanced\nsurface tension forces during reflow to keep components centered on their pads.<\/p>\n\n\n\n
![\"\"](\"https:\/\/www.pcbassemblyexpress.com\/blog\/wp-content\/uploads\/2026\/05\/1-Component-Misalignment.jpg\")
<\/figure>Figure 1. Component Misalignment<\/figcaption><\/figure><\/div>\n\n\n\n <\/div>\n\n\n\nInsufficient Solder Joints?<\/strong>
<\/h4>\n\n\n\nThis occurs when there isn\u2019t enough solder on a pad to properly connect the component lead, resulting in weak or incomplete connections that can cause electrical failures. In the PCB assembly process, a properly designed stencil helps prevent insufficient solder joints by ensuring the correct amount of solder paste is deposited on each pad for strong and reliable connections. <\/p>\n\n\n\n
![\"\"](\"https:\/\/www.pcbassemblyexpress.com\/blog\/wp-content\/uploads\/2026\/05\/2-Insufficient-Solder-Joints.jpg\")
Figure 2. Insufficient Solder Joints
<\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\nSolder Bridging?<\/strong>
<\/h4>\n\n\n\nThis occurs when excess solder unintentionally connects two nearby pads or component leads, creating a short circuit that can cause the circuit to malfunction. In the PCB assembly process, a properly designed stencil helps prevent solder bridging by depositing the right amount of solder paste in the correct locations, reducing the chance of solder flowing between adjacent pads during reflow.<\/p>\n\n\n\n
![\"\"](\"https:\/\/www.pcbassemblyexpress.com\/blog\/wp-content\/uploads\/2026\/05\/3-Solder-Bridging.jpg\")
<\/figure>Figure 3. Solder Bridging<\/figcaption><\/figure><\/div>\n\n\n\n <\/div>\n\n\n\nTombstoning
<\/strong><\/h4>\n\n\n\n<\/strong>This occurs when a small surface-mount component (such as a resistor or capacitor) lifts up on one end during soldering in the PCB assembly process, standing upright instead of lying flat on the printed circuit board, which can lead to a poor or open electrical connection. A properly designed stencil helps reduce tombstoning by ensuring equal solder paste is deposited on both pads, creating balanced forces during reflow and keeping the component flat.<\/p>\n\n\n\n
![\"\"](\"https:\/\/www.pcbassemblyexpress.com\/blog\/wp-content\/uploads\/2026\/05\/4-Tombstoning.jpg\")
Figure 4. Tombstonig <\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\nVoiding Under\nComponents<\/strong><\/h4>\n\n\n\n
This occurs when\nsmall air pockets or gaps form inside the solder joint beneath a component,\nwhich can weaken the connection and reduce heat transfer.
\nA properly designed stencil reduces voiding by controlling solder paste volume\nand using segmented aperture patterns that allow trapped gases to escape during\nreflow.<\/p>\n\n\n\n![\"\"](\"https:\/\/www.pcbassemblyexpress.com\/blog\/wp-content\/uploads\/2026\/05\/5-Voiding-Under-Components.jpg\")
<\/figure>Figure 5. Voiding Under Components<\/figcaption><\/figure><\/div>\n\n\n\n <\/div>\n\n\n\nSolder Skipping<\/strong> <\/p>\n\n\n\n
This occurs when solder does not properly adhere to part of a pad or component lead during the PCB assembly process, leaving areas without solder. A properly designed stencil helps reduce solder skipping by ensuring consistent and sufficient solder paste deposition on every pad, allowing the solder to entirely wet the leads and form complete, reliable joints during reflow.
<\/p>\n\n\n\n![\"\"](\"https:\/\/www.pcbassemblyexpress.com\/blog\/wp-content\/uploads\/2026\/05\/6-Solder-Skipping.jpg\")
Figure 6. Solder Skipping<\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\nSMT Stencil\nTypes<\/strong><\/h4>\n\n\n\n
Choosing the\nright stencil type helps achieve consistent results across different component\nsizes and layout densities.<\/p>\n\n\n\n
Standard\nLaser-Cut Stainless-Steel Stencil:<\/strong>
\nThis stencil has a uniform thickness and is used for most SMT assemblies\nwithout large pitch variation. It also offers the lowest lead time and\nmanufacturing cost.<\/p>\n\n\n\nStep Stencil\n(Step-Up \/ Step-Down):<\/strong>
\nThis stencil has varying thicknesses in specific areas to increase or decrease\nsolder paste volume for mixed component sizes.<\/p>\n\n\n\nElectroformed\nStencil:<\/strong>
\nThis stencil offers very smooth aperture walls for improved paste release,\nmaking it ideal for fine-pitch and high-density designs; however, its thickness\nis not as uniform as that of a standard stencil.<\/p>\n\n\n\nNano-Coated\nStencil:<\/strong>
\nThis stencil has a special surface treatment that enhances paste release by\nreducing solder paste adhesion to the stencil walls. It also reduces the\nfrequency of stencil cleaning.<\/p>\n\n\n\nStencil\nThickness <\/strong><\/p>\n\n\n\n
Stencil thickness\ndetermines how much solder paste is deposited. Choosing the right thickness is\nessential for balancing solder volume across all components.<\/p>\n\n\n\n
- \n
- Small pads require thinner stencils<\/strong> to avoid excess solder <\/li>\n\n\n\n
- Large pads may require thicker stencils<\/strong> for proper solder joints <\/li>\n<\/ul>\n\n\n\n
Typical\nThickness Guidelines<\/strong><\/p>\n\n\n\n
- \n
- 0.004″ (100 \u00b5m)<\/strong> ? Fine-pitch components (e.g., QFNs, BGAs) <\/li>\n\n\n\n
- 0.005″ (125 \u00b5m)<\/strong> ? Most common, general-purpose designs <\/li>\n\n\n\n
- 0.006″ (150 \u00b5m)<\/strong> ? Larger components or higher solder volume needs <\/li>\n<\/ul>\n\n\n\n
Aperture\nDesign & Modifications<\/strong><\/p>\n\n\n\n
Stencil apertures\n(openings) control how much solder paste goes onto each pad. Proper aperture\ndesign helps prevent defects.<\/p>\n\n\n\n
Aperture\nReduction:<\/strong>
\nAperture reduction makes the stencil opening smaller than the pad to control\nhow much solder paste is applied and prevent defects like solder bridging.\nReducing the aperture size by about 5\u201320% helps limit excess solder and is\ncommonly used for fine-pitch components, QFNs, and BGAs.<\/p>\n\n\n\nHome Plate\nApertures:<\/strong>
\nHome plate apertures use a trapezoidal shape to reduce the amount of solder\npaste at the front edge of chip components, helping prevent tombstoning by\ncontrolling how the solder pulls during reflow.<\/p>\n\n\n\nWindow Pane\nApertures:<\/strong>
\nWindow pane apertures divide large stencil openings into smaller sections to\nbetter control solder paste on large pads, such as thermal pads. This design\nhelps reduce voiding, improves paste release, and prevents components from\nfloating during reflow.<\/p>\n\n\n\nRounded\nApertures:<\/strong>
\nRounded apertures improve solder paste release from the stencil by reducing\nsharp corners where paste tends to stick.<\/p>\n\n\n\nOverprint\nApertures:<\/strong>
\nOverprint apertures are slightly larger than the pad to increase solder volume\nwhere additional solder is needed for stronger joints.<\/p>\n\n\n\nStep Aperture Modifications:<\/strong>
Step aperture modifications locally adjust paste volume by varying stencil thickness or aperture design to accommodate mixed component sizes.<\/p>\n\n\n\n<\/p>\n
- 0.005″ (125 \u00b5m)<\/strong> ? Most common, general-purpose designs <\/li>\n\n\n\n
- Large pads may require thicker stencils<\/strong> for proper solder joints <\/li>\n<\/ul>\n\n\n\n
- Small pads require thinner stencils<\/strong> to avoid excess solder <\/li>\n\n\n\n