What is Solder Escape?

Solder escape, also known as solder wicking, is a common issue in PCB Assembly where molten solder flows away from the intended joint and spreads across the board or up component leads. This unwanted solder flow can cause shorts, bridges, and other defects that impact the reliability and functionality of the assembled PCB.

Solder escape typically occurs during the reflow soldering process, when the PCB and components are heated to melt the solder paste and form electrical connections. Several factors can contribute to solder escape, including:

• Excessive solder paste volume
• Incorrect reflow profile
• Poor stencil design or aperture placement
• Inappropriate pad geometry or spacing
• Contamination or poor wetting surfaces

To minimize the risk of solder escape and ensure high-quality reflow soldering results, it’s important to understand the causes, implement preventive measures, and follow best practices throughout the PCB design and assembly process.

Strategies for Preventing Solder Escape

1. Optimize Solder Paste Volume

One of the primary causes of solder escape is applying too much solder paste to the pads. Excessive solder paste can lead to solder balling, bridging, and wicking as the molten solder seeks to minimize its surface area during reflow.

To optimize solder paste volume:

• Use solder paste with appropriate particle size and viscosity for your application
• Design stencil apertures with the right area ratio and aspect ratio for the pad size and pitch
• Ensure proper stencil thickness, typically 4-6 mils (0.1-0.15mm) for standard SMT components
• Maintain stencil cleanliness and alignment during printing
• Monitor solder paste volume and consistency using 3D solder paste inspection (SPI) systems

Here’s a table comparing common stencil thicknesses and their suitability for different Component Types:

Stencil Thickness	Suitable Components
3-4 mils (0.075-0.1mm)	Chip components (0201, 0402), fine-pitch QFPs and BGAs
4-5 mils (0.1-0.125mm)	Standard SMT components (0603, 0805, SOICs, PLCCs)
5-6 mils (0.125-0.15mm)	Large SMT components, connectors, through-hole leads
6-8 mils (0.15-0.2mm)	High-volume solder deposits, mechanical attachments

2. Optimize Reflow Profile

The reflow profile plays a critical role in controlling the solder melting and solidification process. An incorrect reflow profile can cause solder escape issues by exposing the molten solder to excessive temperatures or dwell times.

To optimize the reflow profile:

• Follow the solder paste manufacturer’s recommended profile for your specific alloy and flux system
• Use a multi-stage reflow profile with distinct preheat, soak, reflow, and cooling zones
• Adjust the peak temperature and time above liquidus (TAL) based on the PCB thickness, density, and thermal mass
• Minimize the temperature delta between the top and bottom of the PCB to prevent thermal stress and warpage
• Verify the actual PCB temperature using thermocouples or profile boards

Here’s an example of a generic reflow profile for lead-free solder (SAC305) with key parameters:

Reflow Stage	Temperature Range	Duration
Preheat	25-150°C	60-90 sec
Soak	150-180°C	60-120 sec
Reflow	235-245°C (peak)	30-90 sec
Cooling	< 6°C/sec	–

3. Design for Manufacturability (DFM)

Incorporating DFM principles into your PCB design can help minimize solder escape issues by ensuring proper pad geometry, spacing, and solder mask clearance. Some key DFM guidelines include:

• Use solder mask-defined (SMD) pads instead of copper-defined pads for better solder paste containment
• Provide adequate spacing between pads, typically at least 0.2mm for standard pitches
• Avoid acute angles or sharp corners on pads that can promote solder wicking
• Maintain a solder mask clearance of 50-75um around the pads to prevent solder mask from interfering with solder wetting
• Follow the component manufacturer’s recommended footprint and stencil designs

Here’s a comparison of solder mask clearance and its impact on solderability:

Solder Mask Clearance	Effect on Solderability
< 25um	Insufficient clearance, solder mask may inhibit wetting
25-50um	Marginal clearance, potential for inconsistent wetting
50-75um	Optimal clearance, good solder wetting and containment
> 75um	Excessive clearance, increased risk of solder escape

4. Ensure PCB Cleanliness

Contamination on the PCB surface can interfere with solder wetting and cause solder escape issues. Common contaminants include flux residues, fingerprints, dust, and oxidation.

To ensure PCB cleanliness:

• Handle PCBs with gloves to avoid fingerprints and skin oils
• Store PCBs in clean, moisture-barrier bags to prevent oxidation and dust accumulation
• Clean the PCB surface with isopropyl alcohol (IPA) or specialized cleaners before solder paste printing
• Use no-clean or low-residue flux systems to minimize post-reflow contamination
• Implement a controlled environment with temperature, humidity, and particle control for critical assembly processes

5. Implement Process Controls

Robust process controls throughout the PCB assembly process can help identify and correct potential solder escape issues before they impact the final product quality. Some key process control steps include:

• Implement solder paste inspection (SPI) to monitor paste volume, alignment, and consistency
• Use automated optical inspection (AOI) to detect solder bridges, shorts, and other post-reflow defects
• Perform X-ray inspection for hidden solder joints, such as BGAs and QFNs
• Conduct regular maintenance and calibration of printers, pick-and-place machines, and reflow ovens
• Train operators on proper handling, setup, and troubleshooting procedures
• Document and analyze process data to identify trends and implement continuous improvement initiatives

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nFRutrcRx+RDKhxzvPX3JJqqY9h/wBa6t0ADE/yNOxDZavdP8m3LRxIgzywPzZqO3tXESsbhBgco+OagRJZt6SXBZM8LuIb8AaT7IQ5PzuOys+KWwIvJbo/3lVuhGHyOPYVZXyInzNHDtIGAvOT0yaZpySRtLG9s6h1wuGVgM++c0ssRgkz+6Kf7bZNSaJEitc7neEJuXoEQcqKhe4uJA6ywuc8EkDA96kxPOFECHpyEzyB9KWIS20qpPBJg8kEg5FA2jNVXWQF5MKpyFVOM+manaRzGcP5eTxk4LfhVu8mZc+YfLhH+rCR4I+prO86ANumDugwVwmM1RmkSTuhWNWY5OATgj+VTxxRRplGdsjkt0BPpSW95YM/mADao+5KP5ZFPuJtNdS22UbugV9q/gDRcbiQgyxkspnbP9zGMfjTCwjLEec7MOQzdM0+KTT41LxiXIG7aGLZP41DJPDOynY6BjgqAeffIpkWEd7e28t42Z3c5cNn5PoacL2JTuIO9xgbqVLEDErKdpI++Twv40mpxRBoXt2iZVUF9vzfjVCsVrjzGHCkKQOVb1+lNg02WZDNlNo4+c9/xqS2j3NvhDO2Mkdvfg0+NG807uhOfLZsLnNTystFV4LhNwIUY6FTn9KmgRFVTNgnnPUYq7dTRW6qm2EzPgKqksFz71D9lk2CQtGZjz5ZH48Zo5RMkt/sMau26LGCT5y9PoTWdNLbStII3BQE8x/KppHeXDxzQAA52hhjn2IqssTsWURMoHPCkZ9qpRC5atJYQGU8KeASM09tQMGCgO6MnDY/lUEFrdiISSIxUNngcj8qtQhrrzFijDCMDO8Y5/Gnawtyt9oafdKR8z9Wyc0G4ePy0V5Djn6H61dmkVYvsksUaFuVMafN+dRJt8qSFolHy4VnwD+FJsaiPDyXITO8kcLuxj8KWUXETIuflI5YYGKr2/2eILHO535Ij2sQT71au7JlhEsjzKjfd3t1NK43HQznunEhUKXQHlmcgH8KvxBXC4RRnnrkVSjtoiREwlbzGwGyAq/WrwtYIIxHDKyMvfO7JNO5HKJI1wjgxhRjj7oIxTd2oujhWGw8kKvX8asPBewxiRvmDD5WGMH61Csl1ExDbfmXPyk5/lTUkS6YRT3CARfJEBg72QE/maJm8xt5lEpHdVAIx7CnMqNtLEshOCT2Pvmm+SbcnJQq4yu3BOK1jqYuNiRJLUxFoJn83BEiyJkL71Cvlsfvjcc9BwTUwgfyWlDIkTHGSQCahW0CsWLrt6ghxVWJBFnUkkALnjcMD606VJ+rGIpjnbJyfypDLuJEjsyjAGSKY6wjGznPvRYV7CRGMqUWPa2c7+SR+JqeRGii80yxOpwMr94fWmKJF4HygjkY4pfs7y4HG3OSOMfiKLE3KqPuDvEwY56NSs8p4YDA9BVnyoVONy56EIu0cU5lIUkKCvTJpiuV1IUBznGcckfyFWN0Xq35GqqieJ90UYck5AX5v0NWftOo/wDPs/8A3wKdyrI1Lpkm8ouoLJkEr8qgE56Cm3CxwANH5E0DqCzJklCexqhdSiTaqOyufvbHHA9wajRbvY8UTLGjctJwxP1Fcp0Isq7gMI/s5BP3UzuAqRpbdNqEvHKQCGK8ZNVbSa+kl8mPYxQjayx4BPucVo3BnglBMEEsxjy5fdgH/ZBpF3sNzfFcRRNc7RkMu0cfQVVZdSmDb7aYkDoCBxVu1vGs/MuHhWNnUqwYsCfoBxQ98b512LLCowSxP3vw60ikyjbXYh81JLOUN0y4yQfoantdVtrVpvORmLKViXZyp9eKu7bVyWknOwoQ5kj2yBuxXNZcthbNvdLmZcZKM6qGqkhNlhZrN43aZwZTkgHIIz7GoQwON6ymMkgDjaQfYVHJbWgiTK3MrPjcQu0/nVmJgiBY4nwowA7jPTuTV8orkvlWSbDFFbhmxjZy2T/ezV2RbO2tm8+JGkkwU2Dd+ZFVkWzWAu0CiZjzmTcf0rP220zOk1xcW4PR0G/j2BNTYZazA648lNpP3s/0FSuLC2jx5GyQjIl5/QGs97PTIiTHeTfLgh23qXP0qSFpLzaRcfKnB81ecDjvUOJopFR57lyzRsZiT/F0x9Kmtbi8mkCSeWpxgeYSQB6Yq3JBZttDXW0jr5YVPzxTBb6ZEu5pGkyeFEmCT9afKJzJX+yxRP58yzknAjgQ8Z46is2ays9pmgcq2RlSSGHfpWkbm2t1UW9swLcFPlYfizVnvDdzvJKWULkjaCpYH6CnYzbK8MBkbf8AOBGxGU5Jq7DDFM5KtOvOOnWp7CPyt8akKSCWcjqKGeK3STBkYsTuO8gAfhStcpMvx23kozYfJ4z1JzUGLWLcojbcw5kYBgD9DVVLwAozO0cYHBLlgfzq0+JE87zYzCxzkZP6VPKXzgJBCq7pZwzH71uNuF/DiiJJ7if5ZXMGM+bcyfMPoKqfaADIRMwjVSNpjPJH1qNJxOgYLIzB/l2rxn6CjlDmuWLtZpLhIxlIoxnzZTlHwc5FVjMzySKWikUcAkbQO1TXVxcEwrKwQpgorp8v5U1lurlmZorcZXaNnyj6kVXKJSsyxbWNm22VzCG/2WyM/jUN/aWplh33EaqCOhDKPwqCOC4hUxMImXJI3M3X6irFtYxnzzcABduURPmz+dLlY+ZMgha2MvkE5TOBImACKuLawo+yKX5c8evNLDYwKryoIC4Pyo5ZWAH0oln8mEuYYEYEkkFieOe9UK5NdWiQxbbqaVVdflZj8p/AVlLHNESEPmRj7oTHzD0NPF8t4VNw5mjAxsVtpx7ZqEXMkbN5UYjiHCh/mP6UXFYtraSSr5hElsT0CMOfrTBprEljOp54Lmmw3bPnzsMDkDaG4qaORk3bQCM5APb6CtERK6ImgkjIUBSwwQwApZd5wzsvnYwDkgCp3nL7GSNvMX+IkAHHqKlF5HdFUuktI40GAVKsSffHNURzFFftRAP2yEgfwlQ1XZmg+zx5aNpONzJwQKhn+xRsmzaVyB8in5jVgJIskbDywpGArpnGfWnZId2USsZJEXmGPGeHIBNLAsyOxiiVUP32ycj6in3EskTusjRnccBQgUD6YqCI3MLNcNG8kLdBCwJz7ipbQ7NkrQyNIDKFYZBD9utWLlA01uLdLaQhRuz1J+hqg97Kx2yI8aE5AbHy/XFTQ4LALIsjEZTjnP1qWkwUmhssJll2yRYmBypCYRcdt1JPFfXgxucpGMEAZUfjU25YnZZXkbdyVwSM+lWHs/tEPn291NHEv34FUpyPXNS4WL5zG8qeMvE4lwB8pIyCafHDNIASCh7sen5VZb7UWBWTGBtCsOo9SamEyLCytFAWY/OxlPA9hQo3Fzk6kwwKHuYZCRhVJ5z9Kr+bNI20Id4xyFwuD6GkM1jGYQwcKTyIkDnJ75NTOwmwkN0yMR8i7AG+pJp8gvaDHhcBtyN5ZHzYHANQMBJsFrFKwUYff83TqRitCW5uLKFbaTbKXXlmbrn6VQi+0wxyCN0i8zOVXOSD71STRlJpjGOnBCZEuWmB/wBXt/dgjvUJkEsgzE2wdBjaBUiCdckyDryCQc+1JI8gwPL4boRxWlmRdBKCq5CRbB2zyKhLxFRtimLfxHoop7KzsGfIxx/nFEilsBZSFAyQMc/hQSxqzPOQhG0IMKM9fxpWcwEKAT0ztGePrSJD8wfJ9QDxj3qUo+SwcD04FMm5XkkkMkQQsN55LDoKleNzI8X2gvHgNuAwR7VITOQAWBUdAEGT+NMywJ+Uj3NUkK42AR2swdVaYAHG4kAGrn2+b/nmPzqt5itlUUl/rinbZ/7q/wDfS0WAkZLORm80SW9zj5ijApIvbr3ogWXY0SRAIHBMzup+Uc068+/N9F/9BrPT/US/SuexqmdLb3tqsboLSSVBgGUMqc+oIqC5t5pEe4UlEU53SSBiB6cVkWv/AB6v/vVqWX+om/CpsaoGubgQK6PHPgY8psBvrk01iXUNBJGkjAb4yASpPoRSj/XT/wC7VdP+PtvoKdh7FsWhXP2hopSVyFdsA+xNRv8AZYfLEcCKW4PzhguPSi4/1E/0NV4v+Xb6VSRPMW5poJY4UMRLgnJXKrj15qssLDzGDMgGCFGG3fjV67/1DfhVaD/VP9BViuNjmjVvuJvUbsuBz7VcWbTr2286S22vGxG6NPlJrPb/AFx+gq6v/IPb/fX+dTYOZlNZZpmcKse2NvlV1I3Ury3MhCNZqh+7iLjdV5f9SPwpbf8A4+F+opWKKKQW8W4TW8i8Zbf0/Oqlxb2sTo9tBM6McswyyitvUP8AWv8ASiD/AI9m+ppDM+1a2nZUmilUKOuCucetPeaASHyLMRKDhnc5L9qmPT/PrUE33D+NArjBI0cskkYyjgbg2Sv4Um6eXfDbwW+2UbXeTPyjuRVu0+4f901Gv8f1oSGmFvpNyUZgqTCMYVHHysfbFTxfaomUPZW6LGM+W56/QVpW33Iv94Vn6j/x+j6f0osJsrz3EV/I0Zi8tsYVIlyOPXFV0S4tS5jQKoPzCXg/UCrth/x9x/739a1Nb+430FCjcnmMEToxJcI/puOcfSpEljzsEIYv/wAtOcD6Yqlb/eP1P860l/h+gqrBzMhkdowwTa5zgAoSB+NTAS28Ikk8oPKuAQDhQfSnL/H9TUmof6q3+iU7C5mZ0Ahhd5ZJt2c4yCRz7UTESqzhXZV5wyYB+lSt/qPxoX/Vr9DRyoXtGQ2lzAGQTwQiI8bVG1gPcip4bW3juJJLcvLG5yFbB2+wzUSf6uepLLq/0NRKCNY1GXri3V4xJgQlRyEUZP1rGxbxlnkd2Jyfmzjr6Ctg/cb61Tn6Q/WoijSU9CNBAVDi2kw3JypAaomtdNXLzWiR7uVPKn6jFbWofes/+uI/kKr3nS3/AN0VujBszogsiskbyKitkNt6fQmpmdo0cG53ZHV/vfpWhL90f7grMH+vf/rmatIhydyALAXj850k3jgykhT+NRzSxxyFYowhB4WMsVbHep73/Vw/UVag/wDZKTijSM2UvKiuQHkhlDnnrkfjip4I4I9phCkxk7iex96mh6Sf7xqH+Gf/AHv61m0VzEgUszszDfIeGjB+T3ppkaHdCZjMxHUN/QU5f9S/+7WXF/x+D6f1qB3LjQTyAyM22KPlixxj24psctk+FLoHyQODz9a0pf8AkHXv/AawdO/4+G/GmlYRfkG3AkULGcFJOgNTC7smPlEwjC8Ooyc/U0aj/wAe8f0FZKffj+prREstSBGcmNnkH95ux9qaEuCRtBIPXJ7Vaj7/AEob7z/QVcUYsaI5Ao2/L3OBnBpsgnfAdlPHykDB/GtSz+7c/wDXI1Wk+7H+NVYRRbe2M7cAY/pzURhGQxVcjpipz/x73H1H86j/AOWa/wC4KLCYzMquhHCgg+ucdjVmW4gnIbylRwoGEGASKhH3D9KB2oI6ku11TBYYbnjGQKiZTjqSPfFPHQ1Kv3D/AJ7U0MpbY42zwCevvT/Mt/f86kuPufgaiT7if7q/yoA//9k=” 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FAQ

1. What is the difference between solder paste and flux?

Solder paste is a mixture of tiny solder particles (powder) suspended in a flux medium. The solder particles provide the metal content for forming the solder joints, while the flux removes oxides, improves wetting, and protects the metal surfaces during reflow. Flux, on the other hand, is a chemical compound that is applied separately to the PCB or component leads to prepare the surfaces for soldering and promote solder wetting.

2. How can I tell if my solder paste volume is too high?

Excessive solder paste volume can be detected visually by inspecting the printed PCB for solder paste height, consistency, and alignment. If the solder paste deposits appear tall, uneven, or slumped, it may indicate too much paste. Additionally, solder paste inspection (SPI) systems can provide quantitative measurements of paste volume, height, and area to help identify potential issues.

3. What should I do if I observe solder escape or bridging after reflow?

If solder escape or bridging is detected after reflow, the affected PCBs should be reworked to remove the excess solder and ensure proper joint formation. Rework methods may include manual soldering with a fine-tip iron, hot air rework, or specialized desoldering tools. It’s important to identify and correct the root cause of the solder escape issue to prevent recurrence in future assemblies.

4. Can solder escape be caused by the PCB Surface Finish?

Yes, the PCB surface finish can impact solder wetting and the risk of solder escape. Some surface finishes, such as HASL (Hot Air Solder Leveling), can result in uneven or inconsistent pad surfaces that promote solder escape. Other finishes, like ENIG (Electroless Nickel Immersion Gold) or OSP (Organic Solderability Preservative), provide flatter, more consistent surfaces that can help contain the molten solder during reflow.

5. How can I optimize my reflow profile for different PCB designs?

Optimizing the reflow profile for different PCB designs involves considering factors such as the PCB thickness, component density, thermal mass, and solder paste specifications. It’s important to start with the solder paste manufacturer’s recommended profile and then fine-tune the parameters based on the specific PCB characteristics. Using thermocouples or profiling boards to measure the actual PCB temperature during reflow can help validate and optimize the profile for each design.

Conclusion

Preventing solder escape is crucial for achieving high-quality, reliable solder joints in PCB assembly. By understanding the causes of solder escape, implementing preventive measures, and following best practices for solder paste printing, reflow soldering, and PCB design, manufacturers can minimize the risk of solder escape and improve overall assembly yields.

Key strategies for preventing solder escape include optimizing solder paste volume, fine-tuning the reflow profile, incorporating DFM principles, ensuring PCB cleanliness, and implementing robust process controls. Regular monitoring, inspection, and continuous improvement initiatives can further help identify and correct potential solder escape issues before they impact the final product quality.

By adopting these tips and tricks for avoiding solder escape, PCB assembly professionals can enhance their soldering processes, reduce rework and scrap, and deliver high-quality, reliable electronic products to their customers.