SMT Stencil Printing & Cleaning: A Complete Guide to Consumables
Stencil printing is the most critical process step in surface mount technology (SMT) assembly. The quality of solder paste deposition — volume, consistency, and placement accuracy — directly determines the quality of every solder joint on the board. Maintaining a clean stencil and a clean printing environment is not optional; it is the foundation of a capable SMT process. This guide covers the key consumables used in stencil printing and stencil cleaning, and how to choose the right products for your printer and process.
Why Stencil Cleanliness Is Critical
Solder paste is a suspension of fine metal particles in flux. Any paste residue remaining in or around the apertures from a previous print cycle will contaminate the next deposit — causing bridging, insufficient paste volume, or aperture clogging. Understencil contamination — paste deposited on the underside of the stencil — transfers to the PCB surface during the next print cycle, causing smearing, bridging, and misregistration.
Understencil Cleaning Rolls and Fabric
Understencil cleaning rolls are the primary consumable for automated stencil cleaning in inline stencil printers. The cleaning fabric runs beneath the stencil on a take-up roll system, wiping the underside between print cycles in three modes:
Dry wipe cleaning — Removes loose paste and flux residue without solvent. Fastest cleaning mode for routine between-print cleaning.
Wet wipe cleaning — Applies a controlled amount of cleaning solvent to the fabric, dissolving flux residue and softening dried paste. Used periodically during a production run.
Vacuum cleaning — Applies suction through the stencil apertures while the fabric wipes the underside. Most thorough mode, used for fine-pitch apertures and whenever aperture clogging is a concern.
Understencil cleaning rolls must be matched to your printer's cleaning system width. Common widths include 300mm, 400mm, 440mm, 443mm, 500mm, 515mm, 530mm, and 607mm. ASM (DEK) UFP ECO understencil cleaning rolls are engineered specifically for DEK, Accela, Momentum, and MPM stencil printers.
Stencil Cleaning Wipes
Stencil cleaning wipes are used for manual stencil cleaning — during setup, at changeover, at end of shift, and whenever the stencil is removed from the printer.
Dry wipes — Low-lint nonwoven wipes for wiping away loose paste and final drying after solvent cleaning. Polyester wipes offer the lowest lint for cleanroom and fine-pitch applications.
Pre-saturated wipes — Pre-wetted with cleaning solvent (typically 70% IPA / 30% DI water) for consistent solvent delivery without measuring or mixing.
Stencil cleaner wipes — Pre-saturated with chemistries specifically formulated for solder paste and flux removal, providing more aggressive cleaning for heavily contaminated stencils.
Understencil Cleaning Chemistries
Liquid understencil cleaning chemistries are dispensed onto the cleaning fabric by the printer's solvent delivery system. Key considerations: compatibility with your paste flux chemistry, evaporation rate, and VOC content. ASM DEK Pro and DEK Pro XF chemistries are formulated specifically for DEK stencil printers and are compatible with a wide range of no-clean and water-soluble solder paste formulations.
Stencil Coating
Nano-coating products applied to the stencil surface reduce paste adhesion to the stencil underside, improving paste release from apertures and reducing the frequency of understencil cleaning required. ASM NanoClear stencil coating provides a durable, low-surface-energy coating that improves paste transfer efficiency and extends cleaning intervals.
Solder Paste Nozzles
Solder paste nozzles are the consumable tip components used in automated paste dispensing systems. They must be replaced regularly to maintain consistent paste deposition. Available in lead-free (green) and lead-rich (red) versions to prevent cross-contamination between alloy types.
Building a Complete Stencil Cleaning Program
An effective stencil cleaning program combines automated understencil cleaning during production with manual stencil cleaning at changeover and end of shift:
— Understencil cleaning rolls matched to your printer model and width, changed on a regular schedule
— Understencil cleaning chemistry compatible with your paste flux, dispensed at the correct rate
— Manual stencil cleaning wipes for changeover, inspection, and end-of-shift cleaning
— Stencil coating applied regularly to maintain paste release performance
— Paste nozzle replacement on a scheduled basis
KEM-TRON stocks the complete ASM (DEK) line of stencil printing consumables, including understencil cleaning rolls in all standard widths, DEK Pro cleaning chemistries, pre-saturated and dry stencil wipes, NanoClear stencil coating, and solder paste nozzles for DEK, Accela, Momentum, and MPM printers.