SMT Stencil Printing & Cleaning: A Complete Guide to Consumables
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. During printing, paste is forced through apertures in the stencil onto PCB pads. 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. Over time, uncleaned paste dries and hardens, making it progressively more difficult to remove and increasing the risk of print defects.
Understencil contamination — paste that has been forced through the apertures and deposited on the underside of the stencil — is a particularly common source of print defects. It transfers to the PCB surface during the next print cycle, causing smearing, bridging, and misregistration. Effective understencil cleaning is essential for maintaining print quality across a production run.
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 of the stencil between print cycles — either dry, with solvent, or with vacuum assist, depending on the cleaning mode selected.
Dry wipe cleaning removes loose paste and flux residue without solvent. It is the fastest cleaning mode and is used for routine between-print cleaning where paste has not yet dried or hardened.
Wet wipe cleaning applies a controlled amount of cleaning solvent to the fabric before it contacts the stencil, dissolving flux residue and softening dried paste for more thorough removal. Wet cleaning is used periodically during a production run and whenever paste has begun to dry in the apertures.
Vacuum cleaning applies suction through the stencil apertures while the fabric wipes the underside, pulling paste residue out of the apertures from below. This is the most thorough cleaning mode and is used for fine-pitch apertures and whenever aperture clogging is a concern.
Selecting the right roll size: Understencil cleaning rolls must be matched to the width of your stencil printer's cleaning system. Common widths include 300mm, 400mm, 440mm, 443mm, 500mm, 515mm, 530mm, and 607mm. Roll length determines how many cleaning cycles are available before the roll must be changed — longer rolls reduce changeover frequency in high-volume production.
ASM (DEK) UFP ECO understencil cleaning rolls are engineered specifically for DEK, Accela, Momentum, and MPM stencil printers, providing consistent fabric tension, controlled solvent absorption, and reliable take-up performance across the full production run.
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 for inspection or storage. They are also used for cleaning stencil frames, squeegee blades, and printer components.
Dry wipes are low-lint nonwoven wipes used for wiping away loose paste and for final drying after solvent cleaning. They are available in polyester, polycellulose, and poly-blend constructions — polyester wipes offer the lowest lint and are preferred for cleanroom and fine-pitch applications.
Pre-saturated wipes are pre-wetted with cleaning solvent — typically 70% isopropyl alcohol (IPA) / 30% deionized water, or a proprietary cleaning chemistry — and are ready to use without measuring or mixing. They provide consistent solvent delivery and are convenient for manual cleaning at the printer or at a dedicated cleaning station.
Stencil cleaner wipes are pre-saturated with chemistries specifically formulated for solder paste and flux removal from stencils, providing more aggressive cleaning than standard IPA wipes for heavily contaminated stencils or dried paste residue.
Understencil Cleaning Chemistries
Liquid understencil cleaning chemistries are used in the wet cleaning mode of automated stencil printers, dispensed onto the cleaning fabric by the printer's solvent delivery system. The chemistry must be compatible with the printer's dispensing system and with the solder paste flux chemistry being used.
Key considerations when selecting understencil cleaning chemistry include compatibility with your paste flux (no-clean, water-soluble, or rosin), evaporation rate (faster evaporation reduces residue but may require more frequent dispensing), and VOC content (low-VOC and VOC-free formulations are available for facilities with emission restrictions).
ASM DEK Pro and DEK Pro XF understencil cleaning chemistries are formulated specifically for use in 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. Stencil coatings are particularly beneficial for fine-pitch apertures where paste release is critical and cleaning access is limited.
ASM NanoClear stencil coating is applied by wipe to the stencil surface and 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 on stencil printers. They control the flow and placement of paste during the print cycle and must be replaced regularly to maintain consistent paste deposition. Nozzles are available in lead-free (green) and lead-rich (red) versions to prevent cross-contamination between alloy types.
Adhesive Remover Wipes
Adhesive remover wipes are pre-saturated with solvent formulated to dissolve adhesive residue from stencil frames, printer components, and tooling. They are used during changeover and maintenance to remove adhesive tape residue and other contamination that standard IPA wipes cannot effectively dissolve.
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. The key elements are:
— 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 to maintain consistent paste deposition
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. Additional stencil printing brands will be added as our catalog expands.