By Bob Wettermann
SMT stencils to can be subject to a variety of process defects due to the removal of solder paste and uncured/unreacted SMT adhesives from stencils, misprinted printed circuit boards.
The cleaning of stencils and misprinted PCBs has taken an increasingly important role in surface mount technology as pitches become finer and parts spacing narrows. Paste volume is a critical issue for fine and ultra-fine pitch components, chip-scale, BGA and other miniature components. Insufficient solder due to clogging of stencil apertures is a consistent cause of defects. Clogged stencil apertures results in poor paste printing. Therefore clean apertures in SMT stencils are important to delivering the proper volume of solder paste. Several studies and fishbone diagrams report that approximately 70% of surface mount technology defects are due to solder paste printing problems.
Cleaning agents for SMT stencils must be effective, safe for workers and safe for the environment. Isopropyl alcohol (IPA), commonly used for cleaning SMT stencils, poses both environmental (VOC) and safety (i.e., fire hazard) concerns.
SMT stencils to can be subject to a variety of process defects due to the removal of solder paste and uncured/unreacted SMT adhesives from stencils, misprinted printed circuit boards.
The cleaning of stencils and misprinted PCBs has taken an increasingly important role in surface mount technology as pitches become finer and parts spacing narrows. Paste volume is a critical issue for fine and ultra-fine pitch components, chip-scale, BGA and other miniature components. Insufficient solder due to clogging of stencil apertures is a consistent cause of defects. Clogged stencil apertures results in poor paste printing. Therefore clean apertures in SMT stencils are important to delivering the proper volume of solder paste. Several studies and fishbone diagrams report that approximately 70% of surface mount technology defects are due to solder paste printing problems.
Cleaning agents for SMT stencils must be effective, safe for workers and safe for the environment. Isopropyl alcohol (IPA), commonly used for cleaning SMT stencils, poses both environmental (VOC) and safety (i.e., fire hazard) concerns.
The residue encountered on stencils is generally non-reflowed solder paste. After hours of use, the solder paste may partially dry thereby increasing the difficulty of cleaning.
Proper stencil cleaning needs to remove solder particles and organic flux media from stencil apertures without damaging the stencil, bonding adhesives, and the glue of the frame of the stencil. Solder paste residue on a stencil can result in transferring paste from the bottom of the stencil the next board being printed. These contaminants can interfere with the intimate contact or ‘‘gasketing’’ of the stencil to the board.
Printing of solder paste is a crucial step in the assembly of SMT produced PCBs. The stencils used must be clean and free of residue to avoid the misapplication of solder paste deposits to the PCBs. Additionally, the flatness of the PCB, the positioning of the stencil with respect to the PCB lands, consistency of the squeegee pressure, stencil aperture opening in relation to the PCB land dimensions and wetness of the paste being applied, can all result in solder paste being left on the stencil. There are many more reasons for the misprinting of the stencils but these are the most common related to the SMT stencil cleanliness.
For the reasons stated above, the bottom of the SMT stencil requires inspection, monitoring and wiping throughout the process. There are various methods to wipe the stencil bottom. For printers that do not have a built in means of under stencil wiping, manual methods must be used.
Failure to implement a process that removes residue from stencil apertures contributes to board processing problems.
The following problems are indicators of potential cleaning issues related to SMT stencils
Proper stencil cleaning needs to remove solder particles and organic flux media from stencil apertures without damaging the stencil, bonding adhesives, and the glue of the frame of the stencil. Solder paste residue on a stencil can result in transferring paste from the bottom of the stencil the next board being printed. These contaminants can interfere with the intimate contact or ‘‘gasketing’’ of the stencil to the board.
Printing of solder paste is a crucial step in the assembly of SMT produced PCBs. The stencils used must be clean and free of residue to avoid the misapplication of solder paste deposits to the PCBs. Additionally, the flatness of the PCB, the positioning of the stencil with respect to the PCB lands, consistency of the squeegee pressure, stencil aperture opening in relation to the PCB land dimensions and wetness of the paste being applied, can all result in solder paste being left on the stencil. There are many more reasons for the misprinting of the stencils but these are the most common related to the SMT stencil cleanliness.
For the reasons stated above, the bottom of the SMT stencil requires inspection, monitoring and wiping throughout the process. There are various methods to wipe the stencil bottom. For printers that do not have a built in means of under stencil wiping, manual methods must be used.
Failure to implement a process that removes residue from stencil apertures contributes to board processing problems.
The following problems are indicators of potential cleaning issues related to SMT stencils
- Opens-previously uncleaned apertures clogging the apertures of the SMT stencil
- Insufficients due to not all previous solder paste having been released from the apertures
- Poorly aligned print
- Damages stencil resulting in “smearing” of solder paste
- Bridging due to uncleaned solder paste apertures
- Solder fines or balls on PCB which are loose resulting in a defect created by a poorly cleaned or uncleaned stencil