- Stencil Cleaning Process
Process Overview: Stencil printing solder paste is a well-known and proven process technology for surface mounted components. Solder paste deposition on fine pitch and area array packages offer an attractive, cost-saving, and high volume production alternative. The cleanliness of the stencil is critical to the success of the stencil printing process. Insufficient solder is the primary cause of defects originating from the stencil printing process; therefore, stencil cleanliness is an essential process step for delivering the proper amount of solder paste to the PWA pads. Stencil cleaning must remove all solder particles and flux vehicle from stencil apertures without damaging the stencil, bonding adhesive, and elastomer frame.
The degree of cleanliness required varies with complexity of the board design. Stencils are usually cleaned to a visually clean condition. Cleaning misprinted circuit assemblies require the removal of uncured solder paste and ionic contaminants that could interfere with wetting and bonding. For assemblies, a visually clean appearance provides a satisfactory cosmetic condition, but this condition does not necessarily assure product performance. Semi-quantitative and qualitative ionic contamination testing provides cleaning verification and insures long term field reliability. While cleaning to a set of performance requirements is the primary objective, other objectives must also be set and achieved. The cleaning process must not damage the parts cleaned and cleaning must be accomplished in a practical and cost effective way. The process employed must be safe to operators and environmentally compatible.
Modern stencil cleaning processes integrate mechanical and chemical cleaning forces. Cleaning chemistry suppliers work closely with cleaning equipment manufacturers to provide an integrated process. Stencil cleaning faces the challenge of removing solder paste and SMT adhesive from tiny apertures while not damaging the thin stencil foil. To achieve reproducibility in cleaning, the process requires a mild chemistry integrated with stencil cleaning equipment that provides mechanical scrubbing actionand exceptional residue removal.
In summary, the stencil cleaning objectives are the removal of contaminants in the form of non-reflowed solder paste, flux residues, uncured adhesives, and other process residues. Process engineers are seeking robust processes that provide practical, cost effective, safe, and environmentally friendly methods.
Substrate: IPC-7525 Stencil guidelines document the design and fabrication of stencils for printing solder paste and surface-mount adhesive. The fabrication of stencils combines various metal alloys (stainless steel, copper, aluminum, nickel) as well as plastics in their material of construction. Frames are typically aluminum with the mesh border permanently mounted using an adhesive. The mesh material holding the stencil to the frame is typically polyester fiber. Frames may be tubular or cast aluminum with the border permanently mounted using adhesive.
The basic stencil components include the aluminum frame, mesh to mount the stencil foil to the frame, bonding adhesive used to fasten the soil to the mesh, and aperture pattern. The foil tension is made to 25n/cm2. Final tension depends on frame strength.
The stencil manufacturing processes create different types of apertures due to the methodology. Chemical etching is a “double sided” process and “eats” material away from each side of the stencil, creating an “hourglass” effect in the etched hole. The laser cut stencil is usually “trapezoidal” in shape due to the laser cutting characteristics. Chemical etched stencils are specifically built to incorporate the trapezoidal design.
Chemical etch aperture walls are typically smooth.
Laser apertures are tapered with more textured walls. To smooth the walls, electropolishing can be used.
Nickel additive apertures are tapered and very smooth.
Electroform E-Fab stencils used an additive process. The smooth tapered wall provides better wear and release characteristics. Special gasketing features reduces wiping frequency and solder paste bleed through.
E-Fab
Chemical etch is least costly, and the least accurate in tolerance variables. Laser cut is more costly, though tolerances improve and the default trapezoidal openings improve paste release.Selective stencils are hybrid chemical etch and laser cut combinations. To reduce cost, the bulk of the apertures are chemical etched, while the critical fine pitch parts are later laser cut into the stencil. This gives the best characteristics for paste release on fine pitch while conserving costs overall on the stencil.Electro-formed (E-Fab) are the most costly to manufacture, but will provide the closest tolerances and by nature of the wall characteristics of the apertures will exhibit the best paste release.
In regards to the stencil itself, material compatibility is the critical factor when selecting a stencil cleaning agent. Alkaline cleaning agents are more prone to react with aluminum frame. Chemical attack can roughen the aluminum surface and potentially turn the frame black.
Cleaning chemistry and temperature, used in the process, may affect stencil compatibility. Cleaning chemistry designs must consider stencil materials of construction and address compatibility constraints. Alkaline cleaning agents may chemically react with metals, such as aluminum, causing the surface to tarnish or darken over time. Elevated cleaning temperatures, in excess of 110ºF, may result in the interaction of the cleaning chemistry with the adhesive bond to the frame, causing the stencil to loosen or break from the frame. Compatibility concerns must be considered when selecting a cleaning chemistry.
Tools: Squeegee, spatula and solder paste pots are some of the many tools that require cleaning. Similar to compatibility issues discussed for stencils, the cleaning chemistry must be compatible with tools being cleaned.
Pallets: Some assemblers use their stencil cleaning process for removing flux residue build up on pallets. Pallets passed over the wave see many process cycles before cleaning. This requirement calls for a cleaning chemistry that exhibits enhanced effectiveness for removing baked-on flux residues.
Misprint Circuit Board Cleaning: Section II illustrates common problems that lead to a misprint circuit board. To address these problems, the misprint solder paste or SMT adhesive must be totally removed from the board surface.
Misprinted Solder Paste: Single-sided surface mount assemblies that are misprinted and rejected by vision inspection systems require cleaning of non-reflowed solder paste or SMT adhesive. Hand wiping is not preferred due to solder ball smearing over the surface of the circuit assembly. Tiny solder balls may end up in vias, which may lead to shorts and electrical paths. Therefore, cleaning systems are required.
Doubled sided surface mount assemblies are printed and reflowed on the primary side; then printed and reflowed on the B-side of the circuit assembly. From the cleaning perspective, this scenario is commonly overlooked with double-sided reflowed SMT assemblies, where the primary side is reflowed and B-side is misprinted. In such a situation, the cleaning process must be capable of removing both cured and uncured flux residues. This issue changes the scope of the cleaning process and cleaning chemistry selection. In the manufacturing operations, which have this requirement, the process engineer must consider advanced process conditions.
Single-sided Misprint Boards: Solder ball and particulate removal is often more difficult than dissolving uncured flux residue. The most reliable removal method for removing solder balls is a process that integrates mechanical and chemical driving forces. Solder balls are held in place by the flux composition. Developing an integrated cleaning process, releases the solder balls and allow removal during the wash and rinse cycles.
Solder balls are collected in the cleaning chamber over time. Most cleaning machines use filtration methods to prevent solder balls from being picked up by the pump and resprayed onto the board. Consult the equipment-operating manual for proper maintenance procedures to remove uncured paste from the wash holding tank and filters.
Double-sided Misprint Boards: Double sided surface mount assemblies are printed and reflowed on the primary; then printed and reflowed on the B-side of the circuit assembly. From the cleaning perspective, the scenario that is commonly overlooked is the reflowed flux residue on the B-side of the misprinted board. In such a scenario, the cleaning process must be capabile of removing both cured and un-cured flux residues. This issue changes the scope of the cleaning process and cleaning chemistry selection. In the manufacturing operations which have this requirement, the process engineer must consider developing a new cleaning process that optimizes the static and dynamic cleaning forces.
Misprinted SMT Adhesives: Cleaning uncured SMT adhesive from stencils and boards can be accomplished in an automated cleaning process. Many aqueous cleaning chemistries are not designed for removing SMT adhesives. When removal of SMT adhesives is required, selection of a cleaning chemistry will be necessary to meet this challenge.
Misprint Process Considerations: Misprint board cleaning requires additional process consideration to assure a reliable circuit assembly following the build process. The following list comprises factors for consideration when engineering the process for cleaning misprint boards.
- When using a spray in air cleaning system, both sides of the misprint board need to be cleaned simultaneously.
- Spray-in-air systems should be equipped with a filter in the recirculation wash to remove dislodged particles from the cleaning fluid.
- A clean water rinse prevents wash contamination from re-depositing on the surface of the board.
- Rinse pressure and duration are important process considerations when removing the cleaning chemistry from under components.
- The drying cycle is longer for a populated assembly and may need to be lengthened for misprints versus stencils.
- Holding the assembly properly is important when cleaning a misprint board with high impingement air spray. An adjustable spring loaded board holder secures the board in place and maintains the correct impingement angle for particle removal.
- Machines using ultrasonic technology should orient misprinted boards with uncured solder paste side down to allow gravity to carry the solder paste away from the reflowed side.
- A separate ultrasonic DI water rinse may be necessary when processing boards.
- Studies by the EMPF Laboratory and GEC Marconi suggest an ultrasonic frequency of 40 kHz or higher and should alternate or “sweep” +/- 1 – 3 kHz.
- Sweep technology eliminates hot spots or focused ultrasonic energy in the wash bath.
- The lower the ultrasonic frequency, the more aggressive the scrubbing action (cavitation).
- The power density is the amount of electrical energy delivered to the bath via the ultrasonic generators. This energy is measured in watts per liter of wash solution. For example, if the wash bath contained 100 liters and the generator is rated at 1000 watts, the power density would be 10 watts/liter. The same studies indicate that the power density should be around 10 watts per liter or less.
- Table 1 lists process recommendations when cleaning different types of contaminants from misprinted boards.
PROCESS STEP THAT CAUSED THE CONTAMINANTS
/NATURE OF CONTAMINANTS
/CLEANING PROCESS STEPS
Glue Dispense /- Misprinted, Uncured Glue – Unpopulated or Populated boards
- Do not Scrape the glue – Remove all components carefully if populated
- Make sure no glue residues are present on pads or in the via holes
- Send the board through the Aqueous / Semi-Aqueous Wash System.
Glue Curing /
- Populated with components and Cured
- The board is not salvageable. Scrap the board.
Paste Printing /
- Wet or dry misprinted Solder Paste – No components placed
- Wet or dry misprinted Solder Paste – Components Placed
- Wet or dry Solder Paste + Uncured Glue with populated or unpopulated boards
- Do not scrape wet solder paste or placed components from the boards. It parts have been placed, pick off carefully.
- Brush with alcohol to dissolve the glue and/or solder paste and wash the board in the Automatic Aqueous/Semi-AqueousWash System
- Clean and Inspect per IPC J-Std-001C Class 3 Specifications
Printing, Placement and Reflow Soldering /
- Excessive SMT No Clean Solder Paste Flux Residue
- Clean in the Aqueous/Semi-Aqueous Wash System with appropriate Wash Chemistry.
- >20 solder balls per panel or all over the laminate (>5 mils diameter or >5 solder balls /square inch.
- <20 Solder balls at a fine pitch SMT component
- Micro Solder Balls (> 6 Solder balls to a side of a pad) or >20 solder balls in via holes
Wave Soldered Board /
- Excessive Wave Solder Flux Residues
- Clean in the Aqueous/Semi-Aqueous Wash System with appropriate Wash Chemistry.
- Boards to be conformal coated per customer
- Solder balls around THT component leads
- Brush solder balls off with an anti static brush
- Solder balls on Laminate Surface
- Brush Solder balls off with anti static brush
- Solder balls on the component bodies
- Solder Splash on PWA Laminate
- <5 Solder ball Clusters/Panel on Laminate
- Remove the Solder balls with a brush or a stainless steel dental pick.
- >20 Solder Balls / Panel
- Clean in the Aqueous/Semi-Aqueous Wash System with appropriate Wash Chemistry.
All Completed PWA Assemblies /
- Lint, Dust, Fingerprints/ Grease
- Wipe with Isopropyl Alcohol wipe or comparable solvent if sparsely populated board
- If densely populated, send the board into Aqueous/Semi-Aqueous Wash System.
Table 1: Process Recommendations for Mis-Printed PWAs with Different Types of Contaminants