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Views: 0 Author: Site Editor Publish Time: 2025-11-11 Origin: Site
Your opening statement that "70-80% of defects originate from poor printing" is a golden rule in the SMT industry. This dictates that our quality control focus must be shifted forward, making the printing process the core critical point of quality control. Mastering printing control means addressing the issue at its core.
The six defect phenomena you listed are very typical. Based on this, we will stratify the causes and improvement measures in more detail.
Root Cause Categorization: Almost all printing defects stem from a loss of control over three core elements:
Stencil
Squeegee
Printing Parameters & Environment
| Primary Cause | In-Depth Analysis & Corrective Actions |
|---|---|
| Poor PCB Support/Fixturing | - Check Support: Not only consistent height, but the layout of support points should be uniform to avoid PCB bowing or flexing in the middle. - Reference Plane: Use a spirit level or dial indicator to check if support PINS/Blocks and clamps are coplanar. - Vacuum Assist: Ensure the printer's vacuum assist is activated and has sufficient pressure to hold the PCB flat against the platform. |
| Poor Stencil Cleaning | - Frequency: Set cleaning frequency based on solder paste characteristics (e.g., no-clean types dry faster), typically every 3-10 boards. - Quality: Use high-quality non-woven wiper paper; ensure solvent (usually IPA or specialized cleaner) is sprayed evenly. - Wet + Dry Wipe: Typically use a mode where wet wiping dissolves residue, followed by dry wiping to absorb it. - Inspection: Regularly check if the wiper roll is used up and if the cleaning mechanism moves smoothly. |
| Stencil-to-PCB Separation | - Separation Speed: Speed too fast can cause solder spikes and transfer paste to unwanted areas. Appropriately reduce separation speed (e.g., 0.1-0.5 mm/s). - Separation Distance: Ensure sufficient separation distance so the stencil fully releases from the PCB before moving. |
| Stencil Issues | - Tension: Regularly (weekly/monthly) check stencil tension using a tension gauge; it should be >35 N/cm². Insufficient tension causes stencil sag, leading to bridging. - Aperture Design: Check if stencil apertures lack anti-bridging features (e.g., solder thief pads). For fine-pitch components, use reversed trapezoid (tapered) apertures with electropolished inner walls. |
| Squeegee Issues | - Pressure & Speed: Insufficient pressure or excessive speed prevents paste from rolling, instead pushing it, which can squeeze into gaps between aperture and board, causing shorts. The standard is a clean stencil surface and uniform paste rolling behind the squeegee. - Levelness: Regularly calibrate squeegee levelness using a dial indicator. |
| Primary Cause | In-Depth Analysis & Corrective Actions |
|---|---|
| Vision Alignment System | - Fiducial Mark Quality: Fiducials should be clear, non-oxidized, and uncontaminated. If contrast is poor, adjust lighting brightness and angle. - Camera Cleanliness: Regularly clean the camera lens to prevent dust from causing recognition coordinate shifts. - Coordinate System: Ensure the PCB file coordinate origin is consistent with the stencil fiducial mark coordinate system. |
| PCB Support/Fixturing | Same analysis as "Bridging". Unstable fixturing is the root cause of all misalignment. |
| Program Coordinates | - Initial Setup: Use the first printed board, measure offset at several key points using SPI or a high-precision microscope, then fine-tune Offset X/Y/θ in the printer. - Regular Verification: In mass production, periodically sample and inspect, performing preventive compensation for offset trends. |
| Primary Cause | In-Depth Analysis & Corrective Actions |
|---|---|
| Clogged Stencil Apertures | - Immediate Action: Perform manual stencil cleaning and check the automatic cleaning system. - Aperture Design: For fine-pitch QFP/BGA, an insufficient Area Ratio (>0.66 is target) is a root cause. Consider nano-coated stencils for better release. |
| Excessive Squeegee Pressure | Too much pressure "scoops" paste from the apertures and accelerates stencil and squeegee wear. Adjust to the critical pressure where paste rolls correctly. |
| Improper Separation Parameters | Excessive separation speed stretches and breaks the paste, forming peaks/tails. Appropriately reduce the speed. |
| Solder Paste Quality | - Viscosity: Improper paste viscosity (too low=slump, too high=peaking). Inadequate thawing or stirring causes abnormal viscosity. - Metal Content: Low metal content (e.g., <88%) causes paste to "collapse" after printing, appearing visually like insufficient solder. |
| Phenomenon | Primary Causes & Improvements |
|---|---|
| Too Thick | - Excessive Gap between Stencil & PCB: Check support and fixturing. - Insufficient Squeegee Pressure: Increase pressure until the stencil surface is clean after the squeegee passes. - Incorrect Stencil Thickness: Verify stencil specifications. |
| Too Thin | - Excessive Squeegee Pressure: Reduce pressure. - Excessive Squeegee Speed: Reduce speed to allow time for paste to fill apertures. - Contaminated Stencil Bottom: Clean the stencil. - Poor Solder Paste Fluidity: Check if paste is expired or inadequately stirred. |
Root Cause: The cohesive force within the solder paste is less than its gravity or its fluidity is too high.
Corrective Actions:
Ambient Temperature & Humidity: Control the workshop temperature at 23±3°C, humidity 30-60% RH. High temperature and humidity exacerbate paste moisture absorption and slump.
Solder Paste Quality: Select solder paste with good anti-slump properties.
Prompt Reflow: Reflow should be completed within 4 hours after printing to prevent paste moisture absorption and flux diffusion.
Reflow soldering acts as a "fault mirror"; many latent printing issues become exposed after reflow.
Root Cause: Imbalanced surface tension generated by the molten solder at both ends of the component during reflow.
Relationship to Printing:
Tombstoning: Usually caused by insufficient solder, paste slump, or uneven paste thickness at one end, causing that end to melt first and pulling the component upright.
Misalignment: Usually caused by printing misalignment or paste slump/bridging, where the component's self-alignment capability during reflow cannot correct the excessive offset.
Reflow-Specific Causes:
Temperature Profile: Excessive ramp-up rate in the preheat/ramp zone can cause component ends to reach liquidus temperature at different times. Ensure the ramp-up slope is 1-3°C/s.
Oven Wind Speed: Excessively high airflow can displace small chip components.
Root Cause: Flux/solvent in the solder paste spattering due to rapid heating, carrying tiny solder particles.
Relationship to Printing:
Contaminated stencil apertures, PCB oxidation can lead to poor paste wetting, forming solder balls.
Solder paste slump is a primary cause; slumped paste can coalesce into balls during reflow.
Reflow-Specific Causes:
Excessive Ramp-up Rate in Preheat Zone: This is the most common cause. Solvents volatilize too rapidly, causing "explosive" spattering.
Insufficient Preheat Temperature/Time: Flux is not fully activated.
Improper Paste Thawing/Stirring: Absorbed moisture rapidly vaporizes during reflow, causing spattering.
Root Cause: Solder has not reached a complete molten state or has been oxidized during the process.
Relationship to Printing: Usually minimal, unless the solder paste itself has degraded.
Reflow-Specific Causes:
Insufficient Peak Temperature or Too Short Time Above Liquidus (TAL): Not reaching the solder paste's recommended process window.
Temperature Profile: Cooling rate is too slow, leading to coarse grain structure in the joint, reducing strength.
Root Cause: In the liquid state, solder bridges due to insufficient cohesive force.
Relationship to Printing: 80% of causes originate from bridging already present at the printing stage. Reflow merely solidifies and reveals it.
Reflow-Specific Causes:
Excessive Ramp-up Rate: May cause flux to become exhausted prematurely, failing to provide adequate wetting tension.
Excessively High Peak Temperature or Prolonged TAL: Causes the solder to become overly fluid, reducing cohesion.
Data-Driven Approach: Vigorously promote and utilize SPI (Solder Paste Inspector) data. Correlating printing data (height, volume, area, offset) with AOI (Automated Optical Inspection) and FTT (First-Time Through) yield allows for precise root cause identification.
Preventive Maintenance: Establish a regular equipment maintenance schedule (TPM), including squeegee leveling, stencil tension checks, cleaning system inspection, and temperature profiling.
Standardized Work: Document successful parameter settings and experiences (e.g., stencil design rules for different components, reflow profile templates for different pastes) into standardized work instructions and train operators accordingly.
Man, Machine, Material, Method, Measurement, Environment (6M Analysis): Comprehensively review problems from these six dimensions:
Man: Are operators adequately trained?
Machine: Is the equipment condition stable?
Material: Is the incoming quality of solder paste, PCB, and components acceptable?
Method: Are the operating methods and parameters optimal and being followed?
Environment: Are workshop temperature, humidity, and cleanliness controlled?
Measurement: Are the measurement tools (SPI, AOI) accurate and used correctly?
