In SMT production, insufficient solder paste directly leads to 68% of solder joint defects (IPC data), causing open circuits, virtual soldering, and even component detachment. This article will delve into the seven fundamental reasons and solutions to help you achieve a printing yield of over 97%.
Table of Contents
1、 Design defects in steel mesh: accounting for 55% of the problem of insufficient solder paste
1. Inappropriate opening size and proportion
| Component Type | Recommended Hole Ratio | Error Cases | Consequences |
| 0402 resistor | 1:1 | 0.8:1 (reduced by 20%) | Insufficient tin content, monument risk increases by 40% |
| QFN device | 1.1:1 | 1:1 (without external expansion) | Insufficient solder crawling on the side wings |
| BGA pad | Diameter 1:1 | Hole<90% of pad | Ball and socket filling rate<75% |
Optimization plan:
-Using nano coated steel mesh (reducing demolding resistance by 25%)
-Step steel mesh: locally thickened to 0.15mm (for connector large solder pads)
2. Geometric structure error in opening
-Avoid rectangular angles: Use rounded rectangles (radius ≥ 0.05mm) to reduce hole blockage
-Close spaced component: Diamond shaped opening increases solder paste release by 18% compared to square shaped opening
2、 Printing process parameters out of control: accounting for 30% of the problem
1. Precise setting of key parameters
| Parameters | Standard Range | Deviation Effects |
| Scraper pressure | 0.3-0.5kg/cm | >0.8kg/cm → Steel mesh deformation |
| Printing speed | 10-30mm/s | >50mm/s → Insufficient filling |
| Separation speed | 0.1-0.5mm/s | >1mm/s → Tip pulling/solder paste residue |
| Demolding distance | 0.5-1.5mm | <0.3mm → solder paste tearing on solder pads |
Real time monitoring tool:
-Laser thickness gauge: online detection of solder paste thickness (tolerance ± 15 μ m)
-SPI (3D solder paste inspection machine): Identify area/volume defects (accuracy ± 5 μ m)
2. Environmental control critical point
-Temperature: 23 ± 2 ℃ (with a 10% decrease in solder paste viscosity for every 1 ℃ increase)
-Humidity: 45-65% RH (<40% accelerates flux evaporation)
3、 Solder paste material failure: accounting for 12% of the problem
1. Identification characteristics of solder paste deterioration
-Abnormal viscosity: Initial viscosity of 120-200 kcps →>300 kcps after deterioration
-Metal oxidation: The surface of tin powder is dull (normally bright silver)
-Flux separation: transparent liquid seeps out of the paste
2. Usage standards
| Operational procedures | Standards | Risk points |
| Temperature recovery | 4-6 hours (original tank) | Condensation water pollution |
| Stirring | 1-3 minutes (viscosity restored to 90%) | Metal particles shattered |
| In place time | ≤ 4 hours | Solvent evaporation causes viscosity to soar |
4、 Equipment status and maintenance deficiencies: accounting for 3% of the problem
1. Maintenance of scraper system
-Blade flatness: If the wear is greater than 0.05mm, it needs to be flipped over, and if it is greater than 0.1mm, it must be replaced forcibly
-Angle setting: 60 ° steel scraper vs 45 ° polyurethane scraper
2. Steel mesh cleaning protocol
[Every 5 boards] –>[Vacuum nozzle cleaning]–>[Wet wipe (IPA)]–>[dry wipe]–>[every 2 hours] –>F [deep ultrasonic cleaning]
5、 Ultimate solution: Systematic prevention and control system
1. DFM optimization
-Pad spacing ≥ 0.2mm (avoid bridging steel mesh openings)
-Avoid placing precision components at the edge of the layout
2. Intelligent process control
-SPI system equipped with AI: real-time adjustment of printing parameters (such as Koh Young system)
-Automatic viscosity tester: monitors the rheological properties of solder paste every 30 minutes
3. Failure analysis process
-SEM-EDS analysis: identification of solder paste pollutants
-Frozen slicing: Observing the microstructure of solder joints
Case study: Practical experience in improving the yield of automotive ECU boards
Problem: QFN device virtual soldering rate of 22%
Root cause: 1:1 steel mesh opening (not expanded externally)+scraper pressure of 0.7kg/cm
measures:
-Change the opening to 1.15:1 (15% outward expansion)
-The pressure drops to 0.4kg/cm
-Add stepped steel mesh (0.13mm → 0.15mm)
Result: The virtual soldering rate has been reduced to 0.3%, saving rework costs of $18k/month
→ [Download the “Solder Paste Printing Defect Checklist”]
Including steel mesh design template, IPC-7525 standard parameter table, and SPC control chart
