RayMing PCB: SCREEN PRINT AND REFLOW YOUR PROTOTYPE CIRCUIT BOARDS

Thursday, February 20, 2025

SCREEN PRINT AND REFLOW YOUR PROTOTYPE CIRCUIT BOARDS

Introduction

Screen printing and reflow soldering are essential processes in the prototyping and manufacturing of printed circuit boards (PCBs). This comprehensive guide will walk you through the entire process, from preparation to final inspection, helping you achieve professional-quality results in your prototype PCB assembly.

Understanding Screen Printing for PCBs

What is Screen Printing?

Screen printing is a precision process used to deposit solder paste onto PCB pads where components will be placed. This technique ensures consistent solder paste volume and placement, which is crucial for reliable electrical connections and mechanical strength in the final assembly.

Equipment and Materials Needed

Category Essential Items Optional Items
Printing Equipment Stencil frame, Squeegee, Work holder Automatic printer
Materials Solder paste, Cleaning solutions Paste height gauge
Safety Equipment Nitrile gloves, Eye protection, Ventilation Protective clothing
Tools Spatula, Cleaning wipes Microscope

Category	Essential Items	Optional Items
Printing Equipment	Stencil frame, Squeegee, Work holder	Automatic printer
Materials	Solder paste, Cleaning solutions	Paste height gauge
Safety Equipment	Nitrile gloves, Eye protection, Ventilation	Protective clothing
Tools	Spatula, Cleaning wipes	Microscope

Solder Paste Specifications

Property Typical Range Optimal Value
Viscosity 150-250 Pa·s 200 Pa·s
Metal Content 85-90% 88.5%
Particle Size Type 3-5 Type 4
Shelf Life 6-12 months 8 months

Property	Typical Range	Optimal Value
Viscosity	150-250 Pa·s	200 Pa·s
Metal Content	85-90%	88.5%
Particle Size	Type 3-5	Type 4
Shelf Life	6-12 months	8 months

Stencil Design and Selection

Stencil Thickness Guidelines

Component Type Recommended Thickness Aperture Ratio
0201 Components 100μm 1.5:1
0402 Components 120μm 1.5:1
QFP/SOIC 150μm 1.8:1
BGA 150μm 2.0:1

Component Type	Recommended Thickness	Aperture Ratio
0201 Components	100μm	1.5:1
0402 Components	120μm	1.5:1
QFP/SOIC	150μm	1.8:1
BGA	150μm	2.0:1

Material Selection

Modern stencils are typically manufactured from stainless steel, with thickness variants ranging from 0.1mm to 0.2mm. The selection depends on your specific application requirements.

Screen Printing Process

Preparation Steps

Environmental Control
- Temperature: 20-25°C
- Humidity: 45-65%
- Clean room conditions recommended
Material Preparation
- Allow solder paste to reach room temperature
- Mix paste gently to ensure homogeneity
- Clean stencil thoroughly

Printing Parameters

Parameter Range Notes
Squeegee Angle 45-60° Affects paste release
Print Speed 20-50 mm/s Depends on paste viscosity
Pressure 0.3-0.7 kg/cm Based on stencil width
Separation Speed 0.5-3.0 mm/s Prevents smearing

Parameter	Range	Notes
Squeegee Angle	45-60°	Affects paste release
Print Speed	20-50 mm/s	Depends on paste viscosity
Pressure	0.3-0.7 kg/cm	Based on stencil width
Separation Speed	0.5-3.0 mm/s	Prevents smearing

Component Placement

Placement Considerations

Component Orientation
- Verify polarity marks
- Align components with pad pattern
- Consider thermal requirements
Placement Accuracy

Component Type Placement Tolerance Speed (components/hour)
0201/0402 ±0.1mm 5000-8000
SOIC/QFP ±0.05mm 3000-5000
BGA/QFN ±0.025mm 1000-2000

Component Type	Placement Tolerance	Speed (components/hour)
0201/0402	±0.1mm	5000-8000
SOIC/QFP	±0.05mm	3000-5000
BGA/QFN	±0.025mm	1000-2000

Reflow Soldering Process

Reflow Profile Zones

1. Preheat Zone

Temperature Range: 150-200°C
Duration: 60-120 seconds
Purpose: Gradual heating to prevent thermal shock

2. Soak Zone

Temperature Range: 150-200°C
Duration: 60-120 seconds
Purpose: Thermal equilibrium and flux activation

3. Reflow Zone

Peak Temperature: 230-250°C
Time Above Liquidus: 30-90 seconds
Purpose: Solder melting and joint formation

4. Cooling Zone

Cooling Rate: 2-4°C/second
Final Temperature: <50°C
Purpose: Controlled solidification

Typical Reflow Profile Parameters

Zone Temperature (°C) Time (seconds) Rate (°C/s)
Preheat 150-200 60-120 1-3
Soak 150-200 60-120 0-1
Reflow 230-250 30-90 1-3
Cooling 250-50 120-180 -2-4

Zone	Temperature (°C)	Time (seconds)	Rate (°C/s)
Preheat	150-200	60-120	1-3
Soak	150-200	60-120	0-1
Reflow	230-250	30-90	1-3
Cooling	250-50	120-180	-2-4

Quality Control and Inspection

Visual Inspection Criteria

Defect Type Acceptance Criteria Inspection Method
Bridging None allowed Visual/Microscope
Insufficient Solder <25% pad exposure Visual/X-ray
Misalignment <25% pad offset Visual/AOI
Voids <25% joint area X-ray

Defect Type	Acceptance Criteria	Inspection Method
Bridging	None allowed	Visual/Microscope
Insufficient Solder	<25% pad exposure	Visual/X-ray
Misalignment	<25% pad offset	Visual/AOI
Voids	<25% joint area	X-ray

Common Defects and Solutions

Defect Possible Causes Solutions
Bridging Excess paste, poor separation Adjust print pressure, clean stencil
Tombstoning Uneven heating, pad design Balance thermal mass, adjust profile
Cold Joints Insufficient temperature Adjust peak temperature, time
Voids Outgassing, paste quality Adjust profile, check paste storage

Defect	Possible Causes	Solutions
Bridging	Excess paste, poor separation	Adjust print pressure, clean stencil
Tombstoning	Uneven heating, pad design	Balance thermal mass, adjust profile
Cold Joints	Insufficient temperature	Adjust peak temperature, time
Voids	Outgassing, paste quality	Adjust profile, check paste storage

Process Optimization

Key Process Indicators (KPIs)

Metric Target Action if Out of Spec
First Pass Yield >95% Review process parameters
Defect Rate <1000 DPM Analyze defect patterns
Cycle Time <4 hours Optimize workflow
Material Waste <5% Improve material handling

Metric	Target	Action if Out of Spec
First Pass Yield	>95%	Review process parameters
Defect Rate	<1000 DPM	Analyze defect patterns
Cycle Time	<4 hours	Optimize workflow
Material Waste	<5%	Improve material handling

Cost Considerations

Cost Element Typical Range (%) Optimization Strategy
Materials 40-50 Bulk purchasing, storage optimization
Labor 20-30 Training, automation
Equipment 15-25 Preventive maintenance
Overhead 10-15 Process efficiency

Cost Element	Typical Range (%)	Optimization Strategy
Materials	40-50	Bulk purchasing, storage optimization
Labor	20-30	Training, automation
Equipment	15-25	Preventive maintenance
Overhead	10-15	Process efficiency

Maintenance and Troubleshooting

Preventive Maintenance Schedule

Equipment Frequency Tasks
Stencil Daily Clean, inspect for damage
Printer Weekly Calibrate, clean rails
Oven Monthly Profile verification, clean
Tools Monthly Clean, replace worn items

Equipment	Frequency	Tasks
Stencil	Daily	Clean, inspect for damage
Printer	Weekly	Calibrate, clean rails
Oven	Monthly	Profile verification, clean
Tools	Monthly	Clean, replace worn items

Environmental and Safety Considerations

Safety Requirements

Area Requirement Implementation
Ventilation 3-4 air changes/hour Install exhaust systems
PPE Gloves, glasses, masks Provide and enforce usage
Chemical Storage Controlled temperature Dedicated storage area
Waste Disposal Hazardous waste protocol Contract certified handler

Area	Requirement	Implementation
Ventilation	3-4 air changes/hour	Install exhaust systems
PPE	Gloves, glasses, masks	Provide and enforce usage
Chemical Storage	Controlled temperature	Dedicated storage area
Waste Disposal	Hazardous waste protocol	Contract certified handler

Frequently Asked Questions

Q1: What is the optimal temperature for storing solder paste?

A: Solder paste should be stored at 2-8°C (35-46°F) in a refrigerator. Before use, allow the paste to reach room temperature naturally (typically 4-8 hours) to prevent condensation and ensure proper viscosity.

Q2: How long can PCBs wait between paste printing and component placement?

A: In a controlled environment (20-25°C, 45-65% RH), printed boards should be populated within 4 hours. However, it's best to minimize the wait time to prevent paste dry-out and potential quality issues.

Q3: What causes tombstoning and how can it be prevented?

A: Tombstoning occurs when one end of a component lifts during reflow, typically due to uneven heating or pad design issues. Prevention includes ensuring balanced thermal mass on both sides of the component, proper pad design, and optimized reflow profiles.

Q4: How often should the stencil be cleaned during production?

A: Clean the stencil after every 5-10 prints or when visual inspection shows paste buildup. For high-precision applications, more frequent cleaning may be necessary. Always use appropriate cleaning solutions and lint-free wipes.

Q5: What is the shelf life of opened solder paste?

A: Once opened, solder paste typically has a shelf life of 1-2 weeks when stored properly at room temperature. However, this can vary based on the manufacturer's specifications and storage conditions. Always check the manufacturer's recommendations and maintain proper storage conditions.

Conclusion

Successful PCB prototyping through screen printing and reflow soldering requires attention to detail, proper equipment maintenance, and strict adherence to process parameters. By following this guide and maintaining proper documentation of your process parameters, you can achieve consistent, high-quality results in your PCB assembly operations.