Key SMT Printing and Reflow Soldering Procedures Guide

This chapter will focus on introducing the key control points of the SMT manufacturing process through the SMT printing process, placement process, and reflow soldering process.

I. SMT Process Control Items and Documents

1. SMT production and control operations are carried out according to the specifications at each stage of the process. Pay attention to the separation of HS/HSF materials, processes, and documents. Leaded and lead-free solder paste must be stored in separate areas and labeled with HSF identification for distinction.
2. Special process parameters must be defined in the SOP according to the model and followed accordingly.

• Printing process includes: printing speed, separation speed, wiping speed, separation gap, printing pressure, scraper type, stencil type, etc.
• Reflow soldering process includes: conveyor speed, temperature in heating zones, hot air fan speed, board input spacing, etc.
• In the case of the following process changes, if customer approval is required, they must be notified or approval obtained. If not required, internal approval through a process change request is needed:

a. Tool changes/additions/removals (including changes in design or method), including fixtures, gauges, and test equipment.
b. Process flow changes (e.g., changing a series of operations, adding or removing an operation or inspection), including any chemical, mechanical, or process changes that could affect performance, reliability, safety, suitability, appearance, dimensions, or durability.
c. Testing/inspection changes.
d. Chemical changes in the assembly or repair processes.

II. Printing Process:

1. Solder Paste Usage Control: Follow FIFO. After incoming inspection, solder paste must be labeled and stored at 2~8°C. Use based on expiry date sequence. Warm up in a dry box for over 4 hours before use. Take based on serial numbers in ascending order. Maintain proper records.
2. Refrigerator Temperature Control: Maintain 2~8°C. If abnormal, notify SMT engineer and record for follow-up.
3. Solder Paste Addition Control:

1. Set machine to stop every 20 prints for solder amount check. Record inspection and addition.
2. Add solder paste as needed.
3. Use clean steel blades.
4. Use solder paste within 24 hours after opening. Unopened solder paste can be stored indoors for up to 1 month; expired paste must be scrapped.
5. Stencil Control:

1. Clean stencils before line start, every 6 hours, and after 30 mins of downtime.
2. Inspect cleanliness according to Stencil Operation SOP and Cleanliness Check SOP.
3. Tension control per “Stencil Tension Measurement SOP”: Tension ≤ -0.21mm or ≥ 32N/cm.
4. Scrap stencils after a defined number of uses (e.g., 150,000 times) or if damaged.
5. Track stencil cleaning in SFCS.

5. PCB Positioning Adjustment: During line change, refer to production folder for flow direction, size, etc. Ensure proper PCB clamping and support.
6. Scraper Control:

1. Manage all scraper operations through SFCS.
2. Cleaning/adjustment: omitted.
3. Scrapping: Replace after defined number of uses (e.g., 150,000 times) or if damaged.

7. PCB Cleaning: Clean PCBs if printing/dispenser failure, equipment failure, or over 4-hour delay before reflow.
8. Solder Paste Printing Control: Refer to production data folder during line change and confirm printing quality:

1. SPI automatically inspects each board (thickness, area, volume, no bridging, no missing print, no offset), and records data.
2. If SPI is unavailable or malfunctioning, manual visual inspection must be performed (no bridging, no missing print, no offset or <1/3 PAD).
3. Solder Paste Thickness & Volume Control:
   A. Thickness Control Range:
   0.5 mm Pitch CSP / Micro BGA: T – 0.005mm ~ T + 0.055mm
   0.5 mm Pitch QFP / QFN: T – 0.005mm ~ T + 0.055mm
   0.8~1.0 mm Pitch BGA/ CSP: T – 0.005mm ~ T + 0.065mm
   Normal Chip (RLC & others): T – 0.005mm ~ T + 0.085mm
   (Remark: T = stencil thickness)

B. Volume Control (for SPI measurement):
0.5 mm Pitch CSP / Micro BGA: 0.0016387 ~ 0.008194 mm³
CSP / Micro BGA 0.65–0.8mm: 0.0065548 ~ 0.029497 mm³
CSP / PBGA / Micro BGA 1.0mm: 0.0163871 ~ 0.081935 mm³

Operators must observe and record CPK and average thickness every 2 hours using SPI. Any abnormality must be reported to the on-line technician or engineer immediately.

III. Component Placement Process:

1. During line change, confirm that PCBs are uniformly and stably supported.
2. During line change, confirm component placement offset. Adjust offset so the lead does not extend beyond 1/3 of the pad width.
3. High-speed machine pick/place rate must be ≥ 99.90%, checked and recorded hourly by the operator.
4. Secondary Material Recycling Control:

1. Chip components are not reused; PD disposes of them as defective parts after each shift.
2. For M/O materials, ensure:

a. BGA: Use 10x magnification to check for missing balls, damage, contamination, or appearance defects. Use flat fixtures to check for warpage.
b. IC (QFP, etc.): Use 10x magnification to check for contamination or appearance defects. Use flat fixtures to ensure no warping or bent leads.
c. Record secondary use on SMT Reused Material Usage Form.
d. If customer has specific requirements, follow customer rules.

5. Manual Placement Component Control: For manually placed or reused components, mark polarity side with a gold dot.
6. Temperature and Humidity Sensitive Component Control:

1. For vacuum-packed components (e.g., BGA, QFP), control exposure time after unpacking per MSDC system. Record date and time of opening on a label and affix to tray or reel.
2. Unused sensitive components must be stored in a temporary storage box with door kept properly closed.
3. If unfinished within allowed exposure time, follow MSDC baking conditions. During hold production, return parts with accumulated exposure time and opening label to warehouse for control.
4. For program modification due to special reasons (e.g., non-ECN changes to parts or vendor), changes must be verified by SQM, confirmed by QE, and approved by SMT engineer before updating. Record changes in SMT Program Change Log. Notify IPQA and PD, and begin mass production only after first-piece inspection is OK.

IV. Reflow Soldering Process:

1. Temperature Setting & Measurement: Profile measured by technician and confirmed by engineer.

1. For new models, set according to trial-run suggestions and define production temperature in product data.
2. For first mass production, follow defined product data.
3. If results are inconsistent, verify thermocouple placement, retest, and adjust if needed with engineer confirmation.

2. Temperature Measurement Points: Use board or SOIC depending on PCB structure: BGA body, QFP pin/body, chip, etc.
   Record points during trial or first production run in product data as reference for future runs.
3. Temperature Control: Based on BGA and fine pitch IC solder joint references and model-specific process specifications.
4. Measurement Frequency:

1. Measure profile for each line change and shift (do not exceed 12 hours for same model continuous production). Also, measure if quality issues arise. Notify supervisor.
2. Check thermocouple for damage before measurement; replace if broken.
3. Reflow Control:

1. Oxygen level in reflow oven must be ≤1000ppm (follow customer SOP if different). Record panel temperature every 2 hours.
2. At line change, follow model-specific production conditions and verify via Reflow Changeover Checklist.
3. Record oven panel temperatures every 2 hours.
4. Send temperature testers for regular calibration per calibration room control. Verify with standard fixture during monthly maintenance and record in CPK control chart.
5. Update temperature test board every 3 months or if deviation exceeds ±2°C.
6. Actual vs. set zone temperatures must be within ±5°C. If exceeded, stop production and notify engineer.
7. Reflow Profile Specification Control (Note: Refer to SOP if special components or solder paste changes are involved):

1. Tin-Lead Process:
   a. Ramp rate: < 3°C/sec
   b. Soaking time (150183°C): 60120 sec
   c. Time above liquidus (183°C): 60120 sec
   d. Peak temperature:
   205225°C: for critical components (BGA, CSP, QFP, connectors, etc.)
   205~230°C: for normal chips (RLC, etc.)
2. Lead-Free Process:
   a. Ramp rate: < 3°C/sec
   b. Soaking time (150180°C): 60120 sec
   c. Time above liquidus (217219°C): 4090 sec (Control per customer requirement: 217°C or 220°C)
   d. Peak temperature:
   230250°C: for critical components
   230260°C: for normal chips

V. Rework Operations:

1. For manually repaired ICs, QFPs, PLCCs, mark with a red dot after confirming they are good. (Do not mark if customer prohibits.)

VI. Notes:

1. For quality control, follow process control requirements and SOPs strictly.
2. Follow ESD protection procedures. Wear ESD wrist strap. If handling bare boards or non-ESD sensitive components, gloves must be worn and pads avoided. Use ESD bags when moving PCBs.
3. Workshop temperature and humidity: 23 ± 5°C, 40~60%RH. Notify facility/engineer if out of range.
4. Component exposure time and baking conditions must comply with MSDC regulations.
5. Use visual inspection templates (hole cards) for inspection.
6. OSP PCBs must complete DIP wave soldering within 48 hours after unsealing or within 24 hours after 1st reflow. If not, Control Build must be used and quality confirmed via visual and X-ray inspection.

Disclaimer:

1. This channel does not make any representations or warranties regarding the availability, accuracy, timeliness, effectiveness, or completeness of any information posted. It hereby disclaims any liability or consequences arising from the use of the information.
2. This channel is non-commercial and non-profit. The re-posted content does not signify endorsement of its views or responsibility for its authenticity. It does not intend to constitute any other guidance. This channel is not liable for any inaccuracies or errors in the re-posted or published information, directly or indirectly.
3. Some data, materials, text, images, etc., used in this channel are sourced from the internet, and all reposts are duly credited to their sources. If you discover any work that infringes on your intellectual property rights or personal legal interests, please contact us, and we will promptly modify or remove it.