What are the steps in the lead-free soldering process?

The core steps of the lead-free soldering process are listed below, each of which contains key control points to ensure solder quality:

1. Optimisation of pre-soldering preparation

Introducing automatic optical inspection (AOI) equipment for initial inspection of component pins and PCB pads to reduce the time required for manual visual inspection.

Use ionic contamination tester to check PCB cleanliness to ensure that it meets the low contamination requirements of the lead-free process.

Establish solder paste inventory management system and follow the first-in-first-out (FIFO) principle to avoid waste of solder paste due to expiration.

2. Solder paste printing optimisation

Laser nano-coated or electro-cast stencils are used and the opening edges are nano-polished to reduce solder paste residue.

Introduced an online stencil thickness detection system to ensure that stencil thickness deviation is controlled within ±2μm.

Using a closed-loop control system, real-time monitoring and adjustment of printing pressure, speed and key parameters such as release distance and release speed.

Deploy SPI machine vision system to detect the amount of solder paste deposited in real time, and automatically trigger the alarm mechanism when the deviation exceeds 5 per cent

3. Component placement optimisation

Upgraded to a multi-axis mounter to increase placement speed to 80,000 CPH (components per hour) while maintaining high accuracy of ±30μm.

Adopted vacuum nozzle pressure online monitoring technology to ensure stable component placement pressure and avoid solder paste extrusion.

The introduction of an intelligent feeder that automatically switches component trays according to the BOM (bill of materials) significantly reduces line changeover time.

4. Reflow soldering optimisation

Use reflow oven with adaptive learning function to automatically adjust the temperature setting of each zone according to the difference of heat capacity of PCB.

Deploy infrared and thermocouple composite temperature measurement system to achieve three-dimensional visual monitoring of the temperature field in the furnace.

Adopting local nitrogen protection technology, injecting nitrogen only in the reflow zone to control the oxygen content below 50ppm to enhance soldering quality.

5. Wave soldering optimisation (if applicable)

Replace conventional wave soldering with selective wave soldering to reduce thermal shock to components and PCBs.

Increase through-hole fill rate to over 95% using a dual wave design (turbulent wave combined with smooth wave).

Introduced a flux spray closed-loop control system that automatically adjusts the spray volume according to the size of the PCB and the aperture to ensure even flux coverage.

6. Manual soldering and rework optimisation

Equipped with a rework station with temperature profile preset function, capable of automatically recording and reproducing successful rework parameters.

The temperature of the rework area is monitored in real time using an infrared camera to avoid damage to components or PCBs due to overheating.

7. Post-solder cleaning optimisation

Introducing a cleaning machine with online monitoring of pH value and automatic addition of cleaning agents to ensure that the ionic contamination of residues is less than 1.5μg/cm².

Adopt vacuum vapour degreasing technology to reduce the use of cleaning agent by more than 50% and improve the cleaning efficiency at the same time.

8. Inspection and Test Optimisation

Deployment of AOI equipment based on deep learning algorithms to increase the defect recognition rate to 99.9%, while reducing the false alarm rate to less than 0.1%.

Introduced 3D X-Ray inspection system to achieve quantitative analysis of BGA solder joint voiding rate with an accuracy of ±1%.

Conducted in-line electrical testing using flying probe testers, increasing the number of test points to 10,000 and reducing the test cycle time to 30 seconds/board.

9. Rework and repair optimisation

Introducing laser rework table to realise non-destructive disassembly and replanting of BGA and other tiny components, improving rework efficiency and quality.

Establishing a rework process database to automatically match the best rework parameters and increase the one-time repair rate to over 95%.

Through the implementation of the above optimisation measures, the efficiency and quality of the lead-free soldering process can be significantly improved, and the cost and environmental pollution can be effectively reduced. Enterprises can implement these optimisation solutions in phases according to their actual production scale, product complexity and investment budget, and it is recommended to prioritise upgrading of testing equipment and process control systems in order to quickly obtain improvement results.