THERMAL PROFILING: OPTIMISING SOLDERING OPERATIONS

Soldering operations play an important role in PCB assembly. Advanced monitoring technologies, such as thermal profiling, improve PCB quality and reduce defects. These solutions can be effectively applied across various soldering processes, including reflow, wave, vapour, and selective.

Thermal profiling is the unsung hero in electronics manufacturing, ensuring precision and reliability in every solder joint. Temperature control throughout the soldering process safeguards against common issues like solder joint voids and damage to delicate heat-sensitive micro components. This is critical as the industry shifts towards Pb-free solder paste, which demands higher melting points and brings new challenges to the table. Additionally, thermal profiling ensures proper solder penetration within through-hole joints during wave and selective solder.

We explore the complexities of soldering and how thermal profiling with specialised equipment benefits PCB assemblers.

The thermal profiling process

Thermal profiling involves measuring and analysing the temperature changes across the PCB during the soldering process to achieve consistent and reliable soldering results by fine-tuning the heating and cooling phases. Wave and selective soldering profilers also measure wave height and contact time in a single pass.

Applications:

• Process Development: Essential for developing and optimising the reflow/wave process for new products and matching specific solder paste requirements.
• Quality Control: Used to verify that the reflow oven and wave solder are operating correctly and consistently within the desired temperature ranges.
• Troubleshooting: Helps identify issues in the reflow process, such as hot spots, insufficient heating, and inadequate wave height/contact with the wave or selective soldering.

Advantages:

• Provides a detailed thermal profile essential for fine-tuning the reflow process.
• Helps to ensure that all areas of the PCB reach the required temperatures for proper soldering.
• Enables process optimisation of many machine variables and better-quality control.

Key issues with inadequate temperature control

Thermal profiling ensures that every area of the PCB reaches and maintains the correct temperature for the right duration. The following problems occur when temperatures are not regulated:

Heating Rate: Heating too quickly can cause splatter or solder balls, while heating too slowly may cause the flux to dry out before a good solder joint forms.

Overheating: Control the temperature as excessive heat can cause thermal shocks to the components and damage them.

Insufficient Heating: Failing to heat the board adequately results in poor solder joints.

Understanding a PCB’s thermal profile

A PCB’s thermal profile represents its temperature over time during the soldering process. Several factors influence this profile:

Number of Layers: More layers mean more copper, which absorbs more heat.

Copper Content: Different sections of the same board may have varying amounts of copper, leading to different heating profiles. Areas with more copper heat up slowly.

Solder Paste: A proper thermal profile allows gases and volatile materials to escape before the solder joint solidifies, reducing voids.

Vacuum Control in Soldering

Convection reflow ovens with integrated vacuum chambers such as the Heller MK 5 and MK 7 are becoming more common. Heller asserts that vacuum-assisted reflow has been shown to reduce solder joint voiding by 99%. Modern thermal profiling now integrates both vacuum measurement and temperature control. Here’s how vacuum-assisted reflow works:

Absence of Air: The vacuum stage is after the peak reflow zone exposing the PCB to the vacuum while the solder joint is still liquid. The soldering process takes place under reduced pressure. This reduces void formation, improves wetting, and minimises the risk of solder bridging or other defects.

Advanced Electronics: Beneficial in applications where void-free soldering is crucial, such as high-power electronics, aerospace components, and medical devices.

Vacuum Levels: Incorrect vacuum levels can lead to problems with inadequate void removal or unnecessarily prolonged hold times in the vacuum, resulting in reduced throughput.

Fine-Tuning Soldering Equipment

To achieve the ideal thermal profile for each PCB, fine-tune the soldering oven or wave/selective solder machine using a test board, just as you would during actual production. It’s best to profile with a fully loaded oven as oven characteristics vary between loaded and unloaded. This process involves:

Using a Data Logger: A data logger, housed in a heatshield or carrier case designed for the oven or machine, is essential for capturing temperature data. At the heart of thermal profiling, the data logger is used across reflow, vapour phase, wave and selective processes. Systems such as the Solderstar SLX Datalogger have advanced capabilities to measure and record pressure levels when soldering is performed under a vacuum as well as vibration and O2.

Placing Temperature Probes: Thermocouples hook up on various sections of the board to monitor the temperature in the oven. These must be attached properly to avoid erratic thermocouple readings on the profile graph.

Data Logger Carriers: A protective shield or carrier surrounds the data logger after it’s connected to a PCB. Each soldering process requires a unique data logger carrier to ensure accurate measurements and protection. For instance, a reflow shield safeguards the logger from extreme temperatures in the oven, while a wave pallet measures wave process points such as speed, temperature, parallelism, and dwell times. A selective carrier is designed for measuring specific process points, and the vapour phase process barrier protects the logger from gases and liquids within the system.

Software Tools: Inbuilt libraries with manufacturer’s oven and solder paste profiles assist with profiling. The Solderstar SLX Datalogger also incorporates a wizard to perform common tasks and provide profile improvement solutions if required.

Recording Temperature/Vacuum Pressure Data: As the PCB passes through the oven or wave solder machine, sensors monitor the temperature/vacuum and are recorded by the data logger.

Analyzing Data: Software generates time vs. temperature plus vacuum pressure graphs for detailed analysis. This analysis ensures:

• The temperature did not exceed the maximum allowable temperature for the components.
• The heating rate did not exceed the maximum specified rate.
• All areas of the board reached and maintained the correct temperature for the specified duration.
• The vacuum levels, hold down time, pull down and release rates.
   

Vacuum Measurement Graph: Solderstar Thermal Profiling Solutions. The above graph illustrates the vacuum data reading on the profile with the pressure scale/values displayed down the right-hand side.

Adjusting and Storing Machine Settings: Machine settings can be adjusted and stored for each specific board based on the thermal profile analysis. This step is important for Contract Electronics Manufacturers (CEMs) involved in high-mix manufacturing, where different boards may require unique profiles. This is crucial for consistent quality across multiple production runs.

Benefits of Thermal Profiling

Manufacturers can significantly reduce solder defect rates by measuring, optimising, and recording unique thermal profiles for each board. The result is higher quality and more reliable PCB assemblies with long-term durability. Other advantages include:

Minimised Rework: An optimised thermal profile reduces defects, resulting in fewer boards requiring rework or disposal. This improves the yield of the soldering process.

Traceability: Thermal profiling provides a record of the temperatures experienced by the PCB during the soldering process. Data recording is crucial for traceability, particularly in industries like aerospace, automotive, and medical devices where documentation of manufacturing conditions is required.

Material Savings: Thermal profiling can reduce the amount of solder paste and flux required, leading to material savings.

Energy Efficiency: Proper profiling can reduce energy consumption by optimising the heating and cooling cycles of the reflow oven or wave soldering machine, lowering operational costs.

In summary, thermal profiling is a vital part of the PCB soldering process. PCB assemblers can be more confident with the reliability and efficiency of the final product. Profiling reduces defects, optimises the soldering process, and provides valuable data for continuous improvement, leading to cost savings and enhanced product performance.

Hawker Richardson has a wealth of expertise in SMT production, and our team can help you find the best profiling solutions for reflow, vapour, wave and select soldering.

Top image caption: Vacuum Control in Soldering. Image credit: Solderstar