PCBMASTER takes you through the key role of the preheating phase in PCBA processing



Author: Jack Wang

In the realm of modern electronics manufacturing, the soldering process in PCBA (Printed Circuit Board Assembly) is a critical step in ensuring the quality and reliability of electronic products. Among these processes, the preheating stage, though seemingly basic, holds key factors that determine the success of soldering, and its impact on the overall production process is profound. However, in actual production, especially during rework, prototyping, and PCBA first-article sampling, the importance of preheating is often underestimated or even overlooked, leading to a series of issues that cannot be ignored.

1. Reflow Soldering Preheat Zone: The Foundation of Soldering Quality

In SMT reflow soldering, the temperature profile is the core element for achieving high-quality soldering. The soldering zone typically consists of four main temperature control zones: the preheat zone, soak zone, reflow zone, and cooling zone. These zones work together to form a precise temperature control system, with the preheat zone being the critical starting point.

The primary task of the preheating stage is to gradually and steadily raise the temperature of the components from room temperature to a specific temperature slightly below the melting point of the solder paste. Generally, the melting point of common lead-based solder paste (e.g., Sn63Pb37) is around 183°C, while lead-free solder pastes typically melt between 217°C and 227°C. The preheating temperature is usually set about 50-60°C below the solder paste's melting point, roughly in the range of 150-160°C. This heating process should not be too rapid; the industry generally recommends a heating rate of 2-4°C per second. If the temperature rises too quickly, the circuit board and electronic components may suffer damage due to excessive thermal stress, such as micro-cracks in chip structures or fractures in brittle components like ceramic capacitors. Conversely, heating too slowly will significantly extend the production cycle, reduce efficiency, and increase costs.

During this heating process, volatile solvents in the solder paste gradually evaporate. If too much solvent remains, it can form voids during subsequent soldering, severely affecting the density and mechanical strength of solder joints, leading to issues like cold solder joints or solder detachment. This compromises the electrical connections in electronic products, reducing their reliability. Additionally, preheating activates the flux, which plays a crucial role in removing oxides from the soldering surface, reducing surface tension, and ensuring proper wetting of the solder. If the flux is not fully activated, oxides on the soldering surface cannot be effectively removed, leading to poor soldering and increasing the defect rate.

2. The Domino Effect of Insufficient Preheating

Skipping the preheating stage can have multiple negative impacts on PCBA processing. From the perspective of solder joint quality, insufficiently evaporated solvents and unactivated flux can directly cause numerous defects in solder joints. Voids not only reduce the mechanical strength of solder joints but also affect their electrical performance, increasing resistance and generating excessive heat during prolonged use, accelerating aging and failure. Cold solder joints can lead to intermittent open circuits during device operation, causing malfunctions and severely impacting product stability and lifespan.

From the perspective of component reliability, without the buffer of preheating, circuit boards and components exposed directly to the high-temperature reflow zone may suffer severe thermal shock. This shock can cause irreversible changes in the microstructure of components, such as cracks in the metal interconnects of semiconductor chips or damage to the layered structure of multilayer ceramic capacitors, leading to degraded or complete failure of electrical performance. Even if no immediate failure is observed, latent defects may emerge over time, increasing the probability of failure and significantly raising post-sales repair costs and user risks.

3. Medical Electronics PCBA: The Pursuit of Perfection in Preheating

The medical electronics manufacturing industry, which demands near-perfect product quality and reliability, places particularly strict controls on the preheating process in PCBA manufacturing. Medical diagnostic equipment, such as high-precision blood analyzers and electrocardiographs, relies on every solder joint on the PCB for accurate and reliable data. Minor soldering defects can interfere with electronic signal transmission, leading to deviations in test results and potentially affecting medical diagnoses, delaying treatment, and causing serious consequences.

When producing interconnects between flexible circuit boards and rigid-flex boards, the unique properties of flexible circuit materials make them more sensitive to temperature changes. Improper preheating can cause deformation or delamination of flexible circuits during soldering, compromising both electrical performance and mechanical strength, and reducing the reliability of the entire interconnect structure.

Imaging systems in medical electronics, such as CT and MRI machines, require extremely high image clarity and stability. The soldering quality in their circuit boards directly affects imaging performance. Issues in the preheating stage can lead to soldering defects that cause signal interference, resulting in image noise or blurring, severely impacting diagnostic accuracy.

Similarly, PCBs for high-tech temperature measurement devices used in security and epidemic prevention also require stringent preheating processes. These devices need to be highly accurate and responsive, and any soldering defect can affect the signal transmission and processing of temperature sensors, leading to deviations in temperature readings and failing to meet the strict accuracy requirements for epidemic prevention.

To meet the high demands of medical electronics PCBA, manufacturers must not only possess advanced manufacturing capabilities and the latest technologies but also provide customized services for each PCBA project. Deep communication between customers and engineering teams is crucial, as it helps manufacturers fully understand the unique requirements of medical devices, optimize PCBA processes from the design stage, and ensure every step meets the high standards of the medical industry, providing a solid foundation for the stable operation of medical electronic devices.

If you'd like to learn more, feel free to consult PCBMASTER at www.pcbmaster.com.

Author: Jack Wang