Reflow soldering

Reflow soldering is the most common method used in Surface Mount Technology (SMT) to attach electronic components to a PCB. After solder paste is printed on the board and components are placed, the entire assembly is passed through a tightly controlled convection oven. The heat melts the microscopic solder spheres in the paste, permanently bonding the components to the board.

A proper thermal profile consists of four distinct zones: Preheat (slowly raising the board's temperature to avoid thermal shock), Thermal Soak (holding the temperature steady to activate the flux and normalize heat across the board), Reflow (spiking the heat above the solder's melting point to create the joint), and Cooling (safely dropping the temperature to solidify the solder).

Every PCB design absorbs heat differently based on its layer count, copper weight, and component density. If a board is heated too quickly, components can crack. If it gets too hot, sensitive silicon chips will be destroyed. A custom thermal profile guarantees that the coldest point on the board reaches reflow temperatures without the hottest point exceeding component damage thresholds.

Before running production, we use a "Golden Board"—a sacrificial test PCB outfitted with multiple physical thermocouples. These sensors are attached to known hot spots (edges, small components) and cold spots (heavy copper areas, under large BGAs). We run this board through the oven to mathematically graph the exact heat absorption and adjust the oven zones accordingly.

Traditional leaded solder (SnPb) melts at approximately 183°C, allowing for cooler, less stressful oven profiles. Lead-free solder (like RoHS-compliant SAC305) requires significantly higher temperatures, melting around 217°C, with peak oven temperatures often hitting 240°C to 250°C. This requires stricter process control to prevent damaging heat-sensitive parts.

Pumping inert Nitrogen gas into the reflow oven actively displaces Oxygen. Oxygen causes rapid oxidation of both the solder pads and the melting solder paste at high temperatures. Nitrogen reflow creates a pristine environment that results in brighter, stronger, and more reliable solder joints—which is especially critical for higher-temperature lead-free processes.

Tombstoning occurs when a small, two-pad component (like a 0402 resistor) stands up vertically on one end during reflow. This happens when the solder on one pad melts and pulls the component before the other pad melts. We prevent this by ensuring perfect DFM pad geometries, balanced copper pour routing, and a perfectly engineered thermal soak zone.

If a board contains components that cannot survive a standard reflow cycle (like certain plastic connectors, batteries, or specialized sensors), we leave them off during the primary SMT run. They are later attached using highly controlled selective soldering machines or specialized hand-soldering stations.

Vapor Phase Soldering uses the latent heat of a boiling, inert liquid (Galden) to transfer heat to the PCB. Because the vapor can never exceed the exact boiling point of the liquid, it is physically impossible to overheat the board. It is highly recommended for massively thick backplanes or boards with extreme varying thermal masses.

Yes. We first populate and reflow the top side. We then flip the board, print paste, place components, and run it through the oven a second time. The surface tension of the solidified solder is incredibly strong and typically prevents bottom-side components from falling off. For extremely heavy bottom-side parts, we use specialized SMT adhesives.