Classic PCB temperature curve of reflow soldering process

The temperature profile drawing of a classic printed circuit board (PCB) involves connecting the thermocouples on the PCB assembly to a data recording curve instrument and passing the entire assembly through a reflow soldering furnace. There are two main purposes for creating temperature curves: 1) to determine the correct process settings for a given PCB assembly, and 2) to verify the continuity of the process to ensure reproducible results. By observing the actual temperature (temperature curve) that the PCB passes through in the reflow soldering furnace, the furnace settings can be verified and/or corrected to achieve the best quality of the final product.

The classic PCB temperature curve will ensure the best and continuous quality of the final PCB assembly, effectively reducing the PCB scrap rate, improving the PCB productivity and qualification rate, and improving overall profitability.

reflow process 

During the reflow process, heating inside the furnace brings the assembly to the appropriate welding temperature without damaging the product. In order to verify the reflow soldering process, people use a device that produces a temperature curve to determine the process settings. The temperature curve is a visual dataset of the time and temperature of each sensor during the heating process. By observing this curve, you can visually accurately see how much energy is applied to the product and where it is applied. The temperature curve allows operators to make appropriate changes to optimize the reflux process.

A typical temperature curve includes several different stages - initial ramp, soaking, spike to reflow, reflow, and product cooling. As a general principle, the desired temperature gradient is within the range of 2-4 ° C to prevent damage to the plate and/or components due to rapid heating or cooling.

During the heating period of the product, many factors may affect the quality of assembly. The initial heating is a rapid temperature increase when the product enters the furnace. The purpose is to bring the solder paste to the desired insulation temperature for starting the soldering process. The ideal insulation temperature is just below the melting point of the solder paste material -183 ° C for eutectic solder, and the insulation time is between 30 and 90 seconds.

The insulation zone has two purposes: 1) to bring the board, components, and materials to a uniform temperature close to the melting point of the solder paste, allowing for easy transition to the reflow zone, and 2) to excite the flux on the assembly. At the insulation temperature, the activated flux begins the process of removing oxides from the solder pads and pins, leaving a clean surface where the solder can adhere. The peak temperature formed by reflow is another transition, during which the assembled temperature rises above the solder melting point, and the solder paste becomes liquid.

Once the solder paste is above the melting point and assembled into the reflux zone, it is usually called time above liquid (TAL). The reflux zone is a critical stage inside the furnace, as the temperature gradient during assembly must be minimized and TAL must be maintained within the parameters specified by the solder paste manufacturer. The peak temperature of the product is also reached during this stage - assembly reaches the highest temperature inside the furnace.

Be careful not to exceed the maximum temperature and heating rate of any temperature sensitive components on the board. For example, a typical tantalum capacitor has a maximum temperature of 230 ° C. Ideally, all points on the assembly should reach the same peak temperature simultaneously and at the same rate to ensure that all parts experience the same environment in the furnace. After the reflow zone, the product cools down, solidifies the solder joints, and is ready for assembly in the subsequent process. Controlling the cooling speed is also crucial. Cooling too quickly can damage the assembly, while cooling too slowly can increase TAL and potentially cause fragile solder joints.

Two common types of temperature curves are used in reflow soldering processes, commonly referred to as insulation type (soap) and tent type (tent) temperature curves. In the insulation curve, as mentioned earlier, the assembly experiences the same temperature over a period of time. The tent temperature curve is a continuous temperature rise, starting from assembly into the furnace and continuing until assembly reaches the desired peak temperature.

Typical insulation type temperature curve

Typical tent temperature curve

The desired temperature curve will vary based on the type of solder paste used in assembly manufacturing. Depending on the chemical composition of the solder paste, the manufacturer will recommend the optimal temperature curve to achieve the highest performance. The information on the temperature curve can be obtained by contacting the solder paste manufacturer. The most common types of formulations include water-soluble (OA), rosin moderately activated (RMA), and no clean solder paste.