PCB Blog - Classic PCB board temperature curve system components

A classic PCB board temperature profiling system consists of the following components: a data collection profiler, which passes through the middle of the furnace and collects temperature information from the PCB. Thermocouples, which are attached to key components on the PCB, are then connected to the accompanying curve meter. Thermal insulation, it protects the curve meter from being heated by the furnace. A software program that allows the collected data to be viewed in a format that quickly determines soldering results and/or finds out-of-control trends before they adversely affect the final PCB board product.

Thermocouples
Type K thermocouples are commonly used in the electronics industry. There are various techniques for attaching thermocouples to components on a PCB board. The method used depends on the type of PCB being processed, as well as the user's preference.
1) Thermocouple attachment
High temperature solder, it provides strong connection to the PCB board. This method is typically used for operations where a dedicated reference board can be sacrificed for profiling and inspection processes. Attention should be paid to the guaranteed amount of tin to avoid affecting the curve. Adhesive for fixing thermocouples to PCBs. The use of glue typically results in a rigid physical connection of the thermocouple pair assembly. Disadvantages include the possibility that the glue may fail during heating, leaving residue on the assembly when removed after profiling. Also, care should be taken with the amount of glue used, as increasing thermal mass may affect the results of the temperature profile. Kapton or aluminum tape, it's easy to use, but an unreliable method of fixing. Temperature curves using tape often show very jagged curves because the thermocouple junction is lifted from the contact surface during heating. Ease of use and no residue that can interfere with assembly make capeton or aluminum tape a popular method. Pressure-type thermocouples are clamped to the edge of the circuit board and use spring force to securely contact the thermocouple connection point to the assembly that is being profiled. Pressure probes are quick, easy to use and non-destructive to the PCB.

2) Placement of thermocouples
Because the outer edges and corners of an assembly heat faster than the center, and elements with higher thermal mass heat up more than elements with lower thermal mass, a minimum of four thermocouple placement is recommended. One thermocouple is placed on the edge or corner of the assembly, one on the small element, another in the center of the plate, and a fourth on the larger mass element. It is also possible to add thermocouples on other parts of the board of interest, or on components at risk of thermal shock or thermal damage. Solder paste manufacturers generally have recommended temperature profiles for their solder paste formulations. The manufacturer's recommendations should be used to determine a process-specific curve for comparison with actual assembly results. Steps may then be taken to alter machine settings to achieve results for a particular assembly. For PCB board assembly manufacturers, there are now new tools that make it easy to design target profiles for a specific combination of solder paste and reflow oven. Once designed, the target profile can be simply recalled by the machine operator for this specific PCB assembly, automatically run on the reflow oven.

3) When to make a temperature curve
Temperature profiling is especially useful when starting a new assembly. The furnace settings must be decided to optimize the process for high quality results. As a diagnostic tool, the profilometer is invaluable in helping to identify poor yields and/or high rework.
Temperature profiling can identify inappropriate furnace settings, or ensure that these settings are appropriate for assembly. Many companies or factories make temperature curves on standard reference boards, or use the machine's quality control curve meter on a daily basis. Some plants run temperature profiles at the beginning of each shift to verify furnace operation and avoid potential problems before they occur. These temperature profiles can be stored as a hard copy or in electronic format and can be used as part of an ISO program or to perform SPC statistical process control of machine performance over time. Assemblies used for temperature profiling should be handled with care. The assembly may degrade due to improper handling or repeated exposure to reflow temperatures. Curved plates may delaminate over time, and thermocouple attachments may become loose, which should be expected, and the profiling equipment should be inspected before damage occurs on each run. The key is to guarantee the results that the measuring equipment can get.

4) The temperature curve of the classic PCB board and the quality control curve of the machine
While the common type of profiling involves the use of a running profiler and thermocouple to monitor the temperature of components on the PCB, profiling is also used to ensure that the reflow oven operates continuously at the specified settings. A variety of built-in machine temperature profilers are available that provide daily inspection of key reflow oven parameters, including air temperature, heat flow and conveyor speed. These instruments also provide the opportunity to quickly identify any out-of-control trends before they affect the quality of the final PCB boardassembly.