Precautions for reflow soldering temperature curve testing

The testing of the reflux temperature curve is generally carried out using a temperature collector (i.e. temperature memory device) that can enter the furnace together with the PCB board. K-type thermocouples are used for measurement (various types of thermocouples are made of different materials according to the measurement temperature range and accuracy), and the diameter of the thermocouple wire is appropriate to be 0.1-0.3mm. After the test, the data from the memory device is input into the PC special testing software for curve data analysis and processing, and the PCB component temperature curve is printed out.

1. Installation of Thermocouples a. For thermocouples used for temperature sensing, during use and installation, it should be ensured that there are no short circuits except for the test points, otherwise the test accuracy cannot be guaranteed, and the test points should be as small as possible b. When a thermocouple is connected to a memory device or other testing equipment, its polarity should be consistent with the equipment requirements. The thermocouple converts temperature into electromotive force, so there is a directional requirement when connecting (Currently, the thermocouple plugs we use have positive and negative polarity differentiation)

2. The selection of testing points is generally at least three points, which can represent the temperature changes on the PCB component (reflecting the temperature changes in high, medium, and low temperature parts of the PCB component); In general, the highest temperature is located at the center of the non component edge perpendicular to the transmission direction of the PCB, and the lowest temperature is at the half field terminal of large components near the center of the PCB (PLCC, QFP, etc.). In addition, there should be testing points on the surface of parts with poor heat resistance and customer specific requirements

3. Installation of testing points: The thermocouple should be reliably connected to the testing location, otherwise thermal resistance will occur. In addition, the materials in contact with the thermocouple and the materials used to fix the thermocouple should be minimized, as their insulation or heat absorption effects will directly affect the authenticity of the thermocouple measurement values. Four commonly used thermocouple connection methods:

High temperature solder: melting point above 290 ℃, good thermal conductivity, low thermal resistance between thermocouple and PCB surface, high mechanical strength, reliable connection with small measurement error, and can be continuously tested Welding technology is difficult, changing test points is inconvenient, and it is easy to damage PCB pads or components due to overheating. Thermocouples cannot be connected to non tinned surfaces Suitable for continuous testing at fixed points.

Adhesive B: It can connect thermocouples to non tinned surfaces and withstand several cycles of reflow soldering temperature Curing after bonding, inconvenient to operate, and difficult to remove residual adhesive Suitable for continuous testing at fixed points.

C High temperature tape: It can connect thermocouples to non tinned surfaces, making it easy and convenient to change the testing points As the temperature increases, the adhesive force of the tape decreases, and the thermocouple deviates from the test point, causing testing errors. The thermocouple cannot be fixed in a narrow position Suitable for multi-point testing.

Mechanical connection: The connection is sturdy and reliable, can withstand repeated testing, and can be tested in narrow positions. The mechanical components increase the heat capacity near the thermocouple, resulting in high testing costs Suitable for high-density multi-point continuous testing.

At present, we mostly use high-temperature welding materials. We fix the thermocouple test head of the memory device to the testing points of the PCB using high-temperature welding material patch adhesive or high-temperature tape paper, and then fix the thermocouple wire with high-temperature tape/adhesive to avoid affecting the measurement data due to its movement. When welding and fixing, the welding amount should be as small and uniform as possible, and the fixing adhesive should be as thin as possible

4. Requirements for the test board: a. In principle, the complete reflow product of this model should be used for production to ensure a true reflection of the temperature changes of the product in the reflow soldering furnace. b. Other substitute test boards should meet the following requirements: the substrate material should be the same, the substrate outer dimensions should be the same, the substrate thickness should be the same, the number of SMT parts should be roughly the same, and the heat absorbing or heat-resistant parts should be similar

5. Other precautions: a. When placing the test board and memory device together in the furnace, make sure that the distance between the memory device and the test PCB board is at least 100mm to avoid heat interference b. The relevant experimental data shows that the reflux furnace can only reach the thermal equilibrium of the furnace body after 30 minutes of operation. Therefore, it is required to run the furnace for at least 30 minutes before conducting temperature curve testing and production c. After printing the temperature curve, the equipment will be adjusted to meet the temperature curve requirements based on the preheating temperature time, reflux peak temperature, reflux time, and temperature rise and fall rate. Due to the different heat capacity of the test points and temperature non-uniformity factors that characterize the performance of the reflux furnace, there will be certain differences in the temperature curves of the three test points d. Temperature curve recording: In addition to the printed temperature curve, it should indicate the required range and actual values of each parameter; The set value of the equipment; Distribution of testing points, direction of testing board deployment, measurement time, and result judgment e. Frequency measurement: In principle, it should be conducted once a week (according to customer requirements). When setting or product changes occur, temperature curve testing should be conducted as needed