Detailed explanation of the measurement method for reflow soldering temperature curve

Due to the structure and working principle of reflow soldering equipment, there may be significant differences in the set temperature of the temperature zone when reflected on the produced products. The interval temperature displayed on the reflow soldering equipment is not the actual interval temperature. The displayed temperature only represents the ambient temperature sensed by the thermistor in that temperature range. If the thermocouple is close to the heating source, the displayed temperature will be correspondingly higher than the temperature in other areas in the temperature range. The closer the thermocouple is to the transmission track of the printed circuit board, the more accurately the displayed temperature will reflect the actual temperature of the product. Therefore, after setting the temperature, it is necessary to conduct actual measurements on the product to obtain the actual temperature, and analyze and modify the set temperature to obtain an optimal temperature curve for a certain product. JEENOCE will share with you the following details:

1、 Equipment and auxiliary tools required for measuring the temperature curve of reflow soldering furnace:

Temperature curve instrument: Temperature measuring instruments are generally divided into two categories: real-time temperature measuring instruments, which can transmit temperature and time data in real-time and make graphs. Another type of non real-time thermometer, which collects, stores, and then uploads the collected data to a computer for analysis.

Thermocouple: a medium used to sense ambient temperature. The working principle is that a circuit is composed of two different components of conductors. When there is a temperature difference between the temperature measurement end and the reference end, a thermal current is generated in the circuit, which reflects the change in external temperature through the magnitude of the current. Thermocouples generally require a smaller diameter, as smaller diameter thermocouples have smaller thermal mass and faster response, resulting in more accurate results. Tools for attaching thermocouples to printed circuit boards: solder, adhesive, high-temperature tape.

2、 Selection of testing points for reflow soldering temperature curve:

In the actual production process of a printed circuit board, the temperature of various components carried by each area on the board surface is not the same. The hot air inside the furnace flows through the furnace under the action of the hot air fan (flowing from the upper and lower heating plates towards the transmission track direction. When encountering obstacles, it diffuses along the surface of the printed circuit board towards the edge of the board, making the central area of the board the place with the lowest temperature). The size of the component also determines the temperature, with smaller components having higher temperatures and larger components having lower temperatures. Therefore, in actual measurement, it is particularly important to select the measurement point that reflects the true temperature of the tested product more accurately and comprehensively. Generally follow the following principles.

1) Select as many points as possible under the conditions allowed.

2) The selection of the measured points should be on the same longitudinal axis as much as possible.

3) Components with special temperature requirements.

4) The position with the highest surface temperature.

5) The position with the lowest surface temperature.

3、 Fixation of thermocouple for measuring reflow soldering temperature curve:

After selecting the measured point, thermocouples need to be installed at the measured point. There are generally several methods:

1) Use solder to fix the thermocouple, usually using high-temperature solder with high PB content to fix the thermocouple at the measured point. It is required that the solder joints should be as small as possible, as the weldability of high-temperature solder is not high, so the welding skills are required to be high. Due to high-temperature welding, the thermal impact on the components is also significant. The temperature measurement effect is good.

2) Fix the thermocouple with adhesive, usually using epoxy resin adhesive to fix it at the measured point. Require the adhesive points to be as small as possible. The temperature measurement effect is average.

3) Use high-temperature tape to fix the thermocouple, usually using tape with high temperature resistance and good thermal conductivity to fix the thermocouple at the measured point. It is required to place the thermocouple as close as possible to the measured point. This method is easy to operate, but the measurement effect is the worst. Each method should place the measuring end of the thermocouple as close as possible to the measured point in order to obtain more accurate measurement results.

4、 Measurement method for reflow soldering temperature curve:

1) Perform thermal performance analysis on the tested printed circuit board and select the test point.

2) Attach the thermocouple to the tested printed circuit board. To ensure the accuracy of the measurement results, the printed circuit board being tested should be selected as a fully assembled product.

3) Trigger the thermometer to start recording data and place it together with the tested printed circuit board into the reflow soldering furnace. Two points to note here. The first point: There are two types of thermometers. One is to start recording data immediately after triggering, which requires putting the thermometer into the reflow soldering furnace in the shortest possible time after triggering to ensure the accuracy of the measured reflow time. Another type of thermometer is a temperature sensing type, which starts recording data only when the ambient temperature rises to 40 ℃ (usually higher than the human body temperature, to ensure that data is not recorded during the operation phase) or above. This type of thermometer has no specific requirements for the insertion time. Secondly, in order to maximize the restoration of the actual production environment, it is necessary to ensure the number of passing boards in front of the tested board during normal production. After the tested board is placed in the furnace chamber, the product cannot be placed again to avoid accidents.

4) Finally, connect the thermometer that has collected data from the furnace to the computer to generate a temperature curve.

5、 Analysis of reflow soldering temperature curve and furnace temperature adjustment:

Before analyzing the reflow soldering temperature curve, it is necessary to master the data of several control points:

1) Determine the total time from ambient temperature to peak reflux temperature.

2) Process parameters for each temperature zone (temperature, time, heating rate).

The specific adjustment method for the reflow soldering temperature curve is as follows: first, adjust the conveyor belt speed to match the total time from ambient temperature to reflow peak temperature with the desired heating curve residence time. Next, the deviation of the curve (process order) should be adjusted in order from left to right to ensure that the overall shape of the curve matches the process parameters and given standards of each temperature zone. Example: If there is a difference between the preheating zone and the reflux zone, the difference in the preheating zone should be adjusted correctly first. It is best to adjust one parameter at a time. Before making further adjustments, the adjusted curve should be run first and a new curve should be measured before referring to the new curve for adjustment. Because a temperature change in a given temperature range will affect the subsequent temperature changes in that range. The adjustment of the furnace temperature curve should be carried out in a progressive manner.