Analysis of PCB board extreme temperature test method

Because the PCB assembly board with solder paste printed but no soldering cannot fix the test end of the thermocouple, it is necessary to use the actual soldered product for testing.

In addition, the test sample cannot be used repeatedly, and should not be used more than 2 times. Generally speaking, as long as the test temperature does not exceed the limit temperature, the assembled board that has been tested 1~2 times can still be used as a formal product, but it is absolutely not allowed to use the same test sample repeatedly for a long time.

After long-term high-temperature soldering, the color of the printed circuit board will become darker, even brown. Although the heating method of the full hot air furnace is mainly convection conduction, there is also a small amount of radiation conduction. The dark brown PCB absorbs more heat than the normal fresh light green PCB. Therefore, the measured temperature is higher than the actual temperature. If it is lead-free soldering, it is likely to cause cold soldering.

1. Select test points: According to the complexity of the PCB assembly board and the number of channels of the collector (generally the collector has 3 to 12 test channels), select at least three or more representative temperature test points that can reflect the high (hottest point), medium, and low (coldest point) on the PCB surface assembly board.

The highest temperature (hot spot) is generally in the middle of the furnace, where there are no components or few components, or where there are small components; the lowest temperature (cold spot) is generally at large components (such as PLCC), large areas of copper, transmission rails or furnace edges, and locations that cannot be reached by hot air convection.

2. Fix the thermocouple: Use high-temperature solder (Sn-90Pb, solder with a melting point over 289°C) to solder the test ends of multiple thermocouples to the test points (solder points). The solder on the original solder points must be cleaned before welding; or use high-temperature tape to stick the test ends of the thermocouples to the various temperature test points on the PCB. No matter which method is used to fix the thermocouple, it is required to ensure that it is firmly soldered, glued, and clamped.

3. Insert the other end of the thermocouple into the 1, 2, 3... jacks on the machine table, or into the socket of the collector, and pay attention to the polarity. Number the thermocouples and remember the relative position of each thermocouple on the surface assembly board and record them.

4. Place the surface PCB assembly board to be tested on the conveyor chain/mesh belt at the entrance of the reflow soldering machine (if a collector is used, it should be placed behind the surface PCB assembly board, leaving a slight distance, about 200mm or more), and then start the KIC temperature curve test program.

5. As the PCB runs, a real-time curve is drawn (displayed) on the screen (when the device comes with KIC test software).

6. After the PCB runs through the cooling zone, pull the thermocouple wire to pull the PCB assembly board back. At this time, a test process is completed and the complete temperature curve and peak temperature/timetable are displayed on the screen (if a temperature curve collector is used, take out the PCB and collector from the exit of the reflow soldering furnace, and then read the temperature curve and peak temperature timetable through the software).