PCB Technical - How to test the PCB board temperature limit

In order to print the solder paste, the PCB assembly board without soldering cannot fix the test end of the thermocouple, so it is necessary to use the actual soldered product for testing.

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

Because of long-term high-temperature welding, the color of the printed board will become darker, or even brownish brown. Although the heating method of the all-hot blast stove is mainly convective conduction, there is also a small amount of radiant conduction, and the dark brown absorbs more heat than the normal fresh light green PCB. Therefore, the measured temperature is higher than the actual temperature. If in lead-free soldering, it is likely to cause cold soldering.

1. Select the test point: According to the complexity of the PCB assembly board and the number of channels of the collector (generally the collector has 3~12 test channels), choose at least three or more to reflect the height of the PCB surface assembly board (the hottest spot), Medium and low (coldest point) representative temperature test points.

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

2. Fix the thermocouple: Use high-temperature solder (Sn-90Pb, solder with a melting point of more than 289 degree Celsius) to solder the test ends of multiple thermocouples to the test points (solder joints). The solder on the original solder joints must be soldered before soldering. Clean it up; or use high-temperature tape to stick the test ends of the thermocouples on each temperature test point of the PCB. No matter which method is used to fix the thermocouple, it is required to ensure that the thermocouple is welded, glued, and clamped firmly.

3. Insert the other end of the thermocouple into 1, 2.3... on the machine table. The position of the jack, or the socket of the collector, pay attention not to insert the polarity reversely. Number the thermocouples, remember the relative position of each thermocouple on the surface assembly board, and record it.

4. Place the PCB assembly board of the tested surface on the conveyor chain/mesh belt at the entrance of the reflow welder (if you use a collector, you should place the collector behind the surface PCB assembly board, leaving a little distance, about 200mm or more) ), and then start the KIC temperature curve test program.

5. With the operation of the PCB board, draw (display) a real-time curve 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), Then take out the PCB circuit board and the collector from the outlet of the reflow soldering furnace, and then read the temperature curve and peak temperature schedule through the software).