In order to print solder paste, the unsoldered PCB assembly board cannot hold the test terminal of the thermocouple, so the actual product needs to be used for testing.
In addition, test samples cannot be reused 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 once or twice can also be used as a formal product, but the same test sample is not allowed to be reused for a long time.
Because after long-term high-temperature welding, the color of the printed circuit board will become darker or even brown. Although the heating method of the hot air stove is mainly convection conduction, there is also a small amount of radiation conduction. Dark brown PCB absorbs more heat than regular fresh light green PCB. Therefore, the measured temperature is higher than the actual temperature. If lead-free soldering is used, cold soldering is likely to occur.
1. Selection of test points: Based on the complexity of the PCB assembly board and the number of collector channels (generally, the collector has 3-12 test channels), select at least three representative temperature test points that can reflect high (hot spot) temperatures. ), medium and low (cold spot) PCB surface assembly boards.
The temperature (hot spot) is generally located in the middle of the furnace, where there are no components or there are few components. The temperature (cold spot) is generally located in large components (such as PLCC), large areas of copper distribution, transmission rails or edges of the furnace hall, and where hot air convection is impossible. Arrival location.
2. Fixed thermocouple: Use high-temperature solder (sn-90pb, solder with a melting point exceeding 289°C) to weld the test ends of multiple thermocouples on the test points (solder points). The solder on the original solder points must be the same as before welding. Remove; or use high-temperature tape to stick the test end of the thermocouple to each temperature test point on the PCB. No matter which method is used to fix the thermocouple, it is necessary 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. The location of the jack, or the socket into which the collector is inserted, pay attention to the polarity and do not insert it backwards. Number the thermocouples and record the relative position of each thermocouple on the surface mount board.
4. Place the PCB assembly board on the surface to be measured on the conveyor chain/mesh belt at the entrance of the reflow soldering machine (if a collector is used, place the collector at a distance of more than 200mm behind the surface PCB assembly board), and then start the KIC temperature Curve test procedure.
5. As the PCB is operated, real-time curves are drawn (displayed) on the screen (when the device comes with KIC test software).
6. When 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/schedule are displayed on the screen (if a temperature curve collector is used, take out the PCB and collector from the reflow oven outlet, and then read the temperature curve and peak temperature table through the software ).