Application 2 – IR Reflow Calibration
Q-Flex has developed a proprietary process that converts a conductive plane in Flex or Rigid PCB into a temperature sensing plane. Specific locations on a board are assigned a location number and the real time highly accurate temperature information can be collected and analyzed.
IR Reflow Chamber Thermal Profile System
When the reflow process is set up after the pick and place operation, it is important to know exactly what temperature is received at the PCB level where the solder paste is screened on. If the temperature is too high, considerable damage can occur to the base PCB and/or sensitive components. In a worst case situation, the damage is subtle enough to pass the functional test and then cause inconsistent failures in the field. This makes trouble shooting extremely difficult. If the temperature is not optimum, the solder may not reach eutectic level consistently. This may cause poor quality solder joint and failures later on.
Current process controls do not give accurate thermal profile as received at the board level. Considering the complexity of current PCBs, thermal modeling will be extremely difficult and will vary from part number to part number.
Thermal Profile System:
Using the in situ thermo sensing technology, the calibration system has been developed to address the above issue. The objective is to accurately measure the temperature in real time over the entire PCB area to define the exact thermal profile that will be experienced by the assembly during reflow process.
The system consists of a PCB 12 in. x 18 in. size. Typically the board has 15 measuring points. These points are shaped like SMT pads. There are no components, no solder, no assembly. Just a standard PCB.
Sub layer traces connect these points and direct the signals to a corner of the board where the measurement circuits are located under a thermally insulated covering. The electronic circuit measures the temperatures in real time (response time 0.1 second) and transmit the data by Zigbee transmitter to a laptop outside the IR chamber.
During the set up, this PCB is placed on the conveyor and the chamber is set up with the first setting. As the board passes through the IR zones, the transmitter sends the data out to the computer outside. The temperature profile for each point is plotted in real time and digital data is collected for more analysis.
The settings are adjusted until the desired temperature is reached on the PCB surface. The system will show the point to point variations as well as thermal energy absorbed and the temperature levels (area under the curve). This is an important aspect of the system. While the same thermal energy can be absorbed at lower temperature and longer time vis-à-vis higher temperature for shorter time, the effect on the assembly can be completely different. The system also shows the thermal ramp up before the actual reflow process.
Since the heat absorption can vary due to the material of construction for PCB (FR-4, polyimide, high frequency etc.), corresponding materials are offered for the system board.