SMT007 Magazine

SMT-Nov2017

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34 SMT Magazine • November 2017 IR systems measure temperature empirically at the PCB or component, whereas hot air systems measure hot air temperature near the hot air exit source. This makes for a more-exacting pro- file based on achieved component temperature. In addition, IR heat sources require little tool- ing for specific BGA sizes. Another advantage is the ability to easily take in-process temperature data of the IC for each reflow operation. How- ever, the primary advantage to the IR heating is the ability to not disturb very small compo- nents as no turbulent air flow exists along with the heating source. While the infrared heating source has some distinct benefits over the hot air systems, it too has some shortcomings as a reflow source for BGA rework. There are still some underpowered IR heaters on the market which means that the rework process cycle time is much longer than a properly-designed nozzle and hot air system. This means that for very large boards such as servers, backplanes and other high current-car- rying PCBs, the IR heat source may not be suf- ficient. The user needs to be aware that IR sys- tems using medium wavelength IR resulting in darker-colored components have a different ab- sorption of heat energy than light-colored com- ponents. In some cases, this means that the user must employ an ESD-safe black tape to make sure the component heats up to the right heat energy levels. Process control can be difficult with infrared heating as the absorbency spec- trum and therefore reproducibility from com- ponent to component and board to board re- sults can be an issue. Both hot air and IR heat sources have their place in PCB rework with specific choices of re- work heat source dependent upon the applica- tion at hand. SMT Bob Wettermann is the principal of contract rework and repair firm BEST Inc. TWO PREVALENT REWORK HEATING METHODS—WHICH ONE IS BEST? Figure 3: Focused IR heating technology for PCB rework.

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