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90 SMT007 MAGAZINE I MAY 2021 operator dependent and requires skills practice in order to not damage the PCB. • e solder wick method requires the least amount of equipment investment, but it does require intermediate soldering skills. In addition to the solder wicking method, vacuum extraction is another means for remov- ing the solder from a pad location. is vac- uum can be created by a manual spring-loaded pump, a hand tool, as part of a BGA/leadless device rework system, or as a stand-alone pro- grammable tool (Figure 2). It is not recommended to use a manual pump as there is a lack of continuous suction at the tip as the solder joint is cycled through sev- eral heating and cooling cycles. In the case of the other powered vacuum desoldering tools, a hole in the tip center is used as a vacuum to remove solder which has gone into reflow. Matching the diameter of the tip to the width of the pad is recommended as larger-than-pad- sized tips may burn the PCB laminate. Aer applying flux to the location, place the heated tip onto the pad gently until you sense the sol- der is going into reflow. Do not exert pressure onto the pad as it may cause pad damage. Integrated programmable soldering excava- tion tools usually have a sensor which keeps the distance of the PCB at a fixed height. Reser- voirs which contain the excavated solder need to be cleaned out whether it is a manual hand piece or part of an automated system. e vac- uum solder extraction method, regardless of the type of system used, is fast and requires less skill and experience compared to the sol- der braid approach. e technique which is used the least for sol- der removal in PCB rework is the copper cou- pon method. is method uses a flux-dipped copper coupon to remove all the solder at one time. e coupon is placed onto a BGA rework system nozzle and heated, then slowly low- ered onto the surface of the PCB. As it gets in the vicinity of the BGA or CSP pads, solder is wicked onto the coupon. One of the advan- tages of this approach is the speed of excava- tion. is is offset by potential coplanarity problems for the PCB which may cause dam- age to the PCB laminate or cause it to not be as effective, the cost of the specially tooled single use coupon, and the need to be able to program or stop the coupon from going down onto the PCB. Whether using the solder wick approach, a solder vacuum excavation tool, or a wicking coupon, solder removal from the pads prior to placement of the replacement device is impor- tant from both a reliability and quality of place- ment perspective. e use of the technique depends on the kind of tooling capabilities, the sophistication of the rework equipment, and the experience of the operators. Cycle times and potential risks in damaging the PCB laminate factor into choosing the appropriate method for PCB rework. SMT007 Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact To read past columns or contact Wettermann, click here. Figure 2: A programmable solder excavation tool.

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