SMT007 Magazine

SMT-July2015

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68 SMT Magazine • July 2015 is very small. This means that neighboring de- vices can be disturbed. Resoldering of the shields can be very chal- lenging as the gap between the shield lip and the board needs to be closed so as to not allow for any ingress or egress (in some cases) of RF signals. This means there is little room for error. Reflow Methods There are a variety of reflow methods that are used for single-piece RF shields, including the use of hot air convection, conduction and laser sources. After removal, the shields themselves need to be discarded for a variety of reasons. First, the shields are typically discolored after hav- ing gone through multiple heat cycles. While these discolored, formed pieces are not a reli- ability concern, the end users do not want this inside their product and have augmented their inspection criteria to make a discolored shield a defect. Second, in these rework processes, tin shields can become highly oxidized and nearly unsol- derable surfaces. Finally, from a practical basis, the shield lips soldered to the PCB can warp af- ter reflow (the shields are typically 2–4 mils in thickness) and thereby cannot then sit on the same plane as the PCB, rendering it impossible to close the gap between the PCB and the land of the board. This will make the purpose of the shield, which is to properly attenuate RF sig- nals, a failed one. The most commonly used method for the reflow of these shields involves the use of a hot air system with a customized nozzle. A bot- tom heater is used to first heat the PCB to 100– 125°C. Once this temperature is established, the hot air nozzle will come over the device. The nozzle itself is custom-designed to contour to the shape of the part, with the air baffles blow- ing the hot air towards the periphery of the part where the lip of the shield connects to the PCB land. The removal profile must be carefully thermocoupled as there are parts underneath the shield which should have limited heat ex- posure. In addition these same parts may be un- der filled, which can cause the solder on these components to experience "solder squirt out," an unrecoverable defect in the rework process. These undesired reflow problems along with the extended lead times of the nozzles makes the hot air removal method time consuming and expensive. Another method for the reflow of the RF shields is the use of a conductive soldering iron. In this method a conductive tip, shaped exactly to the outline of the shield, is placed on top of the shield. It is powered by a very generously- powered power supply that can deliver enough thermal energy to the shield to remove it quick- ly. The downside of this method is that the tips themselves are highly customized and subject- ed to both a machining and plating process, re- sulting in an expensive, long lead time in the manufacturing process. The flip side is thatthis method is a very fast way to both de-solder and to re-solder shields. Another method involves the use of a laser source. In this method, a material, which can absorb energy from the wavelength of the laser source being used is placed onto the shield. The laser then directs its beam to the selective area of this material on the shield surface and rap- idly transfers this energy in the form of heat to the shield. This allows the shield to quickly be removed. The benefits of this approach are very rapid removal, thereby limiting potential periph- eral part damage and little chance for neighbor- ing device reflow. The downside of this approach is that the laser source is capital intensive. figure 4: customized rf shield conductive tip. KnoCKinG doWn the Bone Pile A REWORK DIlEMMA: PCB SHIElDS continues

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