SMT007 Magazine

SMT-July2015

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July 2015 • SMT Magazine 69 Site Preparation Several methods can be employed in order to excavate the remnant solder on a PCB af- ter shield removal, including the use of solder wick, non-contact hot air excavation and cop- per coupons. The most common solder excavation meth- od, especially predominant in cases where the volume of rework is low, is the use of solder braid. The keys to making sure all of the remnant solder is wicked off and flat with no neighboring device damage is to use the proper technique and materials. In terms of the materials for this method the use of the correct-sized braid, the proper flux and the correct conductive hand sol - dering tool is important. Using the correct size of braid is critical for correct removal as too large of a braid can damage neighboring components; too small of a braid can lead to inefficiencies in the removal process as well as potentially dam- aging the PCB laminate or solder mask. The proper flux will ensure that the solder wets the braid properly. Finally, making sure the conductive power supply can deliver the power required for the application—especially if the board has a large thermal mass or the shield is connected to a ground plane, is required. In terms of technique the solder wick should be place a perpendicular to the area to be excavat- ed and then lifted off during heating. It should not be dragged across the surface of the PCB as this may damage pads, laminate and mask. A non-contact hot air programmable exca- vation system can also be used to scavenge the solder from a PCB after the RF shield has been removed. In this system, the board is brought up to temperature using a preheater. Next, a solder excavation tool with both a heated nozzle and a vacuum source emanating from the orifice of the tool will be programmed to drive around the periphery of the shield land pattern on the PCB. This is a non-contact method and has the advantage of not being subject to the skill lev- el of the operator for consistency and for not damaging either the land or the laminate of the PCB. While this method is quite slow, it is very repeatable. Like all air driven sources in PCB as- sembly it is imperative to keep the air systems clean or they will not work. The final method being used for remnant solder removal involves the use of a pre-fluxed copper coupon. It is used as a wicking vessel to pull up the solder around the land. Custom flux- coated coupons, cut to the shape of the shield, are placed in to a rework station. Once they come to temperature they are lowered to the surface of the PCB. The remnant solder is then drawn to the coupon for subsequent removal. After this operation the spent coupon is discarded. This is a fairly slow technique, but results in less dam - age than the completely manual solder-wicking approach previously described. Solder Deposition Several methods are used to make sure that a consistent solder fillet is formed between the lip of the shield and land on the PCB. Manual sol- dering, solder dispensing and solder performs are all methods which can be used to close the gap. Manual soldering requires that the proper technique is used to ensure that the entire pe- riphery of the shield has no gaps between shield lip and land. In many handheld consumer de- vices employing these shields, the spacing to neighboring shields or components is less than two millimeters, requiring a very high level of dexterity from the soldering technicians. Parts and neighboring shields can be easily damaged. Robotic solder paste dispensing is a very con- sistent way to get the same volume of solder Figure 5: Example of parts being in very close proximity to shield. KnoCKinG doWn the Bone Pile A REWORK DIlEMMA: PCB SHIElDS continues

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