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88 SMT007 MAGAZINE I MAY 2021 To properly perform rework—the removal and replacement of a component on a PCB— the remnant solder needs to be properly removed aer the component has been desol- dered and removed. is step is important for the following reasons: 1. e component, especially if it is a small outline package or an ultra-fine pitched component, needs to lie co-planar to the surface of the pad in order to get the replacement component properly aligned. 2. e remnant solder may already have a thick intermetallic layer which may cause the solder joint to prematurely fail. ere are several methods in use for the removal of the remnant solder including but not limited to vacuum extraction, solder braid (wick) and coupon use. Each method has its own advantages as well as drawbacks. e solder braid approach is outlined in the IPC 7711 Methods for PCB Rework, section 4.1.3 (Figure 1)—surface solder removal-braid method: • In this method, solder braid, which is a copper mesh, is placed onto the fluxed PCB pad. Some braids are pre-fluxed which increases the wicking action. • e braid size chosen should be slightly smaller than the pad dimension. It is recommended that paste flux be used to make sure the flux is active during the solder removal process. • Make sure the tip temperature corre- sponds to the reflow temperature of the solder alloy being removed. Move the soldering iron tip up and down perpen- dicular to the pad with the soldering braid in between the tip and pad. • When solder wicks up into the braid, remove the braid from the pad advancing it such that a non-solder filled section of the braid can be used on the rest of the pad. • Do not "swab the deck" on multi-leaded components or area array sites moving across the pads as the pads could be lied or the mask damaged. is operation is Solder Excavation and Rework Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: Solder wick being used to remove remnant solder prior to reballing BGA.

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