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December 2015 • SMT Magazine 69 no TIme For A boArD SPIn? SeLecTIVe SoLDer mASK remoVAL ical stripper is applied. The chemical will (most- ly) strip the coating and break it down just like a paint stripper. There are several negatives to this tech- nique. Not only will the stripper remove the solder mask, it may also attack the base mate- rial surface if exposed for too long a period of time. In addition, testing of the stripping agent and its impact on neighboring components to the strip area is critical to proving out this tech- nique. This means that chemical strippers must be used only if and when the alternatives don't make sense, and they must be used with great care, and the stripped and neutralized boards need to be handled carefully. micro Abrasion Another method for selective solder mask removal includes micro ablation. In this meth- od (appropriate for larger areas as the precision without a mask is very poor) an abrasive mate- rial is propelled pneumatically through a pen- cil-shaped hand piece to ablate the coating. A variety of materials (which include, but are not limited to ground walnut shells, glass or plastic beads, and sodium bicarbonate powder) of dif- ferent hardness can be used to break down the mask. The size of the particles and the makeup of the material will determine the desired result on the mask. Air pressure and tip geometries will impact the output air velocity propelling the media, which in turn will remove the sol- der mask. This method, which requires excel- lent dexterity, uses high-velocity abrasive par- ticles accelerated and directed through a nozzle, as shown in Figure 2, and care must be taken to control the depth of the removal effect and avoid undue stresses on device leads. The lack of control in this process means that only the most experienced and skilled operators should perform this technique. One of the downsides of this approach is that the process creates substantial friction, and static charges in the process and there- fore built-in ESD protection is required. When working on circuit boards containing static sen- sitive devices, the micro-blasting system must be designed to eliminate potential ESD damage. Many microabrasion systems have anti-static ionizers and grounding points contained with- in the unit. Significant preparation time includ- ing masking is often needed to control the areas to be removed and help to prevent unwanted laminate of component damage. Finally, a thor- ough cleaning will be required to flush away any blasting material from the circuit board and operator skill and training are essential. Selective Laser Ablation In yet another method, mask can be ablated away using a properly controlled power level, laser source and frequency to expose the copper Figure 3: Solder mask on center ground area of Qfn. Figure 4: Solder mask on center ground area of QFn ablated in "window pane" pattern. knoCking Down THe bone Pile

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