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40 SMT Magazine • November 2014 These methods are manual in nature and are very time-consuming and therefore expensive to perform in high labor-cost countries. A final method relies on a complete circuit frame which is adhered with a specialty epoxy. In this method, all of the replacement traces and pads are adhered to the PCB surface at one time using a replacement copper frame. This makes it a good choice for higher-volume, more complicated repairs, as much of the highly skilled labor element is taken out of the process. These replacement frames are aligned with the board fiducials in a semi-automated process. Af- ter being custom fabricated for the additions of the lands and pads required, the surface of the replacement copper pattern is adhered with B- stage adhesive. Finally, heat and pressure in this lamination process allow the "sandwich" to be adhered to the PCB. Pad additions A variety of processes allow for PCBs to have pads added, much like traces, without affect- ing processing and assembly. One of the meth- ods, if the configuration of the change allows, relies on a solid-state laser to selectively ablate the solder mask in a given area to fabricate or modify a pad location. A second method, like that of the trace modifications, is found in the IPC-7721 Method 4.5.1. (epoxy method) and 4.5.2. (dry film method) for the adherence of a new pad through a replacement circuit frame. As previously stated, these methods are manu- al in nature and are very time-consuming and costly on a mass basis, even with customized frame patterns. The final method relies on a complete circuit frame which is adhered with a B-stage epoxy. In this method all of the replace- ment traces and pads are adhered to the PCB surface at one time making it a good choice for higher volume more complicated repair. Both pads and traces are routinely added using one of the aforementioned techniques. conductor/trace cuts There are several manual and machine- based conductor cutting techniques which can be used to modify either populated or non- populated PCBs. The most common manual methods which are used include the breaking of a conductor run by either hand-scribing with a Dremel, scraping with a sharp dental tool, or small sharp knife (IPC-7721 4.1.3). In these methods, the cut is made by severing the trace. This is followed by gluing down any part of the trace which has been lifted in the cut- ting process and making sure that an electrical open (high resistance) has been created in the process. The cut must be at least as wide as the minimum conductor width. The area is then sealed with epoxy by confirming there is an insulator in the area where the circuit was cut. Alternately, a small ball end mill can be placed in a Dremel tool and can be used to rout an area where the conductor needs to be severed. A gal- vo depth-controlled laser can alternatively cut very precisely (line width of a few microns) the copper trace material without cutting down in to the laminate. Finally, precision mills as seen in Figure 6 can also be used for depth controlled trace cuts. selective solder mask removal For selective solder mask removal there are several techniques that can be employed. For manual solder mask removal a Dremel tool with a ball end mill (IPC-7721-2.3.5); a dental scraper (IPC-7721-2.3.5), as seen in Figure 7; and a mi- croblaster (IPC-7721-2.3.6), as shown in Figure 8, can be used to ablate the solder mask from selective areas. For each of these methods ex- treme care must be taken in order to not dam- age nearby components or the PCB laminate. Figure 6: high-speed routing system in action. (Courtesy lPKf) feaTure bare boarD ecos, ecns anD Design moDiFications continues