Key Uses of Rework Stencils
When a device needs to come off a PCB and be replaced due to improper orientation, wrong part number or value, defective solder fillets the operation is a rework of a PCB. The processes for taking off different parts are governed by the industry workbook IPC 7711/21 Rework and Repair of Printed Circuit Boards.
Some of the processes for reworking devices are such that hand soldering is used for common devices for SMT connectors, passive components, PLCCs and others. There is however other more complex devices, many of which have non inspectable solder fillet areas that require a different rework method. Common devices requiring rework that consists of mimicking the initial assembly process include but are not limited to devices such as QFPs, BGAs and QFNs. When reworking these devices the device is first removed and site prepped and cleaned. In many cases the solder is re-applied using a miniature version of the SMT stencil a rework stencil. This mini sized solder paste stencils controllably and repeatably applies solder paste to the rework location. After application of the solder paste the device is sent through a controlled reflow that mimics the time-temperature characteristic curve of the initial reflow processes. Finally the device location is (in many cases) cleaned and inspected.
Rework stencils come in a variety of configurations. The mini stencils can be made from the same material that most SMT stencils are manufactured from. These stencils in many cases have flaps that protect the rest of the circuit board from being “smeared” with solder paste. The process difficulties with these stencils have led in the last few years with a whole new class of rework stencils. The problems of maintaining co-planarity with the PCB which causes “smearing” of paste underneath the rework stencil and fitting these rework stencils on a very crowded PCB have led to the development of different rework stencils made from plastic film.
Plastic film rework stencils with adhesive backing have eliminated the problems of “smearing” solder paste underneath the stencil location. Permanent stencils such as the BEST StencilQuik™ allows for intimate contact between the stencil and the PCB thereby preventing this problem. In another version the BEST StikNPeel™ stencil uses a repositionable adhesive system that allows the stencil to be applied and removed thereby preventing this problem from occurring. In both versions of the film-based stencils they can be easily fit in to tight locations.
