SMT Magazine

SMT-Sept2017

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30 SMT Magazine • September 2017 As electronic components continue to get thinner, issues related to moisture sensitivity will continue to be front-and-center for elec- tronic assemblies being reworked. In cases where water soluble fluxes are being used in the rework process, in cases where the components (i.e., like in the BGA reballing process) need to have flux residues washed off or in cases where the board needs to be cleaned with water for subsequent conformal coating touchup, prop- er drying and perhaps even bake out procedures need to be performed. This will properly protect the components from moisture ingress and sub- sequent damage There are a variety of factors that influence the need for baking out a printed circuit board post cleaning and after rework has taken place. In the process of diffusion, the amount of time and in what type of environment the board is in determines the maximum allowable "floor time" or open time the board can be in with- out baking. IPC-1601 will provide guidance for the proper handling, the correct packaging ma- terials and methods, environmental conditions, and storage for printed boards. (Click here for bake out tables.) Another factor determining if a bake out is required is whether an in-line or batch wash- er was used for cleaning the assembly (see dis- cussion below). If there are areas of the assem- bly, such as connectors, which can entrap wa- ter, this will also determine whether a board as- sembly needs to undergo the bake out process. Finally, the type of material the PCB is made of and the degree to which these materials are hy- groscopic, will have an impact on the dry out process used. These are the most important, but certainly not all the influences on the need for baking out a board. There are several reasons that drive the need to bake the PCB after washing post rework. If the boards are processed in an inline cleaning system consisting of air knives to blow off the moisture, there are several component types where this process is not sufficient. Moisture cannot easily be blown out from inside of con- nector pins, from under flat ICs, in magnetic windings and other components that can en- trap moisture. If you then place the assembly by Bob Wettermann BEST INC. Drying Boards after Rework Cleaning— To Do or Not to Do? FEATU RE COLU M N: KNOCKING DOWN THE BONE PILE Table 1: IPC/JEDEC J-STD-033 bake conditions for drying SMDs at the user site.

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