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22 DESIGN007 MAGAZINE I JULY 2023 prepreg might look like. is technique will require more careful layer alignment during fabrication but will require only minor milling or cleanup of the cavity edges rather than com- plete material removal. A newer technique has been developed for the formation of a cavity in the PCB. is technique involves the use of a release film being applied to the area of the inner layer where the cavity is to be formed. e board is then fabricated in the usual single lamina- tion process. Aer lamination, laser drilling methods are used to define the perimeter of the cavity. Finally, the plug of material is removed from the board and the cavity is cleaned to remove any residue from the release film before surface finish is applied. is technique requires careful alignment of the layer and release film. Additionally, accurate alignment of the laser cutting of the plug is necessary to assure no release film is le in the board that might cause issues like delamination or moisture ingress, but also close enough to the release film to allow plug removal withoutfracture or other damage to the remaining board material. Fig- ure 4 shows an example of this process. e final area of concern with cavities is in the assembly process. e placement heads in pick-and-place machines in modern assembly lines feature controlled depth travel. For cavi- ties, this depth needs to be altered to ensure that the part is fully inserted into the cavity and not released at the regular board surface and allowed to fall into the cavity. is will require more complex and careful programming of the pick-and-place equipment and may need addi- tional fiducials or other alignment methods to ensure proper placement. Another area of difficulty in the assembly of cavity parts is the application of solder paste. Due to the depth of the cavity, the use of tra- ditional paste screens for the cavity is not pos- sible. is will require the use of direct paste printing or manual paste application in the cavity. While there are several challenges to the use of cavities, with careful planning and con- sideration cavity can be an excellent solution to packaging challenges in modern electronic designs. DESIGN007 Kris Moyer, CID+, is a certified master designer and an IPC instructor. Figure 4: The steps for creating a cavity by using laser drilling to remove a plug of material from the board.