Issue link: https://iconnect007.uberflip.com/i/1491409
56 SMT007 MAGAZINE I FEBRUARY 2023 Maintaining a consistent temperature over the entire component surface area presents another challenge in reworking these large packages. Air flow turbulence in the hot air rework nozzles can make it difficult to maintain a consistent reflow temperature coming out of the nozzle, which limits its ability to uniformly reflow and remove these large packages. Sim- ilarly, many commercially available IR rework systems do not have a spot size which can cover the entire size of these packages heat the com- ponent uniformly. Custom-machined board supports for these larger packages, stay-in-place stencils to open the process window through solder paste print- ing, and a thorough understanding of proper thermal profiling can all be used to allow older machines to accommodate these larger pack- ages—albeit only to a certain extent. Challenge #2: Neighboring Device Damage Components in and around the rework area can be damaged during the rework pro- cess, as the heat sources typically used—both IR and hot air—can thermally damage neigh- boring components. Nearby components such as aluminum, tantalum and ceramic capaci- tors, crystals, oscillators, plastic-bodied com- ponents, and others need to be properly ther- mally shielded. LEDs and cameras can also be damaged at reflow temperatures. Not only can these neighboring components be damaged when exposed to heat, they can also be sent into reflow if heating areas and temperature cycles are improperly controlled. is can cause heavier components to fall off the underside of the PCB during BGA rework. Furthermore, when solder is reflowed, the intermetallic formed in the solder fillet can grow, causing the component to form a weaker mechanical bond to the PCB. With the use of the proper thermal shield- ing materials, these problems can be avoided 1 . Newer materials are now available to help pro- tect components from both damage and reach- ing reflow temperatures. ermal shielding materials such as water-absorbing cool gels, as well as ceramic non-wovens, are the most effective thermal shields. Challenge #3: Underfilled BGA Rework Underfill has made its way into a variety of industries, including automotive, military, and aerospace applications. Underfill boosts the reliability of the component, which is subject to mechanical impacts and shocks by distrib- uting the forces. ermal stresses caused by the coefficient of thermal expansion mismatch between the component and the PCB are also lessened by using underfill. However, the pro- liferation of underfills has increased the need for solutions to underfill component rework. With underfill usage expected to grow over 5% per year through the next five years, finding appropriate solutions for underfill component rework issues will remain challenging 2 . BGA rework locations neighboring the same or opposite side of the PCB can be damaged during the rework process. Since the under- fill soens before reaching the solder's liqui- dus temperature, underfills near the rework location can "push out" solder from its desired location, thereby creating soldering anomalies such as shorts or other soldering defects. Another major rework challenge is break- ing the BGA free from the underfill during the Figure 1: An underfilled BGA.