Issue link: https://iconnect007.uberflip.com/i/1537054
JULY 2025 I SMT007 MAGAZINE 37 Underfilled BGAs Underfill, typically made of epoxy or poly- meric materials, is used to protect BGAs from mechanical stress and issues related to their coefficient of thermal expansion (CTE). While effec- tive at safeguarding components, underfill can introduce significant challenges during rework processes. Hot air reflow methods are generally unsuitable as they can liquefy the underfill, hinder- ing reliable solder joint formation. Effective rework techniques involve controlled reflow to create a defined separation line, requir- ing precise temperature and force application to avoid damage. Another method involves applying specialized softening agents to weaken the under- fill's adhesion, facilitating safer component removal with minimal risk to the PCB. When salvage isn't feasible, cold milling offers an alternative by pre- cisely removing the component one thin layer at a time. Although this process is destructive to the component, it leaves only minor residual solder on the pad sites, which can be easily cleaned for sub- sequent BGA placement. Cold milling requires spe- cialized equipment and skilled operators to ensure reliable results. Component and PCB Warpage Warpage occurs when components expe- rience rapid temperature changes, with factors like heating rate, moisture, and component size influencing its extent. It typically causes dis- placement at the corners, leading to defects such as opens or bridging. To minimize warpage dur- ing rework or assembly, strategies include using custom fixtures or clamps to hold components flat, localized heat application to relax internal stresses, thermal cycling to gradually relieve stress, and applying supportive overlay materials. Following industry guidelines like J-STD-033 and JEDEC for moisture management also helps reduce warpage risks. Combining mechanical, thermal, and proce- dural methods ensures better alignment, connec- tivity, and overall reliability. Mirrored BGAs BGA mirroring—placing BGAs back-to- back on opposite sides of a two-sided PCB—creates rework challenges due to increased density and the risk of damaging the underside component. Conventional preheating methods can expose the bottom BGA to prolonged heat, risk- ing warping, solder defects, and joint failures. To address this, localized heating techniques with precise temperature control, such as IR or hot air rework, combined with thermal shields like alumi- num foil or pads, can protect the underside com- ponent. Using low-temperature solder and closely monitoring temperature profiles further reduces risks, ensuring effective rework while maintaining component integrity. Package-on- package Package-on- package (PoP) involves stacking BGAs, with the logic or CPU mod- 3 4 5 6 Figure 2: Package-on- package (PoP) involves stacking BG As, with the lo gic or CPU mo dule at the b ot tom and memor y on top, enabling compact, high-density designs ideal for p or table devic es. ▼