Issue link: https://iconnect007.uberflip.com/i/1544707
MAY 2026 I SMT007 MAGAZINE 43 PCB damage, which may, in some cases, be ir- reparable. Selecting the best approach for rework- ing an underfilled component requires evaluating multiple factors to identify the lowest-risk method for the specific application. Precision Milling Process The primary goal of the precision milling process is to preserve the printed circuit board assembly, whereas destruction of the underfilled surface mount component is acceptable. Potential Issues Precision milling requires adequate clearance around the rework site to minimize the risk of damage to adjacent components. This method is most effective when the target component is fully underfilled. If the component is only partially underfilled or if voids are present in the underfill, pad damage may occur during removal. Advantages of Precision Milling A key advantage of precision milling for rework- ing underfilled surface-mount components is that it preserves the PCB assembly while avoiding heat, which is why it is often referred to as a "cold removal" process. In a properly controlled milling operation, the component package is either removed completely or milled down until approximately 100–150 mi- crons of material remain above the board surface, along with a thin layer of underfill. This process typically leaves a thin layer of solder visible on top of the PCB pads. Any residual underfill and solder can then be removed by wicking and board clean- ing, preparing the site for placement of a replace- ment component in the same location and allowing the assembly to be salvaged. Precision milling also avoids several challenges commonly associated with heat-based rework methods. Factors such as conformal coatings, ce- ramic packages, and large heat sinks rarely inter- fere with the success of the milling process. Lower milling heights may allow removal of all remaining solder and underfill in a single step, but they also increase the risk of damaging the board. For that reason, the lower the milling height, the flatter and more stable the board must be during processing to prevent unintended contact with the board surface. Conclusion Underfilled surface-mount components are becoming increasingly common in high-technology electronics applications, including aerospace, defense, medical, and other high-reliability prod- ucts. Reworking these components is especially challenging because thermal removal methods can cause physical damage to the PCB, increasing the risk of scrapping expensive multilayer assemblies. Precision milling offers a low-risk alternative to traditional rework methods when the priority is to preserve the assembly rather than salvage the component. One of the key advantages of preci- sion milling is that it can leave a remnant layer of solder on the board, effectively providing pre- tinned pads for placement of a replacement com- ponent in the same location. This makes it possible to salvage the printed circuit board assembly while reducing the risks associated with heat-based rework. SMT007 Nash Bell is president of BEST, Inc. To read past columns, click here. K N O C K I N G D OW N T H E B O N E P I L E Figure 2: Remnant solder on PCB serving as pre-tinned pads for new component.