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Knocking Down the Bone Pile: Precision Milling of Underfilled Surface-mount Components

Underfill is a polymeric material used to fill the gap between a printed circuit board and the underside of surface-mount area-array packages such as BGA, QFP, and QFN devices, thereby surrounding and protecting the solder interconnections. This material increases the component's reliability when subjected to mechanical impacts and shocks by distributing forces. 

Conventional rework methods for underfilled surface-mount components that use heat can result in physical damage to the PCB, including laminate damage, mask destruction, and lifted or missing pads, thereby reducing manufacturing yields and product reliability. In most cases, these defects are irreparable and can result in the scrapping of a very expensive multilayer PCB.

Available Underfill Rework Methods
The downside of using underfill is that it makes the rework process extremely difficult. While some underfills are categorized as “reworkable,” this does not mean that the underfill removal process is without challenges. There are several problems that arise when removing an underfilled surface mount component, regardless of whether it is underfilled with a “reworkable” or “non-reworkable” material.

Each available rework method is tasked with breaking the bond between the PCB and the underfilled surface mount component. These methods include:

• Hot air rework: Utilizes a stream of hot air to heat the surface mount component and PCB.
• Infrared rework: Uses infrared emitters to target the surface-mount component, minimizing thermal impact on surrounding areas
• Manual removal: Typically used only for “reworkable” underfill material and requires skilled and highly trained technical operators
• Chemical agents: Involves the use of a specialized softening agent to reduce the adhesive strength of the underfill material, thereby facilitating easier removal of the underfilled component
• Laser ablation: Utilizes focused laser beams for precise heating, allowing selective reflow while minimizing damage to adjacent components
• Precision milling: Used for removing underfilled surface mount components, this process employs high-precision milling equipment to remove the component one layer at a time

Potential Issues with Underfill Rework
Rework methods that rely on heat are generally not recommended for underfilled surface-mount components. When the underfill is heated to near-reflow temperatures, it can soften or become fluid, interfering with the formation of reliable solder connections and potentially compromising product integrity.

• Hot air rework: Heating the solder beneath the component to a liquidus state can also cause the underfill to soften or reflow. As a result, adjacent underfilled components may be displaced from their pads during removal.
• Infrared rework: Infrared emitters apply heat directly to the component, creating a risk of excessive localized overheating. This can also cause adjacent underfilled components to be displaced from their pads during removal.
• Manual removal: Although highly flexible, this method is generally unsuitable for “non-reworkable” underfill materials and may damage the board if not performed by skilled, highly trained operators.
• Chemical agents: “Non-reworkable” underfills are often difficult to remove through chemical processing. In addition, some OEMs object to the use of chemical softening agents because they may aggressively attack components and/or the board.
• Laser ablation: Effective use of laser ablation requires careful selection of the laser source and wavelength to match the material’s absorption characteristics. If the beam is not properly controlled or focused, underfilled components can be damaged.
• Precision milling: This “cold” removal process offers a significant advantage over heated rework methods for underfilled components. Although it is destructive to the component being removed, precision milling allows for subsequent component placement in the target area and is a low-risk option when component salvage is not the primary objective. 

In general, component removal processes that do not rely on heat are preferred for underfilled components. Heated rework methods introduce greater unpredictability and increase the risk of PCB damage, which may, in some cases, be irreparable. Selecting the best approach for reworking 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 reworking 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 microns 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 cleaning, preparing the site for placement of a replacement 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, ceramic packages, and large heat sinks rarely interfere 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 products. 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 precision milling is that it can leave a remnant layer of solder on the board, effectively providing pre-tinned pads for placement of a replacement component in the same location. This makes it possible to salvage the printed circuit board assembly while reducing the risks associated with heat-based rework.

This column originally appeared in the May 2026 issue of SMT007 Magazine.