Rework and Reball Challenges for Wafer Level Packages

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Rework and reball process yield is influenced by several different factors, and can be classified into four general categories, including: (1) personnel; (2) methods; (3) materials; and (4) machine/ tooling. Figure 3 summarizes the various categories and sub-categories for a typical PCB assembly.

Figure 3: Diagram showing the factors that dictate the yield of rework and reball processes.

The personnel category represents the “human factor,” and includes handling, training, and quality control. The personnel factor is especially important for WLPs and other small form factor devices, as the components are much more fragile than standard flip-chip packages. Proper handling must be used to minimize mechanical artifacts and prevent damage to the bulk Si or dielectric layers. The methods category encompasses the process steps for rework and reball, including sample preparation, package removal from the board, solder removal from the package-side pads, and reball of the package. Prior to demount, the sample must be prepared by removing any heat spreaders, thermal grease, corner glue, or underfill. The package is then demounted from the PCB using either mechanical or thermal methods. Next, solder is removed from the package-side pads using solder wicking with a braided wire and solder tip, or using a no-contact vacuum scavenging technique. Lastly, the package is reballed using either a stencil, preform, or laser jetting method.

Materials—such as package type, PCB design, flux, underfill, and corner glue—also influence the rework and reball process yield. With higher board densities and package miniaturization, it becomes increasingly difficult to selectively heat and remove parts from a small footprint. Since WLPs do not possess a package substrate or solder mask, there is also an increased risk to damage the dielectric or even the metal redistribution layers.

Lastly, rework yield is impacted by the machines and tooling used to handle and process the package. Rework machines can vary greatly in cost and complexity—ranging from a hot air pencil and tweezers, to a fully automated rework station. Semi-automated and fully automated rework tools are expensive, but can greatly minimize the risk for thermal and mechanical artifacts. The latter is especially important for WLPs, as it is difficult to handle and secure the parts.

To read the full version of this article, which appeared in the September 2017 issue of SMT Magazine, click here.



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