Issue link: https://iconnect007.uberflip.com/i/869081
54 SMT Magazine • September 2017 of the package edge. Reball is thus required to align the package and enable good electrical contact with each pin. It is very important to preserve the defect signature prior to testing; accordingly, precautions must be taken to im- prove the reball yield and reduce thermal and mechanical artifacts. Rework and reball process yield is influenced by several different factors, and can be classified into four general categories, including: (1) per- sonnel; (2) methods; (3) materials; and (4) ma- chine/tooling [5] . Figure 3 summarizes the vari- ous categories and sub-categories for a typical PCB assembly. The personnel category represents the "hu- man factor," and includes handling, training, and quality control. The personnel factor is es- pecially important for WLPs and other small form factor devices, as the components are much more fragile than standard flip-chip pack- ages. 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, includ- ing 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 re- moving any heat spreaders, thermal grease, cor- ner glue, or underfill. The package is then de- mounted from the PCB using either mechani- cal or thermal methods. Next, solder is removed from the package-side pads using solder wick- ing with a braided wire and solder tip, or us- ing a no-contact vacuum scavenging technique [6]. Lastly, the package is reballed using either a stencil, preform, or laser jetting method. Figure 4 shows the transformation of a WLP through- out the rework and reball process. Materials—such as package type, PCB de- sign, flux, underfill, and corner glue—also influ- ence the rework and reball process yield. With higher board densities and package miniaturiza- tion, it becomes increasingly difficult to selec- tively heat and remove parts from a small foot- print. Since WLPs do not possess a package sub- strate 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 ma- chines and tooling used to handle and process the package. Rework machines can vary great- ly in cost and complexity—ranging from a hot air pencil and tweezers, to a fully automated rework station. Semi-automated and fully au- tomated rework tools are expensive, but can greatly minimize the risk for thermal and me- chanical artifacts. The latter is especially impor- REWORK AND REBALL CHALLENGES FOR WAFER-LEVEL PACKAGES Figure 3: Diagram showing the factors that dictate the yield of rework and reball processes.