To extend reliability to accommodate larger WLPs, many approaches are being investigated. These include the integration of metal columns, compliant materials and wafer-applied underfills, or the novel application of polymers.
Failure Modes Crack propagation due to solder fatigue is the most prevalent failure mode for WLPs. Much research is being conducted in an effort to understand and inhibit this mechanism. One approach that addresses this mechanism uses a polymer ring that is applied around the neck of the individual solder balls (Polymer Collar(2)), as shown in Figure 2. After the polymer is cured, a rigid support structure (fillet) is created. Since solder-joint cracking is typically observed on the die side, the application of the polymer to this area of the solder ball actually changes how the cracks begin and propagate within the solder joint. This enhancement has been shown to extend Weibull life by over 50 percent. A reinforcement layer that provides more coverage around and between the balls represents the migration of this technology to the next level. This approach is similar to a wafer-applied underfill, yet is still reworkable. Termed a Polymer Reinforcement Layer (PRL), it has been shown to extend the reliability of the WLP well beyond that of the Polymer Collar(3). Figure 3 shows the Weibull life comparison of three thin-film redistributed WLPs designed with a 10x10 array of 320µm diameter solder balls at 0.5mm pitch. For WLP designs that depend primarily upon the solder ball itself to provide the reliability, this package size would not pass most requirements. Solder-Joint Reliability As shown in Figure 3, application of the polymer ring roughly doubles the solder joint fatigue life, and application of the next-generation polymer reinforcement layer more than triples the original solder joint life.(4) We believe that these enhancements will allow certain WLPs to migrate from 4-60 I/O to 12x12 arrays and allow new device types to be introduced to wafer-level packaging.
As with wire-bonded devices, WLPs can also be pad-limited. Designers will not increase the size of a device to accommodate a 0.5mm pitch, since the overall cost savings gained from the use of a WLP would be offset by revenue lost to a reduced die count per wafer. Enhancements, such as those referenced earlier, can be applied to smaller solder balls to keep solder joint reliability at their prescribed levels. Unfortunately, 0.4mm pitch, although desirable, is limited by board technology and assembly capability. Once these areas are successfully addressed, the industry will see more devices offered in a WLP format. |
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