By: Dr. Subodh Kulkarni, CyberOptics President and CEO (as published on 3DInCites)
Advanced packaging (AP) processes are often described as the front-end-like processes that have migrated to traditionally back-end applications for packaging semiconductor devices. However, AP processes also draw on techniques developed in the other direction, downstream from packaging at the assembly level, where packaged devices are connected to each other on printed circuit boards (PCB), typically using surface mount technologies (SMT). Just as features are scaled up from their front-end sizes to connect devices in AP processes, they are scaled down from the SMT/PCB world, ultimately comprising a size range from about 10µm to 100µm that has been called the “middle-end”.
Many of the techniques used in SMT, especially inspection and measurement, can be scaled successfully for AP applications. The evolution of high-resolution, phase shift profilometry (PSP) is one example. The technique’s ability to provide fast, accurate, 3-dimensional characterization has earned it a central role in SMT processes. Now, the latest generation of sensors (Nano Resolution MRS™, CyberOptics Corporation )provides nanoscale sensitivity to height variation and micrometer-scale spatial resolution at production-worthy speeds that are significantly faster than alternative technologies.
In adapting PSP technology to AP sensors, engineers were challenged to increase the resolution of the optical system while preserving the technique’s high speed. Another, less obvious, hurdle was the need to measure specularly reflective surfaces, such as solder bumps, for which accurate 3D measurements of height and coplanarity are critical. Conventional PSP technology requires a diffuse reflection from the measured surface. The new sensor includes a dedicated channel for specular surfaces that provides fast, accurate measurements.