Manufacturing Bits: June 23

Fan-out gas sensors
At the recent IEEE Electronic Components and Technology Conference (ECTC), the University of California at Los Angeles (UCLA) and the Indian Institute of Science presented a paper on the development of a wearable MEMS gas sensor device based on a flexible wafer-level fan-out packaging technology.

Researchers have demonstrated a gas sensor device or a personal environment monitor, which is used to track the pollution levels in a given environment. The device also has wireless communication capabilities. This in turn enables the device to transmit data to the cloud for spatial pollution tracking and mapping.

The wearable device integrates MEMS sensors, transimpedance amplifiers, analog-to-digital converters and a Bluetooth technology in a fan-out package with interconnect pitches of 40μm.

Fan-out is an advanced technology that packages various dies while in a wafer-like format. In one example of fan-out, a DRAM die is stacked on top of a logic chip in the package. Generally, fan-out is limited to rigid packaging applications. In contrast, UCLA has developed a technology called FlexTrate, which uses a biocompatible polydimethylsiloxane (PDMS) as the packaging mold compound in fan-out.

The flexible material has some advantages over traditional fan-out. Generally, in the traditional fan-out flow, a wafer is processed in a fab. The chips on the wafer are diced and placed in a wafer-like structure, which is filled with an epoxy mold compound. Then, the redistribution layers (RDLs) are formed within the compound.

There are several challenges with traditional fan-out. During the flow, the wafer-like structure is prone to warpage. Then, when the dies are embedded in the compound, they tend to move, causing an unwanted effect called die shift. This impacts the yield.

The flexible properties of PDMS reduces die shift and warpage. Others have also developed sensors using flexible substrates. “Though a lot of research has been done in this direction, most of them suffer from minimized flexibility. Hence, the aim of the project is to be able to heterogeneously integrate different sensor components on a very flexible PDMS substrate with a fine interconnect pitch to achieve a low cost, light weight and small form factor system,” said Samatha Benedict, a postdoctoral Researcher at UCLA, in a paper. Others contributed to the work.

In the process, the dies are developed. The MEMS gas sensor itself consists of a microheater. Interdigitated electrodes are patterned on top of the metal oxide sensing material. The other dies include the transimpedance amplifiers, converters and Bluetooth.

The dies are spin coated and then placed face down on the substrate using a pick and place tool. The PDMS undergoes a compression molding step. The dies are connected. “One of the main issues in FOWLP involving the integration of MEMS sensors with a released membrane is the stability of the membrane during the molding process, which results in poor yield. We have optimized the process for integrating released MEMS devices by protecting the membrane prior to the molding process and thus improving the stability of the released membranes and improving the yield by >90%,” Benedict said. “If the membrane is not protected, the stresses induced by PDMS during curing leads to membrane cracking. Simulation studies of the temperature profile of the microheater after protecting the membrane shows that the power consumption for 300 degrees C of heater temperature is 0.1W as compared to 0.091W where the PDMS fills the cavity of the membrane, which is a <10% increase. Thus, this proves that the membrane protection process improves stability without affecting the thermal characteristics of the heater. Furthermore, there is an effort to integrate rechargeable flexible batteries to power the system wirelessly.”

Glass panel embedded packaging
At ECTC, the Georgia Institute of Technology presented a paper on a type of fan-out technology called glass panel embedded (GPE) packaging.

Designed for heterogenous integration, GPE goes beyond the capabilities of traditional fan-out. GPE also addresses the thermal management and cooling issues in packaging.

As stated, traditional fan-out has some challenges. In comparison, GPE is an embedded die process that solves some of the issues. The GPE process starts by embedding a die between two glass structures or panels. This is done using a pick-and-place tool. Redistribution layers are formed on top.

“These processes are followed by stencil printing of silver epoxy paste to form (an) integrated heat spreader, although this thermal material can be varied for performance and reliability. For board-level assembly, a ball grid array is arranged,” said Ryan Wong from the School of Materials Science and Engineering at the Georgia Institute of Technology, in a paper. Others contributed to the work.

“The active sides of these devices are exposed to enable the formation of high-density multi-layered redistribution layer (RDL) wiring and direct board attach of the embedded package, providing two separate paths for heat dissipation – through the board and through the backside glass carrier,” Wong said. “There are concerns on the ability of such a package architecture with glass materials notoriously considered as thermal insulators to handle high heat flux densities. To address this challenge and better understand the thermal performance of GPE packages, a parametric modeling study was carried out in ANSYS, considering a 10mm × 10mm logic chip embedded in a glass substrate.”

Gel substrates
At ECTC, Tohoku University presented a paper. Researchers are developing flexible hybrid electronics using an advanced RDL-first fan-out wafer-level packaging technology.

For this, dies or dielets are embedded in hydrogel substrates. In this work, researchers integrate dies and mini-LED dielets in a hydrogel substrate with gold interconnects.

The technology is designed for wearable and implantable biomedical applications.