Page 28 - Chip Scale Review_January February_2023-digital

Ultra-thin mold cap for advanced packaging

By Nabankur Deb, Xavier Brun [Intel Corporation], Chris Masuyama, Yoshikazu Hirano, Kei Ganbayashi, Hiroki Oshida
[TOWA Corporation]
T o enable advanced, smaller in utilization of granular material in WLP at significantly lower pitches, we chose

and cheaper packages with
assisted face-down compression molding
molding technique. Samples were
finer-pitched structures, development. Similarly, the vacuum- to leverage the use of the compression
wafer-level packaging (WLP) has process developed in the last few years prepared at Intel Corporation and
emerged in recent years as a solution has shown promise for WLP because subsequently compression molded at
[1-2]. For the development of the mold it avoids the drawbacks of some of the TOWA Corporation.
process with WLP, previous research other processes in terms of low-pressure In this process, the wafer is held face
has investigated several different aspects processing and the void-free requirement down on the upper chase, while resin is
of molding. These include carrying out of the mold layer, while minimizing dispensed on the release film to set on
the process with different types of mold die shift [6-7]. Through our current the lower chase. An optimized release
compounds (granular, liquid and sheet studies, we have looked to leverage the film was utilized to ensure proper
mold) [3-5] as well as different molding compression molding process to devise encapsulation of the bumps. Both top
processes (transfer molding, face-up a new ultra-thin mold cap solution for and bottom mold chases can control
and face-down compression molding WLP. By controlling the processing the process temperature independently,
[6], and vacuum imprinting). Sheet-type conditions, we managed to obtain a low which gives good control of temperature
mold material has been gathering some warpage, ultra-thin uniform mold cap during processing.
interest lately [4], but liquid and granular with applications in fine-pitch WLP. The top mold chase can preheat the
materials are still more ubiquitous. While wafer prior to the mold process, then
liquid materials have been shown to be Materials and experimental setup clamp down for the uniform temperature
good for filling finer gaps and easier to The following sections discuss mold process. As a result, uniform resin
flow, granular materials are easier to use various aspects of the materials used, the covers the entire wafer evenly, thereby
and cheaper—and with the right filler experimental setup and process skews. minimizing or eliminating excessive resin
size, also enable better fill of finer pitches, Compression molding. To help flow and/or localized resin overheating
as well as good thickness and warpage achieve a thin and uniform mold cap for a wide range of process parameters
control [2-3]. This has led to more interest with properly encapsulated solder balls (Figure 1a).

Figure 1: a) Stepwise compression molding at the wafer or panel level; b) Wafer stack with thin mold cap and exposed solder balls pre- and post-mold.

26 Chip Scale Review January • February • 2023 [ChipScaleReview.com]
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