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Alternative Low Cost Cu Post Substrate for HB PoP Application

IP.com Disclosure Number: IPCOM000248052D
Publication Date: 2016-Oct-21
Document File: 5 page(s) / 381K

Publishing Venue

The IP.com Prior Art Database

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Alternative Low Cost Cu Post Substrate for HB PoP Application

Background

The Cu posts, commonly known as Cu studs or Cu pillars on substrate are created as parts of the substrate structure used in the high band width (HBW) PoP for the interconnection between the package to package, or package to interposer. The Cu post development by the conventional Cu plating method shown in Figure 1 involved complex process steps from Cu seed layer formation to Cu post surface treatment as given in the Figures 1A to 1G.

Figure 1:Prior art - Cu Post Substrate by means of Cu plating. Refer to Figure 1A to 1G for the process flow.

Figure 1A: Desmear/ Cu seed layer formation

Figure 1B: Dry film resist (DFR) lamination

Figure 1C: DFR exposure and development

Figure 1D: Cu fill plating


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Alternative Low Cost Cu Post Substrate for HB PoP Application

Figure 1F: Cu seed layer removal

Figure 1G:Surface treatment (OSP)

This innovation is employing the bare Cu ball to eliminate some processes as addressed above in Cu plating approach to form the Cu stud on the substrate. It provides simpler alternatives to benefit the overall process system such as improvement in cycle time, usage of resources, as well as process yield.

Using Cu ball as it is also less costly compared to another existing option called for the Cu core solder ball, albeit solder material is required during processing. Having said that, solder material is a part of the key structure constituted the electrically and thermally conductive Cu posts in the new approaches. The conceptualized process flow of the innovative methods is illustrated in the next section.

Description

The first approach is using the bare Cu ball by means of solder paste printing. The solder paste is screen printed onto the solder ball pads, through the patterned solder paste stencil that is aligned with the ball pad on top of the standard substrate as shown in Figure 2A. The Cu ball is then placed on each of the printed solder ball pad as in Figure 2B, through the ball mount stencil during the ball drop process. The substrate is subjected to reflow process to form a sturdy Cu post when the solder paste has fused and adjoined in between the Cu ball

Figure 1E: DFR stripping


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Alternative Low Cost Cu Post Substrate for HB PoP Application

and the ball pad depicted in Figure 2C. This followed by the optional flux cleaning process to clear the remains of flux. Subsequently, the CuOSP coating is applied onto the Cu balls, Flip Chip bump pads and BGA ball pads in the OSP treatment to preserve Cu surface from oxidation as given in Figure 2D.

Figure 2A: Solder paste printing

Figure 2B:Cu ball drop

Figure 2C:Reflow and optional flux cleaning

Figure 2D:OSP treatment (on Cu ba...