Advanced technologies are becoming available to overcome all of the limitations imposed quad flat no lead (QFN) packages by conventional leadframes. These technologies allow the manufacture of Super QFN packages, with lead counts from 2 to 500+. Super QFN packages will be able to replace a significant portion of BGA and other more expensive packages. The newest technology is the sintered approach that provides the full complement of performance advantages and significant cost savings. The sintered approach is also a green technology and eliminates all plating and etching.
Arthur L. Chait, EoPlex, Inc., Redwood City, CA USA
The quad flat no lead (QFN) package is among the most cost effective of semiconductor packages. QFNs are near chip-scale packages with perimeter lands for solder connections to the circuit board. They have many advantages over other package types, including: low cost, small size and weight and good thermal properties. Unfortunately, QFNs have disadvantages that prevent wider application, with the biggest factor being limited lead count (I/O). Because of this, QFNs are restricted to packages with less than about 150 leads. If low cost QFNs could be manufactured with higher I/O, they would be used to replace some Ball Grid Arrays (BGAs) and other more expensive packages. This would result in cost savings and also allow designers to take advantage of the excellent properties of QFNs.
|Figure 1. A partial leadframe sheet of 32 I/O packages.|
The major barrier to increasing I/O in QFNs stems from the leadframe that is the backbone of the package. A leadframe is a thin sheet of plated copper that is etched to remove metal and create the pattern of pads for die attachment and interconnect. A partial leadframe sheet of 32 I/O packages is shown in Fig. 1. After die-attach to the large pads and wire bonding to the smaller ones, the sheet is molded with standard mold compound and then diced into individual packages. As Fig. 1shows, lead frames contain a large amount of metal and all the metal is connected. Much of the metal is only needed to temporarily hold the pads together and this is the major source of the limitations with leadframes. Table 1 summarizes the limitations and cost factors of conventional leadframes.
QFNs have dominated low lead-count applications for several years, but the limitations imposed by the leadframe have prevented the use of the QFN format in higher lead-count applications. As one industry expert said, "…the leadframe has served us well for 40 years, but it is not optimum for what we want to do now with the QFN format, for example, it won't take us beyond 150 leads." An alternative is desired to enable a "Super QFN" package that would open more advanced applications and reduce cost further.
The three most promising technologies for an alternative to the standard QFN leadframe package include the following:
Double-etched. In the double-etched process, the leadframe is prepared with the top surface patterned by etching, but not etched all the way through. Chips are mounted on the die-attach pads, wire bonded to the I/O pads, then molded in the standard way. After molding, the back side of the strip is etched to remove all excess connecting metal. At that point, the packages are electrically isolated and can be tested in the strip before dicing.
Plated. This process starts with a metal substrate. A plating mask is created with the pattern for the die-attach and I/O pads. The pads are built up by electroplating to create the proper structure on the substrate. Then, the plating mask is stripped away. Die attach and wire bonding proceeds in the standard fashion and the sheet of packages is molded in the usual way. After molding, the entire sheet of packages can be peeled away from the metal substrate. After peeling, the packages are electrically isolated and can be tested in the strip before singulation.
Sintered. The newest technology is the sintered process and this also starts with a metal substrate. This substrate must first be treated to create a unique microstructure that will later act as a high temperature release layer. This proprietary release layer is a composite microstructure of oxides, metallics and other inorganics. The release layer is necessary to prevent the bond pads and wire-bond pads from "welding" to the substrate during high temperature sintering.
Sintering process details
The wire-bond and die-attach pads are print-formed on the pretreated substrate, using both proprietary methods and metal pastes that have been designed to work with the release layer. The metal is deposited and cured using high speed print-forming methods. The most common metal used today is silver. However, other metals that can be sintered are candidates for the process. The requirements for the metal paste in this system are rigorous and require precise control of the metal particles, including: grain size, shape, distribution and chemistry. The paste also uses temporary binders and organic additives that give exact control of viscosity and rheology. Once the pads are formed, the substrate is subjected to various heat treatments to sinter the metal to high density.
This strip of sintered pads is referred to as a "lead carrier" and is used directly as a substitute for the leadframe. Die attach, wire bonding and molding are carried out using standard QFN processes. During these processes, the special release layer, which was created on the alloy surface, provides good attachment strength and then allows the strip to be easily and cleanly peeled away after molding. Like the plated method, the sintered method also yields a strip of packages that are electrically isolated and are ready for testing in the strip before singulation.
All three of the advanced options described are functionally superior to the lead frame technology. Both plating and sintering offer better performance, but the sintering technology also provides a significant cost advantage. The sintered technology is also a green process and eliminates all plating and etching chemicals; my company recently developed this technology for use with QFN packages. Table 2 shows a comparison of the three advanced technologies with the current leadframe technology.
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Arthur Chait received his BS in materials engineering from Rutgers U. and an MBA in marketing from the U. of Pittsburgh, and is president & CEO of EoPlex, Inc., 3698-A Haven Avenue, Redwood City, CA 94063 USA; ph.: 650-361-9070; firstname.lastname@example.org .