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Publication Date: 07/1/2009
Archive >  July 2009 Issue >  Special Features: Assembly and Packaging > 

Parameters for Stacking POP Modules
The NPM (Next Production Modular) system provides a completely integrated single-platform solution.

In a never-ending process, designers of handheld products or wireless communication devices continue to build increased functionality into smaller electronic devices. While transistor densities approach the physical limits of miniaturization, the 3rd dimension has become the new frontier for advanced packaging innovation. Fifteen years ago, multi-chip-modules (MCMs) were heralded as the answer to increased packaging density. But their 2-D approach occupies too much space to meet the needs of today's cell phone and PDA wireless devices.

In recent years, there have been steady advances in stacking both wire bond and flip-chip die inside packages by using ever-thinner die and substrates along with finer bump and bond pitches. However, such feats of integration do not necessarily translate into the lowest-cost assembly method, since they require specialized equipment, advanced materials and high yields. Because of this, an alternative to stacked "die-in-package" is package-on-package (POP), a 3-D packaging technique, which is being adopted by SMT assemblers. Industry forecasts show POP growing at a 40 percent clip through 2012.
The CM602 modular high speed multi-functional placement machine.

Why have POP packages appeared in more and more wireless devices? First, there is a need for increased memory in these devices while maintaining small formats. Second, if memory is attached inside the package with the logic device, the usual die-shrinks for the memory can result in delays to perform reliability testing on the logic/memory combination. So by decoupling the memory from the logic chip, the two packages can be individually developed, tested and assembled later in the supply chain by the board assembly house. And because of standardization of the POP designs, multiple sourcing of memory devices is allowed — driving costs even lower. Standardization also helps define the common package footprints for board designers and gives the industry a benchmark to conduct reliability studies on board finishes and solder alloys.

Memory Device on Top
POP packages are typically constructed with a memory device on top and a 0.65mm pitch solder ball pattern. The bottom package houses the logic or an RF device with a matching land area on top for the 0.65mm pitch memory, and a 0.5mm pitch pattern for the PC board lands on its bottom. Each POP package could have multiple die inside of varying chip and wire or flip chip combinations. The trends, of course, are for the POP package pitch and solder ball dimensions to shrink. Packages with 0.4mm bottom/0.5mm pitch top are being developed and the industry roadmaps show a move down to 0.3mm pitch on the bottom package in coming years.
Common problem when mounting POP.

This reduction in pitch also provides a challenge to the device design. The mold cap on the bottom package and the die inside it have to get thinner so the top package solder balls can make contact with the bottom package. Since thinning can result in increased warpage in the packages during solder reflow, close coordination is necessary among material suppliers and the packaging house — not only for the encapsulant and laminate, but to ensure that the devices can be reliably assembled.

There are two approaches to actual assembly: pre-stacking or one-pass assembly. Pre-stacking involves using a matrix-type fixture to house a number of bottom packages which are either manually or machine-loaded, depending on the equipment set that is available. Next, a pick-and-place machine equipped with a flux dipping station picks the top package from a matrix tray, performs a dip step into either flux or solder paste, performs vision recognition of the dipped part and the bottom part fiducials, then places the top part onto the bottom. The entire tray then moves to mass reflow for soldering and the pre-stacking is completed. This approach is suitable when the final assembler may not have the latest equipment for dipping and the POP pre-stacking needs to be outsourced — or if yield is a concern and 100 percent testing of the POP package is desired prior to final board assembly. The drawbacks of pre-stacking include the need for multiple reflow cycles in the oven and added process steps.

Assembling in One Pass
The more common method is to assemble POP in one pass. Here the bottom package is placed along with all the other SMT components into screened-on solder paste. Then the top package is picked, dipped and placed onto the bottom package and the entire assembly is reflowed.

Selection of the right interconnection materials is critical to ensure a high-yielding assembly process. Early in POP development, flux was used to facilitate the soldering of top to bottom. But as packages became thinner and pitches decreased, the tendency for package warp necessitated the use of solder dipping to increase the metallic volume in the joint area to bridge any gaps. These dipping fluxes are specialized formulations, usually higher in tackiness and viscosity compared to screen printable pastes. Their cost will be higher than flux, so the assembler should evaluate the yield with flux vs. a metal-bearing material to minimize the overall cost. Most dipping fluxes are lead-free and available in various tin-silver-copper or SAC formulations.
Mounting sequence for POP process.

There are also a few specialized formulas that are designed especially for severe package warp and bridging of a large gap. These materials use a tin-silver flake metal suspended in flux. It has the added benefits of forming a daisy-chain-like linkage of flake material and a melting point that is slightly higher than the POP package solder balls. This combination creates a bridge of paste to allow the top package ball to wet down to the top of the bottom package.

Board assembly houses considering adding POP to their competencies need to become familiar with several equipment and test methods. Having flux dipping capability on the pick-and-place machine is essential. This unit should be able to consistently provide the dipping material at the pre-set height — depending on the application. Since some products require more than one POP package, possibly at different ball pitches, a unit that permits multiple dip heights is desirable. Understanding package warp is also important, as is familiarity with thermal warpage profiling equipment. Testing services exist for this as well as in-house equipment for characterizing warp under various conditions. Since most handheld devices are required to meet drop test standards, underfill of the entire POP device is sometimes added. Consequently dispensing equipment may be needed and knowledge of the material specifications for POP underfills should be mastered.
Melting timing delay of STAMP vs. solder ball essential to fill ball-pad gap.

With the POP standardization efforts underway and the adoption of POP into many wireless communication devices, the 3-D aspects of board assembly will continue to develop. The requirement for thin and small-sized devices will drive material suppliers, device packagers, and board assemblers to innovate and adapt to meet these challenges. Board assemblers in particular will need to improve their processes for finer-pitch screen printing down to 0.4mm and smaller and update their SMT equipment to add dipping capability and possibly dispensing for POP underfill. A thorough understanding of the tradeoffs among fluxes and dipping pastes along with POP package behavior during reflow is important to deliver quality assemblies at high yields.

Contact: Panasonic Factory Solutions Company of America, 909 Asbury Dr., Buffalo Grove, IL 60089 847-495-6131 fax: 847-495-6093 E-mail: Web: 

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