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May 15th, 2012
Molding Technologies: a closer look
At the recent IMAPS Micro/Nano-Electronics Packaging & Assembly, Design and Manufacturing Forum (MiNaPAD) in Grenoble, in late April, Rolf Aschenbrenner of Fraunhofer IZM gave a detailed presentation on molding technologies for system integration. I-Micronews thought it was worth “A Closer Look”.
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Typical encapsulation technologies include: glob top, underfilling, transfer molding and compression molding. 
i-Micronews has recently looked at underfilling in detail (see “Underfilling in the era of high density / 3D interconnect: A closer look”)
Process conditions for transfer molding and compression molding are shown below.

The majority of encapsulating processes in IC packaging are transfer molded. By applying pressure with a plunger to a heated reservoir of molten molding compound it is transferred into the mold.  The mold contains a series of cavities, one for each package and each cavity has vents that allow air to exit the cavity as the molding compound is forced in. Cycle times under a minute are typical with some operating in less than 15 sec. 

Newer Epoxy molding compounds have been developed which can simultaneously underfill and overmold the Flip Chip die in a single transfer molding process. With suitable mold design, such MUF technology (molded underfill) can utilize the currently installed capital base. The ability to apply pressure during the molding process can reduce the void rate under the die as well as achieving production efficiencies which are typical of transfer molding processes. Molded underfill  compounds offer enhancements in thermal expansion coefficients and moisture absorption levels relative to traditional liquid underfills as well as low levels of package deformation due to cure shrinkage and thermal mismatch effects. These improvements are achieved through the use of proper resin chemistries and filler package compositions. Such solutions are, however, limited in terms of gap (ball size) and pitch that they are capable of.  For further details on MUF capabilities see ref 1.

Compression molding is a forming process in which the epoxy molding compound  is placed directly into a heated mold cavity, is softened by the heat, and forced to conform to the shape of the mold as the mold closes. Compression molding has recently come into  vogue for so called “fan out packaging” , i.e the molded reconfigured wafers of Infineon [2] and their licensees and the redistributed chip package of Freescale [3].

The basic steps towards a molded reconfigured wafer are: a) relocation, b) fixture at a new location, and c) encapsulation to fill the gaps between fixed chips and to cover the backside. As shown in the figure below, 1)  tape is laminated to a carrier plate ; 2) the die are placed onto the mounted tape face down; 3) the mounted chips are encapsulated by compression molding using a liquid molding compound; 4) the molded wafer is released from the carrier plate and the tape is pealed from the molded wafer. The molded wafer is then processed with RDL, balling and singulation.

Such embedded eWLB packages are seen evolving into a  series of SiP and 3D configurations as shown in the roadmap below.
A complete study of fan out / embedded packaging can be found in the Yole report “Embedded Wafer-Level-Packages: Fan-out WLP / Chip Embedding in Substrate “ 

1. Joshi M., Pendse R., Pandey V., Lee T.K., Yoon I.S., Yun J.S., Kim Y.C., Lee H.R., “Molded Underfill (MUF) Technology for Flip Chip Packages in Mobile Applications”, Proceed ECTC, Las Vegas, 2010, p. 1250.
2. M. Brunnbauer, E. Fürgut, G. Beer, T. Meyer, “Embedded Wafer Level Ball Grid Array (eWLB)”, Proceed 2006 EPTC, p. 1.
3. Keser, B.; Amrine, C.; Trung Duong; Hayes, S.; Leal, G.; Lytle, W.; Mitchell, D.; Wenzel, R., “Advanced Packaging: The Redistributed Chip Package”, IEEE Trans on Adv. Pkging., Volume: 31 , 2008 , p. 39



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