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1、Molded Underfill (MUF) Technology for Flip Chip Packages in Mobile Applications Joshi M., Pendse R., Pandey V., Lee T.K., Yoon I.S., Yun J.S., Kim Y.C., Lee H.R., STATS ChipPAC Inc. Abstract Increased functionality requirements coupled with progressively reducing package size have necessitated the i
2、ntegration of flip chip packages into various baseband and application processor products in mobile platforms. Such products use flip chip technology using traditional capillary underfill (CUF) process on a strip based package which is subsequently over molded to finish the end-product assembly. The
3、 growing pricing pressures and competitive landscape in mobile-packaging has made it imperative for assembly subcontractors to drive the flip chip assembly cost down. To achieve this without compromising product reliability requires a fundamental shift in the way these packages are assembled. Molded
4、 underfill (MUF) approach offers such unique solution with promising advantages over CUF; such as lower material cost, higher through put and excellent reliability to meet the overall product needs of todays evolving mobile market; and is discussed in this paper. Capillary Underfill (CUF) has been t
5、he cornerstone of todays flip chip technology in both flip chip BGA and flip chip CSP format. Several advancements in CUF materials and dispense technologies made over years has made CUF the underfill technology of choice for various flip chip applications. However as the need for reducing package a
6、ssembly cost has grown simultaneously; CUF material and underfill process comes under scrutiny due to higher material cost and slow through put process in the flip chip assembly flow. MUF was explored and found to be a viable lower cost alternative for mobile products by virtue of lower material cos
7、t and faster throughput due to batch process operation in strip format. The cost benefit is further complemented by the capability of MUF to enable finer spacing between die-to-die and die-to-passives; as well as smaller keep-out zones to enable reduced die-to-package edge clearance or effectively s
8、hrink the overall package size than that with CUF. Use of vacuum assisted molding was also found to be capable to fill very small gap between die and substrate of the order of 50um without voiding concerns. This paper outlines the multidisciplinary effort undertaken to design, develop, and qualify f
9、lip chip package with MUF technology for mobile application; which was successfully introduced in high volume production with yields and reliability at parity with an equivalent CUF package. MUF material with fine filler size was chosen from a material screening DOE; and was used in series of test v
10、ehicles (TVs) with different package configurations including single die and multi-die flip chip CSP packages. Process and material margin studies were conducted to establish process window for MUF technology with eutectic and Pb-free bump assemblies. Finally MUF technology was intercepted on mobile
11、 application processor product with fcTFBGA-12x12 mm sq. package and 7.5X7.5 mm2 die towards a successful introduction into high volume production. MUF challenges as well as known-limitations are also described along with future plan. Further studies are being conducted to characterize and qualify M
12、UF on larger die sizes and/or with finer bump pitches and to establish the process and reliability margins of MUF with the same. 1. Introduction MUF is inherently conducive process for mobile products as they are often made in CSP format using strip type substrates which can be molded using traditio
13、nal mold systems. Conventional flip chip CSP used in mobile products use 2 step approach; first using capillary underfill process to underfill the gap between die and package substrate using underfill material; and second using standard mold compound to over mold the package. Mold Underfill (MUF) as
14、sembly concept uses a single step approach to both underfill and over mold the die during the same mold shot thus making the process lot simpler and faster than that with CUF. The schematic exemplifying this primary process difference between the two is shown in Figure 1.1 below. Figure 1.1 Comparat
15、ive Package Schematic Illustration of Capillary Underfill and Mold Underfill. Despite continued improvements in both the material properties as well as dispensing systems and mechanisms; underfill process still remains as one of the slowest processes in flip chip assembly flow. This is primarily due
16、 to the fact that it is not a batch process and needs to be done in a unit-by- unit manner with typically up to two dispense heads used in tandem to complete underfilling of multiple units on a boat or in a strip. Secondly; capillary flow of underfill being surface tension driven flow; is also slow being dependent on multiple factors; including the solder mask and die passivation material types; die size, bum
