Development of Low Stress No-Flow Underfill for Flip-Chip Application
No-Flow Underfill (NUF) technology has been studied in the various ways about cure properties, self fluxing performance, and filler loading. Due to the filler entrapment between solder bump and contact pad on board, no-flow underfills are mostly no filler loading or filled with very low filler loading. The high coefficient of thermal expansion (CTE) of the polymer material has significantly lowered the reliability of flip chip assembly using NUF, and has limited its application to large chip assemblies. However, high-end flip-chip packages are heading towards a high pin count and large chip size with lead free issue and low-k’s. [1] It is expected to develop a low stress underfill because of flip-chip such as low-k chip package has a brittle structure. This paper presents a novel approach to develop for low stress NUF. We have realized flip-chip bonding with a high filler loading of NUF using a local reflow flip-chip bonding assembly process. We have evaluated a deformation of flip-chip package using Shadow Moiré Technique during 260 reflow. This technique is one of the warpage measurement techniques that use a shadow of grating glass. Warpage of IC is decreasing during the reflow process after cooling it recovers initial warpage level. The effects of resin, filler size, filler loading level, fillet size is investigated Filler size, loading level, and fillet size is roughly the same result as reference. As a result, NUF is optimized for lead free soldering process with low stress. Introduction The development trend of flip chip package is the use of Cu/low-k interconnection for higher performance and the use of lead free solder bump for environmental request. Due to low-k materials such as black diamond and SiLK possess weaker mechanical property. [2] It is easy to destruction the low-k layer by package stress such as package warpage. Underfill is one of the factors of package warpage. Lower stress underfill will be optimized the new package. Other side lead free solder bumps possess higher modulus than eutectic solder bump. It is very fragile. The bump cracking was caused by different thermal expansion between substrate (around 15ppm/deg.C) and silicon die (3ppm/deg.C) during thermal cycling test. The bump protection is same as reduction of package stress. This is other development drive of underfill. Capillary underfill (CUF) is currently a leading technology. NUF is one of alternative technology that typically has no filler. Figure 1 shows the process flow of NUF. Previous work has been done to make eutectic solder joint using local reflow flip chip assembly process with filler loaded NUF. [3] This paper presents two approaches that is optimization for lead free solder of NUF and investigation of low stress NUF. Experiment 1. No-Flow Underfills (NUF) NUF were prepared by epoxy-based formulation. Nine NUF were investigated. It contained epoxy resin, hardener, and filler. Table.1 lists the ingredients of each formulation and properties in this study. NUFs were cured at each condition as post curing after reflow. 165deg.C for 90min was required as post curing for NUF-D. Other NUF was cured at 150deg.C for 30min as post curing. Figure 1. Assembly Process Using NUF Table 1. Ingredient of Formulation and properties 2. Test Vehicle The substrate consisted of High Tg FR4 with 0.73 mm thickness and gold plated pad. The test die consisted of daisy chained lead free (Sn / Ag 3.5 / Cu 0.5) solder bump with 9.6 mm x 9.6 mm x 0.72 mm size. 240 bumps were arranged peripheral position. NUF-A NUF-B NUF-C NUF-D NUF-E Base resin Epoxy Epoxy Epoxy Epoxy Epoxy Hardener A B C D E Soft Modifier NO NO NO NO NO Filler loading level [wt%] 50 50 55 65 50 Filler average size [um] 2 2 2 2 2 Viscosity [Pa.s] 150 85 60 55 65 Tg ( DMA ) [C] 190 145 80 90 140 CTE [ppm/C] 40 35 30 25 35 Gelling speed at 150C [sec.] 20 40 120 900 80 NUF-F NUF-G NUF-H NUF-I Base resin Epoxy Epoxy Epoxy Epoxy Hardener B B B B Soft Modifier NO NO LOW HIGH Filler loading level [wt%] 0 50 50 50 Filler average size [um] 0.5 2 2 Viscosity [Pa.s] 12 80 95 110 Tg ( DMA ) [C] 145 145 145 145 CTE [ppm/C] 70 40 35 35 Gelling speed at 150C [sec.] 40 40 40 40
High-performance light-emitting diodes encapsulated with silica-filled epoxy materials.
Packaging materials have a great impact on the performance and reliability of light-emitting diodes (LEDs). In this study, we have prepared high performance LED devices through encapsulating LEDs by epoxy materials incorporated with filler powders. A set of evaluation methods have also been established to characterize the reliability of LED devices. No delamination or internal cracking between packaging materials and lead frames has been found for the encapsulated high performance LED devices after the package saturation with moisture and subsequent exposure to high-temperature solder reflow and thermal cycling. Four kinds of inorganic silica fillers, namely, quartz, fused silica, cristobalite, and spherical silica, and one kind of organic filler, that is, spherical silicone powder, were incorporated into the epoxy packaging materials to compare their effects on performance of LED devices. The properties of epoxy packaging materials and LED devices were studied by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), dynamic mechanical analysis (DMA), thermomechanical analyzer (TMA), ultravioletvisible spectrophotometer (UV-vis), scanning acoustic microscopy (SAM), and scanning electron microscopy (SEM). Except the spherical silicone powder filled epoxy materials, all the other filled systems showed lower equilibrium water sorption content and smaller water diffusion coefficient in both water sorption and moisture sorption tests. The coefficient of thermal expansion (CTE) values were also decreased with the addition of fillers, and the systems filled with quartz, fused, and filled with spherical silica gave the best performance, which exhibited the reduced CTE values both below and above Tg. The results of TGA essentially showed no difference between filled and unfilled systems. The glass transition temperature changed little for all the filled systems, except the one incorporated with spherical silicone. The modulus at room temperature increased with the addition of fillers. The light transmittance of filled epoxy materials varied with fillers after UV and thermal aging.
Early Missionaries and the Ethnolinguistic Factor During the ‘Invention of Tribalism’ in Zimbabwe
There is evidence from across the disciplines that at least some of the contemporary regional names of African tribes, dialects and languages are fairly recent inventions in historical terms. This article offers some evidence from Zimbabwe to show that missionary linguistic politics were an important factor in this process. The South African linguist Clement Doke was brought in to resolve conflicts about the orthography of Shona. His Report on the Unification of the Shona Dialects (1931) shows how the language politics of the Christian denominations, which were also the factions within the umbrella organization the Southern Rhodesia Missionary Conference, contributed quite significantly to the creation and promotion of Zezuru, Karanga and Manyika as the main groupings of dialects in the central area which Doke later accommodated in a unified orthography of a unified language that was given the name Shona. While vocabulary from Ndau was to be incorporated, words from the Korekore group in the north were to be discouraged, and Kalanga in the West was allowed to be subsumed under Ndebele. Writing about sixty years later, Ranger focusses more closely on the Manyika and takes his discussion to the 1940s, but he also mentions that the Rhodesian Front government of the 1960s and 1970s deliberately incited tribalism between the Shona and the Ndebele, while at the same time magnifying the differences between the regional divisions of the Shona, which were, in turn, played against one another as constituent clans. It would appear then that, for the indigenous Africans, the price of Christianity, Western education and a new perception of language unity was the creation of regional ethnic identities that were at least potentially antagonistic and open to political manipulation. Through many decades of rather unnecessary intellectual justification, and as a result of the collective colonial experience through the churches, the schools and the workplaces, these imposed identities, and the myths and sentiments that are associated with them, have become fixed in the collective mind of Africa, and the modern nation states of the continent now seem to be stuck with them. Missionaries played a very significant role in creating this scenario because they were mainly responsible for fixing the ethnolinguistic maps of the African colonies during the early phase of European occupation. To a significant degree, these maps have remained intact and have continued to influence African research scholarship.