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
Underfill characterization for low-k dielectric / Cu interconnect IC flip-chip package reliability
Due to low dielectric constant inter-metal-dielectric (low-k IMD) materials possessing weaker mechanical properties and higher coefficient of thermal expansion (CTE) compared with IC silicon substrates, the integrity of advanced IC low-k IMD layer/silicon substrate structures, in a packaged form, becomes of great concern. Flip-chip underfill characterization was conducted to investigate the key factors of underfill material properties that can yield good low-k IC package integrity after reliability tests. A simplified stress-coupling-index concept was proposed and used to select five underfills for evaluation, based on their different property characteristics. 27/spl times/27 mm/sup 2/ FCBGA packages with heatspreader were built using 8/spl times/10 mm/sup 2/ low-k test die, and tested by temperature cycling. The test results showed that an FC package with low Tg underfill yielded good performance for low-k package integrity.
A New Vacuum Pressure Infiltration CFRP Method and Preparation Experimental Study of Composite
In order to prepare a carbon-fiber-reinforced polymer composite (CFRP) with ideal microstructure and properties, a new vacuum pressure infiltration CFRP method is proposed based on an analysis of existing CFRP preparation process methods. Research on composite material preparation systems was carried out by using this new method principle. The system mainly includes a fiber pre-forming module, a vacuum heating infiltration module, a hot-press curing molding module, and a data acquisition control module. Under the conditions of natural curing at 0 MPa + 6 h + 25 °C, vacuum heating curing at –0.05 MPa + 30 min + 80 °C, and hot-press curing at 0.7 MPa + 5 min + 50 °C, a two-dimensional (2D) CFRP with excellent microstructure and properties was successfully prepared. Observing the microstructure of the prepared composite material, it can be found that the inside of the composite material was sufficiently and uniformly infiltrated, and common preparation defects such as holes and delamination were effectively controlled. Through the performance test, the bending strength of the material reached 790 MPa.