Journal of Electronic Materials,
Dec 2003 by
Farooq, Mukta,
Goldsmith, Charles,
Jackson, Ray,
Martin, GregoryFlip-chip carriers have become the preferred solution for high-performance, application-specific integrated circuit and microprocessor devices. Typically, these are packaged in organic or ceramic ball grid array (BGA) packages, which cover a wide range of package input/output (I/O) capabilities required for high-performance devices, typically, between 300 to more than 1,600 I/O. Recently, there has been a move toward Pb-free solders as replacement alloys for standard, eutectic Sn/Pb and other Pb-based BGAs. The leading solder that has emerged from various Pb-free solder evaluations by industry and academic consortia is the Sn/Ag/Cu (SAC) alloy. One of the primary issues with changing solders is the reliability of the joints when these are subjected to thermomechanical fatigue (TMF). This evaluation has previously been conducted on SAC ceramic ball grid array (CBGA) assemblies in a 1.27-mm pitch.1 However, with the need to shrink the I/O pitch to accommodate higher wiring density, it has become increasingly important to conduct TMF reliability assessments in a 1-mm pitch format. This paper describes such an evaluation conducted using SAC BGA assemblies. The results show that, for a 1-mm pitch, the Pb-free SAC CBGA solution provides superior reliability as compared to the standard Sn/Pb CBGA solutions. This finding is an added incentive for a new CBGA offering employing the new Pb-free, SAC, single-alloy, self-aligning system.
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Key words: Ceramic ball grid array, lead-free solder, tin silver copper, accelerated temperature cycling, thermal fatigue reliability
INTRODUCTION
IBM's current standard, ceramic ball grid array (CBGA) offering is a dual-alloy system that employs a eutectic Sn/Pb alloy in conjunction with a high-Pb alloy (90wt.%Pb, balance Sn). During card assembly, the joining alloy is also eutectic Sn/Pb, which effectively creates a sandwich as depicted in Fig. 1a. When this structure is subjected to thermal coefficient of expansion mismatch strain during temperature cycling, it exhibits better reliability than its single-melt cousin, depicted in Fig. 1b. The advantage delivered by the dual-alloy system is that the high-Pb alloy is capable of accommodating higher strain by deforming. This reduces the effective strain at the eutectic fillets, thus, leading to improved reliability.
Additionally, in a Sn/Pb system, it has been found that higher joint height leads to improved reliability.2 Because the high-Pb alloy remains solid during the creation of the structure depicted in Fig. 1a, the final height is dictated to a large extent by the starting diameter of the high-Pb ball. This is not true in Fig. 1b, where the entire joint is composed of one alloy, which undergoes complete liquefaction during the creation of the joint. The final joint height is, therefore, lower than in Fig. 1a, assuming the same starting ball diameter in each case.
At the end of the previous decade (1998-1999), various industry and academic consortia came to a consensus regarding the major candidates for Pb-free alloys for ball grid array (BGA) applications.3-6 The top candidates were Sn/Ag, Sn/Ag/Cu (SAC), Sn/Ag/Bi, and Sn/Sb, with the weight percent of Sn being higher than 90 in each alloy. After further consideration, the Sn/Sb alloy was dropped because of concerns with Sb (antimony). These systems are by definition, single-melt systems, that is, they are comprised of a single alloy of a given nominal composition that yields a final structure similar to the one shown in Fig. 1b.
After early assessment by the IBM team, the SAC alloy with 95.5wt.%Sn/3.8wt.%Ag/0.7wt.%Cu was found to be the forerunner. Several considerations went into this selection, including wettability of the alloy to the input/output (I/O) pads; the nature of the flux systems required for achieving optimum wettability; the microstructure of the alloy after attachment; and after successive reflows, the shear strength of the attached balls; and, last but not least, the thermomechanical fatigue (TMF) behavior of assembled modules subjected to accelerated thermal cycling (ATC).
This early assessment was followed by an indepth evaluation of the chosen SAC alloy as the candidate to replace current Sn/Pb offerings with a Pb-free CBGA solution. This evaluation was conducted on a 1.27-mm pitch 9211 (alumina-based) substrate.1 However, there was an even greater requirement to evaluate the relative TMF reliability of SAC for fine pitch substrates because the industry trend was toward finer pitch and increased wiring within the package. With this in mind, a full-fledged assessment of the SAC alloy for 1-mm CBGA pitch was conducted, with very encouraging results. This assessment is described in detail in this paper.
FEASIBILITY EVALUATION
Test Vehicle Description and Assembly
Two lidless, 32.5-mm square, co-fired multilayer 9211 (alumina-based), stitch pattern, ceramic test vehicles, each with a 31 × 31 array of solder balls, with six solder balls removed from each corner (937 total interconnection) were used for this study. One test vehicle had a thickness of 1.5 mm and the second a thickness of 2.4 mm.
