Jerome A. Rejent

Intermetallic compound layer development during the solid state thermal aging of 63Sn-37Pb solder/Au-Pt-Pd thick film couples

A study was performed which examined the solid state, intermetallic compound layer growth kinetics between 63Sn-37Pb solder and a 76Au-21Pt-3Pd (wt.%) thick film conductor on 96% alumina substrates. A linear, multivariable regression analysis was used to assess the experimental data according to the following empirical relationship: x-x0=At/sup n/ exp(-/spl Delta/H/RT). A time exponent of n=0.78/spl plusmn/0.08 was observed, suggesting that a combination of bulk diffusion and interface reaction mechanisms were responsible for layer growth. The apparent activation energy, /spl Delta/H, was 106/spl ges/8 kJ/mol. Parallel aging experiments were performed on diffusion couples fabricated between 63Sn-37Pb solder and bulk alloy stock having the same Au-Pt-Pd composition as the thick film. Similar growth kinetic parameters were computed. Intermetallic compound layer growth was accelerated under thermal cycling and thermal shock conditions due to residual stresses generated by the thermal expansion mismatch between the solder and the ceramic substrate.

Effect of interface microstructure on the mechanical properties of Pb-free hybrid microcircuit solder joints

Although Sn-Pb eutectic alloy is widely used as a joining material in the electronics industry, it has well documented environmental and toxicity issues. Sandia National Laboratories is developing alternative solder materials to replace traditional Pb-containing alloys. The alloys are based on the Sn-Ag, Sn-Ag-Bi and Sn-Ag-Bi-Au systems. Prototype hybrid microcircuit (HMC) test vehicles have been developed to evaluate these Pb-free solders, using Au-Pt-Pd thick film metallization. Populated test vehicles with surface mount devices have been designed and fabricated to evaluate the reliability of surface mount solder joints. The test components consist of a variety of dummy chip capacitors and leadless ceramic chip carriers (LCCC`s). Intermetallic compound (IMC) layer reaction products that form at the solder/substrate interface have been characterized and their respective growth kinetics quantified. Thicker IMC layers pose a potential reliability problem with solder joint integrity. Since the IMC layer is brittle, the likelihood of mechanical failure of a joint in service is increased. The effect of microstructure and the response of these different materials to wetting, aging and mechanical testing was also investigated. Solid-state reaction data for intermetallic formation and mechanical properties of the solder joints are reported.

Invited article: A materials investigation of a phase-change micro-valve for greenhouse gas collection and other potential applications.

The deleterious consequences of climate change are well documented. Future climate treaties might mandate greenhouse gas (GHG) emissions measurement from signatories in order to verify compliance. The acquisition of atmospheric chemistry would benefit from low cost, small size/weight/power of microsystems. In this paper, we investigated several key materials science aspects of a phase-change microvalve (PCμV) technology with low power/size/weight/cost for ubiquitous GHG sampling. The novel design, based on phase-change material low-melting-point eutectic metal alloys (indium-bismuth, InBi and tin-lead, SnPb), could be actuated at temperatures as low as 72 °C. Valve manufacturing was based on standard thick and thin-film processes and solder technologies that are commonly used in industry, enabling low-cost, high-volume fabrication. Aging studies showed that it was feasible to batch fabricate the PCμVs and store them for future use, especially in the case of SnPb alloys. Hermetic sealing of the valve prototypes was demonstrated through helium leak testing, and Mil spec leak rates less than 1 × 10(-9) atm cm(3)/s were achieved. This confirms that the sample capture and analysis interval can be greatly expanded, easing the logistical burdens of ubiquitous GHG monitoring. Highly conservative and hypothetical CO(2) bias due to valve actuation at altitude in 1 cm(3) microsamplers would be significantly below 1.0 and 2.2 ppmv for heat-treated InBi and SnPb solders, respectively. The CO(2) bias from the PCμV scales well, as a doubling of sampler volume halved the bias. We estimated the shelf life of the SnPb PCμVs to be at least 2.8 years. These efforts will enable the development of low cost, low dead volume, small size/weight microsystems for monitoring GHGs and volatile organic compounds.

Creep Behavior of a Sn-Ag-Bi Pb-Free Solder

Compression creep tests were performed on the ternary 91.84Sn-3.33Ag-4.83Bi (wt.%, abbreviated Sn-Ag-Bi) Pb-free alloy. The test temperatures were: −25 °C, 25 °C, 75 °C, 125 °C, and 160 °C (± 0.5 °C). Four loads were used at the two lowest temperatures and five at the higher temperatures. The specimens were tested in the as-fabricated condition or after having been subjected to one of two air aging conditions: 24 hours at either 125 °C or 150 °C. The strain-time curves exhibited frequent occurrences of negative creep and small-scale fluctuations, particularly at the slower strain rates, that were indicative of dynamic recrystallization (DRX) activity. The source of tertiary creep behavior at faster strain rates was likely to also be DRX rather than a damage accumulation mechanism. Overall, the strain-time curves did not display a consistent trend that could be directly attributed to the aging condition. The sinh law equation satisfactorily represented the minimum strain rate as a function of stress and temperature so as to investigate the deformation rate kinetics: dε/dtmin = Asinhn (ασ) exp (−ΔH/RT). The values of α, n, and ΔH were in the following ranges (±95% confidence interval): α, 0.010–0.015 (±0.005 1/MPa); n, 2.2–3.1 (±0.5); and ΔH, 54–66 (±8 kJ/mol). The rate kinetics analysis indicated that short-circuit diffusion was a contributing mechanism to dislocation motion during creep. The rate kinetics analysis also determined that a minimum creep rate trend could not be developed between the as-fabricated versus aged conditions. This study showed that the elevated temperature aging treatments introduced multiple changes to the Sn-Ag-Bi microstructure that did not result in a simple loss (“softening”) of its mechanical strength.

Compression Creep Behavior of the 95.5Sn-(4.3 3.9 3.8)Ag-(0.2 0.6 0.7)Cu Solders.

The compression creep properties were evaluated for the Pb-free solders 95.5Sn-4.3Ag-0.2Cu (wt.%), 95.5Sn-3.9Ag-0.6Cu, and 95.5Sn-3.8Ag-0.7Cu to determine the effects of small composition differences on time-dependent deformation. The test temperatures were -25°C, 25°C, 75°C, 125°C, and 160°C. The nominal applied stresses were in the range of 2 – 45 MPa. Samples were tested in the as-fabricated condition as well as post-aged at 125°C for 24 hours. Negative creep was recorded for all three alloy compositions. However, the extent of this phenomenon was sensitive to alloy composition and the aging treatment. Creep deformation resulted in the formation of coarsened-particle boundaries within the eutectic regions of the microstructure. The boundaries were comprised of Cu 6 Sn 5 and, to a lesser extent, Ag 3 Sn particles. The minimum creep rate kinetics were evaluated for these solders. The sinh term exponent, n, was 4 – 6 for the Sn-Ag-0.2Cu and Sn-Ag-0.6Cu solders and 1 – 2 for the Sn-Ag-0.7Cu alloy. The apparent activation energy (ΔH) values were in the range of 30 – 70 kJ/mol for all alloys, indicating that a short-circuit or fast-diffusion mechanism controlled creep deformation. The aging treatment did not consistently alter the rate kinetics parameters amongst the alloys. Separating the minimum creep rate data into the low and high temperature regimes, [-25°C, 75°C] and [75°C, 160°C], respectively, showed that bulk diffusion contributed to creep in the higher temperature regime. The ΔH values for the low temperature regime, which indicated that creep was dominated by a fast-diffusion mechanism, were sensitive to solder composition.

Solderability study of 63Sn-37Pb on zinc-plated and cadmium-plated stainless steel for the MC4636 lightning arrestor connector.

Cadmium plating on metal surfaces is commonly used for corrosion protection and to achieve good solderability on the 304L stainless steel shell of the MC4636 lightning arrestor connector (LAC) for the W76-1 system. This study examined the use of zinc as a potential substitute for the cadmium protective surface finish. Tests were performed with an R and RMA flux and test temperatures of 230 C, 245 C, and 260 C. Contact angle, {theta}{sub c}, served as the generalized solderability metric. The wetting rate and wetting time parameters were also collected. The solderability ({theta}{sub c}) of the Erie Plating Cd/Ni coatings was better than that of similar Amphenol coatings. Although the {theta}{sub c} data indicated that both Cd/Ni platings would provide adequate solderability, the wetting rate and wetting time data showed the Amphenol coatings to have better performance. The Zn/Ni coatings exhibited non-wetting under all flux and temperature conditions. Based on the results of these tests, it has been demonstrated that zinc plating is not a viable alternate to cadmium plating for the LAC connectors.