Publications

Environmental and toxicity concerns related to the use of lead have initiated the search for acceptable, alternate joining materials for electronics assembly. This paper describes a novel lead-free solder designed as a ``drop in`` replacement for common tin/lead eutectic solder. The physical and mechanical properties of this solder are discussed in comparison to tin/lead eutectic solder. The performance of this solder when used for electronics assembly is discussed and compared to other common solders. Fatigue testing results are reported for thermal cycling electronics assemblies soldered with this lead-free composition. The paper concludes with a discussion on indium metal availability, supply and price.

This paper reports on a surface mount assembly evaluation with a series of existing lead-free solders. The wettability of the lead-free solders under investigation was measured by the meniscometer/wetting balance technique. This data provided an initial screening assessment of viable candidates for prototype development. Assembly process capability was based on visual, mechanical and metallurgical analyses of prototype circuit boards. The study demonstrated the feasibility of using several of the lead-free solders tested in a surface mount application and identified specific areas (e.g., paste formulation, board finishes, reflow parameters) for improving the manufacturing performance.

The electronics industry has relied heavily upon the use of soldering for both package construction and circuit assembly. The solder attachment of devices onto printed circuit boards and ceramic microcircuits has supported the high volume manufacturing processes responsible for low cost, high quality consumer products and military hardware. Defects incurred during the manufacturing process are minimized by the proper selection of solder alloys, substrate materials and process parameters. Prototyping efforts are then used to evaluate the manufacturability of the chosen material systems. Once manufacturing feasibility has been established, service reliability of the final product is evaluated through accelerated testing procedures.

This paper compares the solderability performance and corrosions ion protection effectiveness of electroless tin coatings versus organic azole films after exposure to a series of humidity and thermal (lead-free solders) cycling conditions. The solderability of immersion tin is directly related to the tin oxide growth on the surface and is not affected by the formation of Sn-Cu intermetallic phases as long as the intermetallic phase is protected by a Sn layer. For a nominal tin thickness of 60{mu}inches, the typical thermal excursions associated with assembly are not sufficient to cause the intermetallic phase to consume the entire tin layer. Exposure to humidity at moderate to elevated temperatures promotes heavy tin oxide formation which leads to solderability loss. In contrast, thin azole films are more robust to humidity exposure; however upon heating in the presence of oxygen, they decompose and lead to severe solderability degradation. Evaluations of lead-free solder pastes for surface mount assembly applications indicate that immersion tin significantly improves the spreading of Sn:Ag and Sn:Bi alloys as compared to azole surface finishes.

A technique was developed for the solder attachment of coaxial cable leads to the silver-bearing thick film electrodes on piezoelectric ceramics. Soldering the cable leads directly to the thick film caused bonds with low mechanical strength due to poor solder joint geometry. A barrier coating of 1.5 {mu}m Cu/1.5 {mu}m Ni/1.0{mu}m Sn deposited on the thick film layer improved the strength of the solder joints by eliminating the absorption of Ag from the thick film which was responsible for the improper solder joint geometry. The procedure does not require special preparation of the electrode surface and is cost effective due to the use of non-precious metal films and the batch processing capabilities of the electron beam deposition technique.

This white paper addresses the issue of banning lead from solders used in electronics manufacturing. The current efforts by legislative bodies and regulatory agencies to curtail the use of lead in manufactured goods, including solders, are described. In response to a ban on lead or the imposition of a tax which makes lead uneconomical for use in solder alloys, alternative technologies including lead-free solders and conductive epoxies are presented. The recommendation is made that both users and producers of solder materials join together as partners in a consortium to address this issue in a timely and cost-effective manner.

This paper describes experiments on the wettability of tin on oxygen free, high conductivity (OFHC) copper using a ″point source″ ultrasonic horn. Ultrasonics are used on such metals as aluminum or stainless steel which are difficult to wet without the use of very strong corrosives. These experiments explore the behavior of acoustic energy transmission in the horn-solder-substrate systems indicated by the solder film generated and explore the use of ultrasonics in actual electronic systems component fabrication and assembly processes.

An evaluation of substitutes for tin-lead alloy solders is discribed. The first part of the evaluation studies the wettability of tin-based, lead free solders. The second part evaluates the solderability. The solders evaluated were commercially available.

The fabrication of electronic systems has relied upon eutectic tin-lead solder for the attachment of components to printed wiring boards. Higher service temperatures are approaching the durability limits of the eutectic solder. The tin-rich, lead-free solders are being actively studied as alternate alloys. Experiments that examined the wettability of 95Sn-5Sb (wt. %), 95.5Sn-4.0Cu0.5Ag, 96.5Sn-3.5Ag, and the control solder, 60Sn-40Pb, on oxygen-free, high conductivity copper were performed. A rosin based, mildly activated (RMA) flux and three water soluble, organic acid fluxes were used in the wetting balance/meniscometer measurements. The 95.5Sn-4.0Cu-0.5Ag and 95Sn-5Sb alloys exhibited good wetting, with contact angles of 35° < θ_c < 55° as compared to the excellent performance of the 60Sn-40Pb material (20°< θ_c <35°). The fair wettability observed with the 96.5Sn 3.5Ag solder (60° < θ_c <75°) was due in large part to the inability of the fluxes to significantly lower the solder-flux interfacial tension. The wetting rates of the 95.5Sn-4.0Cu-0.5Ag and 95Sn 5Sb solders were comparable to those of the control; the 96.5Sn 3.5Ag alloy wetting rate was slower than the other candidates. The solder film formed on the substrate surface by the 95.5Sn-4.0Cu0.5Ag alloy was very grainy. The water soluble fluxes exhibited a larger degree of residue formation than did the RMA flux.

A procedure was developed to use solder technology in the assembly of a single-crystal quartz accelerometer. 87.5Au-12.5Ge (wt.%) solder films 0.5 {times} 10{sup {minus}6}, 1.0 {times} 10{sup {minus}6}, and 2.0 {times} 10{sup {minus}}6 m thick were formed by the electron beam deposition of individual layers of Au and Ge with thicknesses so that the bulk film composition equals the eutectic composition. Interdiffusion of the Au and Ge formed the solder; thermal-physical measurements showed the multilayer films to behave similarly to bulk 87.5Au-12.5Be solder in process thermal cycles. The 2.0 {times} 10{sup {minus}6}m thick quartz/Au-Ge/quartz bonds had an adhesive tensile strength of 17 {plus minus} 2 MPa. The strength increased to 29 {plus minus} 3 MPa and 27 {plus minus} 12 MPa after thermal shock and thermal cycle exposures respectively. The 1.0 {times} 10{sup {minus}6} m thick bonds exhibited strengths of 16 {plus minus} 3 MPa, 16 MPa and 15 {plus minus} 8 MPa in the as-fabricated, post-thermal shock, and post-thermal cycled samples, respectively. The 0.5 {times} 10{sup {minus}6} m joints produced a large degree of scatter in the strength values. Accelerometers assembled with the 2.0 {times} 10{sup {minus}6} m thick joints demonstrated a significant improvement in temperature performance as opposed to units fabricated with a polyimide adhesive. 8 refs., 12 figs., 8 tabs.

The solderability of 60Sn40Pb solder on Kovar was examined as a function of surface-cleaning procedure, flux, and solder-bath temperature. Organic-acid fluxes were more effective at lowering the contact angle than was a mildly activated, rosin-based (RMA) flux on chemically etched Kovar. The contact angles were as low as 29{degree} {plus minus} 5{degree} as compared to 61{degree} {plus minus} 11{degree}, respectively. Varying the solder temperature through the range of 215{degree}C to 288{degree}C caused an insignificant change in the contact angle for the RMA flux and a decrease of the contact angle for a candidate water-based, organic-acid flux. The dilution strength of the flux and the elapsed cleaning time significantly influenced the solder-flux interfacial tension, {sub {gamma}LF}. T-peel strengths of Kovar-60Sn40Pb-OFHC copper joints had a low correlation with the contact angle derived from the solderability experiments. The results of the solderability tests and the T-peel mechanical tests, and subsequence microanalysis of the as-soldered and T-peel samples revealed that the best results for the RMA flux were achieved by using an electropolishing procedure and a solder temperature of 240{degree}C to 260{degree}C. A relatively low contact angle of 31{degree} {plus minus} 2{degree} was observed, with no evidence of cracking or thick-film intermetallic formation at the Kovar-solder interface. T-peel strengths were nominally 9.4{degree} {plus minus} 0.5 {times} 10{sup 6} dyn/cm. 21 refs., 36 figs., 11 tabs.