Search results

Fine pitch (0.4 mm) surface mount assembly studies were conducted with several lead‐free solder pastes formulated with both traditional RMA (∼6% residue level) and low residue (1%) flux vehicles. The lead‐free solder alloys evaluated included the two baseline eutectic binary alloys, Sn‐Bi and Sn‐Ag, and three new lead‐free solder compositions: (1)91.8Sn–4.8Bi–3.4Ag (wt%) developed at Sandia Laboratories, (2) 77.2Sn–20ln–2.8Ag (Indalloy 227) developed at Indium Corporation of America and (3) 96.2Sn–2.5Ag–0.8Cu–0.5Sb (Castin) provided by AIM, Inc. The basic physical properties pertinent to assembly performance (melting temperature and wetting behaviour) were determined for each of the new alloys. Assembly performance was assessed as a function of circuit board surface finishes, thermal reflow profiles and solder paste flux composition. The feasibility of 0.4 mm pitch assembly was established with each of the lead‐free solder alloys investigated. No issues particular to the combined use of low residue flux vehicles and lead‐free solder powders were identified. The circuit board laminates did not suffer any thermal degradation effects (reflow was performed in an inert atmosphere). All lead‐free solders, compared with the Sn‐Pb eutectic solder, exhibited reduced spreading on the circuit board lands after reflow. It was concluded that the performance of the new solder formulations is adequate for surface mount applications. Further differentiation among these solders will have to be based on their long‐term reliability performance. These studies are currently under way.

The purpose of this paper is to investigate the various factors that influence the dissolution of copper in molten solder, paying particular attention to important parameters: temperature, solder composition and flow rate.

To determine the dissolution rate of copper in lead‐free solders, a simple and automated technique is developed. This methodology provides repeatable measurements that allow the various experimental parameters to be isolated. Factors that greatly affect the dissolution rate of copper, such as soldering temperature, flow rate and solder composition, are taken into account. Particular attention is paid to the flow rate of the molten solder. In fact, different alloys at the same temperature can have considerably different flow rates, owing to their different viscosities at that temperature. The dissolution rates of copper in seven lead‐free alloys and the Sn‐Pb alloy are compared at 255, 275 and 300°C.

It is observed that generally the samples with a thicker intermetallic layer are those that exhibit a longer dissolution time.

The transition from tin‐lead to lead‐free increases the tendency for copper dissolution in molten solders, clearly representing a serious risk to circuit reliability. This paper presents the many advantages of a method for comparing the dissolution rate of copper with different solder alloys.

This paper reviews the status of lead‐free solder development works. Some of the solder systems — Bi‐Sn, Bi‐Sn‐Fe, ln‐Sn, Sn, Sn‐Ag, Sn‐Ag‐Zn, Sn‐Ag‐Zn‐Cu, Sn‐Bi‐Ag, Sn‐Cu, Sn‐Cu‐Ag, Sn‐In‐Ag, Sn‐Sb, Sn‐Zn and Sn‐Zn‐ln — are discussed in more detail, while others are briefly commented on. In general, compared with eutectic Sn‐Pb solder, all the lead‐free solder alternatives investigated more or less exhibit some shortcomings, such as price, physical, metallurgical or mechanical properties. Relatively, Sn‐ln‐containing systems are more promising in terms of solder mechanical properties and soldering performance, although the price of ln may be a concern. Eutectic Sn‐Ag solder doped with Zn, Cu or Sb exhibits good mechanical strength and creep resistance, due to refined microstructure. The Bi‐Sn systems doped with other elements may have a niche in the low temperature soldering field. Eutectic Sn‐Cu has good potential due to its good fatigue resistance. The eutectic Sn‐Zn system modified with ln and/or Ag may be promising in terms of mechanical properties. Finding a lead‐free alternative for high temperature solders presents the biggest challenge to the industry.

The purpose of this work is to undertake a comparison of accelerated test regimes for assessing the reliability of solder joints, in particular those made using lead‐free solders.

Identical samples of 1206, 0805 and 0603 resistors were subjected to six different cycling regimes to investigate the effect of thermal excursions, ramp rates and temperature dwells.

The most damage to joints was found to be caused by thermal cycling between −55 and 125°C, with a 10°C/min ramp rate and 5 min dwells. Large thermal excursions were shown to give faster results without compromising the failure mode.

Similar degrees of damage in the lead‐free solder joints were experienced from thermal shock regimes with ramp rates in excess of 50°C/min. However, these regimes, although faster to undertake, appeared to cause different crack propagation modes than observed with the thermal cycling regimes. However, these differences may be small and thermal shock testing may still be used to differentiate between, or enable ranking of, the effects of changes to materials or processes on the reliability of the solder joints. Hence, it is envisaged that if a wide range of conditions are to be tested a first sift can be completed using thermal shock, with the final work using typical thermal cycling conditions.

The difference between the SAC (95.5Sn3.8Ag0.7Cu) and SnAg (96.5Sn3.5Ag) solder alloy results across all types of cycles showed very little difference in the rates of joint degradation.

This paper compares relative reliability (remaining shear strength) of three chip components soldered with two lead‐free alloys based on various thermal cycling conditions.

The purpose of this paper is to investigate the effect of Sb (0, 0.2 and 2 wt.%) on wetting performance of lead-free solder of near eutectic Sn-Cu micro-alloyed with Ni and Ge.

The melting characteristic of the lead-free alloys was studied using differential scanning calorimetry. Wettability was examined using wetting balance test for two liquid fluxes, water based and alcohol based in two temperatures 265°C and 277°C. Also, contact angle was measured using sessile drop test.

It is shown that 0.2 wt.% Sb reduces the melting temperature and pasty range. Moreover, the addition of 0.2 wt.% Sb improves wetting behavior for alcohol-based flux. It is also demonstrated that the effect of Sb on meniscus height in wetting balance test and contact angle in sessile drop test follows the trend of wetting performance.

It is found that adding 0.2 wt.% Sb improves the wettability of Ni-Ge micro-alloyed Sn-Cu solder; however, higher concentration of Sb does not benefit the alloy.

Results for reflow soldering are presented from a three‐year EC funded project “IDEALS” to develop lead‐free soldering solutions. On the basis of fundamental data from the literature, a shortlist of candidate lead‐free solders was selected, and results from tests on physical and soldering characteristics, and wetting balance testing, led to the choice of SnAg3.8Cu0.7, melting at 217°C. Implications for solder paste medium development are discussed. Differences in alloy density, melting point, and surface tension relative to conventional solders were found to give higher levels of internal voids, reduced spread on copper, and rougher, duller joints. Reflow process window studies showed that sound reliable joints could be obtained with a peak temperature as low as 225°C. Reliability was tested on soldered test boards using thermal shock cycling, power cycling, and vibration. Overall the SnAg3.8Cu0.7 gave results approximately equivalent to conventional solders, and different board finishes had no significant effect. The effects of Sb and Bi were also evaluated. No justification was found for minor additions of Sb, but 2‐5 per cent Bi was found to allow a reduction of the peak reflow temperature, though at the cost of reduced reliability if any Pb was present.

This report, presented as the keynote paper at Surface Mount International, is the culmination of joint efforts to assess the use of lead in electronics assembly. The study, which will be presented in two parts, involved the collaboration of the following participants: B. R. Allenby and J. P. Ciccarelli, AT&T, Basking Ridge, New Jersey; I. Artaki, J. R. Fisher and D. Schoenthaler, AT&T Bell Laboratories, ERC, Princeton, New Jersey; T. A. Carroll, Hughes, El Segundo, California; D. W. Dahringer, Y. Degani, R. S. Freund, T. E. Graedel, A. M. Lyons and J. T. Plewes, AT&T Bell Laboratories, Murray Hill, New Jersey; C. Gherman and H. Solomon, GE Aerospace, Philadelphia, Pennsylvania; C. Melton, Motorola Inc., Schaumburg, Illinois; G. C. Munie, AT&T Bell Laboratories, Indian Hill, Naperville, Illinois; and N. Socolowski, Alpha Metals, Jersey City, New Jersey.

The European Commission has decided that from the second half of 2006 only lead‐free solder pastes will be permitted for use in the electronics industry. Earlier results of testing showed that lead‐free solder pastes may not be appropriate for both printed‐circuit‐board (PCB) and hybrid‐circuit applications, because of the materials' compatibility with the soldering process and with the solder pads. The basic properties of the investigated pastes show which of the tested solder pastes can be used for both applications. After selection of the appropriate solder pastes, reliability tests were conducted. The surface insulation resistance was tested for both the hybrid circuits and PCBs, whereas the mechanical strength of the soldered joints of components was only tested for the PCBs.