Tin Pest in Sn-0.5Cu Lead-Free Solder Alloys: A Chemical Analysis of Trace Elements

Scopus: "Two samples of Sn-0.5Cu solder alloys, stored at -18'C for 7 years, were chemically analyzed by an inductively coupled plasma-optical emission spectroscopy method. One of the samples was unaffected by this exposure; the other one had completely transformed into brittle α-Sn.

Ten elements were found to exhibit statistically significant differences in their concentrations between the two samples, with the higher always associated with the untransformed sample. The highest concentrations were found for elements with an appreciable solubility in Sn, i.e., Bi, In, Pb, and Sb"

Open University, UK

Vienna University, Austria

Evaluation of tin-whisker growth during thermal-cycle testing using stress- an

Scopus: "Two models with different crystal-orientation distributions were evaluated. Samples with a higher tin atomic density were found to be more likely to have longer tin whiskers and higher whisker density. It is concluded from these results that the tin-atomic-density distribution calculated with this model can be used as an effective indicator of the propensity to form tin whiskers."

The influence of the Pb-free solder alloy composition and processing parameter

Scopus: "The test results show a direct relationship between characteristic fatigue life and Ag content, with the higher Ag content alloys outperforming those with the lowest Ag content. As might be anticipated, there also was a consistent inverse relationship between fatigue life and dwell time for the Pb free solders. The failure analysis and microstructural evolution is characterized with optical metallography and scanning electron microscopy and the fatigue reliability of the Pb free solders is discussed in terms of the microstructures"

Accelerating the effects of aging on the reliability of lead free solder joint

Scopus: "Three common lead free alloys were selected for the present study: 98.5Sn-1.0Ag-0.5Cu (SAC105), 96.5Sn- 3.0Ag-0.5Cu (SAC305), and 95.5Sn-4.0Ag-0.5Cu (SAC405). The present study did not address effects of solder volume, pad size, pad finish or reflow profile, focusing on 30 mil (760μm) diameter solder spheres reflowed onto solder mask defined OSP coated Cu pads with a typical lead free profile. Isothermal aging was conducted for up to 3,000 hours at temperatures of 70'C, 100'C, and 125'C respectively.

As expected, the resulting room temperature properties all decreased with aging time, and faster so for higher aging temperatures. Some of the acceleration factors extracted for the evolution of the individual properties did, however, differ greatly for a given alloy. The only way to establish the same microstructure, and thus the same combination of properties, faster by annealing at a higher temperature is thus to fully stabilize it. This takes thousands of hours even at 125'C, i.e. it is not practical for real assemblies"

Effects of additional elements (Fe, Co, Al) on SnAgCu solder joints

Scopuss: "One approach to improve the thermal fatigue properties of these alloys is to add a further alloying element SAC X. Until now, further elements have mainly been added by metallurgical alloying, requiring special equipment and conditions. The powder production is even more difficult. In this paper a new way how to add minor amounts around 0.1 wt.-% of a forth element to SAC solder joints is presented. One possibility is to do the doping during the reflow process adding the additional elements via the flux by incorporating Co-, Fe-, Al- or other metal compounds into the flux. Experiments have shown that Co and Fe were successfully added while for certain other elements like for example Al this was not possible until now. In comparison to standard SAC solder the Co containing solder shows a refined microstructure and Co is found in the intermetallic phase Cu6Sn5. Fe which has been found at grain boundaries is not only attractive as alloying element but as metallization layer, too. It is possible to deposit Fe on printed circuit boards using electroplating. During soldering the slowly growing FeSn2 phase forms between the solder and the Fe layer"

Solder Ball Attachment Assessment of Reballed Plastic Ball Grid Array Packages

Scopus: "For area array packages, the practice is referred to as reballing. While reballing has been used for part reclamation, very little information is available on the reliability of reballed parts. This paper presents lead-free ball grid array (BGA) packages subjected to two ball removal and two ball reattachment techniques. Solder attach strength was used as a metric to examine the reballing process."

Lead-free solders with rare earth additions

Scopus: "There have been significant research efforts to improve the properties of globally recognized lead-free solders, such as Sn-Ag-Cu, Sn-Cu, Sn-Bi, etc. Rare earth (RE) elements have been selected in the current research to be added into lead-free solders as alloying elements in an effort to improve the comprehensive properties, such as wettability, microstructural stability, creep resistance, and mechanical strength and integrity of the solders.

This article overviews the background and rationale for the alloy development, the fabrication techniques for RE-containing solders, the roles of RE elements, as well as current status of RE-containing solder research. Examples of several representative RE-containing solders are introduced. The effects of RE element addition on the wetting behavior, microstructure, and mechanical properties of certain lead-free solder alloys are presented and analyzed."

Beijing University, China

Stabilisation of Cu6Sn5 by Ni in Sn-0.7Cu-0.05Ni lead-free solder alloys

Scopus: "Cu6Sn5 exists at least in two crystal structures with an allotropic transformation from monoclinic η'-Cu6Sn5 at temperatures lower than 186 'C to hexagonal η-Cu6Sn5. We recently discovered that the hexagonal structure of Cu6Sn5 in lead-free solder alloys with trace Ni additions is stable down to room temperature using high resolution TEM/ED/EDS.

This report further confirm the phase stabilising effect of Ni by analysing samples of Cu6Sn5 extracted from a Sn-0.7wt%Cu-0.05wt%Ni lead-free solder alloy. Techniques used include X-ray diffraction, transmission electron microscopy and differential scanning calorimetry"

Lead-Free Looking for the Holy Grail

EMS007 Perspectives:: "The continued development of alloys has led to at least two shifts in the industry.

A need exists to rationalize the data from different sources on the properties of the alloys and how the testing for these properties is performed. For example, iNEMI has put its continued effort in lead-free solders into this area and away from alloy specification itself.

In addition, many OEMs are moving away from trying to define the composition of the solder to specifying the performance of the final assembly. They have developed, or are developing, a specification for what they want the assembly to be able to demonstrate, and, as long as this can be demonstrated, the actual solder and process will be left to the EMS. The major remaining concern is that the solder materials be obtained in conformance with international concerns for the environment and human rights."

New Co alloys from Balver Zinn

EMT Worldwide: "The second in the new series is Tin Silver based i-SAC Solder Family i-SAC105. The melting characteristic of silver containing alloys has been improved with the addition of Cobalt (Co). Solder i-SAC105, with silver composition on request between 0.5 – 1.5%, offers the advantages of silver free alloys and the characteristics of silver containing alloys. i-SAC105 produces low priced, bright and shiny solder joints and a fine grain microstructure, even with this low addition of silver content. As with the previous product, Cobalt is responsible for a homogenous microstructure in comparison to standard SnAg alloys. A formulation including Germanium (Ge) again plays the role of antioxidant, preferentially reacting with oxygen to protect the solder from the oxidation that results in the formation of dross. i-SAC105 has a melting point of 217'C to 227'C"

New Alloys Lead to Test Innovations

Surface Mount Technology: "The problem with SAC alloys, particularly SAC105, is that they tend to contaminate contacts faster than SnPb. SAC105, with its reliability in drop tests, has become a favored alloy with mobile electronics manufacturers."

Fine-pitch BGAs are particularly difficult to contact, and fine-pitch spring probes have even higher resistances as a result of their smaller spring force. This, together with the tendency of SAC105 to contaminate spring probes, challenges test applications. When maintenance is required – as often as every 200 cycles for BGAs with SAC105 – cost of test goes up and production test cycles speeds go down. Contamination transfers from the package to the contact tip. The degree of contamination depends on the solder alloy used and is aggravated by high temperature, another hallmark of lead-free assembly.

Some interconnects that are harder than Au show success, including Pd alloys. Pd has higher resistance values and to test, low force and low conductivity are needed. With increased resistance comes reduced first pass yields, so many test engineers simply replace Au-plated contacts more frequently. Other spring probe plunger manufacturers use coatings to help solve contamination and durability problems. Socket manufacturer Yamichi has applied a coating technology to help solve contactor problems with contamination and durability. Their proprietary Conductive Super-Hard (CSH) coating is based on diamond-like carbon technology. The material is conductive and helps mitigate surface contact contamination with intermetallic compounds.