Board Finish Research Articles

Transforming tree topping waste into flooring: a study on the production and evaluation of oriented strand board finish using urban and garden residues.

Oriented strand board (OSB) has become a popular building material for residential construction, but little research has been conducted on its use as a finish floor material. The study investigated the quality and performance of OSB as an alternative to traditional engineered wood products for finish floors. Four types of OSB finish floors using a mixture of garden and urban tree toppings were produced and evaluated, along with different types and levels of resin and mat moisture content. The finish floor panels were subjected to a battery of tests, including concentrated loading, indentation, falling ball impact resistance, abrasion resistance, and surface wettability. The findings showed that urea formaldehyde resin with garden tree toppings performed best in floor surface indentation, abrasion resistance, and falling ball indentation. The phenol formaldehyde resin with garden tree toppings, on the other hand, showed less moisture absorption and swelling during surface wetting tests and better resistance to force application in the concentrated loading test. Our qualitative comparison revealed that OSB finish floor production using 100% garden tree topping strands and 12% urea formaldehyde resin, along with 14% mat moisture content, produced the best results. The study provides valuable insights into the potential use of OSB as a sustainable and cost-effective finish floor material, using waste materials from urban and garden tree toppings.

Wood Cutting Board Finishes and Their Effect on Bacterial Growth

Although wooden cutting boards have been used for centuries, debate remains on how best to coat/seal the boards for microbial protection. A growing body of the literature points to the hygroscopicity of wood—its ability to draw water and bacteria from its surface, deep into the wood, where the bacteria are trapped and die—as the wood attempts to even out its moisture content. There is little research as to how coatings affect this mechanism. In this study, four wood species were coated with either a hardening or nonhardening oil and then, were inoculated with either Listeria or Salmonella, then ‘stamped’ onto sterile agar plates at given time intervals to check microbial growth. The coated woods had significantly more recoverable bacteria on their surface than did the uncoated samples. There was no significant difference in performance between the oils. Remaining bacterial loads did vary significantly by wood species, with European beech having significantly less surface bacteria when inoculated with Salmonella, and the oak species having significantly less surface bacteria with Listeria. Multiple factors appeared to affect the ability of wood to move bacteria from its surface, including porosity of the wood, type of bacteria applied, and time. These results run counter to popular understanding of wood cutting board maintenance and will hopefully impact the way households and professional kitchens use and care for their food-contact wood in the future.

Thermal Aging Effects on the Thermal Cycling Reliability of Lead-Free Fine Pitch Packages

The microstructure, mechanical response, and failure behavior of lead-free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. A direct and deleterious effect on packaging reliability has been observed during elevated temperature isothermal aging for fine-pitch ball grid array (BGA) packages with Sn-1.0Ag-0.5Cu (SAC105), Sn-3.0Ag-0.5Cu (SAC305), and Sn-37Pb solder ball interconnects. Package sizes range from 19 mm with 0.8-mm pitch BGAs to 5 mm with 0.4-mm pitch BGAs with three different board finishes (ImSn, ImAg, and SnPb) previously studied. This paper presents the latest results from an on-going investigation on the aging temperatures were 25 °C, 55 °C, 85 °C, and 125 °C, applied for a period of 12 months. Subsequently, the specimens were thermally cycled from -40°C to 125 °C with 15 min dwell times at the high temperature. Weibull analysis of failures versus cycle number show a ~ 57% reduction in package lifetimes when aged at 125 °C compared to no aging for 19 mm BGAs, for MLFs the degradation is even worse, more than 58% reduction in experiment result. In contrast, the reliability performance of Sn-37Pb is much more stable over long time up to 12 months and temperature. We also study the evolution during isothermal aging, which is one of the major failure modes of solder joints due to its high homologous temperature. The degradation is observed for both SAC alloys on all tested package sizes and board finishes. For the 19 mm SAC105 case, e.g., there was a 53% (57%) reduction of characteristic lifetime at 125 °C for 6 months (12 months) compared to room temperature aging. The trends are in the expected directions; namely, the reliability is reduced when using higher aging temperatures, smaller solder balls, and SAC105. The dominant failure mode can be associated with the growth of Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> intermetallic compounds during the aging, particularly on the pad side.

Impact of Isothermal Aging on the Long-Term Reliability of Fine-Pitch Ball Grid Array Packages With Different Sn-Ag-Cu Solder Joints

A direct and deleterious effect on packaging reliability has been observed during elevated temperature isothermal aging for fine-pitch ball grid array (BGA) packages with Sn-1.0Ag-0.5Cu (SAC105), Sn-3.0Ag-0.5Cu (SAC305), and Sn-37Pb solder ball interconnects. Package sizes ranging from 19 mm with 0.8-mm-pitch BGAs to 5 mm with 0.4-mm-pitch μ BGAs with three different board finishes (ImSn, ImAg, and SnPb) were evaluated. The aging temperatures were 25°C, 55°C, 85°C, and 125°C, applied for a period of 6 mo. Subsequently, the specimens were thermally cycled from -40°C to 125°C with 15-min dwell times at the high temperature. Weibull analysis of failures versus cycle number show a ~ 50% reduction in package lifetimes when aged at 125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C compared to the same at room temperature, with less dramatic but measurable reductions in lifetime at 85°C and even 55°C. In contrast, the reliability performance of Sn-37Pb is much more stable over time and temperature. The degradation was observed for both SAC alloys on all tested package sizes and board finishes. For the 19-mm SAC105 case, for example, there was a 53% (32%) reduction of characteristic lifetime at 125°C (85°C) compared to room temperature aging. The trends were in the expected directions; namely, the reliability was reduced when using higher aging temperatures, smaller solder balls, and SAC105. The dominant failure mode can be associated with the growth of Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> intermetallic compounds during the aging, particularly on the pad side.

Isothermal Aging Effects on the Thermal reliability Performance of Lead-Free Solder Joints

Electronics assemblies containing solder joints are often exposed to elevated temperatures for prolonged periods of time. The time-at-temperature stress impacts the overall package reliability of the assembled circuitry due to evolving materials, microstructural, and mechanical properties. It is especially important to understand the impact of isothermal aging on the long term behavior of lead (Pb)-free solder joints which operate in harsh environments. In this study, we have explored the effects of elevated temperature isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies on board level packages. As the isothermal aging temperature increases, the Weybull characteristic lifetime for SAC 105 and 305 solder joints is drastically reduced compared to Sn-37Pb.In parallel mechanical studies on bulk solder specimens, the creep rate for SAC105, 305 rapidly increases with aging. A full test matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch μBGAs. The test structures were built on three different board finishes (ImSn, ImAg and SnPb). Storage condition temperatures were 25°C, 55°C, 85°C and 125°C with aging over time periods of 0, 6, and 12 months. Subsequently, the specimens were thermally cycled from −40°C to 125°C with 15 min dwell times at the high temperature. It was found that the thermal performance of lead-free fine-pitch packages significantly degrades up to 55–60% after aging at elevated temperature. The dominant failure mode can be associated with the growth of Cu6Sn5 intermetallic compounds (IMC) during the aging, particularly on the pad side.

Corrosion of RoHS-Compliant Surface Finishes in Corrosive Mixed Flowing Gas Environments

Recently, the corrosion resistance of printed wiring board (PWB) finishes has generated considerable interest due to field failures observed in various parts of the world. This study investigates the corrosion issues associated with the different lead-free PWB surface finishes. Corrosion products on various PWB surface finishes generated in mixed flowing gas (MFG) environments were studied, and analysis techniques such as scanning electron microscopy, energy-dispersive x-ray, x-ray diffraction, focused ion beam, and scanning Auger microscopy were used to quantify the corrosion layer thickness and determine the composition of corrosion products. The corrosion on organic solderability preservative samples shows similar corrosion products to bare copper and is mainly due to direct attack of copper traces by corrosive gases. The corrosion on electroless nickel immersion gold occurs primarily through the porosity in the film and is accelerated by the galvanic potential between gold and copper; similar results were observed on immersion silver. Immersion tin shows excellent corrosion resistance due to its inherent corrosion resistance in the MFG environment as well as the opposite galvanic potential between tin and copper compared with gold or silver and copper.

Corrosion of ImAg-Finished PCBs Subjected to Elemental Sulfur Environments

This paper discusses the corrosion process of immersion silver (ImAg) board finish on printed circuit boards subjected to elemental sulfur environments in the presence of high-sulfur content clay and moisture. The tests induced various types of metal migration processes and corrosion products. Migration phenomena ranged from short dendrites of Ag2S, Cu2S, and CuS, which grew from ImAg surface edges without solder masks, to long dendrites of Cu2S growing from ImAg surface edges with solder masks that had copper traces beneath them.

Evaluation of Electrochemical Migration on Printed Circuit Boards with Lead-Free and Tin-Lead Solder

To evaluate the current leakage and electrochemical migration behavior on printed circuit boards with eutectic tin-lead and lead-free solder, IPC B-24 comb structures were exposed to 65°C and 88% relative humidity conditions under direct-current (DC) bias for over 1500 h. These boards were processed with either Sn-3.0Ag-0.5Cu solder or Sn-37Pb solder. In addition to solder alloy, board finish (organic solderability preservative versus lead-free hot air solder leveling), spacing (25 mil versus 12.5 mil), and voltage (40 V versus 5 V bias) were also assessed by using in situ measurements of surface insulation resistance (SIR) and energy-dispersive spectroscopy after testing. It was shown that an initial increase of SIR was caused by consumption of electroactive species on the surface, intermittent drops of SIR were caused by dendritic growth, and a long-term SIR decline was caused by electrodeposition of a metallic layer. The prolonged SIR decline of Sn-3.0Ag-0.5Cu boards was simulated by three-dimensional (3D) progressive and instantaneous nucleation models, whose predictions were compared with experimental data. Sn-37Pb boards exhibited comigration of Sn, Pb, and Cu, while Sn-3.0Ag-0.5Cu boards incurred comigration of Sn, Ag, and Cu. Among the migrated species, Sn always dominated and was observed as either a layer or in polyhedral deposits, Pb was the most common element found in the dendrites, Cu was a minor constituent, and Ag migrated only occasionally. Compared with solder alloy, board finishes played a secondary role in affecting SIR due to their complexation with or dissolution into the solder. The competing effect between electric field and spacing was also investigated.

Thermal Cycling Reliability of Lead-Free Solders (SAC305 and Sn3.5Ag) for High-Temperature Applications

Applications with temperatures higher than the melting point of eutectic tin-lead solder (183°C) require high-melting-point solders. However, they are expensive and not widely available. With the adoption of lead-free legislation, first in Europe and then in many other countries, the electronics industry has transitioned from eutectic tin-lead to lead-free solders that have higher melting points. This higher melting point presents an opportunity for the manufacturers of high-temperature electronics to shift to mainstream lead-free solders. In this paper, ball grid arrays (BGAs), quad flat packages, and surface mount resistors assembled with SAC305 (96.5%Sn+3.0%Ag+0.5Cu) and Sn3.5Ag (96.5%Sn+3.5%Ag) solder pastes were subjected to thermal cycling from -40°C to 185°C. Commercially available electroless nickel immersion gold board finish was compared to custom Sn-based board finish designed for high temperatures. The data analysis showed that the type of solder paste and board finish used did not have an impact on the reliability of BGA solder joints. The failure analysis revealed the failure site to be on the package side of the solder joint. The evolution of intermetallic compounds after thermal cycling was analyzed.

An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics

The worldwide transition to lead-free electronics has increased the usage of several lead-free pad finishes for electronic assembly manufacturers, including immersion silver, immersion tin, electroless nickel-immersion gold, and organic solderability preservatives. This study assesses and compares immersion silver as a circuit board finish in terms of its ease of use, wettability, solderability, shelf life, appearance, solder joint strength, intermetallic and void formation, reliability, and costs.

Electrochemical behavior of metal interconnects in electronic assemblies

The electrochemical and corrosion behaviors of solder alloys—SnAgCu (SAC), SnZnBi, SnPb, and Sn—and printed circuit board finish materials Cu and AuNi were investigated in carboxylic acids (flux) and NaCl solutions using the potentiodynamic scanning technique. The results show that SAC and Sn are passivated in the diluted flux solution, but SnPb, SnZnBi, Cu, and AuNi are under active dissolution when anodically polarized. However, passivation of SAC alloy is not observed in concentrated flux solution. Although a passive film forms on SAC in a 2% NaCl solution, the film is less stable than in the flux solution. In addition, oxidation of the most commonly used lead-free and lead solders, SAC and SnPb, at high temperature was evaluated via sequential electrochemical reduction analysis (SERA). The SERA results revealed that the SAC alloy oxidized more significantly than SnPb under hot, dry conditions.

Forward and Backward Compatibility of Solder Alloys With Component and Board Finishes

The primary objective of the research presented in this paper is to qualify the reliability of mixed assemblies by comparing them to the conventional Sn-Pb assembly and completely Pb-free assembly. The research investigates both forward and backward compatibility in electronic assemblies using a design of experiments (DOE) approach. The investigation utilized a test vehicle containing an area array component (BGA169) and chip components (0603 resistors). Hot air solder leveling (HASL) and organic solderability preservative (OSP) surface finishes were used on the test vehicles to represent Sn-Pb and Pb-free alternatives, respectively. The assembled test vehicles were cut into two panels - one containing a resistor section for isothermal aging and the other containing a BGA and another resistor section for thermal shock. The assemblies were subjected to isothermal aging and thermal shock tests as per Interconnecting and Packaging Electronic Circuits/Joint Electronic Device Engineering Council (IPC/JEDEC) standards. The resistors were sheared in the as-soldered condition, and at various isothermal aging intervals and thermal shock cycles. In order to simulate an intermetallic failure in isothermal aging, a reduced shear height (20 mum) was used for the shear test. The performance of the resistor solder joints were quantified in terms of the shear force. The performance of the ball grid array (BGA) solder joint during thermal shock testing was quantified in terms of the number of cycles to failure. Experimental results from shear analysis of resistor solder joints show that Pb-free alloy assemblies' performance is superior to those assembled using Sn-Pb alloy. Also, the ductile nature of the Sn-Ag-Cu (SAC) alloy provides the joints a better fatigue life. With the area array components, Pb-free assembly joints outlived the traditional Sn-Pb joints when subjected to thermal shock loading. Among the mixed assemblies, backward-compatible assemblies (Pb-free bumps and Sn-Pb solder alloy) showed superior performance. The microstructural analysis of the solder joints indicate a uniform distribution of lead in the solder joint matrix.

Effect of (Au, Ni)Sn$_{4}$ Evolution on Sn-37Pb/ENIG Solder Joint Reliability Under Isothermal and Temperature-Cycled Conditions

The ternary intermetallic compound Au <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> forms at the Sn-37Pb/ENIG solder interface during aging and temperature cycling, leading to increased interfacial cracking and a corresponding decrease in solder joint reliability for 15 mm ball grid array (BGA) structures. (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> forms at both the board finish (bottom) and component side (top) of the solder joint for isothermally aged, temperature-cycled, and (aged + cycled) joints. For control specimens (reflow only), no cracks or interfacial Au are observed. For isothermally aged joints (170 and 340 h at 125degC), a broken, discontinuous layer of (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4 </sub> is present, but no cracking. For temperature-cycled joints, lowered reliability and interfacial cracking occurs along a continuous (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> intermetallic layer on the solder side of the interface after ~450 h of cycling. Aging + cycling did little to inhibit cracking or formation of (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> . Development of a continuous (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> film at the interface is the key failure mechanism. At low cycle numbers where high joint reliability is observed, the (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> layer is discontinuous and not fully developed. At higher cycle numbers and longer aging times, the (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> layer becomes continuous and encourages crack growth along the intermetallic interface and consequent lower reliability. The correlation of interfacial smoothness with lowered reliability is consistent with recent work showing that, when intermetallic compounds form smoothly at the solder interface, the mechanical properties are degraded (compared to a rough intermetallic) due to the decreased resistance to shear along the interface

Are you ready for lead-free electronics?

An expedient transition to lead-free electronics has become necessary for most electronics industry sectors, considering the European directives [The Waste of Electrical and Electronic Equipment (WEEE) directive requires manufacturers to reduce the disposal waste of electrical and electronic products by reuse, recycling, and other forms of recovery. The Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS) legislation restricts the use of lead, as well as cadmium, mercury, hexavalent chromium, and two halide-containing flame retardants, namely polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE), in eight of the ten product categories identified in the WEEE directive. Unlike for WEEE, whereby EU member states are free to set more severe national legislation satisfying the WEEE directive requirements, RoHS is a single market directive. Both the WEEE and RoHS directives will become effective on July 1, 2006] , other possible legislative requirements, and market forces , . In fact, the consequences of not meeting the European July 2006 deadline for transition to lead-free electronics may translate into global market losses. Considering that lead-based electronics have been in use for over 40 years, the adoption of lead-free technology represents a dramatic change. The industry is being asked to adopt different electronic soldering materials , component termination metallurgies, and printed circuit board finishes. This challenge is accompanied by the need to requalify component-board assembly and rework processes, as well as implement test, inspection, and documentation procedures. In addition, lead-free technology is associated with increased materials, design, and manufacturing costs. [The cost of implementing the RoHS directive in the EU has been estimated to be US$ 20Bn . Intel Corporation's efforts to remove lead from its chips have been estimated to cost the company over US$ 100 million so far]. The use of lead-free materials and processes has also prompted new reliability concerns , as a result of different alloy metallurgies and higher assembly process temperatures relative to tin-lead soldering. This paper provides guidance to efficiently implement the lead-free transition process that accounts for the company's market share, associated exemptions, technological feasibility, product reliability requirements, and cost. Lead-free compliance, part and supplier selection, manufacturing, and education and training are addressed. The guidance is presented in the form of answers to key questions.

Lead-free 0201 manufacturing, assembly and reliability test results

The trend towards smaller, faster and cheaper electronic devices has led to an increase in the use of 0201 ( L ∼ 0.02 in.; W ∼ 0.01 in.) and even smaller sized passive components. The size advantages of the 0201 component make it a popular choice among design engineers but not among manufacturing engineers. From a manufacturing perspective, the size of the 0201 package poses significant challenges to the printed circuit board (PCB) assembly process. The many challenges with 0201 assembly can be attributed to the solder paste volume, pad design, aperture design, board finish, type of solder paste, pick-and-place and reflow profile. If these factors are not optimized, they will introduce undesirable manufacturing defects. The small size of 0201 packages and undetected manufacturing defects will also raise concerns about their second level interconnect reliability, especially for lead-free solder alloys and surface finishes, with new processes and higher reflow requirements. To determine the optimum conditions, a design-of-experiment (DOE) study was carried out to investigate the effects of these parameters on assembly defects and solder joint reliability. This paper presents the test results and comparative literature data on the influence of a few key manufacturing parameters and defects associated with the 0201 component using lead-free and tin–lead solder alloys. Data pertaining to component shear strength before and after isothermal aging at 150 °C and intermetallic growth up to 500 h of aging are presented. A number of test vehicles were also subjected to thermal cycling (1500 cycles) in the range of −55/100 °C to determine the solder fatigue behavior. Shear test results for test vehicles subjected to thermal cycling is also presented. In addition, optical microscopy analysis of solder joint behavior during thermal cycling showing the progress of the solder damage and cross-sectional photos taken at 1500 cycles is included.

Real-time scanning electron microscopy and auger spectroscopy of wetting in Sn-3.3Ag-4.8Bi solder paste wetted to Ni-Au

Surface and thin film techniques reveal an immediate and intense surface reaction during wetting of Sn-3.3Ag-4.8Bi solder paste to Ni-Au board finishes. Rutherford backscattering spectroscopy (RBS) shows that a typical, vendorsupplied Ni-Au board finish consists of ∼750 A of Au above the Ni. Stylus profilometry indicates that the finish surface is rough with average vertical height variations of ∼1–2 µm. The board finish seen by the solder alloy during wetting contains a large ∼45% atom fraction of adventitious surface C observed by Auger spectroscopy (AES). The AES and energy-dispersive x-ray spectroscopy (EDX) indicates that Ni has fully intermixed throughout the alloy within seconds of wetting. Dissolution of the Au cap layer into the alloy and Bi and Ag surface segregation is observed. Auger point analysis and maps at the leading edge of the wetting front show no evidence of flux advance ahead of the alloy. Bismuth does not move out with the spreading wetting front. A unique feature of this work is the acquisition of real-time, in-situ electron microscopic observations of the spreading Sn-3.3Ag-4.8Bi alloy front across the Ni-Au substrate during wetting. The observations have been recorded on videotape and converted into computer files accessible on the World Wide Web with standard media players (ftp://131.204.44.20).

Critical Variables of Solder Paste Stencil Printing for Micro-BGA and Fine-Pitch QFP

Stencil printing continues to be the dominant method of solder deposition in high-volume surface-mount assembly. Control of the amount of solder paste deposited is critical in the case of fine-pitch and ultrafine-pitch surface-mount assembly. The process is still not well understood as indicated by the fact that industry reports 52-71% surface-mount technology (SMT) defects are related to the solder paste stencil printing process. The purpose of this paper is to identify the critical variables that influence the volume, area, and height of solder paste deposited. An experiment was conducted to investigate the effects of relevant process parameters on the amount of solder paste deposited for ball grid arrays (BGAs) and quad flat packages (QFPs) of five different pitches ranging from 0.76 mm (30 mil) to 0.3 mm (12 mil). The effects of aperture size, aperture shape, board finish, stencil thickness, solder type, and print speed were examined. The deposited solder paste was measured by an inline fully automatic laser-based three-dimensional (3-D) triangulation solder paste inspection system. Analysis of variance (ANOVA) shows that aperture size and stencil thickness are the two most critical variables. A linear relationship between transfer ratio (defined as the ratio of the deposited paste volume to the stencil aperture volume) and area ratio (defined as the ratio of the area of the aperture opening to the area of the aperture wall) is proposed. The analysis indicates that the selection of a proper stencil thickness is the key to controlling the amount of solder paste deposited, and that the selection of maximum stencil thickness should be based on the area ratio. The experimental results are shown to be consistent with a theoretical model, which are also described.

Wetting characteristics of Pb-free solder alloys and PWB finishes

For a successful transition to Pb-free manufacturing in electronics assembly, it is critical to understand the behavior of Pb-free solders (in bulk and paste form) and their interaction with the Pb-free printed wiring board (PWB) finishes. This paper presents the results obtained from solder paste spread tests and wetting balance experiments with several Pb-free solder alloys and Pb-free PWB finishes. The solder alloys studied were Sn3.4Ag4.8Bi, Sn4.0Ag0.5Cu, Sn3.5Ag and Sn0.7Cu. Eutectic Sn37Pb was used as a reference. The PWB surface finishes were Sn, NiAu, Ag and OSP. Wetting balance experiments were conducted in air while the spread tests were performed in air and nitrogen to understand the effect of reflow atmosphere on the spreading. Surface analysis techniques such as Nomarski phase contrast microscopy, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to characterize the as-received PWB finishes. Sequential electrochemical reduction analysis (SERA) was also performed on the as-received PWB test coupons and on the Sn test coupons after multiple reflow cycles. The effect of multiple reflow cycles on the wetting performance, spreading and the surface composition of the PWB finishes was studied.

Wettability test method for surface mount technology assessment

A study was performed to develop a different experimental methodology to assess wettabilities of solders on various printed wiring board (PWB) finishes, based on a modified spreading test in which solder pastes were heated following temperature reflow profiles representative of those used in surface mount technology (SMT) instead of using a fixed rate temperature ramp. Three solder alloys (Sn63‐Pb37, Sn96.5‐Ag3.5, and CASTINTM: Sn96.2‐Ag2.5‐Cu0.8‐Sb0.5), two fluxes (rosin, mildly activated, RMA, and no‐clean, NC), and seven PWB finishes (Pd, Au/Ni, Ni, Ag, Sn, and organic solderability preservatives: OSP), and bare copper were involved in the study. Better wettabilities were observed in the current study than the results reported in the literature for conventional tests on the same combination of solder alloy, flux, and substrate. The different results in measurement of wettabilities obtained in the current study were attributed to the more adequate heating process allowing flux activation, which reduced reoxidation of solder powders and substrates during the reflow process and thus improved wettabilities of solders. Compared to the results obtained from the popular wetting balance test, the current study demonstrated a more realistic simulation of, and approach to, assessing the wettability of solder for SMT.

ITRI project on electroless nickel/immersion gold joint cracking

A problem exists with electroless nickel/immersion gold (E.Ni/I.Au) board surface finish on some pads, on some boards, that causes the solder joint to separate from the nickel surface, causing an open circuit. The solder joint cracks and separates when put under stress or when it experiences a shock. An ITRI (Interconnect Technology Research Institute) project to investigate this E.Ni/I.Au problem was initiated about a year‐and‐a‐half ago. Since the electroless nickel/immersion gold board finish performs satisfactorily most of the time, a 24 variable experiment was developed to investigate which parts of the chemical matrix are satisfactory to use and which need to be avoided. This paper describes some of the activities that have occurred on the ITRI consortium, from the design of the test vehicle to building hundreds of BGA assemblies, then pulling those BGA assemblies apart and inspecting the results.