Matte Tin Research Articles

Reliability of SMD interconnections on flexible low-temperature substrates with inkjet-printed conductors

This work dealt with potential reliability issues in inkjet-printed electronics. The aim was to increase our understanding of how failures occur in hybrid electronic structures that utilize inkjet printing. A future goal in this field will be to enable expansion of this product area from low-life-expectancy and -performance products to high-life-expectancy and -performance products.In this work the reliability of isotropically conductive adhesive interconnections between 0402- and 0805-sized ceramic resistors and three types of flexible substrates was investigated. The use of low-temperature ink material (curing temperature 150°C) allows new, inexpensive substrates to be introduced into the process. In addition, the interconnection reliability of a more complex low-temperature cofired ceramic module attached to a flexible substrate with ball grid array interconnections was investigated.Reliability testing was carried out in a thermal cycling test chamber in a 0–100°C temperature range. Daisy-chain resistances of the samples were monitored with a datalogger during the cycling. Besides optical inspections, the interconnections were analyzed with X-ray microscopy. Also scanning acoustic microscopy was used to observe crack propagation. Finally, scanning electron microscopy was used to detect failure mechanisms.The two main failure mechanisms of the chip resistor components were loss of adhesion at the ICA/component terminal cladding interface and cracking of matte tin cladding of the component. For the tested LTCCs it was noted that breakdowns of interconnections occurred because of fractures on the module side of the interconnection—in most cases in the solder/intermetallic compound interface—and not because of ICA fractures or ink delamination on the substrate side.

Thermal stress driven Sn whisker growth: in air and in vacuum

Whisker growths from matte tin electroplating have been observed during thermal cycling up to 1,000 cycles either in air or in vacuum. The density, length, and width of thermal stress whiskers depend on the plating thickness of 2 and 5 μm in the present study. Whiskers grown on the 2 μm plating are longer and thinner than those on 5 μm plating. In both cases, whiskers grow thinner and faster in vacuum than in air. These apparent variations come from the grain sizes and the thermal stress distributions in the electroplating, intrinsically different in 2 and 5 μm thick films. The grain structure of whisker root, particularly grain boundary cracks oxidized in air, determines the stress concentration to drive the whisker growth. Cracking caused by oxidation was rarely observed in vacuum hence causes thin and straight whiskers even from thick plating. Our results indicate that the stress concentration at whisker root grain is essential for controlling whisker growth morphology, and has a critical impact on various electronic applications.

Least lead addition to mitigate tin whisker for ambient storage

Long term surface evolution of matte tin electroplating has been investigated under room temperature to understand the tin whisker mitigation by a trace amount of lead addition. No whisker growth has been observed on all the Sn–xPb samples (1 ≤ x ≤ 10 wt%), while at least 3 wt% of Pb addition is required to alter the columnar grain structure of pure Sn plating to equiaxed grains. The mitigation mechanism by such a trace amount of Pb is not caused by the grain texture control, but is due to the less inter-metallic composite (IMC) growth; the segregated Pb at the columnar grain boundaries disrupts the IMC growth, and releases Sn grain boundary migrations to relax the internal stress. This mechanism of stress relaxation and whisker growth suppression suggests that lead-free Sn plating without whisker growth can be realized by co-plating Sn with a Pb-like metal element that precipitates at the grain boundary to interfere with the IMC growth.

Influence of External Strain on the Growth of Interfacial Intermetallic Compounds Between Sn and Cu Substrates

This study used a four-point bending procedure to investigate the influence of compressive and tensile strain on the growth of an interfacial Cu-Sn intermetallic compound (IMC) layer. The test specimens were prepared by depositing 25 μm layers of matte or bright tin atop a copper substrate using electroplating. Samples were then placed in a furnace at 200°C, and external bending strain was applied through a strained substrate. Comparisons were made between samples undergoing tensile strain or compressive strain, and those without strain. We observed the influence of strain levels and aging time on the formation of the IMC. Both tensile and compressive strain influenced the formation of the Cu/Sn IMC. In matte tin samples, the IMC thickness increased under compressive strain and decreased under tensile strain. In contrast, in bright tin samples, the IMC thickness increased under both compressive and tensile strained substrate conditions. The growth rate of IMC was faster in strained bright tin samples than in strained matte tin samples. Moreover, the formation of IMC microscopic structures under external strain differed considerably according to the source of tin.

Ni Barrier-Induced Cracks in Matte Sn Films

A nickel barrier layer between a matte tin film and a C194 substrate has been used to prevent whisker formation on the tin surface by blocking the formation of Cu6Sn5 intermetallics, which is one of the root causes of whisker growth. However, the introduction of the Ni barrier to the matte Sn/C194 system greatly changes the mechanical properties of the bended leads. In this paper, the effect of a Ni barrier on the mechanical stability of matte tin films deposited onto C194 leads of an SOIC-8 package is investigated. Backscattered electron imaging and focused ion beam imaging of the cross-section samples indicate that, during the forming process, the surface cracks in the matte tin films often initiate and propagate from the Ni barrier, which lacks cooperative deformation ability in comparison with the ductile copper alloy substrate and the matte tin film. The thicker Ni barrier could induce considerable mechanical damage to the matte Sn films of the integrated circuit package after reflow treatment due to the coefficient of thermal expansion mismatch. The thermal-humidity testing of the Sn/Ni/C194 samples reveals that the surface cracks formed may be attributed to the built-in tensile stress arising from the formation of Ni3Sn4 intermetallic compounds (IMCs). A thinner Ni barrier layer can withstand the forming stress, thermal mismatch stress, and IMC-induced stress without causing cracks in the matte Sn films and does not promote Sn whisker growth.

Additive-Effects on the Morphological and Structural Characteristics of Matt Tin Electrodeposits from a Methansulfonate Acid Bath

The methansulfonate acid electrolyte is a new type of plating baths which has some advantages such as friendly to environment, stability and outstanding electrical conductivity. In this work, the effect of three organic additives (nonionic surfactant Triton X-100, a weak organic acid (additive A) and an organic sodium salt (additive B)) in stannous methanesulfonate baths on tin electrodeposits were studied by SEM and XRD. The results show that the Triton X-100 plays an important role in improving the coating qualities and promoting the densification of crystalline structure. Under the synergetic action of additive A and additive B in the presence of Triton X-100 a smooth, fine-grained matt tin coating which has certain preferred orientations of lattice plane, could be obtained on copper substrate.

Electrochemical Study of the Additive-Effect on Sn Electrodeposition in a Methansulfonate Acid Electrolyte

The effect of three organic additives in stannous methanesulfonate baths on cathodic overpotential and Faradaic impedance of tin-electrodeposition were studied by measuring cathodic polarization curves, galvanostatic responses and electrochemical impedance spectroscopy. The results show that the composite additive consisting of three organic compounds can significantly increase the cathodic polarization, crystallization overpotential and charge transfer resistance, which is very beneficial to improve coating quality and result in a fine-grained and smooth matt tin coating.

A novel electrolyte for the high speed electrodeposition of bright pure tin at elevated temperatures

A new tin electroplating process capable of producing fully bright, pure tin deposits from an electrolyte operated at temperatures of up to 50°C has been developed. Unlike conventional bright tin deposits, the tin deposit from the new process has an increased grain size and low carbon content. In addition, the deposit demonstrates excellent ductility, solderability, and low tendency for whisker growth. Production trials conducted by various industries have proven that the new pure tin deposit is suitable for connector applications, as well as for corrosion protection and decorative uses. In this article, the characteristics of the tin electrolyte and the bright tin deposit are described. Electrodeposition of bright tin, characterized by its silvery-white decorative appearance, excellent solderabilty, conductivity, good corrosion and tarnish resistance, and non-toxic nature, has been widely used in the electronic industry and protective-decorative applications. The industrial importance of tin electrodeposition has been markedly extended with the introduction of lead-free solder in the electronic marketplace. However, certain drawbacks of commonly used bright tin plating processes have limited the acceptance of electrodeposited bright tin. The operation and control of bright tin plating solutions is more difficult compared to that of matte tin processes, since the additive system of conventional bright tin electrolytes usually contains many volatile organic components that act as brightening agents. Because of the volatile nature of the organic brightening agents, a cooling system is usually required in conventional bright tin plating baths, in order to maintain the bath temperature between 20 to 25°C where most brightening agents are effective. For high-speed plating, the use of a cooling system becomes essential since the bath temperature naturally rises rapidly during plating. A key concern with the use of bright tin in certain applications is the relatively low ductility often associated with bright deposits and the reportedly higher tendency towards tin whisker growth compared to matte deposits. For demanding automotive applications, where the use of bright tin-lead has been permitted for a limited period of time, continuing efforts are being made by both the electronics industry and suppliers of plating chemistry to replace bright tin-lead with bright pure tin. Tin whisker performance has become one of the most critical characteristics in qualification of any bright pure tin plating process for use in the electronics industry. The newly developed plating bath, SOLDERON™ BHT-350 bright tin, has been designed to be operated at temperatures between room temperature and 50°C, while producing ductile, bright deposits with low whisker growth tendency. Production trials conducted by different industries have demonstrated that the new process is able to produce bright tin deposits under high speed reel-to-reel plating conditions, and the simple additive system facilitates automatic or manual dosing during operation. The resulting bright tin deposits are suitable for connectors, contacts, wire and other items requiring a bright tin finish. In this article, the characteristics of the electrolyte and the properties of the bright tin deposit, including microstructure, carbon content, ductility, texture, solderability and whisker performance are described in detail.

Pulse plating of matt tin: effect on properties

The present work was conceived as an investigation on the effects of pulse plating on the microstructure of matt tin coatings from a proprietary acidic methanesulphonate bath. The effects of pulse plating on current efficiency, surface roughness, grain size and orientation of tin deposits were investigated as a function of the duty cycle and the pulse frequency. The impact of pulse plating on the microstructure emerged clearly from the results gathered in this work, though the effect size was relatively limited. The average grain size was found to increase with increasing duty cycle, while the opposite tendency was noticed with increasing frequency. The grain structure, i.e. the cross-section microstructure of the tin deposits, was not influenced to any appreciable degree by pulsed current deposition, remaining columnar irrespective of the pulse plating parameters within our operating range. Grain orientation of direct current as well as high duty cycle pulse plated deposits is a strong (110) texture; this turned out to weaken as either the duty cycle or the frequency was reduced. A relatively stronger impact of pulse plating was that on surface morphology, where surface roughness of pulse plated deposits is reduced with decreasing duty cycle or frequency.

Effect of Grain Size on Pressure-Induced Tin Whisker Formation

This paper discusses the effect of grain size on pressure-induced tin whisker formation. Since a pressure has to overcome the increase in surface energy involved with tin whisker growth, a threshold stress for tin whisker growth exists. Based on traditional nucleation theory, the threshold stress is inversely proportional to the radius of the whisker. For the verification of the proposed theory, nanoindentation tests were carried out on three kinds of finishes with different grain sizes; tin-copper, bright tin, and matte tin. Whiskers and nodules were formed at the contact edge as the applied load increases up to the critical value related to grain size. The finite-element analysis revealed that high pressure concentrates near the contact edge on the tin-copper finish. The value of pressure to whisker formation is up to 40 MPa. This value agrees with the estimation from the proposed theory with a whisker radius of 0.1 μ m. Based on the proposed theory, the difference between whiskers and hillocks can be explained.

Tin Whisker Test Development—Temperature and Humidity Effects Part I: Experimental Design, Observations, and Data Collection

The effects of temperature and humidity on tin whisker growth were investigated through a collaborative project sponsored by the International Electronics Manufacturing Initiative (iNEMI) and its member companies. A broad range of testing conditions was adopted to test a variety of components with matte tin (Sn) plating and copper (Cu)-based leadframes. The primary goal of the study was to collect data that could be used to develop mathematical models (acceleration functions) that describe the dependence of tin whisker growth and corrosion on temperature and humidity. This paper describes the background, experimental design, data collection and reports results. Part II of the study (J. W. Osenbach et a. ?Tin whisker test development-Temperature and humidity effects part II: Acceleration model development,? Electronics Packaging Manufacturing, Vol. 33, no. 1, pp., Jan. 2010) discusses in the data analyses and acceleration model development. Storage testing was performed over a wide range of temperature and humidity conditions from 30?C to 100?C and from 10% to 90% relative humidity (RH). Commercially produced components with both 3 ?m and 10 ?m thicknesses from three sources were evaluated. For components with the 10 ?m-plating, the plating was evaluated in both the as-plated and reflowed (260?C) conditions. These variations resulted in a large experimental matrix that included 13 different Sn platings, aged at ten different temperature and humidity combinations. Further, the aging test was done at five different laboratories with inspections performed at eight different laboratories. The data collected include 1) corrosion incubation time, 2) tin whisker incubation time, and 3) dependence of the maximum whisker length on storage time at each temperature/humidity condition. Data suggest that corrosion is not a unique driving force for whisker initiation and growth. Whisker formation differs in corroded and non-corroded regions. Due to the scope of this work, it is broken down into two papers. The data and experimental observations are discussed in this paper. The mathematical model development, discussion of results and conclusions are included in Part II of this study.

Effect of strain on whisker growth in matte tin

PurposeTo evaluate the role of imposed strain on whisker growth in matte tin, and to identify the appropriate deformation states and environments that will facilitate the development of a reliable, accelerated test methodology to provide results representative of whisker growth under long‐term service conditions.Design/methodology/approachWhisker growth in matte tin coated strips and wires of copper subjected to various outer fibre strains was characterized at three different temperatures under different humidity conditions.FindingsThis study presents findings of the influence of imposed strain and temperature on whisker growth in a matte tin layers present on a copper substrate. A decrease in incubation time and accelerated growth of whiskers occur with critical imposed strain and specific environmental temperature.Originality/valueWhisker growth in tin coatings is becoming a major reliability concern in modern microelectronics. Immersion coating that produces a matte tin layer is being used extensively in electronic applications. The findings of this study facilitate the development of accelerated tests which represent events that occur over long periods of time during service so that suitable mitigation strategies can be evaluated in a reasonable time period.

Observations of ribbon-like whiskers on tin finish surface

The morphology of tin whiskers on a matte tin finish on a phosphor bronze substrate was observed by SEM. The samples were prepared by a sulfate-based bath and held at 328 K/100% RH for several different time increments. The results show that, besides the fluted whiskers, there are tin whiskers with new morphologies, ribbon-like whiskers. The ribbon-like whisker has a section size of about 5 microns in width and 2.5 microns in thickness and it may be straight or bent with kinks to form some irregular shapes. Whiskers with different morphologies can co-exist on the tin finish with the same aging condition. It is suggested that the formation of ribbon-like whiskers takes place when a new recrystallization grain nucleates and abnormally grows in the finish prior to whisker growth.

Length Distribution Analysis for Tin Whisker Growth

The global movement to lead-free electronics has led semiconductor device assemblers to switch terminals and finishes from lead-based to pure tin or high tin lead-free alloys. This transition has resulted in a reliability concern associated with the formation of conductive tin whiskers, which can grow from a device terminal or lead and cause current leakage or short circuits. This paper presents the results of an experimental study on tin whisker growth. Test specimens consisted of matte and bright tin finishes on copper, Alloy-42, and brass substrate materials. The heat treatments included annealing and two types of simulated reflow. Maximum whisker length and whisker density were measured on 24 different types of tin-plated specimens, after three, eight, and 16 months of room ambient storage after various heat treatments

The impact of electrical current, mechanical bending, and thermal annealing on tin whisker growth

This paper presents the results of an eight-month design-of-experiment assessment of whisker growth on bright and matte tin-plated copper, mechanically deformed and unformed coupons, subject to a continuous 50 °C/50%RH environment, with and without the presence of a constant electrical current density magnitude of 0.48 × 10 2 A/cm 2. Whiskers were observed to grow both at the anode and cathode end. The distribution-based data showed a reduction in whisker density due to annealing and/or the application of electrical current for both bright and matte tin. However, the application of electrical current was observed to increase the standard deviation of the length distribution, and to generate longer whiskers.

The Effect of Annealing on Tin Whisker Growth

This paper presents a design-of-experiments study on the effect of annealing and simulated reflow on tin whisker growth. Copper, brass, and alloy 42 coupons plated with either bright or matte tin were subjected to one of three elevated temperature exposures. After the elevated temperature exposures, specimens along with a set of control specimens were then kept in room ambient conditions and monitored periodically using an environmentally scanning electron microscope. Surface observations up to 16 months of room ambient exposure revealed that tin whiskers formed on the surfaces of each specimen. However, various differences in whisker growth between the matte- and bright tin-plated specimens were observed. Columnar-type whiskers grown on the matte tin plated specimens were initiated from one grain at the surface, as opposed to the growth on bright tin which were independent from the surface morphology. Maximum length and length distribution data for matte and bright tin plating for the various exposures are presented. The result of this study shows annealing to be effective in reducing the maximum length of whiskers, particularly on bright finished coupons

Tin whisker risk assessment

PurposeTo present a methodology, including the algorithms, to quantify the risk of failure from tin whiskers and to present a dynamic risk trend based on the distribution of each of the whisker growth parameters, generated from experiments over a period of time. This paper also aims to demonstrate the practical application of the methodology developed.Design/methodology/approachThis paper has been written to provide a methodology to assess tin whisker risk due to fixed whiskers in electronic products. The risk assessment process has been detailed in the paper. To demonstrate the usefulness of the methodology, a tin whisker risk assessment was conducted for a printed circuit board (PCB) in operation.FindingsBased on the experimental tin whisker growth data it is observed that growth rates of mean length and average density decrease with time. Based on the risk assessment, it was estimated that for the common matte tin over copper finish, the failure risk for the circuit card assembly was 4 per cent over 20 years. It was recommended that, for this product, components with bright tin lead finish should not be used. It was also found that the effectiveness of the conformal coating on this PCB is limited by the relative risk of the components on the board.Originality/valueThe paper provides a new methodology to assess fixed tin whisker risk in electronic products. The methodology provides a dynamic risk trend with time because the algorithm incorporates distributional data of whisker growth and the distributional data as a function of time. This type of assessment was lacking in the previous studies.

Effects of Lead on Tin Whisker Elimination: Efforts toward Lead-Free and Whisker-Free Electrodeposition of Tin

Spontaneous tin whisker growth is one of the direct consequences of complete removal of Pb from tin electrodeposits. To pursue a greater understanding of the whisker growth mechanism, the effects of Pb on the properties of matte tin electrodeposition at different alloy compositions were investigated with respect to cathodic polarization, crystallographic and microstructural characteristics of the deposits, and the formation of intermetallic compounds (IMC) under ambient conditions at the interface between the Sn or films and the Cu substrate. Furthermore, internal stress of pure Sn and deposits was measured as a function of storage time using the bent strip method. The observations strongly suggest that Pb prevents tin whisker growth due to the formation of an even IMC. In turn, upon the IMC growth the deposit exhibits lessening of compressive stress. It is evident that the presence of Pb enhances the lattice diffusion of Cu and increases the ratio of IMC formed through lattice diffusion and grain boundary diffusion. Other contributing factors such as predominant tin [220] textures and the equiaxed grain structure may also play supporting roles in the whisker retarding effect of Pb.

A new fine‐grained matte pure tin for semiconductor lead‐frame applications

PurposeTo present a new tin plating process which provides deposits having a fine‐grained structure and stable crystal orientation with a combination of properties which are well suited for use as a lead‐free finish on semiconductor lead‐frames.Design/methodology/approachThe new process was designed to produce tin coatings with a deposit grain size ca 20 per cent that of a traditional matte pure tin finish to ensure that the deposit retains good solderability after steam ageing.FindingsThe enhanced solderability from the finer‐grained deposit has been demonstrated and other functionally important deposit properties have been confirmed. A stable single crystal orientation 〈220〉 indicates low whiskering propensity and measurements have confirmed the excellent whisker performance, even without specific whisker‐preventing countermeasures. The deposit has high ductility. Analytical methods are available for all key components to ensure optimum process control. The process has been tested in a continuous production environment for over 12 months with excellent results.Originality/valueThe novel process described in the paper provides tin deposits having a unique combination of properties and with production‐proven capability to be an ideal lead‐free solution as a solderable finish for electronic components.

Effects of chloride in acid tin methanesulphonate electrolytes on the deposit thickness, morphologies and compositions

The effects of trace amounts (<500 mg dm–3) of chloride ions in acid tin methanesulphonate plating solutions on the thickness distribution, surface roughness, composition and structure of tin deposits are presented. Chloride ion has little effect on the thickness of tin deposited at a given current density and the morphologies of the tin deposits are similar. Tin deposition is inhibited only when the chloride ion concentration is greater than 250 mg dm–3 and only in the very low current densities. X-ray photoelectron spectroscopy showed no residual chloride on the matte tin surface.