Fine Solder Research Articles

Properties of Sn-3.0Ag-0.5Cu solder joints under various soldering conditions: Reflow vs. Laser vs. Intense Pulsed Light soldering1

In this study, the properties of solder joints were compared after bonding process using different energy sources. Various soldering methods were employed, including reflow, laser, and Intense Pulsed Light (IPL) soldering. When reflow soldering was performed for a relatively long time, coarse solder microstructures and interfacial intermetallic compound (IMC) grains were formed. Laser and IPL soldering, which have rapid heating/cooling times, formed fine solder microstructure and thin interfacial IMCs. Then, it was confirmed that Cu6Sn5 and Cu3Sn IMCs were formed at the SAC305/OSP (organic solderability preservative) interface and (Cu,Ni)6Sn5, Ni-Sn-P, and Ni3P were formed at the SAC305/ENEPIG (electroless nickel electroless palladium immersion gold) interface. An observation of the IMC grain morphology under various soldering conditions revealed that (Cu,Ni)6Sn5 was formed on the ENEPIG substrate, which was polygonal and smaller than on the OSP substrate. The shear strength of the joint increased in the order of reflow, laser, and IPL soldering. Additionally, ductile fractures occurred under all conditions. The IPL soldering joint had a thinner IMC compared to reflow soldering, and moderate interface roughness and bonding area compared to laser soldering. Based on the obtained results, a next-generation IPL soldering method is proposed to improve the properties of Sn-3.0Ag-0.5Cu solder joints.

Immersion Tin Plating Enabling Reliable Wettable Flanks on QFN Packages

The automotive market increasingly uses bottom-termination IC packages, like Quad-Flat No leads (QFN) packages, for miniaturization, cost reduction and performance. QFN lead frames are electroplated with 8?12 um of tin for PCB soldering, but subsequent singulation exposes the copper-alloy lead frame at the cut edges. On these exposed lead flanks a solder fillet does not form, making Automatic Optical Inspection (AOI) of solder joints on lead-less packages not reliable. A solder fillet on lead flanks also improves the reliability of solder joints. Therefore, the automotive industry strives for a tin plated wettable flank on lead-less packages. Several approaches, e.g. dimpled or step-cut leads, to create a wettable flank are in use or development. Disadvantage of these methods is that only a part of the flank is tin-plated. Immersion tin plating processes are able to fully plate lead flanks. However, the maximum ~1.5um of standard self-limiting displacement immersion tin processes is too thin to comply with the durability requirements of automotive tin solder finishes. This contribution will discuss an immersion tin-plating process, machine concept and test results for a >2 um tin plated wettable flanks. The immersion tin plating process consist of the following process steps: 1. A high-pressure spray rinse prevents incomplete tin coverage by removing copper burrs formed during singulation. 2. A selective descaling process removes the natural copper oxide film on the exposed copper leads. 3. A selective activation process creates a high tin nucleation density to obtain a fine-grained tin layer. 4. The autocatalytic immersion tin process plates a 2 ? 3um tin layer in 45-60 minutes at 70-75oC. 5. An anti-tarnish process protects the tin layer against oxidation. Finally, rinsing and drying remove all traces of the used chemicals from the packages with tin plated flanks. A machine concept for the immersion tin-plating process is currently under development. The lead-less packages are processed directly on Standard singulation tape rings without repacking or manipulation of the packages. This enables a throughput of 20 tape rings per hour with the full package traceability required for the automotive market. Different types of QFN packages mounted on singulation tape were tin plated and subjected to standard (JEDEC J-STD-002E) solderability tests for component lead durability. Fine-grained 2-3 um thick pure tin deposit of good visual quality were obtained on the exposed QFN copper lead sidewalls with the following properties: • 0.5-1.5um residual tin layer after intermetallic formation in 8 h steam aging and 16 h oven bake. • 100% solder wetting after 6h dry bake at 150C • 100% solder wetting after 8h steam aging (93C and 80-90% RH). • 100% solder fillet height after ceramic glass test.

Innovations in Soldering Materials and Optimization of Solder Paste Printing and Inspection Parameters for System-in-Package Assembly

System-in-Package (SiP) assembly has become a popular solution for the challenges of continued miniaturization in the semiconductor industry. The continued and expected growth of the semiconductor industry in the coming years is driving more advanced assembly methods within System-in-Package and advanced packaging in general, and more advanced semiconductor assembly materials are required to meet those challenges such as solder paste specifically designed for SiP assembly, with fine solder powder and specifically formulated flux vehicles. Fine feature solder paste printing as an assembly method continues to be a challenge for SiP assembly. Small stencil aperture designs have necessitated fine-powder solder pastes, with type 5, type 6, and type 7 being common, as well as there being a need for type 8 for future, more advanced designs. Additionally, as more components are being packed into a SiP, solder paste rheology becomes an important characteristic to consider. Printing transfer efficiency, stencil life, and resistance to slump, are other important paste attributes to consider when evaluating solder paste for SiP assembly. While material properties are important to consider, proper equipment setup for a certain paste formulation is also necessary to maximize throughput and minimize assembly defects. The effects of solder paste printer parameters on fine-feature printing performance will be discussed, such as print speed, separation speed, separation distance, and stencil material. This paper will first focus on common failure modes in SiP assembly, and the technology advancements in both solder paste formulation and substrate design which help eliminate those failure modes. Then, this paper will discuss how proper stencil printer setup can reduce assembly defects, and will highlight designs-of-experiment outlining which stencil printer parameters are statistically significant in improving process yields of SiP assemblies with fine powder solder pastes, both with respect to pre-reflow inspection and post-reflow component placement. This paper will also discuss the importance of solder paste inspection with respect to assembly defects, and will highlight important parameters to optimize to accurately inspect solder paste deposits below 90 microns.

Mixed reaction gold as versatile plating solution for ENIG, ENEPIG and EPAG plating

The surface finish on a printed circuit board (PCB) or substrate serves as surface protection and at the same time enables a variety of assembly techniques. Depending on which final finish is selected it allows for e.g. soldering connections or wire bonding by Al, Au or even Cu-wires. Surface finishes are typically characterized by different metal layers, including tin, nickel, palladium or gold. In this context the electroless or immersion plating of gold has a long history in the electronics industry. The gold plating represents a key step in different finish systems such as electroless nickel/immersion gold (ENIG), electroless nickel/electroless palladium/immersion gold (ENEPIG) and electroless palladium/autocatalytic gold (EPAG). ENIG, ENEPIG and EPAG finishes are characterized by their capabilities to be used as solderable and wire-bondable finish with specific benefits for ENEPIG and EPAG such as enhanced solder joint reliability and gold-wire bondability. In this paper an overview is given on the different types of electroless gold plating electrolytes from fully immersion type and autocatalytic type to mixed reaction type gold electrolytes. The details of the various plating mechanisms are presented, and the different properties and capabilities of these electrolytes are compared and explained based on the respective mechanisms. The presented study investigates the performance of a gold electrolyte of a mixed reaction type looking into the layer properties as well as the soldering and bonding capabilities. The results show that a mixed reaction gold electrolyte can be capable for versatile application- ENIG, ENEPIG and EPAG. The capability of the process was tested with respect to the solder wetting performance, solder joint reliability, wire bonding performance, corrosive attack to the nickel and thickness distribution. It can be shown that this gold process passes or exceeds all related industry standards and target requirements for ENIG, ENEPIG and EPAG final finishes.

Effect of stearic acid coating on anti-oxidation property of Sn-Ag-Cu solder powder

PurposeThe purpose of this study is to develop a monolayer surface coating of stearic acid on Sn-Ag-Cu solder powder to limit oxidation.Design/methodology/approachStearic acid was adsorbed onto Sn-Ag-Cu solder powder through liquid-phase adsorption. The isotherm of adsorption was measured and then the microstructure of coated powder was characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy.FindingsThe adsorption isotherm of stearic acid on the powder was “H” type, which revealed the layer-by-layer adsorption on non-porous surface. When the concentration of solution was in the range of 0.001-0.006 mol/L, with an adsorption amount of 0.12 ± 0.1 mg/g, monolayer stearic acid covered the solder powder completely. Uniform and integrated self-assembled monolayer coating was formed through hydrogen bonds between the oxygen ions in surface lattice of Sn3.0Ag0.5Cu solder powder and the —O—H hydroxyl group of stearic acid. The maximum angle of stability of coated powder also reduced by 2.87° compared with that of non-coated powder. The increase rate of oxygen content of coated powder was much slower than that of non-coated powder when they were exposed to humid air.Originality/valueAs a result, oxidation of fine solder powder was effectively limited. Essentially, this method can also be applied to the coating of other types of solder powder and has reference significance to other coating by liquid-phase method.

Tin Whiskers Formation and Growth on Immersion Sn Surface Finish under External Stresses by Bending

Deposited tin (Sn) layers used as lead-free solderable finish on the semiconductor devices are known to form whiskers. These whiskers are a single crystal of tin that spontaneously grows from the surface of tin within weeks to years. Thus, they can cause shorts and the failure of a whole electronic circuit. In this research, the effect of external stresses applied by bending on the tin whiskers formation and growth was investigated on immersion Sn surface finish. The plating time was 10 minutes to produce 1.4 μm coating thickness on the copper substrate and exposed for 1, 4, and 8 weeks under the environment of 30°C/60% RH for bent and non-bent samples. It was found that the non-bent Sn surface had uniform ts distributed on the entire surface. Unlike the non-bent Sn surface, the surface of bent samples had non-uniform tin whiskers distribution. The tin whiskers formed more and grew longer at the lower stress region of the bent surface as compared to the higher stress region, based on the micrograph observed using the field emission scanning electron microscopy (FESEM).

Influence of Difference Solders Volume on Intermetallic Growth of Sn-4.0Ag-0.5Cu/ENEPIG

In recent years, portable electronic packaging products such as smart phones, tablets, notebooks and other gadgets have been developed with reduced size of component packaging, light weight, high speed and with enhanced performance. Thus, flip chip technology with smaller solder sphere sizes that would produce fine solder joint interconnections have become essential in order to fulfill these miniaturization requirements. This study investigates the interfacial reactions and intermetallics formation during reflow soldering and isothermal aging between Sn-4.0Ag-0.5Cu (SAC405) and electroless nickel/immersion palladium/immersion gold (EN(P)EPIG). Solder diameters of 300 μm and 700 μm were used to compare the effect of solder volume on the solder joint microstructure. The solid state isothermal aging was performed at 125°C starting from 250 hours until 2000 hours. The results revealed that only (Cu,Ni)6Sn5 IMC was found at the interface during reflow soldering while both (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 IMC have been observed after aging process. Smaller solder sizes produced thinner IMC than larger solder joints investigated after reflow soldering, whereas the larger solders produced thinner IMC than the smaller solders after isothermal aging. Aging duration of solder joints has been found to be increase the IMC’s thickness and changed the IMC morphologies to spherical-shaped, compacted and larger grain size.

Manufacturing Advanced Coatings for Future Electronic Systems (MacFest)

With co-funding awarded by Innovate UK (formerly the Technology Strategy Board), the six partner, two year MacFest project is aimed at developing novel solderable finishes that offer all of the ben...

Interfacial reaction and mechanical properties between low melting temperature Sn–58Bi solder and various surface finishes during reflow reactions

Interfacial reactions and joint mechanical reliability of a representative low melting temperature Sn–58 wt%Bi solder with three different surface finishes were evaluated during a reflow process. The surface materials consisted of organic solderability preservative (OSP), electroless nickel–immersion gold (ENIG), and electroless nickel–electroless palladium–immersion gold (ENEPIG). The relationships between the interfacial reaction, surface finish, shear speed, and shear force were elucidated in this study. The interfacial intermetallic compounds (IMCs) on the ENEPIG substrate were sequentially changed during reflowing at 180 °C in the following order: (Pd,Ni)Sn4, (Pd,Ni)Sn4 + Ni3Sn4, and Ni3Sn4. Compared to the Cu6Sn5 IMC on the OSP-finished Cu substrate, the Ni3Sn4 IMCs on the ENIG and ENEPIG substrates were much thinner. The IMCs of each of the different surface finishes grew as the reflow time increased; however, the morphologies of the IMCs were different each other. Layer-type interfacial IMCs created a stable metallurgical interfacial structure during the reflow reaction, resulting in reduced rates of IMC growth and Ni–P consumption. High-speed shear tests were conducted to evaluate the effects of interfacial reactions on the mechanical properties of three Sn–58Bi solder joints. In spite of the different interfacial reactions and IMC formations, there were no large differences among shear forces in the three Sn–58Bi solder joints. Further, the type of surface finish material did not significantly affect the shear force of the Sn–58Bi solder joints under high-speed shear loading.

Effect of Solder Volume on Interfacial Reaction between SAC405 Solders and EN(B)EPIG Surface Finish

The electronic packaging industry is now being driven towards smaller, lighter, and thinner electronic products but with higher performance and more functions. Thus, smaller solder ball sizes are needed for fine solder joint interconnections to fulfill these requirements. This study investigates the interfacial reactions during reflow soldering and isothermal aging between Sn-4.0Ag-0.5Cu (SAC405) and electroless nickel (boron)/ immersion palladium/immersion gold (EN(B)EPIG). Reliability of solder joint has also been investigated by performing solid state isothermal aging at 125 °C for up to 2000 hours. The results revealed that after reflow soldering, (Cu, Ni)6Sn5 IMC is formed between solder and substrate while after aging treatment another IMC was found between (Cu, Ni)6Sn5 and substrate known as (Ni, Cu)3Sn4. Aging time of solder joints resulted in an increase in IMC thickness and a change in morphology into more spherical, dense and with larger grain size. By using optical microscope, the average thickness of the intermetallics was measured and it found that the larger solder balls produced thicker IMC than the smaller solder balls during reflow soldering. However, after aging the smaller solders produced thicker IMC than the larger solders.

Stencil printing behavior of lead-free Sn-3Ag-0.5Cu solder paste for wafer level bumping for Sub-100 μm size solder bumps

Stencil printing for flip chip packaging using fine particle solder pastes is a low cost assembly solution with high throughput for fine pitch solder joint interconnects. The manufacturing challenges associated with both solder paste printing increases as electronic device size decreases due to trend of miniaturization in electronic components. Among multiple parameters, the two most important stencil printing parameters are squeegee pressure and printing speed. In this paper, the printing behavior of Pb free Sn-3Ag-0.5Cu solder paste with a particle size distribution of 2–12 μm for wafer level bumping using a stencil printing method (stencil opening dimension −30 μm) was evaluated by varying the printing speed and squeegee pressure to fabricate solder bumps with a sub 100 μm size. The optimal squeegee pressure and print speed for the defect free printing behavior and fairly uniform size distribution of reflowed paste were found to be 7 kgf and 20 mm/s, respectively. The average size of the reflowed printed paste decreased with the increasing squeegee pressure.

Soldering a 3D Wire Lattice Structure

Soldering the junctions in a novel woven structure constructed from metallic wires is expected to significantly enhance the mechanical properties of that structure. The ideal bond geometry has solder localized at wire junctions and a uniform amount of solder at each junction throughout the structure. By examining the effects of solvent variations, liquid flux concentration, and solder powder concentration on the soldered joint geometry and distribution, we have developed a new procedure to bond these wire junctions using a very fine solder powder suspended in a flux solution.

Lead Free Paste with under 3um Solder Particles for Fine Pitch C4 bumping and pre-coating solder applications

The work to be detailed in this paper is our development of 96.5mass%Sn-3.0mass%Ag-0.5mass%Cu fine solder particles with an average particle size of under 3um (D50), using a chemical reduction method. An evaluation was conducted on the properties of the particles. The average size of particles appeared to be under 3um with a higher yield compared to particles using the conventional gas atomization method. The melting temperature of fine solder particles using this method was its eutectic temperature, which is same as using the gas–atomized particles. 120um pitch solder bumps from the solder paste using the above mentioned fine solder particles were created on the substrate. As a result of property evaluation, it was turned out that the solder paste created a superior printing shape and coplanarity compared to the conventional paste with gas-atomized particles. In order to investigate the superior printing property generated by the paste with fine solder particles, the rheology of the paste was evaluated.It was verified that the anisotropic shape of particles has contributed to prevent the printed paste from slumping, which has resulted in the improvement of printed shape. It also shows that the filling characteristic of the paste was improved by the smaller particles and the better coplanarity was observed. The importance of finer solder particles for finer pitch assembly will be presented.

Ultrasonic Bonding of Anisotropic Conductive Films Containing Ultrafine Solder Balls for High-Power and High-Reliability Flex-On-Board Assembly

New solder anisotropic conductive films (ACFs) consist of a thermosetting polymer resin and fine solder balls instead of the conventional metal particles or metal-coated polymer particles. These solder balls have lower melting temperature than conventional metal conductive particles, which enable them to be melted during ultrasonic (US) bonding, and form intermetallic alloy joints with metal pads of flex on board. In this paper, excellent solder ACF joints are demonstrated using a US-bonding method for high-power and high-reliability flex-on-board (FOB) assemblies. Ultrasonically bonded solder ACF joints were characterized in terms of their electrical properties and reliability. Solder alloy bonding was achieved using two kinds of solder balls, i.e., Sn-58Bi and SAC305 (96.5Sn-3.0Ag-0.5Cu). At the same time, the acrylic ACF resin was completely cured after 5 s of US bonding. Solder alloy ACF joints show about 20% decrease in daisy-chain electrical resistance and 100% increase in the current-carrying capability compared with conventional physical-contact-based Ni ACF joints. Solder alloy ACF joints also show significantly improved reliability with stable electrical resistances up to 24 h in an unbiased autoclave test, whereas conventional Ni ACF joints show severe electrical open failures within 4 h.

Fabrication of a guide block for measuring a device with fine pitch area-arrayed solder bumps

This paper describes the design and fabrication of a guide block and micro probes, which were used for a vertical probe card to test a chip with area-arrayed solder bumps. The size of the fabricated guide block was 10 mm × 6 mm. The guide block consisted of 172 holes to insert micro probes, 2 guide holes for exact alignment, and 4 holes for bolting between the guide block and the housing of a PCB. Pitch and size of the inserting holes were 80 μm, and 90 μm × 30 μm, respectively. A silicon on insulator wafer was used as the substrate of the guide block to reduce micro probes insertion error. The micro probes were made of nickel–cobalt (Ni–Co) alloy using an electroplating method. The length and thickness of the micro probes were 910 and 20 μm, respectively. A vertical probe card assembled with the fabricated guide block and micro probes showed good x–y alignment and planarity errors within ±4 and ±3 μm, respectively. In addition, average leakage current and contact resistance were approximately 0.35 nA and 0.378 ohm, respectively. The proposed guide block and micro probes can be applied to a vertical probe card to test a chip with area-arrayed solder bumps.

Super Fine Lead-Free Solder Powder for Fine Pitch Bump Applications

We have successfully developed super fine lead-free and low alpha solder powder, which contains more than two elements by the method of wet chemical reduction. The size (D50) of super fine powder is around 2–3 micrometer to meet finer pitch assembly in the near future. This new method made it available to synthesize various compositions of solder powder like Sn-Ag, Sn-Cu, Sn-Ag-Cu, etc. Also, this method achieves very high yield compared to a gas atomization method. A solder paste for printing method composed of the fine solder powder has a superior printing ability because of the unique powder shape. The powder shows anisotropic shape, and it can make printed figure excellent after printing without bridge and coplanarity issues for finer pitch applications. With our super fine solder paste, we will be ready for <100um pitch of solder bumps which will come in a few years. Furthermore, the super fine powder is applied to the Cu pad pre-coat. The solder paste for pre-coat composed of the super fine powder shows an excellent coverage and solders flatness on the outer pad after reflow.

ASPIS – a new multi-partner research project to enhance the performance of nickel-gold solderable finishes

AbstractA new European research project has recently commenced in order to undertake a multi-faceted approach for developing novel and improved nickel–gold solderable finishes. The three-year project is known as ASPIS, which is an abbreviation of advanced surface protection for improved reliability printed circuit board (PCB) systems, and it is being supported via the European Commission’s 7th Framework programme under the FP7-SME AG-2008-2 call. It is a ‘Research for SME Associations’ project that has 12 partners from across Europe, including four key research organisations who will undertake much of the research programme on behalf of the project’s SME members. There are two UK-based research providers, namely ITRI Ltd and the University of Leicester, along with TNO from Holland and the Center for Physical Sciences and Technology in Lithuania. The other partners represent a wide cross-section of organisations from the PCB sector that cover the whole of the requisite industry supply chain. These organisat...

ASPIS – a new multi-partner research project to enhance the performance of nickel-gold solderable finishes

ASPIS – a new multi-partner research project to enhance the performance of nickel-gold solderable finishes

ASPIS – a new multi-partner research project to enhance the performance of nickel–gold solderable finishes

A new European research project has recently commenced in order to undertake a multi-faceted approach for developing novel and improved nickel–gold solderable finishes. The three-year project is known as ASPIS, which is an abbreviation of advanced surface protection for improved reliability printed circuit board (PCB) systems, and it is being supported via the European Commission’s 7th Framework programme under the FP7-SME AG-2008-2 call. It is a ‘Research for SME Associations’ project that has 12 partners from across Europe, including four key research organisations who will undertake much of the research programme on behalf of the project’s SME members. There are two UK-based research providers, namely ITRI Ltd and the University of Leicester, along with TNO from Holland and the Center for Physical Sciences and Technology in Lithuania. The other partners represent a wide cross-section of organisations from the PCB sector that cover the whole of the requisite industry supply chain. These organisations are Atotech GmbH, Graphic Plc, Merlin Circuits Technology Ltd, Scionix Ltd, Somacis S.p.a and Global Interconnect Services. In addition, the European Institute of Printed Circuits and the Institute of Circuit Technology (ICT) are partners in the project and they are undertaking the dissemination activities to bring the technology developed to the attention of the PCB industry in the UK, the rest of Europe and further afield. The project is being managed by the ICT.

The Reliability Impact of Reballing COTS Pb-Free BGAs to Sn/Pb for Military Applications

The electronics assembly market has experienced a material shift from lead (Pb) based solders to Pb-free solders. This is a result of the widespread adoption of Reduction of Hazardous Substances (RoHS) legislation and practices in commercial industry. As a result, it is becoming increasingly difficult to procure commercial off-the-shelf (COTS) components with tin-lead (SnPb) solder balls or finish. There are essentially three responses to the scarcity of acceptable SnPb parts: custom order, post process or adapt. Custom ordering parts with SnPb finishes negates the benefits of COTS based acquisition; however, has a reduced reliability risk because the material and processes are known. Reprocessing parts once in house saves money because the parts are COTS, but expends money and resources by performing post processing on them. Also, the additional touch labor and handling increases the risk of damaging the part. Finally, adapting to Pb-free finishes is the preferred long term approach because it preserves the cost benefits of using COTS parts and does not require post processing. It is the riskiest approach due to the lack of historical data in the DoD environment. This paper presents results regarding reballing 208 I/O Ball Grid Array (BGA) parts from tin-silver-copper (SAC305) solder to SnPb eutectic solder. It is important to understand the reliability risks associated with the reballing procedure, particularly as it relates to thermal cycling, shock and vibration environments. Three major efforts will be presented to answer these concerns. First, a survey of reballing vendors was performed to better understand the processes and variables associated with that industry. The results of that survey were used to down-select to five vendors that were used for the physical testing portion of the effort. Finally, physical testing consisting of thermal cycling, shock, and vibration was performed. The physical testing was performed on parts from the five different reballing vendors as well as native SnPb parts and native SAC305 parts. The results of these activities will be presented.