A Comparative Study of Reactive Wetting of Lead and Lead-Free Solders on Cu and (Cu6Sn5/Cu3Sn)/Cu Substrates

Reactive wetting of solders on Cu and Cu₆Sn₅/Cu₃Sn/Cu substrates was investigated using both (1) the wetting balance, and (2) the hot-stage real time, in situ visualization of the triple-line movement. To understand the phenomenology of the spreading behavior better, comprehensive real-time in situ observations were performed. It was found that the wetting time during the wetting balance tests for both the lead solder (63SnPb) and lead-free solder systems (Sn0.7Cu and Sn3.5Ag) is shorter on Cu substrates than it is on Cu₆Sn₅/Cu₃Sn/Cu substrates. The wetting force was not remarkably different on these two substrates for the same solder system. The hot-stage tests indicate a more pronounced spreading of 63Sn-Pb on Cu₆Sn₅/Cu₃Sn/Cu substrates, along with a much larger spreading area. Spreading of lead-free solders in terms of the triple-line kinetics studied by using the hot-stage visualization shows no significant difference in the spreading evolution either over Cu or over Cu₆Sn₅/Cu₃Sn/Cu substrates.     

The effect of solder paste composition on the reliability of SnAgCu joints

As the electronics industry is moving towards lead-free manufacturing processes, more effort has been put into the reliability study of lead-free solder materials. Various tin–silver–copper-based solders have become widely accepted alternatives for tin–lead solders. In this study, we have tested three different SnAgCu solder compositions. The first consisted of a hypoeutectic 96.5Sn/3.0Ag/0.5Cu solder, the second of a eutectic 95.5Sn/3.8Ag/0.7Cu solder, and the third of a hypereutectic 95.5Sn/4.0Ag/0.5Cu solder. A eutectic SnPb solder was used as a reference. The test boards were temperature-cycled (−40 to +125 °C) until all samples failed. The results of the temperature cycling test were analyzed, and cross-section samples were made of the failed joints. Scanning electron and optical microscopy were employed to analyze the fracture behavior and microstructures of the solder joints. The reliability of lead-free solders and the effect of microstructures on joint reliability are discussed.     

Mechanical Properties and Electrochemical Corrosion Behavior of Al/Sn-9Zn-<Emphasis Type="Italic">x</Emphasis>Ag/Cu Joints

The effect of Ag content on the wetting behavior of Sn-9Zn-xAg on aluminum and copper substrates during soldering, as well as the mechanical properties and electrochemical corrosion behavior of Al/Sn-9Zn-xAg/Cu solder joints, were investigated in the present work. Tiny Zn and coarsened dendritic AgZn₃ regions were distributed in the Sn matrix in the bulk Sn-9Zn-xAg solders, and the amount of Zn decreased while that of AgZn₃ increased with increasing Ag content. The wettability of Sn-9Zn-1.5Ag solder on Cu substrate was better than those of the other Sn-9Zn-xAg solders but worse than that of Sn-9Zn solder. The wettability of Sn-9Zn-1.5Ag on the Al substrate was also better than those of the other Sn-9Zn-xAg solders, and even better than that of Sn-9Zn solder. The Al/Sn-9Zn/Cu joint had the highest shear strength, and the shear strength of the Al/Sn-9Zn-xAg/Cu (x = 0 wt.% to 3 wt.%) joints gradually decreased with increasing Ag content. The corrosion resistance of the Sn-9Zn-xAg solders in Al/Sn-9Zn-xAg/Cu joints in 5% NaCl solution was improved compared with that of Sn-9Zn. The corrosion potential of Sn-9Zn-xAg solders continuously increased with increasing Ag content from 0 wt.% to 2 wt.% but then decreased for Sn-9Zn-3Ag. The addition of Ag resulted in the formation of the AgZn₃ phase and in a reduction of the amount of the eutectic Zn phase in the solder matrix; therefore, the corrosion resistance of the Al/Sn-9Zn-xAg/Cu joints was improved.     

Spreading Kinetics of Liquid Solders over an Intermetallic Solid Surface. Part 2: Lead-Free Solders

This study is devoted to a phenomenological and quantitative investigation of␣the physics of spreading of lead-free solder systems (Sn, Sn-0.7Cu, and Sn-3.5Ag) over Cu₆Sn₅/Cu₃Sn/Cu substrates. Wetting kinetics was studied using real-time in situ monitoring of the triple-line kinetics under a controlled atmosphere. Three different intermetallic (IMC) substrates were used for this investigation. It was found that the grain size of the deposited IMC compound coating has a sizable influence on the triple-line movement of the liquid solders considered. More pronounced spreading of solders on IMC substrates was observed for all lead-free solders over an intermetallic with the smallest grain size. Lead-free solder spreading over IMC surfaces exposed to aging at elevated temperatures, hence featuring larger grain size, did not feature a significant improvement of wettability when compared with the behavior of the same solders on virgin copper substrates.     

A viable tin-lead solder substitute: Sn-Ag-Cu

Rising concern over the use of lead in industry provides a driving force for the development of improved lead-free industrial materials. Therefore, a new lead-free base solder alloy Sn-4.7Agl.7Cu (wt.%) has been developed upon which a family of lead-free solders can be based. This solder alloy exhibits a ternary eutectic reaction at 216.8 ± 1°C (L ↠ η+ ϕ + β-Sn; η = Cu₆Sn₅, θ = Ag₃Sn). Preliminary tests of solderability demonstrate intermetallic phase formation on model solder joint interfaces and good wettability in a fluxed condition suggest technological viability and motivates much more extensive study of this solder alloy.     

Effects of thermomechanical cycling on lead and lead-free (SnPb and SnAgCu) surface mount solder joints

Accelerated reliability tests have been performed on leadless and leaded lead-free and lead containing SMT component assemblies. Results so far have shown that lead-free reflow soldering is a viable alternative for conventional lead based reflow soldering. The selected ternary eutectic solder alloy SnAg3.8Cu0.7 requires higher processing temperatures which could restrict the use of certain board and component types, but other than that no major modifications seem necessary. Although better SnAg3.8Cu0.7 bulk mechanical properties were obtained compared to the near eutectic lead bearing bulk solder properties, reflowed solder joints did not reflect this difference. In general, quite similar reliability results were obtained as found for the lead based solders. Dependent on board and component metallisations and use environment, the reliability of the lead-free solders could perform better or worse than the lead based solders. Temperature dependent aspects such as solderability and mechanical behaviour of the lead-free assemblies could play a role in this. Although microstructural differences can be seen between the lead-free and lead bearing solder joints, similar joint failure mechanisms occur. Resistor solder joint cracks propagate from underneath the component through either transgranular (lead-free) grains or along intergranular (lead) grain boundaries between lead-rich and tin-rich areas and into coarsened regions near the component terminations. Gullwing lead cracks were seen propagating from the heel fillet along the lead/solder interfacial intermetallic mostly (some cracks started in the heel fillet and propagated through the solder body dispersed with coarsened spherical Ag₃Sn intermetallic particles). Package design and leadframe material seem to play a more important role in the fatigue mechanism than the change in microstructure of the solder joint.     

Microstructure characterization of SnAgCu solder bearing Ce for electronic packaging

The creep-rupture lives of Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce lead-free solder joints for electronic packaging were investigated, respectively. And the relationship between creep behavior and intermetallic compound (IMC: Ag₃Sn, Cu₆Sn₅, CeSn₃) particles in SnAgCu/SnAgCuCe solder joints has been obtained. Meanwhile, rare earth Ce concentration gradient and retardation effect of Ce on the IMC layer have been observed at the solder/Cu interface. Moreover, aging reaction of Sn and Cu, and the effect mechanism of rare earth Ce on two IMCs (Cu₆Sn₅ and Cu₃Sn) are reported.     

Microstructural evolution of a lead-free solder alloy Sn-Bi-Ag-Cu prepared by mechanical alloying during thermal shock and aging

In a previous study, a lead-free solder, Sn-6Bi-2Ag-0.5Cu, was developed by mechanical alloying. The alloy shows great potential as a lead-free solder system. In the present work, the microstructural evolution during thermal shock and aging was examined. In the as-soldered joints small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles were finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ phase in the bulk of solder. During thermal shock and aging microstructural evolution occurred with Cu-Sn intermetallic compound (IMC) layer growth at interface, bismuth phase coarsening and Ag₃Sn phase coarsening. The microstructure of the solder appeared to be stable at high temperature. The shear strength of the present solder joint is higher than that of Sn-37Pb and Sn-3.5Ag solders. Shear failure occurred Cu-Sn IMC layer-solder interface and in the bulk of solder.     

Morphology and growth kinetics of intermetallic compounds in solid-state interfacial reaction of electroless Ni-P with Sn-based lead-free solders

A comparative study of solid/solid interfacial reactions of electroless Ni-P (15 at.% P) with lead-free solders, Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn, was carried out by performing thermal aging at 150°C up to 1000 h. For pure Sn and Sn-3.5Ag solder, three distinctive layers, Ni₃Sn₄, SnNiP, and Ni₃P, were observed in between the solder and electroless Ni-P; while for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders, two distinctive layers, (CuNi)₆Sn₅ and Ni₃P, were observed. The differences in morphology and growth kinetics of the intermetallic compounds (IMCs) at the interfaces between electroless Ni-P and lead-free solders were investigated, as well as the growth kinetics of the P-enriched layers underneath the interfacial IMC layers. With increasing aging time, the coarsening of interfacial Ni₃Sn₄ IMC grains for pure Sn and Sn-3.5Ag solder was significantly greater than that of the interfacial (CuNi)₆Sn₅ IMC grains for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders. Furthermore, the Ni content in interfacial (CuNi)₆Sn₅ phase slightly increased during aging. A small addition of Cu (0.7 wt.%) resulted in differences in the type, morphology, and growth kinetics of interfacial IMCs. By comparing the metallurgical aspects and growth kinetics of the interfacial IMCs and the underneath P-enriched layers, the role of initial Cu and Ag in lead-free solders is better understood.     

Shear strength and fracture surface analysis of Sn58Bi/Cu solder joints under a wide range of strain rates

The influences of the strain rate on the shear strength and failure mode of Sn58Bi/Cu solder joints were investigated. After reflowing, some Kirkendall voids were observed at the neighborhood of the Cu₃Sn/Cu interface or in the inner Cu₃Sn layer. In addition, another type of void could also be observed inside the Sn58Bi eutectic solders, and its size was much larger than that of Kirkendall voids. Some Bi particles were obviously found to segregate at the interface between the Cu-Sn IMC and the Sn58Bi solder. The single lap shear test results indicated that the strain rate had an important influence on the shear strength and failure mode of Sn58Bi/Cu solder joints. The shear strength of joints demonstrated increment at first and then decrement as the strain rate increased from 3.33 × 10⁻⁴ s⁻¹ to 3.33 s⁻¹. It was observed that all Sn58Bi/Cu solder joints broke in a mixed-type fracture mode under a wide range of strain rates. Additionally, more broken IMC grains were exposed on the fracture face and more fracture occurred within the IMC layer with increasing strain rate. Furthermore, the fracture path gradually moved from the solder side to the inner IMC side as the strain rate increased.     

Metallurgical reactions in composite 90Pb10Sn/lead-free solder joints and their effect on reliability of LTCC/PWB assembly

Use of 90Pb10Sn solder as a noncollapsible sphere material with 95.5Sn 4Ag0.5Cu and SnInAgCu lead-free solders is investigated. Practical reflow conditions led to strong Pb dissolution into liquid solder, resulting in >20 at.% Pb content in the original lead-free solders. The failure mechanism of the test joints is solder cracking due to thermal fatigue, but the characteristic lifetime of 90Pb10Sn/SnInAgCu joints is almost double that of 90Pb10Sn/95.5Sn4Ag0.5Cu in a thermal cycling test (TCT) over the temperature range from −40°C to 125°C. It is predicted that this is mainly a consequence of the better fatigue resistance of the SnPbInAgCu alloy compared with the SnPbAgCu alloy. Indium accelerates the growth of the intermetallic compound (IMC) layer at the low temperature co-fired ceramic (LTCC) metallization/solder interface and causes coarsening of IMC particles during the TCT, but these phenomena do not have a major effect on the creep/fatigue endurance of the test joints.     

Effects of limited cu supply on soldering reactions between SnAgCu and Ni

The volume difference between the various types of solder joints in electronic devices can be enormous. For example, the volume difference between a 760-μm ball grid array solder joint and a 75-μm flip-chip solder joint is as high as 1000 times. Such a big difference in volume produces a pronounced solder volume effect. This volume effect on the soldering reactions between the Sn3AgxCu (x=0.4, 0.5, or 0.6 wt.%) solders and Ni was investigated. Three different sizes of solder spheres (300, 500, and 760 μm in diameter) were soldered onto Ni soldering pads. Both the Cu concentration and the solder volume had a strong effect on the type of the reaction products formed. In addition, (Cu,Ni)₆Sn₅ massively spalled from the interface under certain conditions, including smaller joints and those with lower Cu concentration. We attributed the massive spalling of (Cu,Ni)₆Sn₅ to the decrease of the available Cu in the solders. The results of this study suggest that Cu-rich SnAgCu solders can be used to prevent this massive spalling.     

Microstructural Variation and Phase Evolution in the Reaction of Sn-<Emphasis Type="Italic">x</Emphasis>Ag-Cu Solders and Cu-<Emphasis Type="Italic">y</Emphasis>Zn Substrates During Reflow

This study aims to investigate the reaction of Sn-xAg-0.5Cu (x = 1.0 wt.% and 3.0 wt.%) solders on Cu-yZn (y = 0 wt.%, 15 wt.%, and 30 wt.%) substrates at 250°C for 0.5 min, 2 min, and 10 min, respectively. Cu and Zn atoms dissolve from the Cu-yZn substrates into the molten solders during reflow, leading to variation of the solder composition. It was revealed that such composition variation altered the microstructure of the solders. The coarsening of the eutectic region and the decrease of large-sized Cu₆Sn₅ compounds inside the Sn-1.0Ag-0.5Cu solder on both Cu-15Zn and Cu-30Zn substrates were correlated with this elemental redistribution. In addition to the solder matrix, the interfacial reaction was also affected by Zn dissolution. For a Zn concentration of 15 wt.% to 30 wt.% in the Cu-Zn substrate, formation of Cu₃Sn was suppressed. An increase of the Zn content in Cu₆(Sn,Zn)₅ at the solder/Cu-30Zn interface resulted in the formation of a new Cu(Zn,Sn) phase. It was demonstrated that the microstructural variation and the phase evolution in the solder joints were controlled by the reflow time and the Zn concentration in the Cu-yZn substrate.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Interfacial reactions and shear strengths between Sn-Ag-based Pb-free solder balls and Au/EN/Cu metallization

The morphological and compositional evolutions of intermetallic compounds (IMCs) formed at three Pb-free solder/electroless Ni-P interface were investigated with respect to the solder compositions and reflow times. The three Pb-free solder alloys were Sn3.5Ag, Sn3.5Ag0.75Cu, and Sn3Ag6Bi2In (in wt.%). After reflow reaction, three distinctive layers, Ni₃Sn₄ (or Ni-Cu-Sn for Sn3.5Ag0.75Cu solder), NiSnP, and Ni₃P, were formed on the electroless Ni-P layer in all the solder alloys. For the Sn3.5Ag0.75Cu solder, with increasing reflow time, the interfacial intermetallics switched from (Cu,Ni)₆Sn₅ to (Cu,Ni)₆Sn₅+(Ni,Cu)₃Sn₄, and then to (Ni,Cu)₃Sn₄ IMCs. The degree of IMC spalling for the Sn3.5Ag0.75Cu solder joint was more than that of other solders. In the cases of the Sn3.5Ag and Sn3Ag6Bi2In solder joints, the growth rate of the Ni₃P layer was similar because these two type solder joints had a similar interfacial reaction. On the other hand, for the Sn3.5Ag0.75Cu solder, the thickness of the Ni₃P and Ni-Sn-P layers depended on the degree of IMC spalling. Also, the shear strength showed various characteristics depending on the solder alloys and reflow times. The fractures mainly occurred at the interfaces of Ni₃Sn₄/Ni-Sn-P and solder/Ni₃Sn₄.     

Effects of Solder Volume on Formation and Redeposition of Au-Containing Intermetallics in Ni/Au-SnAgCu-Ni(P) Solder Joints

SnAgCu is one of the most promising candidates for lead-free solders to replace conventional eutectic SnPb solders. The effects of solder volume on interfacial reactions and microstructure evolution in Ni/Au-SnAgCu-Ni(P) solder joints have been investigated under soldering and thermal aging conditions. The results show that solder volume has a strong effect on the formation of Au-containing intermetallic compounds (IMCs) and their redeposition at the interfaces. The size and volume fraction of Au-containing IMCs dispersed in the solder bulk increased with decreasing solder joint dimensions. For the small solder joint with 300-μm solder ball, the (Au,Ni)Sn₄ IMCs redeposited to the interfaces after thermal aging at 150°C for 9 days, but this was not observed for the other two solder joints with large solder volume. These results also indicate that the redeposition of (Au,Ni)Sn₄ is closely associated with the migration of Cu toward the interfaces.     

Interfacial reaction between multicomponent lead-free solders and Ag,Cu, Ni,and Pd substrates

The interfacial reaction between two prototype multicomponent lead-free solders, Sn-3.4Ag-1Bi-0.7Cu-4In and Sn-3.4Ag-3Bi-0.7Cu-4In (mass%), and Ag, Cu, Ni, and Pd substrates are studied at 250°C and 150°C. The microstructural characterization of the solder bumps is carried out by scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis. Ambient temperature, isotropic elastic properties (bulk, shear, and Young’s moduli and Poisson’s ratio) of these solders along with eutectic Sn-Ag, Sn-Bi, and Sn-Zn solders are measured. The isotropic elastic moduli of multicomponent solders are very similar to the eutectic Sn-Ag solder. The measured solubility of the base metal in liquid solders at 250°C agrees very well with the solubility limits reported in assessed Sn-X (X=Ag, Cu, Ni, Pd) phase diagrams. The measured contact angles were generally less than 15° on Cu and Pd substrates, while they were between 25° and 30° on Ag and Ni substrates. The observed intermediate phases in Ag/solder couples were Ag₃Sn after reflow at 250°C and Ag₃Sn and ζ (Ag-Sn) after solid-state aging at 150°C. In Cu/solder and Ni/solder couples, the interfacial phases were Cu₆Sn₅ and (Cu,Ni)₆Sn₅, respectively. In Pd/solder couples, only PdSn₄ after 60-sec reflow, while both PdSn₄ and PdSn₃ after 300-sec reflow, were observed.     

回流次数对Sn3.8Ag0.7Cu-xNi/Ni焊点的影响

研究了复合无铅焊料Sn3.8Ag0.7Cu-xNi(x=0.5,1.0,2.0)与Au/Ni/Cu焊盘在不同回流次数下形成的焊点的性能.结果表明,Ni颗粒增强的复合焊料具有良好的润湿性能,熔点小于222℃;X为0.5的焊料界面IMC由针状(CuNi)6Sn5演化为双层IMC,即多面体状化合物(CuNi)6Sn5和回飞棒...     

Retarding Electromigration in Lead-Free Solder Joints by Alloying and Composite Approaches

The voids induced by electromigration (EM) can trigger serious failure across the entire cathode interface of solder joints. In this study, alloying and composite approaches showed great potential for inhibiting EM in lead-free solder joints. Microsized Ni, Co, and Sb particles were added to the solder matrix. Cu and Sn particles were added to the melting solder to form in situ Cu₆Sn₅, which formed a barrier layer in the underbump metallization of flip-chip solder joints. The polarity effect induced by EM was observed to be significantly inhibited in the alloyed and composite solder joints. This indicates that the Sn-Ni, Sn-Co, Sn-Sb, and Cu₆Sn₅ intermetallic compounds may act as barriers to obstruct the movement of the dominant diffusion species along phase boundaries, which in turn improves the resistance to EM. However, Sb particles could induce crack formation and propagation that might lead to joint fracture.     

Wettability of electroless Ni in the under bump metallurgy with lead free solder

This study investigates the wettability of several lead-free solders, including Sn, Sn−Ag, and Sn−Bi, on electroless Ni (EN) with various phosphorus content. The role of phosphorus on solder wettability is studied. Microstructure evolution in the lead-free solder/EN joint is investigated with the aid of electron probe microanalyzer (EPMA) to relate metallurgical reactions between the solder and the EN. The SN solder exhibits better wettability on EN, while the Si−Bi solder has a larger contact angle. Wettability degrades as the phosphorus content in EN decreases. The dependence of wetting angle on the phosphorous content can be attributed to the surface roughness and density of EN, along with the interfacial reaction between the solders and EN. An EPMA analysis reveals the presence of a Sn−Bi−Ni−P solid solution at the interface of solder/EN joints due to the interdiffusion of major constituent Ni and Sn. The interaction zone of the solid solution increases with increasing temperature. Wettability of Pb-free solders on EN degrades with the presence of NiO due to oxidation or the existence of Ni₃P due to precipitation after annealing. For an adequate wetting behavior in the Sn (Sn−Bi, Sn−Ag)/EN joint, EN deposited with phosphorus contents in the range of 9 to 12 wt% is suggested.