Growth Of Intermetallic Compound Layer Research Articles

Influences of Ag and In alloying on Sn-Bi eutectic solder and SnBi/Cu solder joints

In this study, the Ag and In were alloyed to the Sn-58Bi solder, and their individual and combined influences on the SnBi solder and the SnBi/Cu solder joints were investigated. The results indicate that the In can effectively decrease the melting temperature and improve the wettability of the solder, while the Ag shows slight impacts on the melting behavior and the wetting properties. During the isothermal aging at 100 °C, growth rate of the intermetallic compounds (IMCs) layer at the Ag-bearing solders/Cu interfaces are lower, while that at the In-bearing solders/Cu interfaces are much higher, because the substitution of Sn in the Cu6Sn5 IMC by In alters the interfacial diffusion behavior and assist in faster growth of IMCs layer. The Bi segregation at the long-term aged SnBi/Cu interface is eliminated in the aged SnBiAg/Cu, SnBiIn/Cu and SnBiAgIn/Cu solder joints. Tensile strength of all the solders joints decreases with increasing aging time, while the sharp decrease in the strength of the long-term aged SnBi/Cu solder joints is inhibited once the Bi segregation is eliminated, and fracture occurs mainly around the IMCs/solder interface. For the In-bearing solder joints, the Cu6(Sn,In)5 decrease the adhesion force between the solder and the interfacial IMCs, while the SnBiAg/Cu joints shows the highest strength. Based on the results, a more practicable method to inhibit the Bi embrittlement at the aged SnBi/Cu solder joints is proposed.

Non-parabolic Al3Ti intermetallic layer growth on aluminum-titanium interface at low annealing temperatures

Abstract Intermetallic compound (IMC) layer growth at dissimilar metal interfaces, controlled by diffusion, usually follows a parabolic law. However, in the present study, the growth of Al3Ti layer on the Al-Ti interface at a low annealing temperature (TA, 500 °C) obviously didn’t follow a parabolic law. A very long incubation stage of Al3Ti layer growth was observed in the Al-Ti diffusion couple annealed at 500 °C. Residual oxide layers and an oxygen rich layer were experimentally observed on Al-Ti and Al3Ti-Ti interface regions using high resolution STEM-EDS. The very long incubation stage is probably caused by the oxide layers and the oxygen rich layer on the interface region, which act as a diffusion barrier and as a source of oxygen atoms. The length of the incubation stage became shorter and even unobservable with increasing temperature probably due to the higher reaction rate between titanium oxides and aluminum at higher temperatures.

The Effect of Multi-Walled Carbon Nanotubes Reinforcement and Multiple Reflow Cycles on Shear Strength of SAC305 Lead-Free Solder Alloy

Abstract In this study, the effect of multi-walled carbon nanotubes (MWCNT) reinforcement on joint shear strength and microstructural development of tin-3.0silver-0.5copper (SAC305)/copper solder joint subjected to multiple reflow cycles was investigated. The MWCNT-reinforced SAC305 solder systems (SAC305-x MWCNT; x = 0.01, 0.05, 0.1, and 0.5 wt.%) were developed by a mechanical dispersion method. The microstructural, mechanical, and melting properties of SAC305 composite solders were evaluated as a function of different wt.% of MWCNT addition. The melting behavior of composite solders was analyzed using differential scanning calorimetry. The morphology and intermetallic compound growth at the solder joint interface were studied using scanning electron microscopy. The copper/solder/copper micro-lap-shear solder joint specimens reflowed for multiple reflow cycles were systematically characterized to evaluate the joint shear strength. The results showed that the reinforcement in the range of 0.01–0.05 wt.% of MWCNT resulted in the improvement of joint shear strength and better wettability compared to plain SAC305 solder alloy. Amongst all compositions analyzed, SAC305-0.05MWCNT nanocomposite suppressed the intermetallic compound layer growth effectively leading to improvement in the joint shear strength under multiple reflow cycles.

Impact of different isothermal aging conditions on the IMC layer growth and shear strength of MWCNT-reinforced Sn–5Sb solder composites on Cu substrate

In this study, the effect of multi-walled carbon nanotubes (MWCNTs) reinforcement on the intermetallic compound (IMC) layer growth and shear strength of Sn–5Sb-xCNT/Cu composite solder joints subjected to different isothermal aging conditions has been investigated. A series of plain and composite lead-free solder systems (Sn–5Sb-xCNT; x = 0, 0.01, 0.05 and 0.1 wt%) was successfully developed through the powder metallurgy method. Isothermal aging process was performed on the solder/Cu joints at 120 °C, 150 °C and 170 °C temperatures in order to investigate the evolution of the interfacial IMC layers and the shear strength property. Experimental results showed that the thickness of the total IMC layer increased with rising aging temperature. While the Cu3Sn IMC maintained a layer-type morphology for all aging conditions, morphology of the Cu6Sn5 IMC transformed from a scallop-type to a layer-type after aging at intermediate (150 °C) and high (170 °C) temperatures. Given the potential of MWCNTs as a reinforcement material, significant suppression in IMC layer growth was demonstrated by the composite solder joints relative to the plain counterpart. Comprehensive investigation on the growth kinetics showed that the presence of MWCNTs in the composite solder joints was effective in slowing down the diffusion mechanism responsible for IMC growth. Overall, the Sn–5Sb-0.05CNT composite solder joint exhibited the lowest diffusion coefficient within the range of 0.09 × 10−14- 1.03 × 10−14 cm2/s and 0.95 × 10−14- 9.8 × 10−14 cm2/s for Cu6Sn5 and Cu3Sn IMC layers respectively. Moreover, the strengthening effect of the MWCNTs reinforcement was well marked in the composite solder joints as the maximum shear strength within a range of 24.2–15.2 MPa was exhibited by the Sn-0.01CNT/Cu composite solder joint subjected to reflow soldering and isothermal aging conditions.

Growth and evolution kinetics of intermetallic compounds in Sn-0.7Cu-10Bi-0.15Co/Cu interface

The morphology and composition of intermetallic compounds (IMCs) formed between substrate and solder play a vital role in the service life and failure of the solder joint, which is the most important part in the research of electronic packaging. Therefore, it is very important to investigate the growth and evolution of IMCs layer in the aging process. In this paper, there are particular phenomena in the interfacial reaction between the Sn-0.7Cu-10Bi-0.15Co solder and Cu substrate. The 0.15 wt% Co added to the matrix solder causes the abnormal transformation in the thickness and morphology of the IMCs. Because the CoSn3 phase is rapidly growing in a porous structure, so the intermetallic compounds are planar in morphology with an increasing thickness. In addition, the thickness of IMCs layer decreases because the CoSn3 with porous structure promotes the dissolution of the IMCs during aging for 200 h at 70 °C. Analytical results indicate that the Co element changes the diffusion channel and mechanism of the IMCs growth at the interface, and affects the growth kinetics of the CoSn3 phase in products; it in turn impacts the morphology and thickness of IMCs and reliability of solder joint.

Effect of Ni on the kinetics of intermetallic compounds evolution on the Sn-0.7Cu-10Bi-xNi/Co interface during various reflow

In this paper, the interface reaction between the Sn-0.7Cu-10Bi-xNi (SCB-xNi, x = 0 and 0.20, in wt%) solders and Co substrate during multiple reflows was studied. The results indicated that the morphologies and thickness of intermetallic compounds (IMCs) layer were changed after addition of 0.20 wt% Ni. The thickness of IMCs increased with the increasing reflow cycles on the SCB-xNi/Co interface. It was found that the formation of Ni-Co-Sn IMCs layer effectively inhibited the diffusion of Co atoms from Co substrate to solder/IMCs interface, and the primary diffusion of Sn atoms in the interface reaction led to the growth of IMCs along with Co/IMCs interface. Moreover, the IMCs layer was stratified obviously and there was a significant difference in compositions of upper and lower IMCs layers on the SCB-0.20Ni/Co interface with the increasing reflow cycles. When the reflow reached to ten cycles, the IMCs layer was dissolved into solder matrix. The results further suggested that the growth and evolution kinetics of IMCs layer was changed at the SCB-xNi/Co interface during multiple reflow cycles. In addition, the mechanism of atomic diffusion and IMCs growth in the interface reaction between solder and Co substrate was improved.

Abnormal effect of temperature on intermetallic compound layer growth at aluminum-titanium interface: The role of grain boundary diffusion

The Al3Ti intermetallic compound (IMC) layer growth kinetics at Al-Ti interface was investigated. Surprisingly an abnormal effect of temperature was observed with a thicker IMC layer formed at lower annealing temperature due to the much faster Al3Ti grain coarsening at a higher temperature, the IMC layer growth at 630 °C became slower than that at 600 °C after a certain annealing time. The quantitative influence of grain size on effective diffusion coefficient was analysed. Calculations showed that the lattice diffusion makes little contribution to the effective diffusion coefficient. Grain boundary diffusion, therefore, dominates for all the annealing conditions studied.

Growth behavior of intermetallic compounds in Fe/Sn diffusion couples

In this work, Fe/Sn diffusion couples were manufactured to study the growth behavior and kinetic parameters of intermetallic compounds (IMCs). The diffusion couples were aged at four temperatures, namely at T = 300, 350, 400, and 500 °C, for various times up to 14,400 s. EPMA and XRD were carried out to study the Fe/Sn interface. The layer of IMCs consisted of only FeSn2 with an irregular microstructure. The growth of IMC layer predominantly took place in the Sn specimen. The mean thickness of the layer was proportional to the power function of the aging time. The growth exponent was close to 0.5 at T = 400 and 500 °C, taking values of 0.479 and 0.503, respectively. Hence, the volume diffusion was the rate-controlling process for the layer growth of IMCs at 400–500 °C. The growth exponent was equal to 0.422 at T = 350 °C and 0.406 at T = 300 °C. Herein, the grain boundary diffusion and volume diffusion contributed to the rate-controlling process. The growth rate constant, K0, was 3.114 × 10−10 m2 s−1 and activation energy was 46.76 kJ mol−1. A mathematical model was established to predict the thickness of IMCs layer as a function of aging time and aging temperature by using Arrhenius equation and including a growth correction factor, ycoor. The prediction results were consistent with the experimental results.

Growth behavior of intermetallic compounds on Sn-10Bi-0.7Cu-0.15Co/Co interface under multiple reflows

The interface reaction between the Sn-0.7Cu-10Bi-0.15Co and Co substrate during multiple reflow cycles and the growth and evolution rhythm of intermetallic compounds (IMCs) were investigated. The results showed that as the reflow cycles increased in number, different growth morphologies and evolution rules of the IMCs layer including its primary growth, dissolution and secondary growth appeared. In the primary stage of multiple reflows, Co could be continuously diffused from the Co substrate to the IMCs/solder interface to provide enough Co for the growth of IMCs layer. As the IMCs layer became thick, the diffusion of Co to the interface was inhibited, and the dynamic equilibrium of IMCs growth was broken. When the growth rate was higher than the dissolution rate, the IMCs layer grew as a whole and whereas the IMCs were dissolved. And under the action of adsorption force, the dissolved IMCs particles re-aggregated and grew.

Effect of flux doped with Cu6Sn5 nanoparticles on the interfacial reaction of lead-free solder joints

In this paper, flux doped with 0.05, 0.1, and 0.2 wt% Cu6Sn5 nanoparticles (NPs) were reasonably believed to affect the morphology and growth rate of the intermetallic compounds (IMC) between Cu substrate and Sn–3.0Ag–0.5Cu solder. Reflowing was performed at 250 °C, then isothermal aging was conducted at 150 °C up to 360 h. The experimental results show that the thickness of the IMC layer increased with the increment of aging time. When the aging time extended to 120 h or more, the Cu3Sn layer appeared on the side of the Cu substrate and also thickened as time increased. The additions of Cu6Sn5 NPs into the flux did not change the type of IMCs while the total thickness of the IMC changed visually. It was calculated that the corresponding growth rate constant of interfacial IMCs in solder joints with flux contained 0, 0.05, 0.1, and 0.2 wt% Cu6Sn5 NPs were 0.14766, 0.14719, 0.14578 and 0.14726 μm/h1/2, respectively. It means that adding Cu6Sn5 NPs into flux could effectively inhibit the growth of IMC layer. The strongest inhibition effect on the growth of IMC layer could be achieved when the content of Cu6Sn5 NPs was 0.1%. Flux with Cu6Sn5 NPs could also effectively inhibit the coarsening of the interfacial IMC grains, and adding 0.1% NPs to the flux has the best inhibition effect.

Suppression of the Growth of Intermetallic Compound Layers with the Addition of Graphene Nano-Sheets to an Epoxy Sn⁻Ag⁻Cu Solder on a Cu Substrate.

This study investigated the suppression of the growth of the intermetallic compound (IMC) layer that forms between epoxy solder joints and the substrate in electronic packaging by adding graphene nano-sheets (GNSs) to 96.5Sn–3.0Ag–0.5Cu (wt %, SAC305) solder whose bonding characteristics had been strengthened with a polymer. IMC growth was induced in isothermal aging tests at 150 °C, 125 °C and 85 °C for 504 h (21 days). Activation energies were calculated based on the IMC layer thickness, temperature, and time. The activation energy required for the formation of IMCs was 45.5 KJ/mol for the plain epoxy solder, 52.8 KJ/mol for the 0.01%-GNS solder, 62.5 KJ/mol for the 0.05%-GNS solder, and 68.7 KJ/mol for the 0.1%-GNS solder. Thus, the preventive effects were higher for increasing concentrations of GNS in the epoxy solder. In addition, shear tests were employed on the solder joints to analyze the relationship between the addition of GNSs and the bonding characteristics of the solder joints. It was found that the addition of GNSs to epoxy solder weakened the bonding characteristics of the solder, but not critically so because the shear force was higher than for normal solder (i.e., without the addition of epoxy). Thus, the addition of a small amount of GNSs to epoxy solder can suppress the formation of an IMC layer during isothermal aging without significantly weakening the bonding characteristics of the epoxy solder paste.

Reliability and Temperature Resistance of Solder–Metallic Mesh Composite Joints

Cu-to-Cu and Si-to-direct bonded copper (DBC) assemblies were bonded with SAC305 solder (reference) as well as with composite layers consisting of SAC305 solder matrices with integrated thin Cu and Ni meshes. The microstructural changes at the interfaces, such as the growth of intermetallic compound (IMC) layers and void formation, were monitored during annealing of the Cu-to-Cu samples at constant temperature of 150°C for 1000 h. All three types of sample exhibited sufficient shear strength above 40 MPa with preferable ductile fracture characteristics after annealing. The solder–mesh composite joints in the Si-to-DBC samples provided superior reliability against short-time temperature cycling between 80°C and 200°C. The cracking path was strongly distracted and branched through the mesh insert. The IMC growth rate was found to be reduced in the solder–mesh joints at the Si chip interface. Thermomechanical finite element simulations revealed that the mesh interacted with the solder matrix through plastic deformation and stretching, which could result in reduced thermal expansion of the composite overall.

Interfacial reaction, microstructure and hardness between graphene-coated Cu substrate and SAC305 solder doped with minor Ni during isothermal aging

This study investigated the influences of Cu, high temperature-treated Cu (H-Cu) and graphene-coated Cu (G-Cu) substrates on interfacial reaction, microstructure and hardness of Sn-3.0g-0.5Cu (SAC305) solder alloy. Intermetallic compound (IMC) layer evolution and mechanical property of Sn-3.0g-0.5Cu-0.3Ni (SAC305-0.3Ni) solder joints were also studied under different aging duration. A continuous scallop-like IMC layer was observed at SAC305/Cu, SAC305/H-Cu, SAC305/G-Cu interfaces during reflow and isothermal aging. After adding Ni in the SAC305-0.3Ni solder alloy, the roughness of IMC layer on Cu, H-Cu substrates increased. In contrast, the addition of Ni had a limited impact on the roughness of IMC layer on G-Cu substrates. The total thickness of IMC layer grew as aging time increases, proportionated to the square root of aging duration. The addition of Ni in the solder alloy promoted the growth of IMC layer on Cu and H-Cu substrates, but it was restrained on G-Cu substrate. The amount of the IMC phases in SAC305 and SAC305-0.3Ni solder bulks on the three substrates increased significantly as aging time prolonged. Thus, the hardness of SAC305 and SAC305-0.3Ni solder bulks on the three substrates rose. The addition of Ni in the solder bulks on the three substrates sharply enhanced the formation of [Formula: see text]-Sn phases and increased the quantity of the IMCs. Consequently, the hardness of SAC305-0.3Ni solder bulks was higher than that of SAC305 solder bulk on the three substrates under same aging condition. In addition, the graphene-coated layer on G-Cu substrate could improve the hardness of SAC305 and SAC305-0.3Ni solder bulks.

Thermomigration induced microstructure and property changes in Sn-58Bi solders

Thermomigration (TM) has become a critical reliability issue in advanced electronic packaging because of Joule heating. A temperature gradient is required to conduct heat away, and only 1 °C of temperature difference across a 10 μm thick microbump produces a temperature gradient of 1000 °C/cm, which can cause TM, especially in low melting eutectic Sn-Bi solder interconnects. We report here that Bi atoms moving from the hot end to the cold end of the temperature gradient, are the dominant diffusing species. Under the assumption of constant volume, the Sn atoms are squeezed by the Bi atoms at the cold end and have to accommodate for the Bi atoms, which makes them move to the hot end. Consequently, the opposing fluxes of Bi and Sn are found to be about the same. Moreover, the growth of Cu-Sn intermetallic compound (IMC) layers at the cold and the hot end were symmetrical, and were unaffected by TM. Additionally, finite-element-method (FEM) simulations showed that the phase separation of Bi and Sn reduced the current crowding regions which affect the electromigration of the eutectic Sn-Bi solder interconnects.

The effect of CNT and Cu on interfacial intermetallic growth of Sn-Ag-Cu solder

In this study, the effect of copper and carbon nanotubes (CNT) composition on interfacial growth of various Sn-Ag-Cu (SAC) solder joints were investigated. SAC is proven to have good reliability and mechanical properties but lacking in terms of melting point and wettability. Thus, Sn-4.0 wt%Ag-0.3wt%Cu (SAC403) and Sn-4.0 wt%Ag-0.7 wt%Cu (SAC407) were reinforced with CNT with various weight percentages of 0 to 0.1wt%. The wetting angle and intermetallic compound (IMC) thickness is measured using optical microscope (Olympus BX5/M) and Olympus Stream Essentials software. The high Cu content alloys formed a long Cu6Sn5 whisker which act as an obstacle to crack propagation. CNT improves wettability and IMC layer growth for both SAC403 and SAC407 by lowering the wetting angle and IMC thickness compared to the plain solder.

Effect of aging temperature on microstructure and mechanical properties of Sn–9Zn–xZrC solder joints

The effect of aging temperature on microstructure and mechanical properties of Sn–9Zn–xZrC (x = 0, 0.06) solder joints was investigated. The results showed that the wettability of Sn–9Zn based alloys was improved by ZrC nanoparticles, the spreading areas of Sn–9Zn–xZrC (x = 0–0.12) were increased firstly then decreased with increasing ZrC content. The spreading area of Sn–9Zn was about 143.64 mm2 and reached maximum about 190.92 mm2 of Sn–9Zn–0.06ZrC. The microstructure of Sn–9Zn–0.06ZrC solder alloy consisted of β-Sn, Sn–Zn eutectic and intermetallic compounds (IMCs), and the microstructure of Sn–9Zn alloy was refined by adding appropriate amounts of ZrC nanoparticles. The interfacial IMCs at the β-Sn boundary in the Sn–9Zn–0.06ZrC solder joint was Cu5Zn8, and partial Cu5Zn8 was transformed to Cu6Sn5 during aging treatment. The IMCs layer thickness was dominated by aging temperature, and ZrC facilitated the IMCs layer growth. The tensile strength of Sn–9Zn based solder joints was enhanced by adding ZrC particles, and then are decreased simultaneously with increasing aging temperature. The fracture surface of Sn–9Zn joints after aging was mainly composed of uniform dimples, and the fracture mechanism of Sn–9Zn based solder joints was ductile fracture.

Geometrical effects on growth kinetics of interfacial intermetallic compounds in Sn/Cu joints reflowed with Cu nanoparticles doped flux

In this study, Cu nanoparticles prepared by chemical reduction method, were doped into flux (0–2 wt%) and disseminated to the pure Sn solder ball at 250. The enhanced spreading rate due to use of nanoparticles, increased the base diameter (W) and decreased the height (H) of the solder at constant volume. The finite element analysis for Cu concentration, temperature and velocity; in relation to the magnitudes of W and H show that larger W is responsible for enhancement of supersaturation and radial thermal gradient, whereas smaller H is responsible for reduction in flow velocity. The growth kinetics of interfacial Cu 6Sn 5 film during isothermal reflow is proportional to W 2/3 during isothermal reflow whereas linearly dependent on H during air cooling. As the ripening at isothermal stage and precipitation at cooling stage contribute to the gross growth behavior of Cu 6Sn 5 intermetallic compounds layer growth, the combined geometrical effect of base diameter and height of the solder specimen renders solder corresponding to flux with 2.0 wt% nanoparticles to have the overall thickest intermetallics.

Influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of solderable anisotropic conductive adhesives (SACAs)

In the present work, the influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of carbon nanotube (CNT)-filled solderable anisotropic conductive adhesives (SACAs) containing low-melting-point-alloy (LMPA) fillers was investigated. To measure the reliability-related properties of SACA assemblies, six types of SACAs with different MWCNT concentrations (from 0 to 2 wt%) were formulated, and two types of reliability tests, thermal shock (TS) and high-temperature and high-humidity (HTHH) tests, were conducted on these samples. All of the CNT-filled SACA assemblies with different MWCNT concentrations showed proper and stable electrical reliability during each aging test, due to the molten LMPA fillers forming a metallurgical interconnection between the corresponding metallization regions. Although the mechanical strength of the CNT-filled SACA joints degraded after aging tests due to excessive intermetallic compound (IMC) layer growth, the CNT-filled SACAs with an MWCNT concentration below 0.1 wt% exhibited better mechanical reliability properties than those of SACAs without MWCNTs due to the initially achieved high bonding strength of the SACA joints, enabled by the reinforcing effects of the MWCNTs within the polymer composites.

Intermetallic compounds (IMCs) formation during dissimilar friction-stir welding of AA5005 aluminum alloy to St-52 steel: numerical modeling and experimental study

In this research, AA5005-O aluminum-magnesium alloy and St-52 low carbon steel sheets were friction-stir welded in a butt-dissimilar joint design. Effects of different processing parameters including tool rotational speed (w), traverse velocity (v), plunge depth, and offset distance on the solid-state weldability of these dissimilar materials were assessed in terms of formation of intermetallic compound (IMC) layer at the interface. A 3D thermo-mechanical finite element modeling procedure was employed to predict the nucleation and growth of IMC layer. Formation of various FeAl, FeAl3, and Fe2Al5 IMCs at the interface, layer morphology, and thickness were experimentally studied as well, by using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis techniques. A good agreement between the simulated thermal results and experimental data was noticed. The results showed that the thickness of IMC layer at the interface as the main controlling parameter in transverse tensile property and fracture behavior of produced dissimilar joints can be varied extremely as a function of processing parameters. By decreasing the heat input and suppressing the kinetics of IMCs layer formation, the tensile performance of dissimilar welded joints is improved, considerably. However, the soundness of these dissimilar welds played another main role as a restriction mechanism against this trend. The maximum joining efficiency is attained around 90% at an optimized working window of w = 1200 rpm, v = 90 mm/min, and a plunge depth of 0.3 mm with an offset distance of 0.5 mm toward the Al side. The hardness of this optimized dissimilar weld is enhanced even more than the steel base metal caused by the formation of IMC layer at the interface as well as the dispersion of reinforcing intermetallic particles (IMPs).

Interfacial microstructure and bonding strength of copper/aluminum clad sheets produced by horizontal twin-roll casting and annealing

The copper/aluminum (Cu/Al) clad sheets were produced on a horizontal twin-roll caster and annealed. The effects of annealing temperature on the interface and peel strength of the Cu/Al clad sheets were investigated. The interfacial microstructures of the as-cast and annealed reinforced clad sheets were analyzed. With the increase of annealing temperature, the growth of total intermetallic compound (IMC) layer accelerated, and the peel strength slightly increased and then decreased rapidly. For the as-cast and annealing reinforced clad sheets, the thicknesses of interface layers were less; the interfaces were mainly composed of two or three IMC grain layers; there are the almost coherent interfaces of Al2Cu/Al4Cu9, Al2Cu/Al and Al4Cu9/Cu. These structural characteristics of the interfaces can improve the peel strength of the clad sheets.