Increasing Reflow Temperature Research Articles

Influence of TiO2 and Al2O3 Nanoparticles Addition on Thermal, Wettability and IMC Growth of Sn-3.0Ag-0.5Cu Lead-Free Solder at Different Reflow Temperature

This study investigated the influence of adding 0.50 wt% and 1.0 wt% titanium dioxide (TiO2) and aluminium oxide (Al2O3) nanoparticles on the properties of Sn3.0Ag0.5Cu (SAC305) lead-free solder alloy. In terms of thermal properties, SAC305 lead-free solder with or without addition of TiO2 and Al2O3 nanoparticles, the melting temperature is very similar and comparable. The wetting performance of the SAC305 nanocomposite solder was determined by the contact angle. The best contact angle recorded is 31.65° at reflow temperature of 250°C for 0.50 wt% addition of TiO2 and Al2O3 nanoparticles. The thickness of the IMC layer was observed using an Optical Microscope and measured. Results show that the thickness of the IMC layer decreased when TiO2 and Al2O3 nanoparticles were reinforced into the SAC305 solder system, which shows significant decrease when 0.50wt% of TiO2 and Al2O3 nanoparticles were added. However, beyond 0.50 wt% the thickness of IMC layer increases along with increasing reflow temperature. The results reveal that the adsorption effect of TiO2 and Al2O3 nanoparticles will restrict the growth of the IMC layer under controlled concentration of the nanoparticles and reflow temperature. The optimal parameters to enhance the performance of SAC305 solder by reinforcing TiO2 and Al2O3 nanoparticles are reflowed at 250°C at 0.50 wt% concentration of TiO2 and Al2O3 nanoparticles.

STUDY OF Co/Sn MULTILAYER SYSTEM WITH VARIOUS TIN LAYER THICKNESS AND REFLOW TEMPERATURES

Co–Sn–Co multi-layer films with various Sn-layer thicknesses have been deposited by using the electrodeposition technique. The deposition was performed at room temperature on gold coated silicon substrates. The thickness of Sn layer was kept [Formula: see text]4.5, 2.5, 1.5 and 1[Formula: see text][Formula: see text]m, while the thickness of each Co layer was fixed to [Formula: see text]0.75[Formula: see text][Formula: see text]m. The Sn layer was sandwiched between two Co layers. The total thickness of the films was [Formula: see text]6, 4, 3, 2.5[Formula: see text][Formula: see text]m, respectively. The intended ratio of Sn was about 25 at.% while Co was about 75 at.%. The separation inside the Sn-layer has been observed at higher Sn-layer thicknesses with increasing reflow temperature. It may be due to the combined effect of thermal stress arising during cooling from elevated reflow temperatures and solidification shrinkage in the thick Sn layer due to the formation of IMC CoSn2. However, 1[Formula: see text][Formula: see text]m has been found to be a critical thickness, which remains intact when sandwiched between two cobalt layers. The structure of these multi-layer films was studied as a function of temperature. It has been observed from X-ray diffraction (XRD) that the as-deposited films exhibit strong peaks belonging to elemental Co and Sn. At temperatures of 270–290∘C, CoSn2 begins to appear and grow at the interfaces and in the middle of Sn layer with the decrease in elemental Sn. However, sufficient amount of Co is still present in pure form as is evident by the XRD and field emission scanning electron microscopy images at all temperatures. This study confirms that only one reaction product, viz. CoSn2, formed in as-deposited during reflow at temperatures 270–290∘C for a fixed time of 10[Formula: see text]min, although several other stable IMCs, e.g. Co3Sn2, CoSn, CoSn2, CoSn3 exist in the Co–Sn system at 250∘C according to phase diagram. It is not uncommon that all the thermodynamically stable IMCs do not form in the system due to kinetic reasons. Accordingly, the formation of IMCs through interfacial reaction has been discussed in this paper.

Voiding Control at Preform Soldering

Abstract Assembly of components with large pads such as high brightness LEDs or high power dies often is soldered with preform, mainly due to a lower voiding and lower flux fume generated when compared with solder paste, and also because of its better thermal and electrical conductivity compared with Ag epoxy. This is particularly true when the joints are to be formed within a cavity. Although lower than solder paste, the voiding in the solder joint is still a concern for high reliability and high performance devices. In this study, voiding at high power die attach reflow soldering using preform was simulated with the use of Cu coupons to mimic both die and substrate. The voiding behavior was studied by varying solder alloy type, flux quantity coated on preform, oxidation extent of Cu coupon, reflow peak temperature, and weight applied on the top of simulated die. For SAC305, with increasing weight, the bondline thickness (BLT) maintained constant initially due to solder surface tension, then reduced rapidly at weight higher than 50 g. The voiding area % increased with decreasing BLT first, then levelled off at lower BLT, although the voiding volume decreased with decreasing BLT due to constrained lamellar solder flow. Voiding was the highest for SAC305, followed by 57Bi42Sn1Ag, with 63Sn37Pb being the lowest, and increased with increasing oxidation of Cu coupon. With increasing flux quantity, voiding increased for SAC305 and 63Sn37Pb, but decreased for 57Bi42Sn1Ag, mainly due to the different temperature range at reflow. Voiding increased with increasing reflow temperature up to 170°C due to increasing vaporization, decreased with further increase in reflow temperature up to 210°C due to increasing flux activity, and increased again at temperature beyond 210°C due to rapid flux outgassing.

Deformation and fracture behaviour of electroplated Sn–Bi/Cu solder joints

This work utilizes the lap shear test to investigate the shear strength and fracture behaviour of electroplated and reflowed Sn–Bi/Cu lead-free solder joints. Particular emphasis is given on the effects of reflow temperature on the interrelationships among the interfacial intermetallic compound (IMC) morphology, shear strength and the fracture mechanism of the solder joints. Single-lap shear specimens are prepared by joining two commercially pure Cu substrates with electroplated Sn–Bi solder of about 50 µm thickness. The geometry of the lap shear specimen is designed to minimize the differences between far-field and actual responses of the solder. Three reflow temperatures (200, 230 and 260 °C) are used to investigate the effects of reflow temperature on the microstructure and shear strength of the solder. The specimens are loaded to failure at a strain rate of 4 × 10−4/s. Elemental mapping of the fracture surface is performed with field emission scanning electron microscope coupled with energy dispersive X-ray spectroscopy. A reflow temperature of 200 °C yields prism-like interfacial IMC morphology, while higher reflow temperatures of 230 and 260 °C yield scallop-like interfacial IMC morphology. The shear strength and elastic energy release, U, of the solder joints increase with increasing reflow temperature. Fractographs of the failed joints suggest that the fracture mechanism is dependent on the interfacial IMC morphology, where solder joints with prism-like interfacial IMC fail within the bulk solder and solder joints with scallop-like interfacial IMC failed with a mixture of bulk and interfacial fracture.

Effect of strain rate on interfacial fracture behaviors of Sn-58Bi/Cu solder joints

The interfacial reactions and mechanical properties of Sn-58Bi/Cu solder joints reflowed at different temperatures ranging from 180 to 220 °C for constant time of 10 min were investigated with various strain rates. Only a continuous Cu6Sn5 intermetallic compound (IMC) layer was formed at the interface between the Sn-58Bi solder and the Cu substrate during reflow. The equivalent thickness of the Cu6Sn5 layer increased with increasing reflow temperature, and the relationship between Cu6Sn5 layer equivalent thickness (X) and reflow temperature (T) is obtained by using method of linear regression and presented as \( X = 0.01 \times T + 0.187 \). For the tensile property, the tensile strength of solder joint gradually decreased as the reflow temperature it increased, whereas it increased with increasing strain rate. Moreover, the fracture behavior of Sn-58Bi/Cu solder joint indicated the ductile fracture with low strain rate (5 × 10−4 and 1 × 10−3 s−1), while toward brittle fracture with high strain rate (2 × 10−3 and 1 × 10−2 s−1). The strain rate sensitivities of the solder joints fractured with various modes were also investigated, and it is found that the tensile strength of the solder is more sensitive to the strain rate than that of the IMC layer.

Interfacial reactions of sequentially electroplated Au/Sn/Au films on Si chips

Au−30 at-%Sn eutectic alloy was fabricated by sequentially pulse electroplating Au and Sn films on Si chips. Three kinds of Au/Sn/Au triple layer films were prepared in the present work: Au/Sn/Au (6/6/1 μm) films, Au/Sn/Au (6/6/6 μm) films and Au/Sn/Au (8/6/1 μm) films. The microstructure and phase transformation in Au/Sn/Au films during aging and reflow soldering were investigated. For Au/Sn/Au (6/6/1 μm) films during aging at 100 and 150°C, the layered AuSn/AuSn2/AuSn4 structure formed in the reaction region. Furthermore, the Sn film was completely consumed, and AuSn4 finally transformed into AuSn and AuSn2 after aging at 150°C for 15 h. For Au/Sn/Au (6/6/6 μm) films during aging at 150°C, the electroplating sequence had an important effect on the formation of Au−Sn phases. An Au5Sn layer was present at the Au II/Sn interface but not at the Au I/Sn interface. For Au/Sn/Au (8/6/1 μm) films, the micropores that formed preferentially along the Au5Sn/AuSn interface remarkably decreased with increasing reflow temperature from 280 to 310°C. After reflowing for 10 s, the microstructure was not an Au−Sn eutectic; however, after reflowing for 60 s, coarsened primary Au5Sn phase and typical Au−30 at-%Sn eutectic microstructure of fine eutectic phases (AuSn+Au5Sn) formed.

Transition of Bi embrittlement of SnBi/Cu joint couples with reflow temperature

Abstract

Mechanical property and fracture behavior characterizations of 96.5 Sn–3.0 Ag–0.5 Cu solder joints

Abstract In this study, 96.5 Sn–3.0 Ag–0.5 Cu solder balls were joined to Au/Ni–P/Cu pads by a thermal reflow process. The interface reactions, solderablity and mechanical reliability of the solder joints were investigated. At the interfaces between the solder balls and pads, intermetallic compounds are formed. With increasing reflow temperature from 220 to 260 °C, the thickness of the intermetallic compound layer increased from 1.1 to 2.5 μm, and the wetting force of molten solder to the pads consequently increased from 0.4 to 0.5 mN. The shear strength of the solder joints decreased from 9.8 to 7.5 MPa because a thick intermetallic compound layer resulted in brittle fracture. The mechanical properties of constituent phases at the solder joints, including primary Sn, Sn–Ag3Sn eutectic phase, intermetallic compound layer and Ni–P underlayer, were measured by nanoindentation. The elastic modulus and hardness of the intermetallic compound layer were obtained as high as about 181 and 6.5 GPa, respectively. Cracks were formed around the indented regions and verified the brittleness of the intermetallic compound layer.

Comparative study of wetting behavior and mechanical properties (microhardness) of Sn–Zn and Sn–Pb solders

In this paper the solder balling, wetting, spreading, slumping and microhardness testing of the Sn–Zn based solders have been compared with the Sn–Pb solder. Two types of solders (Sn–9Zn and Sn–8Zn–3Bi) have been investigated along with Sn–37Pb solder for reference. The variation of these tests has been done as a function of reflow temperature from 220–250°C. Solder balls of these three solder pastes after 15min heating at 230°C show no ball formation surrounding the central ball. Spread test shows that above 240°C Sn–9Zn is very good and can be comparable to Sn–37Pb. The wetting angle of Sn–9Zn (39°) at 250°C is even lower than the Sn–37Pb solder (41°). In case of Sn–8Zn–3Bi, the wetting angle is very high (77°) at 220°C, which is unacceptable but it drops down to 48° at 250°C. Line profiles of slump test show that after preheating at 160°C, Sn–9Zn behaves similar to Sn–37Pb with better distinction in the finer pitch (120μm). Microhardness shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn–37Pb and Sn–9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn–8Zn–3Bi (non-eutectic) increases with increasing reflow temperature. Microstructural characterization at 220 and 250°C shows grain coarsening in Sn–37Pb and Sn–9Zn solders, which cause the hardness to drop a little. For Sn–8Zn–3Bi, with increasing temperature the amount of hard Bi segregation increases which is the main cause of the rise in hardness. SEM images show the formation of Pb rich islands in Sn–37Pb, formation of Zn rod from spheroids in Sn–9Zn and precipitation of Bi-rich phase in Sn–8Zn–3Bi are the important features that contribute to different hardness nature.

Effects of reflow and cooling conditions on interfacial reaction and IMC morphology of Sn–Cu/Ni solder joint

The interfacial reaction and morphology change of intermetallic compound (IMC) between eutectic Sn–Cu solder and Ni substrate under various reflow and cooling conditions were investigated. The reaction between the solder and Ni layer resulted in the formation of (Cu,Ni) 6Sn 5 IMC at the interface. Reflow and cooling conditions changed the morphology of the IMCs formed at the interface. At low temperatures (≤255 °C), only a needle-type (Cu,Ni) 6Sn 5 IMC with a hexagonal cross-section was formed. On the other hand, the morphology of the (Cu,Ni) 6Sn 5 IMC changed from needle-type to dodecahedron-type with increasing reflow temperature and time. The morphology of the (Cu,Ni) 6Sn 5 IMC was also affected by cooling condition. Even though the morphology of the IMC changed with the reflow and cooling conditions, these IMCs had a similar chemical composition.

Investigations on microhardness of Sn–Zn based lead-free solder alloys as replacement of Sn–Pb solder

The microhardness of the Sn–Zn based solder pastes have been compared with the Sn–Pb solder paste. Two types of solder such as Sn–9Zn and Sn–8Zn–3Bi have been investigated along with Sn–37Pb solder for reference. The variation of microhardness with reflow temperature from 220 to 250 °C shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn–37Pb and Sn–9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn–8Zn–3Bi (non-eutectic) increases with the increasing reflow temperature. Microstructural characterization at 220 and 250 °C shows grain coarsening in Sn–37Pb and Sn–9Zn solders, which cause the hardness to drop a little. For Sn–8Zn–3Bi, with increasing temperature the amount of hard Bi segregation increases which is the main cause of the rise in hardness. SEM images show the formation of Pb rich islands in Sn–37Pb, formation of Zn rod from spheroids in Sn–9Zn and precipitation of Bi rich phase in Sn–8Zn–3Bi are the important features that contribute to the different hardness nature. Again the effect of cooling rate on hardness was also studied and compared among these three solder pastes. The result shows that the Sn–9Zn is the most sensitive to cooling rate. With the change in cooling rate, the hardness increase in Sn–9Zn is greater (58% after water cooling) than in Sn–37Pb (30%) and Sn–8Zn–3Bi (33%). The hardness profile along the distance from the centre shows that the hardness gradient is also found highest on Sn–9Zn solder paste whereas similar trendlines were found in Sn–37Pb and Sn–8Zn–3Bi solder. SEM images of air- and water-cooled microstructures show preferential Zn formed at the edge in Sn–9Zn solder while Pb rich islands at the centre (for Sn–37Pb) and precipitation of Bi at the edge (for Sn–8Zn–3Bi) was found along with grain refinement.

A self-aligned offset polysilicon thin-film transistor using photoresist reflow

A simple fabrication method for a self-aligned offset structure, which uses photoresist reflow, is developed to reduce the leakage current of polysilicon thin-film transistors (poly-Si TFTs). The reflow of photoresist can be controlled by varying photoresist thickness and reflow temperature. It is found that the reflow length increases in proportion to the photoresist thickness, and increases with increasing reflow temperature at less than 200/spl deg/C for the AZ5214A photoresist. Poly-Si TFTs are successfully demonstrated with offset lengths of 0.4 and 0.6 /spl mu/m, which show apparent reduction of the leakage current.

Reliability studies of surface mount solder joints - effect of Cu-Sn intermetallic compounds

Cu-Sn intermetallic compounds (IMCs), formed at the interface between the solder and the copper substrate are found to play a dominant role in determining the thermal fatigue life of surface mount solder joints fabricated from a conventional infrared reflow process. In order to predict the growth of this IMC layer during the operating life of the solder joint and its effect on the thermal fatigue life, the formation characteristics of the IMC's in 0805 and 1206 LCCC solder joints are systematically studied in this investigation. Only the stable Cu/sub 6/Sn/sub 5/% /spl eta/-phase intermetallic compound was observed in all as-solidified solder joints as confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The mean layer thickness was found to increase almost linearly with reflow time up to about 200 s. The thickness of the interfacial IMC layer increased with increasing reflow temperature for 0805-type solder joints up to around 250/spl deg/C and reached a saturated thickness of 2.5 /spl mu/m beyond this temperature. Additional intermetallic formation due to higher reflow temperature or longer reflow time would appear as Cu-Sn whiskers in the bulk solder of the joint. The copper land pad size and quality of component lead metallization were also found to greatly affect the formation of Cu-Sn IMC in surface mount solder joints, and hence its reliability in terms of thermal fatigue life and mechanical properties.

Voiding in BGAs

Voiding in BGA assembly using SN63 solder bumps is primarily introduced at board‐level assembly stage. On the pretinned PCBs, voiding of BGA joints increases with increasing solvent volatility, increasing metal content and increasing reflow temperature, and with decreasing powder size. This can be explained by a viscosity dictated flux‐exclusion‐rate model. In this model, a higher viscosity in the fluxing medium at reflow temperature could hinder the exclusion of flux from the interior of the molten solder, hence increase the chance of outgassing due to the increasing amount of entrapped flux, and consequently result in higher voiding in BGA assembly. Flux activity and reflow atmosphere appear to have a negligible effect on voiding when the solderability of the immobile metallisation is not a concern. An increase in void content is accompanied by an increase in the fraction of large voids. This suggests that, similar to voiding phenomena in the SMT process, factors causing voiding in BGA will have an even greater impact on joint reliability than shown by the total‐void‐volume analysis results.