Microstructure Influence of SACX0307-TiO2 Composite Solder Joints on Thermal Properties of Power LED Assemblies

The effect of the microstructure of solder joints on the thermal properties of power LEDs is investigated. Solder joints were prepared with different solder pastes, namely 99Sn0.3Ag0.7Cu (as reference solder) and reinforced 99Sn0.3Ag0.7Cu–TiO2 (composite solder). TiO2 ceramic was used at 1 wt.% and with two different primary particle sizes, which were 20 nm (nano) and 200 nm (submicron). The thermal resistance, the electric thermal resistance, and the luminous efficiency of the power LED assemblies were measured. Furthermore, the microstructure of the different solder joints was analyzed on the basis of cross-sections using scanning electron and optical microscopy. It was found that the addition of submicron TiO2 decreased the thermal and electric thermal resistances of the light sources by 20% and 16%, respectively, and it slightly increased the luminous efficiency. Microstructural evaluations showed that the TiO2 particles were incorporated at the Sn grain boundaries and at the interface of the intermetallic layer and the solder bulk. This caused considerable refinement of the Sn grain structure. The precipitated TiO2 particles at the bottom of the solder joint changed the thermodynamics of Cu6Sn5 formation and enhanced the spalling of intermetallic grain to solder bulk, which resulted in a general decrease in the thickness of the intermetallic layer. These phenomena improved the heat paths in the composite solder joints, and resulted in better thermal and electrical properties of power LED assemblies. However, the TiO2 nanoparticles could also cause considerable local IMC (Intermetallic Compounds) growth, which could inhibit thermal and electrical improvements.

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.

Microstructural Evaluation of Bi-Ag and Bi-Sb Lead-Free High-Temperature Solder Candidates on Copper Substrate with Multiple Reflow Number

An impetus has been provided towards the development of lead-free solders by worldwide environmental legislation that banned the use of lead in solders due to the lead toxicity.This study focus on Bi-Ag and Bi-Sb solder alloys, in compositions from 1.5 to 5 wt % Ag and Sb. The effects of Ag and Sb amount, and reflow number on the microstructure and morphology of solder bulk were analysed by optical microscope and scanning electron microscope-energy dispersive X-ray. Based on the results, the grain boundary grooving was observed in all samples except Bi-5Sb in all three reflows. Metallurgical and chemical reaction between interface and solders were found in Bi-5Sb solder alloys in different reflow numbers which lead to appearance of Cu3Sb intermetallic compound layer at the interface. Reflow numbers had a significant effect on the size of Cu-rich phase. Also it was observed that, with increasing reflow number Bi-Cu phase found in Bi-2.5Sb solder dissolves into the solder bulk.

Bismuth-Antimony as an Alternative for High Temperature Lead Free Solder

The development works on high temperature lead free solder are mostly discussed nowadays. To replace the current high temperature lead free solders, further research need to be done. A great deal of effort has been put into the development of lead free solder alloys. Bi (Bismuth) and Sb (Antimony) solder system proved as one of the promising candidates for electronic assembly. Melting temperature of three Bi-Sb solder alloys studied in this research enhanced their potential as the alternative solder candidates for high temperature lead free solder. At interface, Cu3Sb IMC layer was formed for 95Bi-5Sb solder alloy. Spallation of Cu3Sb IMC layer took placed with the results of Cu3Sb IMC also found in the solder bulk. Analysis of 97.5Bi-2.5Sb solder alloy classified as no metallurgical reaction at the interface and only the mechanical joining existed at the interface. The dissolution of Cu from subtrate affected the formation of Cu rich phase and the unstable Bi-Cu rich phase phenomena act as the isothermal product found in solder bulk. Mechanical grain boundary grooving observed in 98.5Bi-1.5Sb solder alloys at interface. Different compositions of Bi-Sb solder alloys resulted in different types of microstructures at interface and in solder bulk after reflow.

A study of high Cu behavior on electrolytic Ni and electroless Ni pad finish

It has been used various pad finish materials to enhance the reliability of solder joint and Electroless Ni Immersion Gold (the following : ENIG) pad has been used more than others. This study is about reliability according to being used in commercial Electrolytic Ni pad and ENIG pad, and was observed behavior of various Cu contents. After reflow, the inter-metallic compound (IMC) between solder and pad is composed of Cu6Sn5 (Ni substituted) by using EDS, and in case of ENIG, between IMC and Ni layer was observed the dark layer (Ni3P layer). Additional, it could be controlled the thickness of dark layer according to Cu contents. Investigated the different fracture mode between electrolytic Ni and ENIG pad after drop shock test, in case of soft Ni, accelerated stress propagated along the interface between 1st IMC and 2nd IMC, and in case of ENIG pad, accelerated stress propagated along the weaken surface such as dark layer. The unstable interface exists through IMC, pad material and solder bulk by the lattice mismatch, so that the thermal and physical stress due to the continuous exterior impact is transferred to the IMC interface. Therefore, it is strongly requested to control solder morphology, IMC shape and thickness to improve the solder reliability.

The Effect of Intermetallic Growth on Bump Pull Test Responses of SAC105 Solder Bumps

Cold bump pull tests performed on wafer level chip scale packages using SAC105 solder bumps show an increase in the occurrence of brittle failure modes with aging temperature and time. Fast intermetallic growth at 0–1000 h can be attributed to (Cu,Ni)6Sn5, while the decrease in intermetallic growth rate at t>1000 h can be attributed to diffusion processes leading to (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 formation and growth. Ni diffuses toward the solder bulk and saturates at 175–200°C, while Cu diffuses from the under bump metallization (UBM) toward the solder bump at 125–150°C. Interactions between Cu and Ni atoms lead to saturation of their atomic % gradients due to intermetallic formation. Sn diffusion from the solder toward the UBM occurs at 125–150°C. The activation energy for total intermetallic growth was calculated at 0.2 eV.

Electrical resistance of Sn–Ag–Cu ball grid array packages with Sn–Zn–Bi addition jointed at 240 °C

Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously at 240 °C on Cu/Ni/Au metallized ball grid array substrates. The joints without Sn–Zn–Bi addition (only Sn–Ag–Cu) were studied as a control system. Electrical resistance was measured after multiple reflows and aging. The electrical resistance of the joint (R1) consisted of three parts: the solder bulk (Rsolder bulk, upper solder highly beyond the mask), interfacial solder/intermetallic compound (Rsolder/IMC), and the substrate (Rsubstrate). R1 increased with reflows and aging time. Rsolder/IMC, rather than Rsolder bulk and Rsubstrate, seemed to increase with reflows and aging time. The increase of R1 was ascribed to the Rsolder/IMC rises. Rsubstrate was the major contribution to R1. However Rsolder/IMC dominated the increase of R1 with reflows and aging. R1 of Sn–Zn–Bi/Sn–Ag–Cu samples were higher than that of Sn–Ag–Cu samples in various tests.

A Theory of Fatigue: A Physical Approach With Application to Lead-Rich Solder

A fatigue theory with its failure criterion based on physical damage mechanisms is presented for solders. The theory applies Mura’s micromechanical fatigue model to individual grains of the solder structure. By introducing grain orientation (Schmid factor m) into the fatigue formula, an m-N curve at constant loading, similar to a fatigue S-N curve, is suggested for fatigue failure of grains with different orientations. A solder structure is defined as fatigued when the ratio of its failed grains reaches a critical threshold, since at this threshold the failed grains may form a cluster, according to percolation theory. Experimental data for 96.5Pb-3.5Sn (wt. %) solder bulk specimens showed good agreement with the theory and its associated failure criterion. The theory is anisotropic, and there is no size limitation to its application, which could be suitable for anisotropic small-scale (micron scale or smaller) solder joints.

Intermetallic formation with indium solder

This study investigates metallurgical interactions between indium solder and thin film metal layers. Intermetallic formation is an important reliability concern with solder joints because the compounds are typically brittle phases that act as stress concentrators during thermal and power cycling of a device [1], Experiments were carried out for the thin film structure shown schematically in Figure 1. For convenience, 80 nm of Cr and 1.0 μm of Cu were e-beam evaporated on a Si substrate, followed by electroless plating ∼1μm Niklad 752 nickel (1%B). The substrate was diced, then tinned on a hot plate @ 180+/- 5*C using Blackstone #1452 flux. The solder was molten for times of 30,60,120, and 240 sec. Indium was etched away in concentrated HC1 to reveal the intermetallic formation. Shown in Figure 2 is a rod-like eutectic found in the solder bulk. Debye-Scherrer x-ray powder diffraction and energy dispersive x-ray spectrometry were used to identify the phases as In/ϕ-InCu.