Low-melting-point Alloy Fillers Research Articles

Conduction Path Formation Characteristics of Solderable Polymer Composite Filled with Low-Melting-Point and High-Melting-Point Alloy Fillers

In this study, low-melting-point alloy (LMPA) and high-melting-point alloy (HMPA) filler-filled solderable polymer composite (LH-SPC) system was proposed to enhance the mechanical and thermal properties of SPC with LMPA fillers. To identify the conduction path formation characteristics of LH-SPC according to the mixing ratio of LMPA and HMPA, four types of LH-SPC with different mixing ratios of LMPA and HMPA fillers (100:0, 80:20, 30:70, and 0:100) were formulated. Furthermore, a chip resistor interconnection test was conducted. The results indicated that LH-SPC with only HMPA did not form a conduction path because of the excessively cured polymer composite before melting HMPA. Meanwhile, LH-SPC with LMPA and HMPA fillers showed different conduction path formation mechanisms according to the mixing ratio of LMPA and HMPA fillers. In LH-SPC filled with lower HMPA content than LMPA, the conduction path was formed by the flow, coalescence, and wetting behaviors of molten LMPA containing solid-state HMPA at the melting range of the LMPA filler. On the other hand, in LH-SPC containing higher HMPA content than LMPA, the conduction path was formed by the wetting behavior of the molten HMPA at a lower temperature range than the melting temperature of HMPA because of the decreased melting temperature of HMPA owing to the chemical composition change of HMPA by the diffusion of Bi into HMPA.

Conduction Path Formation Mechanism of Solderable Polymer Composites with Low-Melting-Point Alloy/High-Melting-Point Alloy Mixed Filler

A novel interconnection mechanism using a solderable polymer composite (SPC) with a low-melting-point alloy (LMPA)/high-melting-point alloy (HMPA) mixed filler was proposed to enhance the properties of the LMPA filler and establish a conduction path containing the HMPA filler. To investigate the feasibility of the proposed interconnection mechanism, three types of SPC with a different mixing ratios of LMPA/HMPA filler (100:0, 50:50, and 0:100) were formulated. The SPC with only an HMPA filler showed weak conduction path formation due to excessive curing of the polymer composite before melting of the HMPA filler. In contrast, the SPC with an LMPA/HMPA mixed filler formed a stable and wide conductive path due to the suitable flow-coalescence-wetting behaviors of the molten LMPA filler containing solid-state HMPA filler, which came into effect before excessive curing of the polymer composite.

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.

Ball Grid Array Interconnection Properties of Solderable Polymer–Solder Composites With Low-Melting-Point Alloy Fillers

In this work, a novel ball grid array (BGA) interconnection process has been developed using solderable polymer–solder composites (SPCs) with low-melting-point alloy (LMPA) fillers to enhance the processability of the conventional capillary underfill technique and to overcome the limitations of the no-flow underfill technique. To confirm the feasibility of the proposed technique, a BGA interconnection test was performed using four types of SPCs with a different LMPA concentration (from 0 to 5 vol %). After the BGA interconnection process, the interconnection characteristics, such as morphology of conduction path and electrical properties of the BGA assemblies, were inspected and compared. The results indicated that BGA assemblies using SPC without LMPA fillers showed weak conduction path formation, including open circuit (solder bump loss) or short circuit formation because of the expansion of air voids within the interconnection area due to the relatively high reflow peak temperature. Meanwhile, assemblies using SPC with 3 vol % LMPAs showed stable metallurgical interconnection formation and electrical resistance due to the relatively low-reflow peak temperature and favorable selective wetting behavior of molten LMPAs for the solder bumps and Cu metallizations.

Thermo-mechanical reliability of a multi-walled carbon nanotube-incorporated solderable isotropic conductive adhesive

Carbon nanotubes (CNTs) are considered as ideal candidates for the reinforcement of polymer composites due to their superior physical properties. In this paper, in order to investigate the influence of multi-walled carbon nanotubes (MWCNTs) on the reliability properties of solderable isotropic conductive adhesives (SICAs) with a low-melting-point alloy (LMPA), two types of SICAs (with 0.03wt.% MWCNTs and without MWCNT) were formulated. Thermal shock (−55 to 125°C, 1000cycles) and high-temperature and high-humidity (85°C, 85% RH, 1000h) tests were conducted on these samples. The SICA assemblies with and without MWCNTs showed stable electrical reliability properties during reliability testing; this stability was due to the formation of excellent metallurgical interconnection between corresponding metallization by the molten LMPA fillers. Although the mechanical pull strength of SICA assemblies decreased after thermal aging, due to the excessive layer growth and planarization of the IMCs, the SICA with MWCNTs showed enhanced mechanical reliability properties compared with the SICA samples without MWCNTs. This improvement in performance was caused by the enhancement effect of the MWCNTs. These results demonstrate that MWCNTs within SICAs can enhance the reliability properties of SICA joints due to their outstanding physical properties.

Through-hole filling characteristics of solderable polymer composites with low melting point alloy fillers

In this paper, we describe the development of a novel through-hole filling technique using solderable polymer composites (SPCs) with low-melting-point-alloy (LMPA) fillers. Three types of SPCs with different LMPA concentrations (40, 50 and 60 vol%) were formulated, and through-hole filling tests were conducted under different environmental reflow conditions (atmospheric pressure and decompression conditions). The results indicated that the through-hole filling assemblies showed weak filling characteristics with huge defects under atmospheric pressure reflow conditions because polymer trapped in the hole. On the other hand, the through-hole filling assemblies fabricated under decompression reflow conditions showed favorable filling characteristics and excellent, stable electrical properties due to the effective removal of polymer from the through-hole.

Influence of multi-walled carbon nanotube (MWNT) concentration on the interconnection properties of solderable anisotropic conductive adhesives (SACAs)

In this work, a novel CNT-filled solderable anisotropic conductive adhesive (SACA) containing a low melting-point alloy (LMPA) and multi-walled carbon nanotubes (MWNTs) was proposed. To evaluate the influence of MWNT concentration on the interconnection properties of SACA joints, six types of CNT-filled SACAs with different chemically functionalized MWNT content (from 0 to 2 wt%) were formulated. At a low MWNT concentration, a stable conduction path was formed due to the excellent coalescence and wetting behavior of the molten LMPA filler, and the electrical and mechanical properties of the CNT-filled SACA joints were also enhanced due to the reinforcing effect of MWNTs. However, the conduction path area decreased with increasing MWNT content due to increased polymer viscosity and diminished molten LMPA mobility, which attenuated the bonding properties.

An investigation of the reliability of solderable ICA with low-melting-point alloy (LMPA) filler

Conductive adhesives play a major role in the electronic packaging industry as an alternative to solder due to their potential advantages that include mild processing conditions and superior thermo-mechanical performance. In a conductive adhesive interconnection, adequate mechanical and electrical performance and long-term reliability are critical.In this paper, the reliability of solderable isotropic conductive adhesive (ICA) interconnections was investigated. Reliability testing was performed via thermal shock (−55 to 125°C, 1000 cycles) and high-temperature and high-humidity tests (85°C, 85% RH, 1000h). The interfacial microstructure of the solderable ICA was also investigated. Additionally, the fracture mode was investigated via mechanical pull strength testing before and after the reliability test. The electrical resistance of the solderable ICA interconnection showed improved stability compared to conventional ICAs, and similar stability to conventional solder paste (Sn–3Ag–0.5Cu and Sn–58Bi) due to the metallurgical interconnection formed by the molten LMPA fillers between the corresponding metallization layers. After the reliability tests, the grown IMC layer was composed of Cu6Sn5 (η-phase) and Cu3Sn (ε-phase), and the scallop-type IMC transformed into a layer-type IMC. The fracture propagated along the Cu–Sn IMC/SnBi interface and the fracture surface showed a semi-brittle fracture mode mixed with cleavage and ductile tear bands.

Reliability Properties of Solderable Conductive Adhesives with Low-Melting-Point Alloy Fillers

A solderable conductive adhesive (SCA) using low-melting-point alloy (LMPA) filler was developed to overcome the limitations of conventional electrically conductive adhesives (ECAs), which include their low electrical conductivity, increased contact resistance, and low joint strength. The SCA formed good metallurgical conduction path between the corresponding electrodes due to the rheology-coalescence-wetting behaviors of molten LMPA fillers in SCA. This study examined the reliability of SCA assembly through the thermal shock test (218 to 398 K, 1000 cycles) and the high temperature and high humidity test (358 K/85%RH, 1000 h). The electrical resistance of the SCA assembly with metallurgical interconnections was much more stable than those with conventional ICAs. Before the reliability tests, a scallop-type Cu6Sn5 (η-phase) intermetallic compound (IMC) layer was formed on the Sn-plated Cu lead/LMPA and LMPA/Cu metallization interface based on the results of interfacial microstructure observations of quad flat packages (QFPs) that were assembled with SCA. After the reliability tests, the thickness of IMC layer increased with time, and Cu6Sn5 (η-phase) and Cu3Sn (ε-phase) were formed. In addition, the fracture surface exhibited a cleavage fracture mode with the fracture propagating along the Cu–Sn IMC/Sn–Bi interface. These results demonstrate that SCA assembly with metallurgical interconnection has stable electrical and mechanical bonding characteristics.

Effects of novel carboxylic acid-based reductants on the wetting characteristics of anisotropic conductive adhesive with low melting point alloy filler

A low viscosity epoxy resin with carboxylic acid-based novel reductants was introduced to improve the process ability and reliability of an anisotropic conductive adhesive (ACA) resin with a low melting point alloy (LMPA) filler system. The curing degree of the ACA resin and the melting of the LMPA filler were investigated using differential scanning calorimetry (DSC) conducted in the dynamic mode in order to control the curing conditions such as the reaction temperature and a melting temperature of solder. The temperature-dependent viscosity characteristics of the ACA resin were investigated using a rheometer. The compatibility between the viscosity of a polymer matrix and a melting temperature of solder was characterized to optimize the processing cycle. Three different types of carboxyl acid-based reductants were added to remove the oxide layer on the surfaces of the filler particles and the conductive pad. Good wetting properties were achieved between the LMPA filler and the Cu pad in the epoxy resin using these carboxylic acid-based reductants.

Numerical Analysis of Coalescence Characteristics of Low Melting Point Alloy Fillers Using a Non-Equilibrium Phase Field Model

A non-equilibrium phase field model (NPFM) is proposed to examine the three-dimensional coalescence characteristics of low melting point alloy (LMPA) fillers, widely used for self-organized interconnection applications. This model sufficiently considers the non-equilibrium process during phase transitions, resulting in accurate prediction of interfaces between two different phases with a high density ratio. The preliminary simulation is carried out for validation of the present model and numerical predictions are in good agreement with analytical solutions for a one-dimensional solidification problem. The proposed NPFM successfully predicts the phase transition, the heat transfer from solid to liquid, and the coalescence behaviors of LMPA fillers, whereas the conventional enthalpy method fails to describe the non-equilibrium phase transition. Moreover, the results show that there exists grid dependency in the mush zone, suggesting that special care should be taken of the mush zone for better prediction of interfaces between different phases.

Self-Organized Interconnection Process Using Solderable ACA (Anisotropic Conductive Adhesive)

In this paper, we discuss our design of a new class of low-melting-point alloy (LMPA)-filled anisotropically conductive adhesives (ACA) and a self-organized interconnection process for using these adhesives, and we demonstrate a good electrical conduction for the process. Flow, melting, coalescence, and wetting characteristics of LMPA fillers in the ACA facilitate this process. In order to exploit good coalescence and wetting characteristics of LMPA fillers, the polymer matrix has a sufficient fluxing capability against oxide films of both LMPA fillers and electrode materials. Furthermore, it is essential that the polymer have a low viscosity level around the melting point of the incorporated LMPA to achieve a good electrical conduction path. In order to study coalescence and wetting characteristics, we formulated two types of ACA with different volume fractions of LMPA filler (10 vol% and 40 vol%). Our test board had an 18-mm thick Cu line-type pattern (10 mm � 0:1 mm) and an area array-type pattern (square type: 0:1 mm � 0:1 mm, circle type: ’, 0.1 mm) with five different pitches (50 mm to 250 mm). We measured thermal properties of the ACA by differential scanning calorimetry (DSC), and we determined a temperature profile for the interconnection process. We then monitored coalescence and wetting characteristics of LMPA fillers and morphology of conduction path in ACA by using a microfocusing X-ray inspection system and an optical microscope. We determined that the developed LMPA-filled ACA have good coalescence and wetting characteristics regardless of pattern types. In addition, we were able to achieve a good electrical conduction path because of the coalescence and wetting characteristics of the LMPA fillers in the ACA. [doi:10.2320/matertrans.MF200918]

Hybrid Interconnection Process Using Solderable ICAs (Isotropic Conductive Adhesives) with Low-Melting-Point Alloy Fillers

In order to overcome several crucial limitations to the traditional electrically conductive adhesive bonding used in the electronics field, a novel low-melting-point alloy (LMPA)-filled ICA (Isotropic Conductive Adhesive) was synthesized. Also, a hybrid interconnection process using it was developed, which can be achieved by melting-coalescence-wetting behavior of LMPA fillers in ICA. In order to ensure excellent coalescence and wetting characteristics of the LMPA filler particles, the fluxing capability of the resin should have been sufficient to remove the oxide layer on the surfaces of the filler particles and the electrode materials. The effect of reductant on the hybrid interconnection formation was examined. ICAs with and without reductant were formulated with a 40% volume fraction of filler. The QFP chip had a size of 14 � 14 � 2:7 mm and a 1 mm lead pitch. The test board had a 18 mm thick Cu daisy-chained pattern. Thermal characteristic of the ICA was observed by differential scanning calorimetry (DSC), and the temperature-dependant viscosity characteristic of the polymer matrix was observed by torsional parallel rheometer. Based on the results, the reflow profile of the interconnection process was determined. The dipping interconnection method was applied in the QFP bonding process. After the QFP bonding process, the electrical characteristic of ICA was measured with a multimeter. The wetting, bondability, coalescence characteristics of the LMPA filler particles and the morphology of the conduction path were observed by microfocus X-ray inspection systems and optical microscopy. As a result, the LMPA-filled ICAs with reductant had stable contact resistance, forming the metallurgical interconnection. A good electrical conduction path was achieved with the coalescence and wetting characteristics of the molten solder particles in the ICA. [doi:10.2320/matertrans.M2009109]

Numerical Investigation on Self-Organized Interconnection Using Anisotropic Conductive Adhesive with Low Melting Point Alloy Filler

The present study is devoted to develop a new simulation code for the numerical investigation on coalescence characteristics in self-organized interconnection using anisotropic conductive adhesive (ACA) with low melting point alloy (LMPA) fillers. In particular, this article examines the coalescence mechanism during ACA process for different resin viscosity, particle volume fractions, and provides estimated results of conduction path growth fraction for different conditions. Starting from the motion equation of filler particles, moreover, a theoretical approach is presented to find out dominant factors affecting the formation of interconnection. The new simulation code is based on the finite volume method with the use of the continuum surface force (CSF) model and the cubic interpolated propagation (CIP) method. It is confirmed that conduction path formation depends on volume fraction, viscosity, and surface tension during the process. It is also found that the pressure-driven force effect becomes not dominant in the path formation at high resin viscosity and high filler density, the particle drift velocity becomes slow due to the increase in resin viscosity, conduction path growth fraction increases with the volume fraction, and the time for path formation is shorter with the increase of the volume fraction.