Research And Development Of High-performance Solder Pastes For Soldering Al And Its Alloys In LED Chip Module Assembling

With the rapid development of electronics industries and LED-based lighting industry, there has been an increasing trend of using aluminum and its alloys as substitute for Cu due to their obvious advantages such as low density, high strength-to-weight ratio, high thermal and electrical conductivity, and most importantly, being cost-effective. However, it is well-known that soldering of aluminum and its alloys has long been difficult, in particular when using soldering pastes to assemble LED lighting components made of aluminum and/or its alloys. A practical aluminum soldering paste should possess desirable processing properties such as excellent printing performance, good solderability with less residue, and good storability. It should be pointed out that the designing and synthesizing of aluminum soldering pastes are much more demanding than that used for soldering Cu substrate(pad) or other under-bumpmetallization(UBM) in conventional electronic packages due to the strong corrosivity of the flux in aluminum soldering pastes, which reacts strongly with alloy powders and leads to some serious problems, such as drying out of the pastes and weakening of the paste’s activity. Nevertheless, in recent years there has been an increasing demand for aluminum soldering pastes used for automated assembly of LED lighting components. Thus, it is imperative to develop appropriate soldering pastes for soldering aluminum and its alloys, which are widely employed in LED lighting industry as heat sinks, supporting frames of LED modules and so forth.In the present study, single component or multicomponent orthogonal experiment were design for different components of activator, solvents, surfactants, thixotropic agent and other additives which were added in flux, in order to make qualitative and quantitative research for each type of aluminum solder paste flux component and improve the formula. After the composition of alloy powder and flux were determined, the solder and flux ratio was adjusted to find out the best proportion of aluminum solder paste. Then, aluminum solder pastes were evaluated base on experimental results of solderability and mechanical properties of solder joints. Finally, two aluminum solder pastes have been developed which can be applied to slow heating process and rapid heating process respectively.Results show that the compound of organic amine and inorganic acid is a good active matrix, which makes the compact oxide film on aluminum surface cracking(or chapping) during the soldering process. The addition of the active salt can make aluminum oxide film further cracking and strip it from aluminum surface, leading to improvement of the solderability of solder pastes. The spreading area of solder joints and unstable time of solder pastes decrease with decreasing the solder powder particle size. Soldering heating rate has a significant influence on the solderability of solder joints. There is a substantial enhancement of the spreading area when the soldering heating rate is increased from 1.5°C/s to 20.0°C/s. And there is significant influence of the solder joint height on tensile strength of joints. The tensile strength is decreased due to the existence of a large number of voids when the height of solder joints is 0.5 mm.