Characterization and Analysis of High Efficiency Aqueous-Based Flux Cleaning Process for High Voltage Power Device Packages

In the world of power electronics, products with a large die-to-package ratio are sought-after since customers receive the maximum product performance within the package rating. The complexity of assembling large dies and clips with solder in high-voltage products increases the risk of Electrochemical Migration (ECM). Organometallic flux residue is hazardous and can enter thru the passivation into silicon dioxide if they are not cleaned. Thus, this phenomenon causes leakage current in power devices. The ion gains mobility when moisture in the delaminated interface is energized by potential differences and temperature. The failure duration of a product becomes short when the pitch between two terminals reaches the design limit, which is common among high-voltage packages. Even though there are alternative cleaning applications, aqueous-based cleaning is by far the preferred enabler with economies of scale. This research aims to investigate 3 commercial aqueous-based flux cleaning chemistry, WB-1, WB-2, WB-3. The chemical were used to clean 8 x 8mm clipbonded package containing no clean flux solder system. Cleaning performance of each chemical were evaluated by visually inspecting clip attached product after flux test. The residue level on cleaned product is studied using Fourier transmission infrared (FTIR) spectroscopy and Energy Dispersive X- ray (EDX). The chelating behaviour were analyzed using Inductively Coupled Plasma Mass Spectrometry (ICPMS). In this work, final product were used to examine package delamination and reliability using Highly accelerated Stress Test (HAST). Visual inspection and EDX shows WB-1 passed both resin and flux test, while WB-2 have high organic residue trapped at locking feature of the leadframes. Phosporus rich dendrite is detected on sample cleaned with WB-3. FTIR shows WB-2 have spectra matching to flux activator used in solder at intricate location of clip leadframe. Cummulatively, WB-1 have lowest level of introductory ions from incoming and highest amount of ion extracted after flux cleaning. High content of Potassium detected on incoming chemical of WB-2 susceptible to metal cross contamination to package. HAST test shows both WB1 and WB-3 passed 192 hours, while WB-2 had premature leakage fails after preconditioning MSL. Overall samples exhibit good delamination after preconditioning, minor delam on WB-2. Incoming impurity level of the chemical is a critical factor in selecting a suitable chemical for high-voltage power device application. Type of chelating ligand with high complexation formation constant with electronic packaging metal is key for metal ion removal. For an efficient removal of residue, chemistry of Flux cleaning should cater 3 main function; Dissoluting, chelating and passivating. Other additives such as surface activation compound, dispersion and wetting agent to enhance overall cleaning function. Within this study, it is observed that the power devices are more reliable in chemical with this features. This work enables new chemical characterization on package and define requirements for high-voltage power device.