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Soldering is a dominating process for semiconductor packaging. For electronic manufacturing tin based solders play a key role. The surface of most solder alloys is oxidized under an oxygen containing atmosphere. Usually reducing chemicals, called fluxing agents, are used to enable the formation of solder contacts. However, standard liquid flux leaves aggressive residues on the electronic devices. Clean processes were developed using gaseous flux, i.e. formic acid vapor. Despite the competitiveness of the reducing effect of formic acid vapour on many solder alloys, only little is known about the corresponding reaction mechanism, especially at the surface. An oxidized copper powder and a tin silver copper alloy were investigated using thermogravimetric and mass spectra analysis under formic acid flow. Details on the adsorbed and desorbed species and the formation of intermediates and decomposition products are presented. Activation temperatures are estimated and correlated with heating processes

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Practices of IC package reliability testing are reviewed briefly, and the application of transient thermal analysis is examined in great depth. For the design of light sources based on light emitting diode (LED) efficient and accurate reliability testing is required to realize the potential lifetimes of 10 5 h. Transient thermal analysis is a standard method to determine the transient thermal impedance of semiconductor devices, e.g. power electronics and LEDs. The temperature of the semiconductor junctions is assessed by time-resolved measurement of their forward voltage (V f). The thermal path in the IC package is resolved by the transient technique in the time domain. This enables analyzing the structural integrity of the semiconductor package. However, to evaluate thermal resistance, one must also measure the dissipated energy of the device (i.e., the thermal load) and the k-factor. This is time consuming, and measurement errors reduce the accuracy. To overcome these limitations, an innovative approach, the relative thermal resistance method, was developed to reduce the measurement effort, increase accuracy and enable automatic data evaluation. This new way of evaluating data simplifies the thermal transient analysis by eliminating measurement of the k-factor and thermal load, i.e. measurement of the lumen flux for LEDs, by normalizing the transient V f data. This is especially advantageous for reliability testing where changes in the thermal path, like cracks and delaminations, can be determined without measuring the k-factor and thermal load. Different failure modes can be separated in the time domain. The sensitivity of the method is demonstrated by its application to highpower white InGaN LEDs. For detailed analysis and identification of the failure mode of the LED packages, the transient signals are simulated by time-resolved finite element (FE) simulations. Using the new approach, the transient thermal analysis is enhanced to a powerful tool for reliability investigation of semiconductor packages in accelerated lifetime tests and for inline inspection. This enables automatic data analysis of the transient thermal data required for processing a large amount of data in production and reliability testing. Based on the method, the integrity of LED packages can be tested by inline, outgoing inspection and the lifetime prediction of the products is improved.