Thermo-mechanical reliability is one major issue in solid-state lighting. Mismatches in the coefficients of thermal expansion (CTE) between high-power LED packages and substrates paired with temperature changes induce mechanical stress. This leads to a thermal degradation of LED modules by crack formation in the solder interconnect and/or delamination in the substrate, which in turn increases junction temperature and thus decreases light output and reduces lifetime. To investigate degradation and understand influence of LED package design and solder material, a reliability study with a total of 1800 samples − segmented in nine LED types and five solder pastes − is performed. First of all, in this paper a state-of-the-art review of high-power LED packages is performed by analyzing and categorizing the packaging technologies. Second, the quality inspection after assembly is realized by transient thermal analysis (TTA), scanning acoustic microscopy (SAM) and X-ray. For TTA, a new method is introduced to separate the thermal resistance of the LED package from solder interconnect and substrate by applying the transient dual interface method (TDI) on samples with different solder interconnect void ratios. Further measurement effort is not required. The datasheet values for thermal resistance are verified and the different LED package types are benchmarked. The void ratio of the solder interconnects is determined by X-ray inspection combined with an algorithm to suppress disruptive internal LED package structures. TTA and TDI revealed that initial thermal performance is independent of solder paste type and that voiding is more critical to smaller LED packages. In addition, lower silver proportion in the paste is found to increase voiding. SAM is less sensitive for initial void detection than X-ray, but it’s applied to monitor crack propagation while aging in combination with TTA. The results of the reliability study, i.e., the crack growth under temperature shock test for the different SAC solders, will be presented in a second independent paper.

Thermo-mechanical reliability is one major challenge in solid-state lighting for automotive applications. Mismatches in the coefficients of thermal expansion (CTE) between high-power LED packages and substrates paired with temperature changes induce mechanical stress. This leads to thermal degradation by crack formation in the solder interconnect and/or delamination in the substrate, which in turn increases junction temperature, thus decreasing light output and reducing the lifetime. A reliability study with a total of 1,800 samples -segmented in nine LED types and five solder pastes -is performed to investigate degradation and understand the influence of solder material and LED package design. The results are presented in two papers. Initial characterization of the LEDs was handled in the first paper. This second paper focuses on degradation and lifetime. Overall, more than 40,000 transient thermal analysis (TTA) and 9,000 scanning acoustic microscopy (SAM) measurements were taken to evaluate degradation during accelerated aging of 1,500 thermal shock cycles. Six different failure modes were observed, which were distinguishable by only using TTA data. For the reliability evaluation, crack ratio was determined by SAM images while thermal degradation as well as mean lifetime were determined using TTA data. Multiple observations were made within this study. First: SAM and TTA data correlated very well; Second: Higher silver content and additives in the solder paste reduce crack growth and increases lifetime; Third: Thick film ceramic LEDs reach significant longer lifetimes than thin film ceramic LEDs, and copper lead-frame LEDs reached by far the longest lifetimes; Fourth: A pad design with a greater pad size, smaller gaps and balanced size ratio between electrical and thermal pad is advantageous; Fifth: Voiding (below 10%) has no significant influence on the reliability.

This paper introduces a new measurement method for noise suppression in transient thermal analysis (TTA) which is used to measure the transient thermal impedance of power semiconductors and LEDs. An additional short pulse sequence is added to the standard TTA measurement procedure. The thermal response of this additional sequence is converted to the original single pulse response of standard TTA by superposition of previous pulses in the sequence. Applying this method, the noise before 1 ms can be significantly reduced without averaging over multiple measurement repetitions that would increase the measurement time accordingly. The noise reduction for times below 1 ms is important to resolve the thermal resistance layers close to the junction of a semiconductor. The measurement method and data analysis algorithm are developed for TTA measurements, but the principle is suitable for all linear time invariant (LTI) systems where the system response is measured over several time decades. The algorithm was evaluated on simulated and experimental data of a power LED. With an additional sequence of only 100 ms, a signal to noise ratio (SNR) improvement of 15.1 dB for simulative and 14.7 dB for experimental data was achieved. This is equivalent to averaging over 30 repetitions in the standard TTA procedure. Therefore, the new method achieves a reduction of measurement time of 97 % for applications where low noise levels are required for times below 1ms.

Residual-free eutectic Au80Sn20 soldering is still the dominant assembly technology for optoelectronic devices such as high-power lasers, LEDs, and photodiodes. Due to the high cost of gold, alternatives are desirable. This paper investigates the thermal performance of copper-based sintering for optoelectronic submodules on first and second level to obtain thermally efficient thin bondlines. Sintered interconnects obtained by a new particle-free copper ink, based on complexed copper salt, are compared with copper flake and silver nanoparticle sintered interconnects and benchmarked against AuSn solder interconnects. The copper ink is dispensed and predried at 130 °C to facilitate in situ generation of Cu nanoparticles by thermal decomposition of the metal salt before sintering. Submounts are then sintered at 275 °C for 15 min under nitrogen with 30 MPa pressure, forming uniform 2–5 µm copper layers achieving shear strengths above 31 MPa. Unpackaged LEDs are bonded on first level using the copper ink but applying only 10 MPa to avoid damaging the semiconductor dies. Thermal performance is evaluated via transient thermal analysis. Results show that copper ink interfaces approach the performance of thin AuSn joints and match silver interconnects at second level. However, at first level, AuSn and sintered interconnects of commercial silver and copper pastes remained superior due to the relative inhomogeneous thickness of the thin Cu copper layer after predrying, requiring higher bonding pressure to equalize surface inhomogeneities.