Gordon Elger

Inline thermal transient testing of high power LED modules for solder joint quality control

Heat management and reliability is essential for high power LED packages, e.g. for high temperature application like automotive lightning or application with very long lifetimes like street lightning. To reduce the LED junction temperature a low thermal resistance is realized by mounting the LED packages on heat spreaders or boards with good heat conduction. The joint between package and heat spreader, very often a solder joint due to the good thermal conductivity of the solder material, need to be void and gap free to achieve a good heat conduction and high reliability. The quality of solder joints of LED packages is usually controlled in production by X-ray and acoustic microscopy (CSAM). From a good solder joint, i.e. detection of no bad soldered area, a good thermal performance is concluded. The Thermal Transient Testing provides a method to measure the thermal resistance by measuring the forward voltage Vf(t) time dependent after a thermal power step, i.e. switching the drive current from high drive to low drive current. However, the k-factor, the linear dependence between Vf(t) of the LED and the real thermal power step needs to be measured to obtain the correct thermal resistance. We have developed an algorithm to enable inline thermal transient testing of LED modules without the need to measure the k-factor and the thermal power step. Instead of calculating the structural response function from the transient forward voltage, we evaluate the forward voltage in the time domain. For the development of the method we have set up a finite element (FE) model for our LED packages and performed transient thermal simulation. The FE model was fitted to the experimental data. We simulate the influence of void sizes and positions, gaps and joint thickness on the transient temperature curves. By comparing the measured sample with a known good sample we can evaluate the quality of the solder joint and calculate the thermal resistance. We apply the measurement method for quality control of the solder joints of our high power LED packages. The measurement method targets to replace X-ray or CSAM inspection within production. We compared CSAM inspection with the thermal resistance measurements. Thermal resistance and non soldered area are correlated for larger bad soldered areas. We achieved a detection limit of roughly 30% of bad soldered area.

High power LED subassemblies for automotive front light application

A high power LED platform for automotive front lighting is described. Based on small footprint ceramic LED packages customized designs can be realized in a highly flexible way. The diverse automotive applications have different requirements in terms of optical performance, i.e. assembly tolerance and package density, but also regarding the thermal performance of the boards, which are used as substrates for the LEDs. For a future design of a low beam function with a distributed multi cavity reflector concept the optical tolerances are evaluated which are required to form a beam which provides the sharp cut-off line needed to illuminate the road without glaring the oncoming traffic. For such optical concept an overall positioning tolerance below 100μm is required. The tolerances of the LED assembly process on advanced PCB solutions, including reflow soldering, are investigated and compared with alternative assembly solutions. Additionally, the thermal performance of the LED subassemblies is investigated by T3Ster measurements and finite element simulations. The advantage of special Cu-IMS is demonstrated compared to Fr4 based and Al-IMS solutions.

Solder Process for Fluxless Solder Paste Applications

Fluxless soldering, i.e. residue free soldering with the aid of gaseous activation is known for many years, but only well established in the field of opto-and microwave electronics. In low cost high volume applications this technology has not yet become mainstream. In the fluxless soldering the wetting of the solder is made possible by means of an activating process gas. After the soldering process, no cleaning process is necessary because no corrosive residues are left on the circuit boards and components. Therefore soldering using solder paste without aggressive chemical ingredients has a high market potential. Expensive preforms could be replaced by paste dispensing or paste printing. In this paper, a residue free SnAgCu solder paste and a suitable soldering process is developed and presented. It is applied on high power ceramic LED packages and unpackaged flip chip LEDs which can be assembled directly on a substrate. In this paper the main paste properties such as printability of a commercial flux solder paste with those of the fluxless solder paste are compared. Likewise, the soldering results after a reflow process of these different paste systems are evaluated and compared. Different solder joint analysis methods like X-Ray, the transient thermal analysis, cross section and shear strength tests are presented in this paper. Finally a measurement technique to detect contamination on a soldered module is shown. There is a possibility that the concept and process proposed and demonstrated in this paper can reduce the production costs of electronic systems and improve the quality of soldered electronic modules.

LED Matrix light source for adaptive driving beam applications

An adaptive driving beam consists out of many LED segments which are switched on and off or dimmed to adjust the beam to the specific driving situation, e.g. in case of oncoming traffic to avoid glaring. LED Matrix light sources have a great advantage for automotive adaptive front lighting application: No mechanical parts are required to shape the beam. They have the potential to spread from high end car platforms to the high volume cars within the next years but compact and cost saving solutions are prerequisite to make this development happen. In the paper we evaluate different architectures of LED light engines for adaptive driving beams. We discuss optical concepts to realize the required angular resolutions for the matrix segments. We present the beam performance of a solution using a matrix of 3×11 ceramic LED packages assembled on one board with one near die collimator as primary optic and a projection lens as secondary optic. An angular resolution of 1.5° is achieved for the individual addressable segments of the high beam on the street. Due to the dense LED array the electrical routing is performed on two layers to realize individual addressability of the LEDs. The potential board technologies, i.e. double layer IMS boards or advanced Fr4 boards with copper inlay, are discussed for this kind of LED matrix application. The thermal performance is evaluated by FE simulation. The solution realized in our application is a special IMS with two electrical layers and vias through the metal core. In dependence of the required beam up to 50W thermal load is dissipated.

Thermal analysis of high power LED subassemblies for automotive front light application

High power LEDs for front lighting have been introduced on the market during 2007 and cost effective solutions are still challenging due to the high operating temperature, long lifetime and harsh environmental condition. We present LED subassemblies based on ceramic LED packages which provide up to 250lum per package. The subassemblies are LED matrices designed for adaptive front lighting and modern light guide based day running light / turn indicator combination. The LED package has two electrical and one additional thermal pad. An Rth of 4°C/W / Rth_real=5°C/W is measured for the LED package by applying the JESD51-14 standard using special IMS boards for controlled variation of the thermal interface. Different board materials, i.e. Fr4, Al-IMS, Cu-IMS and special Cu-IMS, are evaluated. The advantages of special copper IMS with direct thermal contact between the LED package and the metal core are demonstrated.

New Method to Separate Failure Modes by Transient Thermal Analysis of High Power LEDs

A high reliability of light emitting diode (LED) light sources is essential for general and automotive lighting applications, where exchange of LED components is expensive. Thermal management of modern high power LEDs is crucial for their lifetime. An important aspect is the thermal path for heat conduction. Many different defects can have an influence on this path of an electronic system: on the one hand process failures during production, e.g. voids inside the solder joint, on the other hand typical failures induced by thermo-mechanical stress during their lifetime, like cracks in the solder joint or delamination in the package. The transient thermal analysis (TTA) is a powerful tool to detect changes in the thermal path. Due to improvements in the TTA method during the last years, not only cracks can be detected but also failure modes can be separated, and the root cause can be analyzed by support of transient finite element analysis. In this paper, transient thermal testing is applied and further developed, to monitor the structural integrity of new wafer level LED packages during thermal stress testing. Failure modes are defined and separated. For failure analysis the different defects are simulated by transient finite element analysis and correlated to the TTA results. The simulation results, that solder cracks increase the peak height of the derivative of the transient thermal curves (b(z)). A delamination of an inner layer of the LED package creates additionally to the increase of the peak height also a separation of the b(z) curves between 1 µs and 5 µs. Therefore a transient thermal measurement equipment with a dead time

High yield thermode bond process under activated atmosphere and inline thermal interface measurement for high power optoelectronic devices

The paper reports two equipment development projects in the field of die bonding and die bond testing. Part one deals with the a new confinement system for flux free die bond processes under formic acid enriched process gas on thermode bonders. Part two describes a new inline test equipment for controlling the solder joints by applying the transient temperature test method under inline condition.