October, 2017


www.us-tech.com Die Attach Voiding in LED Assemblies Continued from previous page


of the material and Acontact is the total area of contact between the LED and


the substrate. This area, A, is given by:


A is the total area between the


LED and the substrate and Avoid is the total voiding area with no ther-


mal contact between the LED and the substrate. The temperature dif- ference, DT, between the LED and the substrate can be calculated as:


where F is the heat flow measured in Watts. This relationship between the


LED junction temperature, Tj, and the substrate temperature, Ts, is shown in Figure 2. Thus, the LED junction tem- perature is represented by:


have on the thermal conductivity of the die attach. As seen in Figure 3, the void acts as a thermal barrier. Heat generated by the LED cannot easily reach the thermal sink (sub- strate). The consequence of having a diminished thermal path is an increase in operating temperature.


The Experiment The team inspected a total of


1,000 LEDs and measured the die attach voiding of each of the devices. A TruView Fusion X X-ray inspection system was used. This system is capable of inspecting boards up to 20 x 48 in. (51 x 122 cm), collects data and processes the void measure-


Figure 2: temperature distribution in the LED assembly, from substrate to junction.


ments automatically. This machine allowed the team to tackle the daunt- ing task of inspecting so many LEDs. The large number of devices


inspected lead to a normal distribu- tion of the data with a mean void area of 49.7 percent and a standard deviation of 9.8. The Creative Electron team


pressed on to determine the real cost of voids. Would the distribution they found be appropriate to the cus- tomer’s pricing model? And was the customer charging enough for the extended warranty? Based on the results from the


1,000 LED sample, the cost of the die attach voids could be determined. The next step in the analysis was to use the LED manufacturer’s data on the lifespan of the LED as a function


Continued on next page


Page 71


This shows that the tempera-


ture of operation of the LED is going to be higher than the substrate tem- perature by DT. This is an important equation, as it allows us to determine the junction temperature as:


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This equation also shows us the


critical impact that voids have on the operating temperature of LEDs. All parameters in this equation are con-


stants, except for the void area, Avoid. The heat flow, F, is a function of the power being dissipated by the LED, which in turn is determined by the current and voltage. The thickness, x, is fixed, as is


the total area between the LED and the substrate, A. The thermal con- ductivity of the material, k, is also a constant. For this analysis, we con- sider the temperature of the sub-


strate, Ts, to be a constant as well. This allows us to further simplify this equation as:


This shows us that the operating


temperature, Tj, of the LED grows exponentially with the void area, Avoid. For this reason, it is critical to keep


voiding area to a minimum. X-Ray Inspection The first step in this research


was to understand the quality parameters of the customer’s produc- tion line. The need for a benchmark was critical, so that the Creative Electron team could understand what was working and what needed to be improved. The key metric the team meas-


ured was the void area between the LED die and the substrate. This was selected because it could be meas- ured in a non-destructive way, using an X-ray inspection system. The size and shape of the void, combined with its location in the luminaire, also provided insight into the type of manufacturing issue that the cus- tomer had. Further analysis was done to better visualize the impact that voids


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