One of largest obstacles to the widespread utilization of high brightness light emitting diodes (LED) for general illumination is thermal management. Localized heat generation is characteristic of the semiconductors utilized in these devices. For maximum lumen output, color consistency and device lifetime, this heat must be removed efficiently to allow the LED junction temperature (TJ) to remain stable. Maintaining a stable junction temperature over long periods of operation is the main goal of LED thermal management.
“You can’t manage what you don’t measure: Each 1% increase in void area can decrease the lifespan of an LED by up to 2,000 hours”
The rate at which an LED module will age is highly dependent on the temperature at the p-n junction and will accelerate over time. As a result, service life values are only valid provided that certain p-n junction temperatures (Tj) are not exceeded. Service life values are statistical values determined during test runs undertaken by LED manufacturers and do not reflect the precise behavior of individual LEDs.
The same applies to LED brightness values. The heat generated at the p-n junction impairs the efficiency of the light generation process and results in a measurable drop in brightness.
To better illustrate the economics of LED die attach voiding, we analyzed a real life scenario. One of our customers is a large contract manufacturer new to the LED luminaires market. They offer two warranty options to their customers: a standard 38,000-hour warranty for free or an extended 50,000-hour warranty for extra cost. It is understood that a single LED failure results in a luminaire fail.
For this analysis we will assume the following pricing model:
– Price of a single luminaire: $100
– Price of 50,000-hour extended warranty: $20
– Production and distribution costs: $35
– Warranty cost: $5
The pricing model is based on the expectation that within the standard warranty period the company will receive up to 5% in returns, and up to 20% in returns within the extended warranty period. This LED manufacturer wanted to better understand if the amount allocated to warranty costs was enough. As an entrant in the LED market they understand the sensitivity of price competitiveness. They can currently fabricate LED luminaires at a reasonable profit margin, but fear that these margins will be destroyed by return of product within warranty.
100% X-Ray Inspection
The first step in this research was to understand the quality parameters of the customer’s production line. The need to set a benchmark is critical in this analysis so that we can understand what is working and what needs to be improved. The key metric we measured was the void area between the LED die and the substrate. We selected void area as a key metric because it can be measured in a non-destructive way by using an x-ray inspection system. Furthermore, the size and shape of void, combined with its location in the luminaire, provided us with important insight on the type of manufacturing issue we may be dealing with.
For this analysis we inspected a total of 1,000 LEDs and measured the die attach voiding of each of these devices. To accomplish this task we utilized a TruView 200 X-Ray inspection system, as seen in Figure 4. This is an x-ray inspection system capable of collecting the data and processing the void measurements automatically. The automated process was paramount to the processing of the daunting task of inspecting 1,000 LEDs.
The results of this analysis showed that this customer far underestimated the expected cost of return-products. Not only the pricing model utilized by the company had a deficit within the standard warranty; it had an even larger deficit within the extended warranty period. In consequence, the company had to temporarily change its pricing structure to compensate for product shipped with the deficient manufacturing process. Furthermore, a complete overhaul of the manufacturing process was done to bring it to tighter standards. The void area is now utilized as the key metric to measure process control quality. The TruView 200 X-Ray is used daily to measure the void of sample luminaires. Current data shows that the mean void area dropped to 35% with a standard deviation of 4.5. Thus, they were able to reduce the pricing for their luminaries. This reduction greatly increased their competitiveness in the market.
The Complete White Paper
Please feel free to download this white paper for the complete analysis here.