Two sets of tests were done on the experimental models. In the first set, the test samples were held vertically (leads down) in free air and powered by applying a constant current of 400 mA to each array. The four thermistors were measured by automated test equipment every three seconds for 12 minutes (until the temperature stabilized). At this point, the power was cut and the thermistors recorded for a further 2.5 minutes during the cool down phase.
In the second, an infrared (IR) camera recorded the heat emissions from the assembly during ramp-up and ramp-down so that the isotherms on the rear of each substrate type could be studied. For this test only, the heat sink was not attached.
Results and Conclusions
While a large amount of data was collected and analyzed, some general conclusions became apparent:
• ALN is by far the best substrate material of the three. IMS is second and Alumina third (See Figure 3).
• SAC 305 solder for die-attach is superior to both conductive epoxies used even though these were designated as “best in class” by their manufacturers and superior to solder.
• Although there are performance gains given by thermal greases as heat sink compounds over thermally-conductive epoxies, this is not as major a contributor to the thermal performance as the substrate or die attach selections.
• The data derived from the thermistor measurements and from the IR camera pictures shows that the transverse heat flow across ALN and IMS significantly spreads the heat more evenly across the substrate surface reducing the centrally located “hot-spot” seen on the Alumina substrate. (See Figure 4)
While the three substrate types tested have far superior thermal conductivities to glass/epoxy laminates, often used to interconnect LEDs, there are cost implications and these have to be weighed against the required light intensity output and thermal performance of any actual product.
This article was written by Andy Proudfoot, Vice President of Technology at TASK Micro-Electronics, Inc., in Montreal, Canada. For more information, visit http://info.hotims.com/40439-162.