Design Considerations That Influence LED Solder Joint Reliability

Michael Blattau, M.S.
Ansys
MD, USA

LEDs are the dominant lighting solution for many automotive applications. Their efficiency and increased design flexibility make them ideal for electrical vehicles over traditional incandescent lights. Designers can select a variety of substrates when designing an LED printed circuit board and many times designers only consider thermal performance. An insulated metal substrate or IMS board is commonly chosen as they are the highest thermally conductive type of circuit board.

For maximum thermal conductivity more copper, thinner dielectric and maximum pad attachment is commonly chosen. Solder fatigue is often not considered when selecting the IMS dielectric material. The design parameters such as copper thickness, dielectric thickness, and pad routing can all influence solder fatigue. In this paper we examine the influence of three factors on thermal-mechanical solder joint reliability of LEDs mounted on an IMS PCB using FEA.

The results show that using soft dielectric, thinner copper, and routed pads increase solder fatigue reliability. However, these parameters could influence the thermal performance of the board resulting in the LEDs operating at a higher temperature. A FEA conduction analysis was done to predict the potential temperature rise between the various pad routing options with the results shown in Figure 12.

Figure 12 illustrates that a 4°C temperature rise can be expected between a routed attachment versus a direct pad attachment. Designers need to consider this small temperature increase and the impact it could have on LED life. However, for solder joint fatigue, a temperature increase of 4°C changes the predicted life of the joint for the routed pad layout from 1710 cycles to 1555 cycles which is still 2X greater than the predicted cycles to failure for the plane configuration of 735 cycles.