LED Lighting: Part 2, First Level Connection Challenges in LEDs
Mechanical Engineers handle first level connection challenges
Mechanical engineers typically work with two types of connections when assembling electronic packages. These levels, known as first and second level connections, present mechanical engineering teams with different sets of challenges. The following blog will cover first level connection challenges and how they pertain to Light Emitting Diodes (LEDs).
In first level connections, mechanical engineers’ main responsibility is connecting a chip to an intermediate package, also known as a substrate. Due to the complexity of this process, many companies devote large resources to fund internal research and develop improved methodologies. The chip itself is fragile, especially an LED die, which may contain thin layers of epitaxial films about 20mm thick.
Mechanical engineers connect these fragile chips to usable carriers such as an fr-4 substrate, similar to high performance microprocessor packaging. When packaging engineers require high field reliability and/or minimized thermal mismatches for high power LEDs, ceramic substrates are used. Frequently, mechanical engineers use an intermediate carrier or submounts to ease wafer handling. Mechanical engineers do this prior to connecting devices to lead frames if the active layers are thin and fragile.
The structural assembly process requires a complex balancing act. The rigid mechanical connection between a crystalline semiconductor and a package, distributes signal and power connections to the usable spacing at board level. For example, when selecting first level package materials for a high performance silicon microprocessor, the dielectric layers need ultra low k materials for fast internal signaling. However, many material candidates cannot survive thermo-mechanical stresses during assembly and field use. In the case of LED lighting, epitaxial layers and general fragility of substrates, present a different set of challenges. A mechanical engineering team may find a material promising, but if it doesn’t survive in the connection, they will have to reject it. Also, there are difficult IC thermal management issues to solve.
LED lighting connections
LED lighting currently uses two types of first level connections, wire bonding and flip chip configuration. Wire bonding requires a LED die that faces up and connects to a package using wires. In contrast, a flip chip faces down and uses solder bumps to connect to the package. While wire bonding tends to be more reliable, mechanical engineers are also adopting flip chip configuration to help reduce material processing and optics engineering restrictions. Mechanical engineering teams can widen their portfolio of usable packaging configurations for producing optimal lighting solution.
Stay tuned for the following blog that will explore the challenges engineering teams face in second level connection assemblies.