Underfill

Underfill is a rugged Exascend technology that addresses some of the main disadvantages of the high-bandwidth ball grid arrays (BGAs) used to connect components such as memory integrated circuits (ICs) and NAND flash controllers to the printed circuit board (PCB) in modern SSDs. While seldom a problem in static applications such as workstations and servers, mechanical stress factors such as flexing, vibrations and shocks or thermal fluctuations can cause severe malfunction in SSDs used in environments where such stress factors are present. Storage applications where such stresses are common and underfill is popular include mission critical, industrial automation, edge computing and transportation.

The solder balls that connect the PCB substrate and the integrated circuit are prone to fracturing when subject to mechanical and thermal stress, which in turn may cause a complete device failure. Tin whiskers, a phenomenon where metal hairs project from the solder material over time, can also cause short circuits in a BGA and is particularly common in high-stress environments.

Underfill is a rugged SSD technology that dramatically reduces the likelihood of BGA-related malfunction by bonding the integrated circuits to the PCB with epoxy. With underfill, a layer of epoxy surrounds the solder balls that make up the connection between the IC and PCB, resulting in a solid connection significantly less prone to friction, shocks and thermal expansion. It also leaves little room for tin whiskers to form, thus all but eliminating the risk of short circuits caused by tin whiskers.

In Exascend SSDs with underfill, the epoxy resin is applied next to all ICs attached to the PCB with BGA using a specialized dispenser. Capillary action then carries the epoxy across the entire BGA, covering all connections. Lastly, the Exascend SSD with underfill applied is cured with heat using special equipment, ensuring that the epoxy resin reaches a solid state. Once the epoxy is fully cured, the Exascend SSD is ready for further processing, for example applying a conformal coating, or final deployment in a customer’s application.