Chip-on-board (COB) is a fairly new packaging technology that combines the low-cost benefits of PWAs with the high densities of hybrid microcircuits. According to COB, bare chips are wire bonded or flip-chip interconnected to a PCB designed and fabricated with finer dimensions than normally used. The active chips requiring protection are then selectively encapsulated by dispensing a filled epoxy over the device, a process called glob-topping. In the board design, several ICs can be partitioned closely together and encapsulated as a unit. Although there is always a concern in using plastic encapsulants as the prime method for moisture protection, epoxy glob-top materials on the market have performed very well in many applications and environments. Reliability is enhanced by assuring a
good primary passivation at the die level and a low-stress, low-moisture-absorption encapsulant. Even so, overcoating the entire COB with a high-performance coating such as parylene produced improved results. In one report, DRAM devices glob-topped with either Hysol-Dexter FP-4402, FP-4450, or Dow Corning silicone HIPEC Q14939 and then overcoated with parylene, all passed 1,000 hours of 85/85 THB while control parts without encapsulant began to fail at 250 hours. Tests also carried out on epoxy-encapsulated ATC-01 triple track resistors (Sandia National Laboratory test chip) survived 1,000 temperature cycles from -55° to 125°C.[46]
In other work, unpassivated ATC-01 chips were used to accelerate
failures. After 1,000 hours of 85/85 followed by 100 hours of PCT, the
uncoated controls had 86% failures while those coated with Parylene C
had only 6.8% failures. Devices coated with a trilayer coating
consisting of Parylene C, Cyclotene, and e-beam-deposited silicon
dioxide further reduced the failures to approximately 3%. Under the
same test conditions, passivated ATC-01 chips without any overcoating
had failures of 45%.[47]