A low temperature fabricated ceramic moisture sensor for electronic packaging
by Chow Lok Wan
M.Phil. Mechanical Engineering
1 v. (various pagings) : ill. ; 30 cm
Moisture induced plastic package defects have long been primary concerns for design and reliability engineers. It is estimated that over 40 % devices' failures, such as delamination, corrosion and popcorning, are induced by moisture....[ Read more ]
Moisture induced plastic package defects have long been primary concerns for design and reliability engineers. It is estimated that over 40 % devices' failures, such as delamination, corrosion and popcorning, are induced by moisture.
Weight gain method which is the most commonly used to investigate moisture uptake and diffusivity in bulk, is convenient, but, not specific. Therefore, the need to observe the moisture content as a function of time at that interface for package reliability study drives the motivation of the development of the moisture sensing chip (MoistChip).
Most popular humidity sensors are polymeric; however, they normally could not withstand prolonged hot wet environment (>60 ℃ /60 %RH). Existing high performance ceramic humidity sensors such as porous silicon, sol-gel and powder sintering either require complex and sophisticated process control, or heat treatment at around 1000 ℃, so they are all not compatible to IC fabrication process.
Process design of MoistChips makes use of additional negative DC bias at the substrate during sputtering and annealing time, such that the sensing film could be fabricated at below 500 ℃. with optimized conditions, MoistChips could have 4 order of magnitude change in current sensitivity and improved hysteresis. Modified electrode pattern design helps to bring signal magnification to a milli-ampere level and linearization with curve fitting coefficient, R2, to greater than 0.98.
MoistChip-packaging encapsulates the bond-wires, but not the sensing element. Small drift in signal response shows good performance after encapsulation at high temperature, about 200 ℃, and minimal poisoning by the out-gassing of the encapsulant curing process.
MoistChips packaged inside Chip-On-Board (COB) package provided a real-time and in-situ moisture diffusion measurement under JEDEC Level 1 condition. The diffusivity obtained is greater than that calculated from weight gain method of plane sheet encapsulant. This suggests there are other diffusion paths in the package. MoistChips give the total moisture content at the interested interface, hence, implementation of MoistChips in packages provide a realistic platform to study package reliability.