Impact of FPC Fabrication Process on SMD Reliability

ABSTRACT The functionality of electronic devices continues to increase at an extraordinary rate. Simultaneously consumers are expecting even more and in ever smaller packages. One enabler for shrinking electronics has been the flexible circuit board that allows the circuit board to fit a wide variety of shapes. Flexible printed circuits (FPC) have the capability to be very thin and can have unpackaged components directly attached using surface mount technology (SMT) and flip chip on flex technologies. Bare die can also be thinned and attached very close to the circuit board. However one caveat of high density flexible circuit boards with thin die is that they can be very fragile. The use of back side films and underfill can protect the die making circuits more robust. For underfill to work well it requires good adhesion to the circuit board which can mean that flux residues under the die normally must be removed prior to underfilling. The flux cleaning process can require harsh cleaning chemistries when high temperature solder is used and when die have a very low standoff from the circuit board surface. Some typical vapor degreasing solvents have been found to attack flexible circuit boards at layer interfaces and reduce circuit board reliability. These solvents remain trapped in the circuit layers and can result in blistering and delamination of the circuit boards during subsequent assembly steps. Eliminating or reducing the occurrence of delamination on flexible circuit boards leads to an overall more robust circuit. Certain board suppliers manufacture more robust boards that do not delaminate, but fabrication processes and material selection can be proprietary and some suppliers are unwilling to alter their processes. Consequently, an alternative method to prevent delamination that does not require specific assembly or materials information needed to be established. This paper described a process developed to circumvent or eliminate delamination caused by chemical solvent absorption that uses a post-assembly laser excising fabrication process.