Repair

"Impact of PCB Pad Site Dress Methods on Pad Array Damage"

Authors: Laurence A. Harvilchuck, Brian Roggeman, Raiyo F. Aspandiar, James M. Wade and Gaurav Godbole; harvilch@uic.com.

Abstract: The thermal profile experienced by the pad array can have a profound impact on latent PCB damage, including the presence or absence of pad cratering. In this exercise, pad array damage is evaluated as a function of assembly preheating, pad site dress method, and applied desoldering temperature to offer insight when choosing between simple wick-and-iron solder removal and the more sophisticated vacuum-assisted solder scavenging methods. High-resolution 16-channel thermal profiles were obtained of both the wick-and-iron and vacuum scavenging operations across a single 34 mm square pad array of variable pitch on a 0.060" thick Pb-free ATX motherboard, revealing the nature of the thermal profile at the pad surface and through the board section to the cores. The shortcomings of current repair thermometry methods are also documented in the context of the impact of thermocouple placement on profile accuracy. Process variations inherent in the primarily manual wick-and-iron solder removal methods are readily apparent in the thermal profiles experienced by the pad array, while significantly reduced in the thermal profile generated by the vacuum scavenger. Wick-and-iron scavenging operations can subject the pad array to ramp rates approaching 200[degrees]C/sec. during the brief excursion above solder liquidus, while vacuum scavenging of the same site exhibited a maximum ramp rate nearly a full order of magnitude less, but of much greater times above liquidus. The impact of the thermal profile on the pad array was characterized by bump pull, pad fatigue and dye-and-pry techniques. Results from the present study showed no solder mask damage in the vicinity of the pad array resulted from any of the scavenging processes. Damage to innerlayer circuit board structures (per IPC-A-610) beneath the pad array was also absent in all cases under study. Use of substrate preheating during solder scavenging has a definite impact on reducing the substrate damage that can result from the repair of lead-free laminates. For both the wick-and-iron and vacuum methods, higher applied desoldering temperatures resulted in weaker pad adhesion than the corresponding solder removal operations at lower applied desoldering temperatures. Further pad fatigue testing of the same samples indicated this change in pad adhesion strength may be related to a change in the ductility of the laminate directly beneath the pad. The laminate choice also can have a significant impact. (SMTA Pan Pac Microelectronics Symposium, February 2009)