Development of outer lead bonding apparatus with a focused YAG laser beam for high quality TCP bonding

The recent trend of size reduction in various kinds of electronic equipment demands the high density mounting of high lead count, fine lead pitch components. Conventional hot-bar soldering causes problems such as solder bridges and weak bonds, and is not adequate for the mounting of fine pitch components like tape carrier packages (TCPs). We have developed an outer lead bonding (OLB) apparatus with a focused YAG laser beam for high quality TCP bonding. This apparatus solders leads one by one by scanning the focused YAG laser beam, which does not damage the printed circuit board (PCB). The apparatus uses two galvano mirrors to rapidly scan the laser beam and a flat-field lens (fθ lens) to focus the laser beam to as small as the lead width regardless of the deflection angle. A through the lens (TTL) monitoring system, consisting of the above galvano mirrors and fB lens, is used to monitor the laser beam irradiation area. This construction ensures the monitored area coincides with the irradiation area at all times, and facilitates the precise directing of the laser beam on a single lead and its pad on PCBs. The system measures the displacement between the lead and matching pad by image processing and when the displacement is larger than one third of the lead width, soldering is not conducted. The test results show that the apparatus has realized high quality soldering with no-clean flux for TCPs having 576 leads and a 250 μm pitch