It used to be that tape-automated bonding (TAB) on a printed-circuit board was the preferred way to package driver chips in an LCD module, but over the years, other techniques have also been called into duty.
A few years back, there was a "big rush" to chip-on-glass (COG), said Rob Harrison, vice president of the passive-LCD group at Three-Five Systems Inc. But COG's limitations have brought many programs back to TAB or chip-on-flex, he said.
Because of space constraints, he said, "COG can only be used at a certain resolution level where the lines aren't too fine." At very fine pitches, "COG becomes difficult to test, and TAB is the preferred approach." Chip size can also present a problem for COG, he said. "It can require that the ledge on the display be much greater, and more glass area means higher costs."
Jeff Wainwright, vice president of sales and marketing at Optrex, said the driver-packaging issue is customer-driven. "You're going to see continued usage of COG, especially if there are unique packaging or integration requirements or a need to really get cost down," he said. The trade-off is that the designer "may have to integrate a few passive components on the motherboard, but in many applications, it's a better solution."
Wainwright added that it may be more cost-effective to go to TAB or chip-on-board (COB) "if you have to integrate a keypad or indicator around the display."
Kyocera has historically used COB and COG because of "a weakness in the bond area" with TAB, said distribution manager Joe Maurer. "But one problem with those techniques is that they make dead-space areas, which means the outline dimension has to be bigger for a given active area. It also generally means the active area is not centered within the outline but offset, because of the big area where the circuits are."
That's why Kyocera has developed a rear-bonding system. "We came up with a way to put all drivers on the back of the LCD and still use wear-bonding technology to make all the attachments," Maurer said. "That way, we don't have the problem of stress that can cause reliability problems if the bond breaks, like we have with TAB."