TABLE OF CONTENTS One of the most cost-effective packaging technologies is DCA, such as the solder-bumped flip chip on PCB. However, because of the thermal expansion mismatch between the silicon chip and epoxy PCB, underfill encapsulant is usually needed for solder joint reliability.1 6 However, due to the underfill operation, the manufacturing cost is increased and the manufacturing throughput is reduced. In addition, the rework of underfilled flip chip on PCB is very difficult, if not impossible. This further complicates the known good die (KGD)-related issues.
There is another reason why DCA is not yet very popular. Usually, the pitch and size of the pads on the peripheral-arrayed chip are very small and impose great demands on the supporting structure the PCB. The high density and the fine-line width/spacing PCB with microvia sequential built-up circuits are not yet commonly available at a reasonable cost, as discussed in Chaps. 1 through 8. Thus, alternative low-cost and high-performance electronic packaging techniques are needed.
WLCSP provides a solution to these problems.1 4 The unique feature of most WLCSPs is the use of a metal layer to redistribute the very fine-pitch peripheral-arrayed pads on the chip to much larger-pitch area-arrayed pads with much taller solder joints on the PCB. Thus, with WLCSPs, the demand on the PCB is relaxed, the underfill may not be necessary, and the KGD issues are much simpler. From the system houses point of view, WLCSP is just like another solder-bumped flip chip surface-mount component, except for the following: (1)...
