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www.us-tech.com Underfilling 3D Stacked PoP Devices Continued from previous page
layer. The dispense pattern was an “I” pass (single side). The flowout time is less when
the underfill reservoir utilizes multi- ple sides of the package (L-pass);
that the center of the DJ-9000 Dispense Jet Nozzle was 0.3mm from the edge and 0.5mm above the top of the component for each weight-con- trolled line. The wet-out area was studied utilizing one, two, or three dispense passes. The total amount of underfill was divided equally between passes. The wet- out area respective to the number of passes on the PSvfBGA PoP was quali- fied for underfilling both interconnect
layers PSvfBGA PoP.
however, the testing was designed to only look at a single side as that incorporates the longest flow-out time and the highest probability for a
simultaneously as well as underfilling only the bot- tom interconnect layer; the reservoir for the TMV PoP was only qualified
for underfilling both interconnect layers simultaneously. Testing also qualified the time
required for the underfill to flow under the component with each of the passes. The dispenser’s multi- pass wait timers were set to ensure that subse- quent passes were not dispensed prior to the fluid reservoir being almost depleted.
Through Mold Via (TMV) PoP.
void-free underfill. The parts were prebaked to drive out any residual moisture in the organic substrate and 90°C substrate heat was used to ensure proper flow of the underfill. The dispenser was programmed so
Component Spacing The wet-out area,
and thus the distance
that surrounding components can be placed from the underfilled compo- nent, is directly proportional to the number of passes that the material is dispensed and if one or both intercon- nect layers of the PoP package are
being underfilled. Increasing the number of passes
results in decreasing the wet-out area allowing for tighter surrounding component density. The reduced wet- out area from multiple passes slightly in creases the dispense time. This study showed
that the 2nd generation TSV PoP re quires slightly more wet-out area than 1st generation PSvfBGA when us ing a single pass. The slower flow on
the 2nd level interconnects on the TSV PoP (due to the smaller gap be tween pack- ages) caused the underfill material to accumulate into a larger reservoir when compared with the PSvfBGA when all the material was dispensed in a single pass. Both packages showed similar wet-out areas if multiple passes were used.
underfilled. The underfill wet-out is greater
than the underfill fillet; the fillet is the underfill that is visible around all sides of the component in a cured
July, 2011
Minimum height to underfill both interconnect layers.
The wet-out area is heavily dependent on the number of inter-
package. The dimensions of the fillet are determined by the underfill material contact angle, the height of the solder joints, and the amount of material. Wet-out areas can become relatively close to the fillet dimen-
For manufacturing and rework requirements, the underfill should only come in contact with the component being underfilled. If the underfill comes in contact with other components, surface tension pulls the underfill away from the PoP and onto the other components.
connect layers underfilled. When comparing the wet-out distance on the bottom interconnect vs. both interconnects we see that the small- est wet-out area occurs when only the bottom level interconnect is
sions with numerous dispense pass- es. In this study the fillet on the non- dispensed sides was 0.5mm when both interconnect layers were dis-
Continued on next page
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