July, 2011
www.us-tech.com
Page 57
Underfilling 3D Stacked Package-on-Package (PoP) Devices
By Brad Perkins, Jared Wilburn, and Steve Adamson, Nordson ASYMTEK, Carlsbad, CA I
ndustry trends are driving mobile electronics, such as cell phones, to smaller form factors. This
trend is a driver for 3D packaging such as stacked package-on-package (PoP) devices. These mobile products are expected to work even after being dropped. To provide robustness, underfill
is applied to stacked PoPs to provide a mechanical connection between the PCB and package layers. The under- fill material bonds substrate and component so there is no separation as the PCB flexes upon impact of a
Underfill is needed in mobile devices to protect against failure from mechanical shock, so a dropped cell phone can continue to work after it takes a fall.
drop. This prevents the solder joints from fracturing which could result in electrical open circuits. Underfill is dispensed along one
or two sides of a component and then capillary action draws the underfill to the other side of the component encapsulating the solder joints under the component and holding them in hydrostatic compression once cured. Dispensed underfill forms a
fluid reservoir along the side of a PoP device. The fluid reservoir is depleted by capillary forces pulling the mate- rial to the other side of the compo- nent. Where the fluid was deposited for the underfill reservoir a wet-out area is created. With 2-layer stacked PoP devices, both interconnect layers are underfilled simultaneously from the same fluid reservoir. The size of the wet-out area determines the proximity of neighboring compo- nents.
For manufacturing and rework
requirements, the underfill should only come in contact with the compo- nent being underfilled. If the under- fill comes in contact with other com- ponents, surface tension pulls the underfill away from the PoP and onto the other components. This can result in incomplete underfill of the desired component, wastes material, and eliminates the ability to rework the adjacent components. The use of equipment with inte-
grated weight scales allows for closed loop processing of the dispensed mass ensuring that the appropriate amount of underfill is dispensed for each component. If too little material is dispensed there is an incomplete underfill, which can lead to a failure of solder joints because of lack of mechanical support. When the appropriate amount
of underfill is dispensed, there is a direct correlation between the amount of material dispensed in a single pass and the size of the wet- out area. The more material dis- pensed at one time, the larger the wet-out area.
PoP Package Types A study was made focused on
See us at Semicon West Booth 5368 North Underfill reservoir and flow fronts.
two different PoP package types: the current generation PSvfBGA PoP
and the next generation Through Mold Via (TMV) PoP. The underfill
used in this test had a viscosity of, 3,000 centipoises (cps), with 50 per- cent filler content. Underfill was dis- pensed using an Asymtek S-920 dis- penser with integrated weight scale and a DJ-9000 Dispense Jet. It was found that 120mg was required to completely underfill both layers while only 65mg was required to completely underfill the bottom
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