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Publication Date: 07/1/2011
Archive >  July 2011 Issue >  Special Features: SMT and Production > 

Underfilling 3D Stacked PoP Devices
Underfill reservoir and flow fronts.

Industry 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 underfill material bonds substrate and component so there is no separation as the PCB flexes upon impact of a 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 material to the other side of the component. 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 components. 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. 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 integrated 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 dispensed at one time, the larger the wet-out area.

PoP Package Types
A study was made focused on 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 percent filler content. Underfill was dispensed using an Asymtek S-920 dispenser 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 layer. The dispense pattern was an "I" pass (single side). The flowout time is less when the underfill reservoir utilizes multiple sides of the package (L-pass); 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 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 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-controlled 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 qualified for underfilling both interconnect layers simultaneously as well as underfilling only the bottom interconnect layer; the reservoir for the TMV PoP was only qualified for underfilling both interconnect layers simultaneously.
Through Mold Via (TMV) PoP.

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 subsequent passes were not dispensed prior to the fluid reservoir being almost depleted.

Component Spacing
The wet-out area, and thus the distance that surrounding components can be placed from the underfilled component, is directly proportional to the number of passes that the material is dispensed and if one or both interconnect 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 increases the dispense time.

This study showed that the 2nd generation TSV PoP requires slightly more wet-out area than the 1st generation PSvfBGA when using a single pass. The slower flow on the 2nd level interconnects on the TSV PoP (due to the smaller gap between packages) 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.
Minimum height to underfill both interconnect layers.

The wet-out area is heavily dependent on the number of interconnect layers underfilled. When comparing the wet-out distance on the bottom interconnect vs. both interconnects we see that the smallest wet-out area occurs when only the bottom level interconnect is 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 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 dimensions with numerous dispense passes. In this study the fillet on the non-dispensed sides was 0.5mm when both interconnect layers were dispensed and 0.2mm when only the bottom level interconnects were dispensed.

Wet-Out Area
Underfill wet-out area is greater the fewer the number of passes and if both interconnect layers are being underfilled.

When laying out a board with a PoP device the keep-out zone should not necessarily be symmetrical as the dispense side(s) require a wet-out area and the filleted sides of the package are primarily dependent on the height of the interconnect layer and contact angle of the material. This study showed that the wet-out area on a PoP was approximately 6-10 times the fillet dimensions (3-5mm vs 0.5mm). The fluid reservoir needs to reach as high as the second interconnect area, otherwise no underfill will flow into this area, the higher the reservoir the larger the wet-out area.

The proximity to other components determines the percentage of the total amount of material required to be dispensed in each pass. Dispensing less fluid in a pass enables the wet-out areas to be closer to the fillet dimensions because the fluid doesn't spread as much and can flow under the component more quickly. Obviously, more passes take more time. It is advantageous from a cost and reliability standpoint to design the boards with an appropriate wet-out area for either single level interconnect or multiple level interconnect underfill.

Contact: Nordson ASYMTEK, 2762 Loker Ave. West, Carlsbad, CA 92010 800-279-6835 or 760-431-1919 fax: 760-431-2678 E-mail: Web:

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