Mating top and bottom side BGAs to form package-on-package PCBs.

Miniaturization has driven logic and memory integration using vertically integrated devices in a stacked configuration, where the top and bottom BGAs are mated to form package-on-package (PoP) assemblies (Figure 1). The intrepid nature of this technology has spread beyond the package realm, spawning new stacked die and SoC (system on chip) technologies that integrate a similar vertical stacking approach, while retaining the needed functionality associated with more conventional designs.


Advantages of the PoP architecture are such that one-step reflow can be used in the concurrent processing of both the top and bottom BGAs. This approach to assembling PoP BGAs uses a conventional paste process for the bottom BGA and a flux transfer dip for the top BGA. The topside of the bottom BGA, where the pads are typically a plated NiAu finish, provides a wettable surface pad area for the top BGA. The reflow process employed will depend largely on the solder paste composition and the top and bottom BGA alloy. In situations where the paste alloy and BGA balls are of varying compositions, reflow parameters should be adjusted to achieve the liquidus temperature of the higher melt alloy.

To explore various process options, the EMPF is conducting an experiment that will eventually produce thermal cycling reliability data on PoP assemblies. Using mixed and Pb-free SAC 305 (SnAg3.0Cu0.5) compositions, the experimental matrix will also be designed to include assemblies that will be underfilled, along with the more conventional approach of non-underfilled PoP assemblies. The underfilled PoP will be further divided into bottom package only underfill, and top and bottom underfill. Typically, most PoP architectures that integrate underfill will underfill only the bottom package.

One potential outcome of the experiment was an attempt at differentiation in reliability under thermal cycling conditions. The experiment was designed to use various materials and processes encountered in PoP manufacturing. Materials were selected for their ability to withstand Pb-free reflow temperatures. Solder pastes were composed of no-clean flux and solder balls with a particle mesh of 300. The underfill material was a high temperature, low CTE polymer, usually consigned for flip-chip applications and environmental stress screening conditions where CTE is critical. The material set included Multicore MP200 no-clean paste, Multicore LF320 no-clean SAC alloy, and Hysol FP 4548FC underfill.

The test vehicle (Figure 1) included a daisy-chained PCB with an OSP (organic solderability preservative) coating, and top and bottom BGAs composed of SAC 305 alloys. The top package was a 152-ball BGA with 0.65 mm pitch on a 14 mm square package footprint. The bottom package was a 353-ball BGA with 0.5 mm pitch on a 14 mm square package footprint with a AuNi top pad finish. The vehicle contained 15 independent daisy chain sites where the top and bottom BGAs could be continuity tested while isolating the particular location and BGA on the PCB. 

Fixtures. The screen stencil was designed to apply a suitable amount of paste to obtain adequate wetting and contact between the BGA and substrate. A flux transfer plate was used for assembling the top BGA to the top pads of the bottom BGA. The tacky flux was appropriate for Pb-free processing and used the flux transfer plate as a shallow reservoir designed to hold a volume of flux that comprised half the diameter of the top BGA balls. The top BGA stencil was about 0.12 mm thick with an opening of 0.28 mm. The bottom BGA application had the flux transfer plate machined to a height of approximately 0.150 mm.

A total of 112 assemblies were processed for the experiment.

The assembly process identified in the manufacturing process matrix (Table 1) with an “X” indicates the operation was performed for the specific manufacturing process. The underfilling for manufacturing process “3-6” incorporated a two-step reflow that permitted the bottom package underfill to reflow after the top package assembly. The manufacturing process “7-8” underfilled the bottom package after assembly of both top and bottom packages.


Initial results. Data collected thus far indicate any recurring failures are attributable to missing solder balls. When this issue was discovered, BGAs were closely inspected for missing solder balls (Figure 2), so the packages that had dislodged balls were separated. Even after the segregation process, and subsequent step-by-step inspection, opens occurred occasionally on different sites due to missing balls. A particular concern during PoP assembly processing is the propensity of package warpage that would result in opens, particularly in the bottom BGA. Coplanarity of the bottom package against the substrate becomes an issue as the corner edges of the BGA warp in a concave manner, causing loss of contact between the BGA ball and substrate. This was also discovered in previous studies.


In some cases (Figure 3), the solder ball displaces itself as the liquidus stage is reached and coalesces in other areas. This occurs prior to the underfilling stage, even if the underfill is subjected to reflow. Once the underfill is cured, especially high filler content formulations, the cross-linked polymer would prevent solder from wicking into adjacent areas, since the underfill would restrain it.


Wetting. In the majority of cases where package sites have continuity, good wetting is evident, even among the assemblies using a mixed solder system and a two-step reflow process. PoP assembly x-ray inspection for opens and shorts presents a challenge, because the staggered patterns of the top and bottom BGAs can potentially mask defects, or give a false negative reading due to the unusual appearance. Oblique angle viewing for verification of shorts, opens or acceptable ball collapse is sometimes necessary.

Processing PoP assemblies requires carefully controlled measures at each step of the process. Monitoring PCB warpage and BGA ball height will help mitigate the effects of the warpage caused by the CTE mismatch of package and substrate. Applying a controlled amount of tacky flux is also critical to ensuring adequate wettablility to the pads, without creating a bridging effect. The use of underfill and its effect on reliability for PoPs is not fully understood, but a low CTE underfill is recommended for thermal cycling conditions.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.