Circuits Assembly - December 2007 - (Page 34)

Tech Tips Assemblies Go PoP Mating top and bottom side BGAs to form package-on-package PCBs. iniaturization has driven logic and memory intenon-underfilled PoP assemblies. The underfilled PoP will gration using vertically integrated devices in a be further divided into bottom package only underfill, and stacked configuration, where the top and bottop and bottom underfill. Typically, most PoP architectures tom BGAs are mated to form that integrate underfill will package-on-package (PoP) underfill only the bottom assemblies (Figure 1). The package. intrepid nature of this techOne potential outcome nology has spread beyond of the experiment was an the package realm, spawnattempt at differentiation ing new stacked die and SoC in reliability under thermal (system on chip) technolocycling conditions. The gies that integrate a similar experiment was designed to vertical stacking approach, use various materials and while retaining the needed processes encountered in functionality associated with Figure 1. PoP PCBs used for the experiment. PoP manufacturing. Matemore conventional designs. rials were selected for their Advantages of the PoP ability to withstand Pb-free architecture are such that reflow temperatures. Solone-step reflow can be used der pastes were composed in the concurrent processof no-clean flux and solder ing of both the top and botballs with a particle mesh of tom BGAs. This approach to 300. The underfill material assembling PoP BGAs uses was a high temperature, low a conventional paste process CTE polymer, usually confor the bottom BGA and a signed for flip-chip appliflux transfer dip for the top cations and environmental BGA. The topside of the botstress screening conditions tom BGA, where the pads are where CTE is critical. The typically a plated NiAu finish, material set included Multiprovides a wettable surface Figure 2. BGA showing missing ball. core MP200 no-clean paste, pad area for the top BGA. Multicore LF320 no-clean The reflow process employed SAC alloy, and Hysol FP will depend largely on the 4548FC underfill. solder paste composition and The test vehicle (Figure the top and bottom BGA 1) included a daisy-chained alloy. In situations where the PCB with an OSP (organic paste alloy and BGA balls solderability preservative) are of varying compositions, coating, and top and bottom reflow parameters should BGAs composed of SAC be adjusted to achieve the 305 alloys. The top package liquidus temperature of the was a 152-ball BGA with higher melt alloy. 0.65 mm pitch on a 14 mm To explore various prosquare package footprint. cess options, the EMPF is The bottom package was a conducting an experiment Figure 3. X-ray showing solder displacement. 353-ball BGA with 0.5 mm that will eventually produce pitch on a 14 mm square thermal cycling reliability package footprint with a data on PoP assemblies. Using mixed and Pb-free SAC 305 AuNi top pad finish. The vehicle contained 15 independent (SnAg3.0Cu0.5) compositions, the experimental matrix daisy chain sites where the top and bottom BGAs could be will also be designed to include assemblies that will be continuity tested while isolating the particular location and underfilled, along with the more conventional approach of BGA on the PCB. M 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. 34 Circuits Assembly DECEMBER 2007 circuitsassembly.com http://www.aciusa.org http://www.aciusa.org http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - December 2007

Circuits Assembly - December 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing Metalization Options for COB Assembly A Test Comparison of SAC and Non-SAC Pb-Free Solders An A-to-Z Guide to X-Ray Inspection BEST: A ‘Funky Chicken’ with an EMS Niche Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Approaches Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - December 2007

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