Circuits Assembly - December 2007 - (Page 35)

Table 1. Manufacturing Process Matrix Manufacturing Process Process Steps Stencil printing Bottom BGA placement Reflow Underfill bottom Cure underfill Apply tack flux Top BGA placement X-ray verification Reflow Underfill bottom Underfill top Cure underfill X-ray verification Electrical test 12345678 XXXXXXXX XXXXXXXX XXXX XXXX XXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XX XX XXXX XXXXXXXX XXXXXXXX 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 Pbfree 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 crosslinked 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. ■ Tech Tips Looking for suppliers? Look here! The 2007 CIRCUITS ASSEMBLY Directory of EMS Companies • More than 1,250 EMS facility listings • Detailed info on sales, size, contacts, and more! • Price: Only $999 Order online at circuitsassembly.com/dems Circuits Assembly DECEMBER 2007 35 circuitsassembly.com http://circuitsassembly.com/dems 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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