In case you missed it.

Component Reliability

"Solder Joint Reliability in High Reliability Applications for QFN Type Packages"

Authors: Alex Chan, Aman Khan and Robert Kinyanjui; [e-mail].

Abstract: Leadless packages represent higher risk in solder joint attachment reliability. There is a much lower standoff (solder joint) compared to BGAs. Solder joint final geometry is sensitive to solder paste volume, land pattern design and process control. As such, it becomes difficult to design new, high-reliability products without using QFNs. Different suppliers have different package standards and materials sets. A very small proportion of second-level test data are available to fulfill IPC-9701 requirements for telecom applications. This study looked at various sizes and versions of QFNs using SnPb37 and SAC 305 under IPC-9701 test conditions and accelerated thermal cycling. (SMTA International Conference on Soldering & Reliability, April 2007)

Design

"Via in Pad Guidelines"

Author: Duane Benson, [e-mail].

Abstract: In the assembly world, putting vias directly in surface mount pads is often not recommended. But for routing large or fine-pitch BGAs, bypass capacitors, thermal management, and grounding on high-frequency parts, via-in-pad can be an effective, if demanding, solution. (Company white paper, July 2007)

PWB Solderability

"Impact of Inert Atmosphere Quality on Wettability of Printed Wiring Board (PWB) Finishes for Surface Mount Soldering Applications"

Authors: David Hillman and Don Cullen; [e-mail].

Abstract: The objective of this investigation was to determine the impact of reflow soldering inert atmosphere quality on the wettability of various printed wiring board surface finishes. It was found that an inert atmosphere quality of 1000 ppm [O.sub.2] provides acceptable wetting characteristics applicable for reflow soldering processes using ENIG, immersion silver or immersion tin finishes using SnPb37 solder. Further, an inert atmosphere quality of 100 ppm [O.sub.2] provides acceptable wetting characteristics applicable for reflow using ENIG, immersion silver or immersion tin using a SAC 305 alloy. (SMTA International Conference on Soldering & Reliability, April 2007)

"A Unique Tin Whisker Growth Study of Nine Lead-Free Alloys"

Authors: Karl Seelig and Tetsuro Nishimura; [e-mail].

Abstract: Several groups, including iNEMI and NASA, have performed in-depth studies of tin whiskers. Conformal coating, annealing (reflowing tin) at 150[degrees]C have been suggested to mitigate tin whisker growth. Mechanically stressing the finish coat has not been tested in the studies reviewed. In prior studies, there was similar testing using thermal cycling (-40[degrees]C or -55[degrees] to +85[degrees]C), ambient soak (30[degrees]C, 60% RH), and high temperature soak (60[degrees]C, 87% RH). All tests were on as-processed finishes, or in an annealed state. Some coupons were conformal-coated to mitigate whisker growth. Additionally, all tests have looked at surface finishes for lead coatings. Alloys are believed to be acceptable since they contain enough containments and additives not allowing the growth of whiskers. In alloy testing, the criteria involved bulk casting, not as a coating over copper. In actual use, Pb-free solder will become a thin coating due to wicking on leaded devices. It will also become a thin coating on wire soldered into a board. In both applications, the overall product life will be mechanically stressed. Many studies state that whisker growth stems from stress of intermetallic growth or compression because of scratches. Other studies seem to refer to whisker growth resulting from stress during compression. iNEMI found non-repeatable results in whisker growth. Similar results were seen in this study. Many of the studied alloys are almost identical; however, they exhibited slightly different growth densities and lengths. (SMTA International Conference on Soldering & Reliability, April 2007)