Circuits Assembly - October 2008 - (Page 31)

Cover Story ! Figure 10. IMF and grain structure of con- vection reballed BGA. ! Figure 11. IMF and grain structure of initial ball attachment using proprietary laser process on ENIG. Figure 13. Microsection of laser-attached ball on ENIG pad (initial ball attachment) using proprietary laser process. ! tion with benefit of added nitrogen, and laser. While major differences were seen in the grain structure and IMF, the microsections showed no sign of innerlayer substrate damage or resin recession cause by the localized heating of the pads and balls with laser (Figures 8 and 9). No substrate anomalies were seen in the crosssections of either convection reballed or laser reballed samples (Figures 10 to 14). Shear force test results. Shear testing was used as an indicator for comparison between reballing methods. Shear force testing of all three methods indicated a range from 1.2 to 2.9 Kg. The as-received BGAs failed under the lowest force range; convection and laser reballed BGAs were in the middle of the force range, and originally attached balls soldered with laser in an oxygen-free chemical bath were at the high end of the range. Shear force testing indicated there is virtually no difference in the shear strength for spheres reballed using convection with nitrogen blanket and laser processes (Figure 15). However, initial laser attachment shear forces were higher than those of reballed spheres using both the convection process and the laser attachment with the proprietary process, circuitsassembly.com Figure 14. Typical appearance of solder ball on BGAs stripped and reballed using laser in oxygen-free chemical bath. ! and much higher than as-received BGAs with the original attachment done using convection. Summary Reballing BGAs due to rework and conversion from RoHS-compliant solders to SnPb (or vice versa) typically requires the component be subjected to additional reflow cycles beyond the component manufacturer’s recommendations. Laser attachment or laser reballing in an oxygen-free bath eliminates two additional temperature excursions and provides improved component reliability by facilitating the use of standard assembly reflow temperatures without mixed alloys. n Ed: For a list of references, please see the online version of this article. Joshua Muonio is a senior process engineer at Analog Technologies/Lumagine Inc. (analog-tech.com); jmuonio@analog-tech.com. Richard D. Stadem is a principal process engineer at General Dynamics (gd.com); richard.stadem@gd-ais.com. Circuits Assembly OCTOBER 2008 31 http://www.seho.de http://www.analog-tech.com http://www.gd.com http://www.circuitsassembly.com http://www.seho.de

Table of Contents Feed for the Digital Edition of Circuits Assembly - October 2008

Circuits Assembly - September 2008 Contents Letters Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing 'Checking Up' on Medical Electronics Solder Ball Attachment Using Laser Soldering Improving QFN Reliability Reflow Soldering Tech Tips Test and Inspection Process Doctor Pb-Free Lessons Learned Alternative Energies Eastern Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - October 2008

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