Circuits Assembly - October 2008 - (Page 30)

Cover Story ! Figure 9. IMF and grain structure of BGA stripped and reballed using laser process. This internal profile during laser shows spike from 22.2° !Figure 5.seen only during the ball attach inball attachment vicinity aofsinglethermocouples. to 22.8°C., the immediate the ! Figure 12. View of solder ball on BGA that was stripped and reballed using convection without benefit of nitrogen atmosphere. Figure 15. Shear force distribution. ! and a fourth to reattach the BGA to on, the assembly. The result is inevitable, a further decrease in solder joint and BGA substrate via reliability, as well as additional cost.4,5 Rework causes. Regardless of the RoHS-instigated alloy mixing issues, there are many situations where rework is required on a BGA component no matter which process or alloy is used: Pb-free or standard 63/37. Rework is often required because of hiccups such as misalignment during placement or reflow, excessive voiding after reflow, solder bridging, improper reflow profiles, poor solder paste stencil registration, improper solder paste handling practices, wrong components loaded into the placement machine, and many other factors. BGAs often are 30 replaced to upgrade the performance of the assembly by using one with a higher operating speed or some other improved feature. Retaining the old components for other applications is often desired. Laser Reballing As stated, if a BGA needs to be unsoldered, the original solder balls are lost. In many cases these components can be very costly; some BGAs on the market today cost several thousands of dollars each. To salvage these components, new solder balls are attached with convection heat. Thus begins the vicious downward spiral of the component’s reliability. Hence, it is advantageous to perform initial ball attachment and reballing using laser energy to heat only the BGA pad and solder ball, not the component or internal die or bonds. During qualification of the oxygenevacuated laser ball attachment process, thermocouples were embedded within the package to compare the temperature gradients seen with convection reflow versus laser (Figures 3 to 5). Methods have been developed using laser energy to remove old solder and perform attachment of new solder balls without heating the component itself, in an oxygen-free chemical bath without nitrogen. This process results in smooth, perfectly round, void-free solder spheres completely wetted to the BGA pads. These spheres are optimal for solder connection to the PWB pads in subsequent processes. When comparing laser-attached solder balls with balls attached using a convection soldering process, even those soldered within a nitrogen blanket, the difference is immediately noticeable (Figures 6 and 7). Microsections were made to determine if there were any appreciable differences in initial solder attachment of BGA solder balls received from different vendors where the attachment was done using convection, on BGAs with solder stripped off as part of rework and new balls attached with convection, conveccircuitsassembly.com Circuits Assembly OCTOBER 2008 http://www.circuitsassembly.com

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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