Circuits Assembly - October 2008 - (Page 29)

Cover Story more than one or two rework cycles, especially at higher temperatures. Other components on the assembly, and the board itself, are usually not made to handle these higher reflow times and temperatures, which F ! igure 4. This profile shows the standard convection ball-attachment may result in diftemperature profile. Compare with laser profiles below.` ferent reliability issues.3 To avoid both a higher reflow temperature and a these issues, a reballing process should be longer dwell at that higher temperature. used to remove the unleaded solder balls This is well documented, even within and replace them with SnPb37 solder many of the BGA component manufacspheres, or vice versa as the case may be. turers’ specification sheets. Some specifiFor a rework situation, four additional cally state soldering with mixed alloys or thermal cycles are required if convection at higher temperatures than recommendis used: one to remove the BGA from the ed voids the warranty on the component. assembly, the second to strip off the old Most warn the package will not withstand solder balls, a third to solder the new balls ! Figure 3. Embedded thermocouple measures internal temperatures. ! Figure 6. Solder balls attached in a convec- tion oven with nitrogen. Irregular shape is caused by some oxygen presence (~75 ppm). ! Figure 7. Typical untouched pristine appearance of laser-reflowed solder balls using oxygen-free bath on the same BGA. NE W ! Figure 8. Coarse grain structure seen on BGA from component vendor with solder balls attached to ENIG pads. Precision Cleaning Solvents / Flux Removers Miller-Stephenson offers a wide range of precision cleaning solvents/flux removers that are non ozone-depleting, nonflammable, with a low order of toxicity. These HFC and HFE cleaners are excellent for removing particulates, flux, light and heavy oils and grease. All solvents are available in aerosol and bulk liquid. For technical information (8 AM - 4 PM E.T.): call 800.992.2424 or 203.743.4447. In Canada, call 800.323.4621. having Pb-free solder balls with a SnPb37 solder paste or profile (or vice versa) will nearly always negatively affect BGA solder joint reliability.1,2 This is because the solder joint formed by this mixed alloy is seldom homogenous; the alloy blend of tin and lead must be mixed thoroughly to avoid regions rich in lead (Figures 1 and 2). Complete homogeneity can be achieved with mixed alloys, but requires circuitsassembly.com m s California - Illinois - Connecticut - Canada 203.743.4447 Fax 203.791.8702 e-mail: support@miller-stephenson.com www.miller-stephenson.com Circuits Assembly OCTOBER 2008 29 http://www.miller-stephenson.com http://www.miller-stephenson.com 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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