Another good but sophisticated procedure would be to do planar EDS at different voltages and angles combined with software models for the depth of penetration. Sometimes, I like it better than section, as Sn is soft and always can smear in a section. Vladimir SENTEC Testing Laboratory Inc. 11 Canadian Road, Unit 7. Scarborough, ON M1R 5G1 Tel: (416) 899-1882 Fax: (905) 882-8812 www.sentec.ca -----Original Message----- From: Stewart McCracken <[log in to unmask]> Sender: TechNet <[log in to unmask]> Date: Tue, 23 Nov 2010 16:12:30 To: <[log in to unmask]> Reply-To: TechNet E-Mail Forum <[log in to unmask]>, Stewart McCracken <[log in to unmask]> Subject: Re: [TN] ImSn finish, BGA solderability Hi Chris, I've uploaded a top-down SEM shot showing a good example of the Imm Sn wetting defect where we found excess conversion of the original unconverted Sn to Cu-Sn IMC's. Once we had cleaned the flux residues, SEM inspection showed islands of unconverted Sn on an otherwise fully converted Cu-Sn "cobblestone" background. I have some micro-section shots also, I'll post these shortly. http://www.themcsgroup.co.uk/images/gallery/042_immersion_tin_solderability_defect_-_non_wet_pad_surface_sem_5kx.jpg Kind regards, Stewart Stewart McCracken MCS Ltd. Centre House Midlothian Innovation Centre Roslin Midlothian EH25 9RE m. +44(0)7711 541735 t. +44(0)131 440 9090 f. +44(0)131 440 9085 e. [log in to unmask] w. www.themcsgroup.co.uk -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna Sent: 23 November 2010 15:00 To: [log in to unmask] Subject: [TN] ImSn finish, BGA solderability Due to the holiday time crunch, I'm looking for some brainstorming on ImSn solderability forensics. I'm thinking that there may be some low hanging fruit that I'm missing because we just have little experience with ImSn. We've found random one-up Pb-free BGA balls not wetted to the lands. Finish is ImSn. Basic question in my mind is Sn oxides, SnCu intermetallics, or misprint. Representative bare boards passed J-STD-003 solderability with flying colors, but it is a double sided SMT. Haven't tried after reflow simulation yet. Both the assemblies and representative bare boards are now naturally aged, so SERA wouldn't quantify the problem as it existed. Would it? I've noted the coincidence of eggshell voids with the non-wet balls. Doesn't that imply some paste/flux? Does it imply anything else? Are the volatiles more likely to form eggshells when they are burning through Sn oxides? Here are some pics: http://ipc-technet.groupsite.com/gallery/16789 Thanks! Chris ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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