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July 1997

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Tue, 29 Jul 1997 08:48:28 +0100
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Your lifted wedge bonds could have a number of causes. You don't
say if it is the gold, the aluminium wedges, or both that lift.
What is the gold thickness and what metal is underneath it. Very
thin immersion deposited gold (0.1 to 0.3 microns) are not gold
ball/wedge bondable with reasonable yields. I've not tried
wedge/wedge bonding gold, but this may well be the same. 



         Wire bonding requires cleanliness from the substrates. The          residues form no clean paste are difficult to remove. The alcohol          wash probably isn't making much of an impact. If these residues          are getting on the wire bond sites they can interfere with the          bond. Can you reverse the process flow and do the COB die attach          wire bond and glob top before attaching the SMD components?          If you do this and the COB components are on the same side of the          board as the SMT then you can't stencil print solder paste. You          will have to dispense solder paste instead. Reversing the flow          could also interfere with your test strategy as well, so it needs          some thinking about. Given the choice I would COB first, then          attach the surface mount parts.                         Plasma cleaning can be used to improve COB yields by removing          organics from the metal surfaces. Epoxy die attach materials can          out gas during cure, the molecules depositing themselves on the          wire bond sites. I don't know if plasma will remove no clean flux          residues.                         Look at the surface roughness of the wire bond sites. If the          surface is too rough then you will get some bonds lifting.                         A final set of causes could be the wire bond process itself.          Process parameters, wire and capillary selection, or a machine          fault could all contribute to the problem.                    Hope this helps.                    Jeremy Drake          Design to Distribution Ltd          Stoke on Trent          England.

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