Black oxide to make handling easier, piling up around nailheads, proving cleanliness of surfaces !!!!!!!!! Time to stop old wives tales in their tracks before more people who don't fab boards start believing this c**p. Multilayer boards require a key for the inner layer copper to adhere to the B stage prepreg RESIN (glass is not sticky). In the early days this key was provided by either brushing the cores or by using double treat copper. Double treat copper has had a roughened surface grown onto the copper by electroplating means, and due to that extra processing is more expensive than the standard foil, which has a rough side (used for the same purpose by the laminators) and a shiny side or 'drum' side. Handling of double treat cores is a problem because finger grease is not removed as no further surface prep is needed (otherwise you wouldn't use double treat!). Brushing the shiny side gives a rougher surface and creates a mechanical bond during multilayer lamination. Brushing has a down side, - as cores got thinner, the material became more difficult to handle through a brushing machine, and the copper on those thin cores then stretched, which caused problems with material shift at bonding. The next option was to use pumice, but this had similar problems with thin core handling and machines self destructing. A chemical solution was the obvious choice. Oxidising the layers gives a surface roughness which encourages a mechanical bond to the resin system, and a weak atomic bond between the resin and the copper oxide. The process of black oxide also includes a preclean stage which removes grease and resist residues. There is a problem with black oxide, in as much as it will continue to grow in the bath and if you have too much, the crystals break during the pressure part of the bonding cycle and suddenly you have poor adhesion again! Brown oxide was brought about specifically for bonding high Tg materials such as polyimide, cyanate ester and BTepoxy which have a different affinity to standard epoxy. It is basically a different crystal structure which is what gives it a brown appearance. Pink ring hit us all in the late 80's partly due to drilling smaller holes through solid planes, but the solution was to change to reduced black oxide ( reduced back to copper). The reduction process maintained the crystal structure grown onto the copper but would not be attacked in the same way as the oxide. Other systems have also been used for creating bond adhesion including Durabond which is an immersion tin process - this time trying to improve adhesion by using chemical bonds, and today there are a number of microetch systems which create the mechanical surface on the copper surface by purely chemical means, but allow for a long hold time between processing and bonding - reduced oxide reoxidises over time. So, the purpose of oxide was not to prove surface cleanliness or to improve handling, nor does it 'pile' up around nailheads causing reliability issues. It is there to promote adhesion of copper foil to resin by creating a mechanical structure. Lastly, a word about peel strength. Multilayer boards delaminate in a vertical direction, peel strengths are performed in a horizontal direction. If peel strength really did correlate to delamination tendency, then nobody would ever use black oxide. Peel strength was used as a marketing tool because it allowed certain suppliers to show how their system was better than someone elses. Unfortuneately, like many of these tests, end users believe the marketeers and specify a figure, without fully understanding the issues. I hope all this stops any further myths evolving on the net! Dougal Dougal Stewart Product Development Manager Viasystems Selkirk Ltd Selkirk, Scotland, TD7 5EJ Tel: +44 1750 21601 Fax:+44 1750 22513 email [log in to unmask] ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################