Hi Paul - And you thought solderability was a simple subject! The solderability comparison data you were looking at was developed using a wetting balance. During the wetting balance test the fluxed test sample is lowered into the solder pot (or some equipment uses a solder globule). As solder "wets" the surface in the case of a tin/lead finish or the finish diffuses in the case of gold (silver, etc.) an force measurement device in the wetting balance test head records the force values - basically Young's equation. The buoyancy of the test sample (the amount of molten solder the test sample bulk displaces) as it enters the solder bath does give the instrumentation a negative force but that can be calculated out of the measurement leaving you with a the wetting force. If a test sample is not solderable the end result would be a negative force as describe in the paper you read - no wetting occurred thus no force pulling down on the specimen to be recorded. An analogy would be dipping your finger coated with oil into soapy water - you wouldn't get the water to wet your finger until the soap dissolves the oil allowing the water to wet up your finger creating a positive wetting force. Take a look at chapter 7.2.1 Wetting Balance Method in Klein Wassink's book (ISBN 0-901150-14-2) for a better description and more information. In the gold case you mention if once the gold diffuses into the solder pot and the underlying substrate is not wettable then a negative force would result. Hope this helps. Dave Hillman Rockwell Collins [log in to unmask] PAUL BROWN <[log in to unmask]> on 06/15/99 02:42:17 PM Please respond to [log in to unmask] To: [log in to unmask] cc: Subject: [TN] negative force Hi all, I was just reading a paper which was comparing the solderability of different surface finishes. In it the show electroless nickel/ immersion gold to have a wetting force of 0 uN/mm at "2 sec" just after fabrication. this value becomes negative i.e -0.1 to -0.2 after reflow ect.... What does this mean? Does this mean that solder is actually repelled by Ni/Au finish? Is this only due to the short duration (2sec.) that the dip occurred? Does the value become positive; at say 5 seconds ? How does this affect a surface mount assembly, will parts be displaced prior to soldering? I was under the impression that gold was highly solderable......................I need help, but will settle for answers...............Paul Brown ################################################################ 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################