Hi again, to continue Dave's idea, and Werner's statement "there is no question that SMD SJ fail first", should we worry about voids? Did anybody see BGA joints failing because voiding? Thanks, Ioan > -----Original Message----- > From: Werner Engelmaier [SMTP:[log in to unmask]] > Sent: Thursday, October 17, 2002 1:05 PM > To: [log in to unmask] > Subject: Re: [TN] Z-axis BGA solder joint verification? Addendum - > Stress Riser Question > > Hi Dave, > For once your insight has deserted you. > You are right, that the failure location is the same whether you have SMD > or > NSMD. However, it is not the failure location per se, but the > time/stress-to-failure that determines whether there is a reliability > impact. > And there is no question that SMD SJ fail first, provided of course that > you > have good wetting. > Below s a solder joint where because of misregistration one side is SMD > and > the other is NSMD. The consequences of the SMD-stress concentration are > obvious. Depending on the severity of loading, NSMD SJs will give fatigue > lives of 1.25 to 3 times longer than SMD SJs, everything else being equal. > [Unable to display image] > As I have indicated in my previous e-mail, I am in agreement with you > regarding the pad geometry. > On the other hand, it is not so much "the minimal cross-section area of > the > solder joint is the real culprit," as is the difference between the > cross-section area near the interface relative to the cross-section area > for > the rest of the ball. In other words, a column is always better than a > ball > of equal height. > You write: "We have even attempted to force a stress riser generated crack > in > BGA solder joints by "dimpling" the center of the solder balls on a BGAs > outer row and then > subjected them to thermal cycling. The solder joints still cracked at the > solderball/component pad interface!" Unfortunately, because of time/money > constraints people do "Highly Accelerated Tests." In many cases these > tests > are not accelerating the dominant product damage mechanism(s), but create > earlier failures by making another damage mechanism or loading scheme > dominant. Also unfortunately, in most cases that does not result in a dif > ferent failure mode/location. Thus, improper conclusions are drawn > regarding > product behavior that are not justified. Your observation is a case in > point. > If you do T-cycling [forget about T-shock altogether, and be careful about > bend testig] at loading conditions similar to what actually happens with > product, you will come to different conclusions. I have run T-cycle > studies > with more than 100,000 cycles than ran for over 3 years and where not > everything failed even than. Nowadays nobody can afford to do this--this > was > only possible in the Bell Labs which ceased to exist around 1990. But > having > to do things 'quick and dirty' [emphasis is on 'dirty'] should not > encourage > the industry to draw unwarranted conclusions--thiis can be very dangerous. > > Werner Engelmaier > Engelmaier Associates, L.C. > Electronic Packaging, Interconnection and Reliability Consulting > 7 Jasmine Run > Ormond Beach, FL 32174 USA > Phone: 904-437-8747, Fax: 904-437-8737 > E-mail: [log in to unmask], Website: www.engelmaier.com > > --------------------------------------------------- > Technet Mail List provided as a free service by IPC using LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with following text in > the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send e-mail to > [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to > [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: http://listserv.ipc.org/archives > Please visit IPC web site http://www.ipc.org/html/forum.htm for additional > information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 > ext.5315 > ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------